The distribution and topographical organization in the thalamus of anterogradely-transported horseradish peroxidase after spinal injections in cat and raccoon

Neural origin of evoked potentials during thalamic deep brain stimulation

Closed-loop deep brain stimulation (DBS) systems could provide automatic adjustment of stimulation parameters and improve outcomes in the treatment of Parkinson's disease and essential tremor. The evoked compound action potential (ECAP), generated by activated neurons near the DBS electrode, may provide a suitable feedback control signal for closed-loop DBS. The objectives of this work were to characterize the ECAP across stimulation parameters and determine the neural elements contributing to the signal. We recorded ECAPs during thalamic DBS in anesthetized cats and conducted computer simulations to calculate the ECAP of a population of thalamic neurons. The experimental and computational ECAPs were similar in shape and had characteristics that were correlated across stimulation parameters (R(2) = 0.80-0.95, P < 0.002). The ECAP signal energy increased with larger DBS amplitudes (P < 0.0001) and pulse widths (P < 0.002), and the signal energy of secondary ECAP phases was larger at 10-Hz than at 100-Hz DBS (P < 0.002). The computational model indicated that these changes resulted from a greater extent of neural activation and an increased synchronization of postsynaptic thalamocortical activity, respectively. Administration of tetrodotoxin, lidocaine, or isoflurane abolished or reduced the magnitude of the experimental and computational ECAPs, glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphonopentanoic acid (APV) reduced secondary ECAP phases by decreasing postsynaptic excitation, and the GABAA receptor agonist muscimol increased the latency of the secondary phases by augmenting postsynaptic hyperpolarization. This study demonstrates that the ECAP provides information about the type and extent of neural activation generated during DBS, and the ECAP may serve as a feedback control signal for closed-loop DBS.

Distribution of trigeminothalamic and spinothalamic lamina I terminations in the cat

The distribution in the thalamus of terminal projections from lamina I neurons of the trigeminal, cervical, and lumbosacral dorsal horn was investigated with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in the cat. Iontophoretic injections were guided by single- and multi-unit physiological recordings. The injections in particular cases were essentially restricted to lamina I, whereas in others they spread across laminae I-III or laminae I-V. The trigemino- and spinothalamic (TSTT) terminations were identified immunohistochemically. In all cases, regardless of the level of the injections, terminal fibers were consistently distributed in three main locations: the submedial nucleus; the ventral aspect of the basal ventral medial nucleus and ventral posterior nuclei; and, the dorsomedial aspect of the ventral posterior medial nucleus. The terminal fields in the submedial nucleus and the ventral aspect of the ventral posterior group were topographically organized. Terminations along the ventral aspect of the ventral posterior group extended posterolaterally into the caudal part of the posterior nucleus and anteromedially into the ventromedial part of the ventral lateral nucleus. In several cases with trigeminal lamina I injections, a terminal labeling patch was observed within the core of the ventral posterior medial nucleus. In cases with spinal lamina I injections, terminations were also consistently found in the lateral habenula, the parafascicular nucleus, and the nucleus reuniens. Isolated terminal fibers were occasionally seen in the zona incerta, the dorsomedial hypothalamus, and other locations. These anatomical observations extend prior studies of TSTT projections and identify lamina I projection targets that are important for nociceptive, thermoreceptive, and homeostatic processing in the cat. The findings are consistent with evidence from physiological (single-unit and antidromic mapping) and behavioral studies. The novel identification of spinal lamina I input to the lateral habenula could be significant for homeostatic behaviors.

Central lateral thalamic neurons receive noxious visceral mechanical and chemical input in rats

Thalamic intralaminar and medial nuclei participate mainly in affective and motivational aspects of pain processing. Unique to the present study were identification and characterization of spontaneously active neurons in the central lateral nucleus (CL) of the intralaminar thalamus, which were found to respond only to viscerally evoked noxious stimuli in animals under pentobarbital anesthesia. Responses to noxious colorectal distention, intrapancreatic bradykinin, intraperitoneal dilute acetic acid, and greater splanchnic nerve electrical stimulation were characterized. Electrophysiological recordings revealed activity in most CL neurons (93%) was excited (69%) or inhibited (31%) in response to noxious visceral stimulation of visceral nerves. Expression of c-Fos observed in CL nucleus after intensive visceral stimulation confirmed the activation. However, excited CL neurons did not have somatic fields, except in 3 of 43 (7%) CL neurons tested for responses to somatic stimulation (innocuous brush and noxious pinch). Intrathecal administration of morphine significantly reduced the increased responses of CL neurons to colorectal and pancreatic stimuli and was naloxone reversible. High-level thoracic midline dorsal column (DC) myelotomy also dramatically reduced responses, identifying the DC as a major route of travel from the spinal cord for CL input, in addition to input traveling ventromedially in the spinothalamic tract identified anatomically in a previous study. Spinal cord and lower brain stem cells providing input to medial thalamus were mapped after stereotaxic injections of a retrograde dye. These data combined with our previous data suggest that the CL nucleus is an important component of a medial visceral nociceptive system that may mediate attentional, affective, endocrine, motor, and autonomic responses to noxious visceral stimuli.

Brainstem and thalamic projections from a craniovascular sensory nervous centre in the rostral cervical spinal dorsal horn of rats

To examine the ascending projections from the headache-related trigeminocervical complex in rats, biotinylated dextran amine (BDA) was injected into the ventrolateral dorsal horn of segments C1 and C2, a region previously demonstrated to receive input from sensory nerves in cranial blood vessels. Following injections into laminae I-II, BDA-labelled terminations were found bilaterally in several nuclei in the pons and the midbrain, including the pontine reticular nucleus, the parabrachial nuclei, the cuneiform nucleus and the periaqueductal grey. In the diencephalon, terminations were confined to the contralateral side and evident foremost in the posterior nuclear group, especially its triangular part, and in the ventral posteromedial nucleus. Following injections extending through laminae I-IV, anterograde labelling was more extensive. Some of the above regions are likely to be involved in the central processing of noxious signals of craniovascular origin and therefore putatively involved in mechanisms associated with primary headaches.

Projections of somatosensory cortex and frontal eye fields onto incertotectal neurons in the cat

The goal of this study was to determine whether the input-output characteristics of the zona incerta (ZI) are appropriate for it to serve as a conduit for cortical control over saccade-related activity in the superior colliculus. The study utilized the neuronal tracers wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and biotinylated dextran amine (BDA) in the cat. Injections of WGA-HRP into primary somatosensory cortex (SI) revealed sparse, widespread nontopographic projections throughout ZI. In addition, region-specific areas of more intense termination were present in ventral ZI, although strict topography was not observed. In comparison, the frontal eye fields (FEF) also projected sparsely throughout ZI, but terminated more heavily, medially, along the border between the two sublaminae. Furthermore, retrogradely labeled incertocortical neurons were observed in both experiments. The relationship of these two cortical projections to incertotectal cells was also directly examined by retrogradely labeling incertotectal cells with WGA-HRP in animals that had also received cortical BDA injections. Labeled axonal arbors from both SI and FEF had thin, sparsely branched axons with numerous en passant boutons. They formed numerous close associations with the somata and dendrites of WGA-HRP-labeled incertotectal cells. In summary, these results indicate that both sensory and motor cortical inputs to ZI display similar morphologies and distributions. In addition, both display close associations with incertotectal cells, suggesting direct synaptic contact. From these data, we conclude that inputs from somatosensory and FEF cortex both play a role in controlling gaze-related activity in the superior colliculus by way of the inhibitory incertotectal projection.

Thermally identified subgroups of marginal zone neurons project to distinct regions of the ventral posterior lateral nucleus in rats

Spinal marginal zone (MZ) neurons play a crucial role in the transmission of nociceptive and thermoreceptive information to the brain. The precise areas to which physiologically characterized MZ neurons project in the ventral posterior lateral (VPL) nucleus of the thalamus have not been clearly established. Here, we examine this projection in rats using the method of antidromic activation to map the axon terminals of neurons recorded from the MZ. Thirty-three neurons were antidromically activated using pulses of < or =30 microA in the contralateral VPL. In every case, the most rostral point from which the MZ neuron could be antidromically activated was surrounded by stimulating tracks in which large-amplitude current pulses failed to activate the examined neuron, indicating the termination of the spinothalamic tract (STT) axon. Each of 30 examined neurons responded to noxious but not innocuous mechanical stimuli applied to their cutaneous receptive fields, which ranged in size from two digits to the entire limb. Of 17 thermally tested neurons, 16 responded to innocuous or noxious thermal stimuli. Among STT neurons that responded to thermal stimuli, 50% responded to innocuous cooling as well as noxious heat and cold, 31% responded to noxious heat and cold, and 19% responded only to noxious heat. Axons from cells responsive to innocuous cooling terminated in the core region of VPL, significantly dorsal and medial relative to other thermally responsive subgroups. In rats, thermally responsive subgroups of MZ neurons project directly to distinct regions of VPL.

Segmental and laminar organization of the spinothalamic neurons in cat: evidence for at least five separate clusters

The spinothalamic tract (STT), well known for its role in the relay of information about noxe, temperature, and crude touch, is usually associated with projections from lamina I, but spinothalamic neurons in other laminae have also been reported. In cat, no complete overview exists of the precise location and number of spinal cells that project to the thalamus. In the present study the laminar distribution of retrogradely labeled cells in all spinal segments (C1-Coc2) was investigated after large WGA-HRP injections in the thalamus. The results show that this distribution of STT cells differed greatly between the different spinal segments. Quantitative analysis showed that there exist at least five separate clusters of spinothalamic neurons. Lamina I neurons in cluster A and lamina V neurons in cluster B are mainly found contralaterally throughout the length of the spinal cord. Cluster C neurons are located bilaterally in the ventrolateral part of laminae VI-VII and lamina VIII of the C1-C3 spinal cord. Cluster D neurons were found contralaterally in lamina VI in the C1-C2 segments, and cluster E neurons were located mainly contralaterally in the medial part of laminae VI-VII and lamina VIII of the lumbosacral cord. Most spinothalamic neurons are not located in the enlargements and most spinothalamic neurons are not located in lamina I, as suggested by several other authors. The location of the spinothalamic neurons shows remarkable similarities, but also differences, with the location of spino-periaqueductal gray neurons.

Distribution of trigeminothalamic and spinothalamic lamina I terminations in the macaque monkey

Thalamic terminations from trigeminal, cervical, and lumbosacral lamina I neurons were investigated with Phaseolus vulgaris leucoagglutinin (PHA-L) and labeled dextrans. Iontophoretic injections guided by physiological recordings were restricted to lamina I or laminae I-II. PHA-L-labeled trigemino- and spinothalamic (TSTT) terminations were identified immunohistochemically. TRITC- and FITC-labeled dextrans were injected at different levels to confirm topography. Terminations consistently occurred in two main locations: a distinguishable portion of posterolateral thalamus identified cytoarchitectonically as the posterior part of the ventral medial nucleus (VMpo) and a portion of posteromedial thalamus designated as the ventral caudal part of the medial dorsal nucleus (MDvc). In addition, isolated fibers bearing boutons of passage were observed in the ventral posterior medial and lateral (VPM and VPL) nuclei, and spinal terminations occurred in the ventral posterior inferior nucleus (VPI). Isolated terminations occasionally occurred in other sites (e.g., suprageniculate, zona incerta, hypothalamic paraventricular n.). Terminations in MDvc occurred in concise foci that were weakly organized topographically (posteroanterior = rostrocaudal). Terminations in VMpo consisted of dense clusters of ramified terminal arbors bearing multiple large boutons that were well organized topographically (anteroposterior = rostrocaudal). Terminations in VMpo colocalized with a field of calbindin-immunoreactive terminal fibers; double-labeled terminals were documented at high magnification. This propitious marker was especially useful at anterior levels, where VMpo can easily be misidentified as VPM. These findings demonstrate phylogenetically novel primate lamina I TSTT projections important for sensory and motivational aspects of pain, temperature, itch, muscle ache, sensual touch, and other interoceptive feelings from the body.

Differentiation of lamina I spinomedullary and spinothalamic neurons in the cat

We characterized spinomedullary neurons that project to the ventrolateral portion of the medulla that receives lamina I terminations in two sets of experiments in the cat. First, their distribution was examined using single unilateral iontophoretic injections of cholera toxin subunit B. The injection sites were characterized by microelectrode recordings from nociceptive- and thermoreceptive-specific units, indicative of lamina I input. The spinomedullary neurons were symmetrically distributed bilaterally, predominantly (63-69%) in lamina I but also in laminae V-VIII and the thoracic lateral horn (intermediolateral cell column). In horizontal sections, spinomedullary lamina I neurons included all three main morphological types described earlier. Second, spinomedullary and spinothalamic neurons were compared in retrograde double-labeling experiments. Different combinations of tracers were injected in the right thalamus and the left or right ventrolateral medulla (guided by recordings). The numbers of spinomedullary and spinothalamic neurons on the left side were comparable, and the segmental and laminar distributions were similar, except that a greater proportion of spinomedullary neurons originated from thoracic segments. However, the proportion of double-labeled neurons was consistently approximately 1%, indicating that spinomedullary and spinothalamic pathways arise from separate subpopulations. Spinomedullary neurons were more ventrally located within lamina I than spinothalamic neurons. A significantly greater proportion of spinomedullary neurons had fusiform somata (49% vs. 36%). These observations indicate that lamina I is the major source of spinal input to this portion of the ventrolateral medulla, that the projection includes several morphological types of inputs, and that this projection is distinct from the spinothalamic projection. These findings are consistent with the concept that lamina I projections constitute an ascending homeostatic afferent pathway relating the physiological condition of the body.

A comparative reappraisal of projections from the superficial laminae of the dorsal horn in the rat: The forebrain

Projections to the forebrain from lamina I of spinal and trigeminal dorsal horn were labeled anterogradely with Phaseolus vulgaris-leucoagglutinin (PHA-L) and/or tetramethylrhodamine-dextran (RHO-D) injected microiontophoretically. Injections restricted to superficial laminae (I/II) of dorsal horn were used primarily. For comparison, injections were also made in deep cervical laminae. Spinal and trigeminal lamina I neurons project extensively to restricted portions of the ventral posterolateral and posteromedial (VPL/VPM), and the posterior group (Po) thalamic nuclei. Lamina I also projects to the triangular posterior (PoT) and the ventral posterior parvicellular (VPPC) thalamic nuclei but only very slightly to the extrathalamic forebrain. Furthermore, the lateral spinal (LS) nucleus, and to a lesser extent lamina I, project to the mediodorsal thalamic nucleus. In contrast to lamina I, deep spinal laminae project primarily to the central lateral thalamic nucleus (CL) and only weakly to the remaining thalamus, except for a medium projection to the PoT. Furthermore, the deep laminae project substantially to the globus pallidus and the substantia innominata and more weakly to the amygdala and the hypothalamus. Double-labeling experiments reveal that spinal and trigeminal lamina I project densely to distinct and restricted portions of VPL/VPM, Po, and VPPC thalamic nuclei, whereas projections to the PoT appeared to be convergent. In conclusion, these experiments indicate very different patterns of projection for lamina I versus deep laminae (III-X). Lamina I projects strongly onto relay thalamic nuclei and thus would have a primary role in sensory discriminative aspects of pain. The deep laminae project densely to the CL and more diffusely to other forebrain targets, suggesting roles in motor and alertness components of pain.

Response of neurons in the thalamic nucleus submedius (Sm) to noxious stimulation and electrophysiological identification of on- and off-cells in rats

Previous studies have indicated that thalamic nucleus submedius (Sm) is involved in nociceptive modulation and plays an important role in an endogenous analgesic system (a feedback loop) consisting of spinal cord (Sc)-Sm-ventrolateral orbital cortex-periaqueductal gray-Sc. However, the function of different types of Sm neurons in nociceptive modulation is unclear. For this reason, on the basis of further studies of properties of the Sm neurons responding to noxious stimuli, the different effects of systemic morphine on the Sm neurons were examined and two classes of nociceptive modulatory neurons, named as off- and on-cells, in this region were identified in lightly anesthetized rats. The results showed that (1) most (84%, 132/157) of the Sm neurons responded to peripheral noxious stimuli. Of these neurons, 66% (n = 87) were inhibited, 34% (n = 45) excited. All neurons had very large and bilateral, even all body receptive fields. No neuron was found to be responsive to innocuous stimulation; (2) systemic morphine increased the firing rate of neurons inhibited by noxious stimulation, but decreased that of neurons excited by the same stimulation. Furthermore, the effects of morphine could be reversed by systemic naloxone; (3) 45 of Sm neurons examined could be divided into three different classes: off-cells that decreased the firing rate from tail heating just prior to occurrence of the tail-flick (TF) reflex (3140 +/- 167 ms, n = 27), on-cells that increased the firing rate just before the TF reflex (1720 +/- 240 ms, n = 8), and neutral-cells that did not respond to any stimuli and neuronal activities were not related to the TF reflex (n = 10). Findings of this study provided electrophysiological evidence for involvement of Sm neurons, as those in the rostral ventromedial medulla, in the opioid-receptor-mediated descending nociceptive modulation.

Differential projections of thermoreceptive and nociceptive lamina I trigeminothalamic and spinothalamic neurons in the cat

The projections of 40 trigeminothalamic or spinothalamic (TSTT) lamina I neurons were mapped using antidromic activation from a mobile electrode array in barbiturate anesthetized cats. Single units were identified as projection cells from the initial array position and characterized with natural cutaneous stimuli as nociceptive-specific (NS, n = 9), polymodal nociceptive (HPC, n = 8), or thermoreceptive-specific (COOL, n = 22; WARM, n = 1) cells. Thresholds for antidromic activation were measured from each electrode in the mediolateral array at vertical steps of 250 microm over a 7-mm dorsoventral extent in two to eight (median = 6.0) anteroposterior planes. Histological reconstructions showed that the maps encompassed all three of the main lamina I projection targets observed in prior anatomical work, i.e., the ventral aspect of the ventroposterior complex (vVP), the dorsomedial aspect of the ventroposterior medial nucleus (dmVPM), and the submedial nucleus (Sm). The antidromic activation foci were localized to these sites (and occasional projections to other sites were also observed, such as the parafascicular nucleus and zona incerta). The projections of thermoreceptive and nociceptive cells differed. The projections of the thermoreceptive-specific cells were 20/23 to dmVPM, 21/23 to vVP, and 17/23 to Sm, whereas the projections of the NS cells were 1/9 to dmVPM, 9/9 to vVP, and 9/9 to Sm and the projections of the HPC cells were 0/8 to dmVPM, 7/8 to vVP, and 6/8 to Sm. Thus nearly all thermoreceptive cells projected to dmVPM, but almost no nociceptive cells did. Further, thermoreceptive cells projected medially within vVP (including the basal ventral medial nucleus), while nociceptive cells projected both medially and more laterally, and the ascending axons of thermoreceptive cells were concentrated in the medial mesencephalon, while the axons of nociceptive cells ascended in the lateral mesencephalon. These findings provide evidence for anatomical differences between these physiological classes of lamina I cells, and they corroborate prior anatomical localization of the lamina I TSTT projection targets in the cat. These results support evidence indicating that the ventral aspect of the basal ventral medial nucleus is important for thermosensory behavior in cats, consistent with the view that this region is a primordial homologue of the posterior ventral medial nucleus in primates.

Morphological features of cat cervicothalamic tract terminations in different target regions

Using biotinylated dextran amine to label cervicothalamic tract terminations of cats, three types of terminal arrangements were recognized. The ventral posterior lateral nucleus contains the largest proportion of the cervicothalamic tract terminals (79%) and most (72%) of these are type I terminals (form compact clusters of 5-30 boutons). In contrast, type II (form less compact clusters of 3-10 boutons) and type III (widely spaced boutons along thin axons) terminals dominate in the medial nucleus of the posterior complex (78%) and in the ventral periphery of the ventrobasal complex (86%). In the magnocellular medial geniculate nucleus, type I terminals (38%) are found close to medially located clusters of Cat-301 immunoreactive neurons, whereas type II and type III terminals locate in the surrounding Cat-301-negative regions. These findings indicate a high degree of synaptic security in the transmission between cervicothalamic tract fibers and neurons in the ventral posterior lateral nucleus and highlight the role of this nucleus in faithful transmission of cervicothalamic tract input to the cerebral cortex. Also, the Cat-301-positive neurons in the magnocellular medial geniculate nucleus may faithfully transmit cervicothalamic tract signals. The domination of type II and type III terminals in the medial nucleus of the posterior complex and in the ventral periphery of the ventrobasal complex indicates a more divergent cervicothalamic input to these regions, in line with the large receptive fields and multimodal responses of neurons in the posterior complex.

Behavioral thermosensitivity after lesions of thalamic target areas of a thermosensory spinothalamic pathway in the cat

The ability of 17 cats to discriminate floor temperatures 2-4 degrees C below the ambient temperature was tested before and after unilateral electrolytic thalamic lesions. The lesions were made contralateral to the paws showing better performance in the temperature discrimination task. They were aimed at one or more of the three main target areas of thermoreceptive-specific lamina I spinothalamic neurons [i.e., the nucleus submedius, the dorsomedial aspect of the ventral posterior medial nucleus, and the ventral aspect of the basal ventral medial nucleus (vVMb)], following microelectrode mapping of somatosensory thalamus. The thermosensory consequences of each lesion were measured in postoperative testing, beginning 6-8 days after the final preoperative test session. A mild but definite thermosensory deficiency was found in five cats, in which the response behavior on the contralateral side was reduced below the 69% criterion level for several sessions. Histological analysis indicated that these cats differed only by the inclusion in the lesion of all or part of vVMb. Consequently this area appears to be important for cats' thermosensory behavior. Nevertheless even large lesions of virtually all of the thermoreceptive lamina I spinothalamic projection areas produced only this mild thermosensory deficit in stark contrast with the massive defect observed previously after spinal lesions of the middle of the lateral funiculus, where lamina I axons ascend. Accordingly such spinal lesions were added at the C(4) level, on the same side as the thalamic lesions, in six cats 3 mo after the thalamic surgery. These lesions caused severe contralateral defects (i.e., chance level performance). Thus the present findings are taken to indicate that contralateral ascending projections to vVMb in the thalamus participate in cats' thermosensory discrimination but that ascending projections to the brain stem must play an important role in their behavioral thermosensitivity.

The effect of morphine on responses of nucleus ventroposterolateralis neurons to colorectal distension in the rat

In 71 halothane-anesthetized rats, we characterized the responses of single neurons in the nucleus ventroposterolateralis (VPL) of the thalamus to a noxious visceral stimulus (colorectal balloon distension; CRD) and studied the effects of intravenous morphine on these responses using standard extracellular microelectrode recording techniques. One hundred nine neurons were isolated on the basis of spontaneous activity. Sixty-four (59%) responded to CRD, of which 52 (81 %) had excitatory and 12 (19%) had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 80 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 95 of the VPL neurons isolated. Eighty-three of these neurons (48 CRD responsive and 35 CRD nonresponsive) (87%) had cutaneous receptive fields, of which 96% were small and contralateral and 4% were large and contralateral or bilateral. Ninety-four percent of these neurons responded to both noxious and innocuous cutaneous stimulation, and 6% responded to only noxious stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.125, 0.25, 0.5, 1, and 2 mg/kg, i.v) produced statistically significant dose-dependent attenuation of neuronal responses to CRD. Naloxone (0.4 mg/ kg, i.v.) reversed the effects of morphine. Morphine and naloxone had no significant effects on spontaneous activity. These data support the involvement of VPL neurons in visceral nociception and are consistent with a role of VPL in sensory-discriminative aspects of nociception.

The effect of morphine on responses of mediodorsal thalamic nuclei and nucleus submedius neurons to colorectal distension in the rat

In halothane-anesthetized rats, we characterized the responses of single neurons in the nuclei of medial thalamus (MT), specifically the mediodorsal thalamic nucleus (MD) and the nucleus submedius (Sm), to a noxious visceral stimulus (colorectal balloon distension, CRD), and studied the effects of intravenous morphine (Mor) on these responses using standard extracellular microelectrode recording techniques. 62 MD and 46 Sm neurons were isolated on the basis of spontaneous activity. 47 of the MD neurons (76%) responded to CRD, of which 70% had excitatory and 30% had inhibitory responses. 38 of the Sm neurons (83%) responded to CRD, of which 89% had excitatory and 11% had inhibitory responses. Responses of MD and Sm neurons excited by CRD were related significantly to distension pressure (20-100 mmHg), with maximum excitation occurring at 60 and 100 mmHg, respectively. MD neurons inhibited by CRD also had graded responses to graded CRD, with maximum inhibition occurring at 80 mmHg. The responses to noxious (pinch, heat) and nonnoxious (tap, brush) cutaneous stimuli were studied in 59 of the MD and 44 of the Sm neurons isolated. 22 of the MD neurons (37%) studied had cutaneous receptive fields, of which 59% were large and bilateral, 41% were small and usually contralateral receptive fields. 55% of these neurons were nociceptive-specific, 45% responded to both noxious and nonnoxious cutaneous stimulation. 29 of the Sm neurons (66%) studied had cutaneous receptive fields, of which 72% were large and usually bilateral, 14% were small and bilateral, 14% were small and contralateral receptive fields. 90% of these neurons were nociceptive-specific, 10% responded to both noxious and nonnoxious stimulation. No MD or Sm neurons responded exclusively to nonnoxious cutaneous stimulation. Mor (0.125, 0.25, 0.5 and 1 mg/kg I.V.) attenuated MD and Sm neuronal excitatory responses to CRD in a dose-dependent fashion, abolishing evoked activity with a dose of 0.5 mg/kg (p < 0.05) and 1 mg/kg (p < 0.05), respectively. Naloxone (0.4 mg/kg I.V.) reversed the effects of Mor. Mor and naloxone had no effects on spontaneous activity. These data support the involvement of MD and Sm neurons in visceral nociception, and are consistent with a role of Sm in affective-motivational, and MD in both sensory-discriminative and affective-motivational aspects of nociception.

Insular cortex and neighboring fields in the cat: a redefinition based on cortical microarchitecture and connections with the thalamus

The insular areas of the cerebral cortex in carnivores remain vaguely defined and fragmentarily characterized. We have examined the cortical microarchitecture and thalamic connections of the insular region in cats, as a part of a broader study aimed to clarify their subdivisions, functional affiliations, and eventual similarities with other mammals. We report that cortical areas, which resemble the insular fields of other mammals, are located in the cat's orbital gyrus and anterior rhinal sulcus. Our data suggest four such areas: (a) a "ventral agranular insular area" in the lower bank of the anterior rhinal sulcus, architectonically transitional between iso- and allocortex and sparsely connected to the thalamus, mainly with midline nuclei; (b) a "dorsal agranular insular area" in the upper bank of the anterior rhinal sulcus, linked to the mediodorsal, ventromedial, parafascicular and midline nuclei; (c) a "dysgranular insular area" in the anteroventral half of the orbital gyrus, characterized by its connections with gustatory and viscerosensory portions of the ventroposterior complex and with the ventrolateral nucleus; and (d) a "granular insular area", dorsocaudal in the orbital gyrus, which is chiefly bound to spinothalamic-recipient thalamic nuclei such as the posterior medial and the ventroposterior inferior. Three further fields are situated caudally to the insular areas. The anterior sylvian gyrus and dorsal lip of the pseudosylvian sulcus, which we designate "anterior sylvian area", is connected to the ventromedial, suprageniculate, and lateralis medialis nuclei. The fundus and ventral bank of the pseudosylvian sulcus, or "parainsular area", is associated with caudal portions of the medial geniculate complex. The rostral part of the ventral bank of the anterior ectosylvian sulcus, referred to as "ventral anterior ectosylvian area", is heavily interconnected with the lateral posterior-pulvinar complex and the ventromedial nucleus. Present results reveal that these areas interact with a wide array of sensory, motor, and limbic thalamic nuclei. In addition, these data provide a consistent basis for comparisons with cortical fields in other mammals.

Diencephalic connections of the superior colliculus in the hedgehog tenrec

Using different tracer substances the pathways connecting the superior colliculus with the diencephalon were studied in the Madagascan hedgehog tenrec (Echinops telfairi), a nocturnal insectivore with tiny eyes, a small and little differentiated superior colliculus and a visual cortex with no obvious fourth granular layer. The most prominent tecto-thalamic projection terminated in the ipsilateral dorsal lateral geniculate nucleus. The entire region receiving contralateral retinal afferents was labeled with variable density. In addition, there was a widespread, homogeneously distributed collicular input to the lateralis posterior-pulvinar complex and a distinct tectal projection to the suprageniculate nucleus. The latter projections were bilateral with a clear ipsilateral predominance. Among the intra- and paralaminar nuclei the centralis lateralis complex was most heavily labeled on both sides, followed by the nucleus centralis medialis. The paralamellar portion of the nucleus medialis dorsalis and the nucleus parafascicularis received sparse projections. A clear projection to the nucleus ventralis medialis could not be demonstrated but its presence was not entirely excluded either. There were also projections to medial thalamic nuclei, particularly the reuniens complex and the nucleus paraventricularis thalami. The main tecto-subthalamic target regions were the zona incerta, the dorsal hypothalamus and distinct subdivisons of the ventral lateral geniculate nucleus. These regions also gave rise to projections to the superior colliculus, as did the intergeniculate leaflet. The pathways oriented toward the visual or frontal cortex and the projections possibly involved in limbic and circadian mechanisms were compared with the connectivity patterns reported in mammals with more differentiated brains. Particular attention was given to the tenrec's prominent tecto-geniculate projection, the presumed W- or K-pathway directed toward the supragranular layers.

Direct spinal projections to limbic and striatal areas: anterograde transport studies from the upper cervical spinal cord and the cervical enlargement in squirrel monkey and rat

With the anterograde tracers Phaseolus vulgaris-leucoagglutinin (PHA-L) and biotinylated dextranamine (BD), direct spinal connections from the upper cervical spinal cord (UC; C1 and C2) and the cervical enlargement (CE; C5-T1) were demonstrated in various striatal and limbic nuclei in both squirrel monkey and rat. Within each species and from each spinal level, the total number of terminals seen in the limbic and striatal areas was approximately 50-80% of the number seen within the thalamus. Labeled terminal structures were seen in the hypothalamic nuclei, ventral striatum, globus pallidus, amygdala, preoptic area, and septal nuclei. In both species, the number of labeled terminals in limbic and striatal regions was larger from UC than from CE, although the distributions to each nucleus varied with the specific lamina injected. In both species and from both UC and CE, approximately one-half of the projections to striatal and limbic areas terminated in the hypothalamus. The only region that demonstrated a topographical organization was the globus pallidus, where terminals from the CE were located dorsomedially to those from the UC. In the rat, UC and CE injections into the lateral dorsal horn and pericentral laminae resulted in the largest number of limbic and striatal terminations. The proportion of ipsilateral terminations was greatest when the medial laminae in the UC or the lateral dorsal horn in the CE received injections. Analysis of the morphology of these spinohypothalamic and spinotelencephalic terminals showed that, in the squirrel monkey, terminals from CE injections were larger than terminals from UC injections; no such size difference was evident in the rat. However, limbic and striatal terminals in the rat were generally larger than those in the squirrel monkey following injections into the UC or CE. The exact function of these direct spinal projections to various striatal and limbic areas in primates and in rodents remains to be determined. These findings, however, support recent imaging studies that suggest that the limbic system plays an important role in the mediation of chest pain, perhaps directly through these spinolimbic and spinostriatal pathways.

Changes in the organization of the ventroposterior lateral thalamic nucleus after digit removal in adult raccoon

The ventroposterior lateral nucleus of the thalamus was studied in seven raccoons that had undergone amputation of the fourth digit between 2 and 5 months previously. Extracellular recordings were made in a series of closely spaced penetrations through the thalamus in chloralose anesthetized animals. The responses to cutaneous stimulation of the forepaw were used to reconstruct the somatotopic organization of the thalamus and to identify recording sites believed to be located in the digit zone that had lost its peripheral input. Twelve penetrations that passed through both of the adjacent fifth and third digit regions were analyzed in detail to delineate this deafferented region. None of the recording sites in this region were completely silent, indicating that the deafferented thalamus had undergone significant reorganization of its inputs. At most sites, the neurons had receptive fields on the skin surrounding the amputation wound and including one of the adjacent digits. Approximately half of the sites had low thresholds in the range of normal thalamic neurons. These results indicate that the ventroposterior thalamus is capable of substantial reorganization, which may account for much of the reorganization seen in somatosensory cortex.

The raccoon spinocervical and spinothalamic tracts: a horseradish peroxidase study

The locations of the cells of origin of the spinocervical tract (SCT) and spinothalamic tract (STT) were examined in relation to the somatotopic and laminar organization of the cervical enlargement of the raccoon dorsal horn (DH). In different animals, either the lateral cervical nucleus or the lateral thalamus was injected with a 2% solution of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Following 24 h or 4 days, respectively, the animals were sacrificed and both injection and target sites (spinal cord segments C6-T2) were processed using the TMB method. All labelled cells were counted in every fifth 50 microns section. Following injection of WGA-HRP into the lateral cervical nucleus, all labelled SCT cells were located ipsilateral to the injection sites. Most (84%) were in laminae III and IV, laminae known from other studies to contain cells preferentially responsive to light tactile stimulation, with very few (3%) in lamina I. Nearly 50% of labelled cells were located in the medial 1/3 of the DH, the region of representation of the glabrous surfaces of the raccoon forepaw. The mean number of labelled SCT cells per section was 4.19. After tracer injections of the lateral thalamus, more than 75% of STT cells were located contralateral to the injection sites. Forty-three percent were located in lamina I and 24% were in lamina V, laminae whose cells have been shown to be responsive to more intense forms of stimulation, as well as to light touch. Only 22% were located in the medial 1/3 of the DH. The mean number of labelled STT cells per section was 0.83. The results suggest that the SCT may play a more critical role in relaying discriminative light tactile and nociceptive information from the glabrous surfaces of the forepaw, but that there may be a greater role for the STT in relaying nociceptive information from the forelimb as a whole.

Evidence for glutamate as neurotransmitter in trigemino-and spinothalamic tract terminals in the nucleus submedius of cats

The nucleus submedius in the medial thalamus of cats is an important termination site for lamina I trigemino-and spinothalamic tract (TSTT) neurons, many of which are nociceptive-specific, and the nucleus submedius has been proposed to be a dedicated nociceptive substrate involved in the affective aspect of pain. In the present study, the distribution of glutamate was examined by immunocytochemical methods in order to evaluate the possible role of this amino acid as a neurotransmitter in TSTT terminals in the nucleus submedius. TSTT terminals were identified by anterograde transport of horseradish peroxidase and wheatgerm agglutinin-horseradish peroxidase conjugate from the spinal cord or the medullary dorsal horn. Quantitative analysis of immunogold labelling revealed that TSTT terminals contain about twice the tissue average of glutamate-like immunoreactivity. A strong positive correlation was found between the density of synaptic vesicles and the density of gold particles in these terminals, whereas no relationship was seen between these variables in GABAergic presynaptic dendrites. Enrichment of glutamate-like immunoreactivity (approximately 250% of the tissue average) was also observed in terminals of presumed cortical origin. Presynaptic dendrites and neuron cell bodies in the nucleus submedius were found to contain relatively low levels of glutamate-like immunoreactivity, at or below the tissue average. These observations provide evidence that glutamate is a neurotransmitter in lamina I TSTT terminals in the nucleus submedius. The findings also suggest glutamatergic neurotransmission between cortical afferents and nucleus submedius neurons. Glutamate is therefore likely to be an important mediator of nociceptive processing in the medial thalamus.

Relationship of thalamic basal forebrain projection neurons to the peptidergic innervation of the midline thalamus

To better understand the input-output organization of the midline thalamus, we compared the distribution of its peptidergic and monoaminergic afferents, which were visualized by using immunocytochemistry, with the distribution of neurons projecting to different basal forebrain structures, which were mapped using retrograde fluorescent tracers. Serotonin and most of the peptides were found throughout paraventricular thalamic nucleus (PV) and in other midline and intralaminar nuclei (type 1 pattern). Neuropeptide Y, alpha MSH and the catecholamine synthetic enzymes were largely restricted to dorsolateral PV (type 2 pattern). Vasopressin was found in dorsomedial PV and intermediodorsal nucleus in a pattern complementary to the type 2 distribution (type 3 pattern). Neurons projecting to accumbens core were present in paraventricular, intermediodorsal, and other midline nuclei. Neurons projecting to accumbens shell and to central amygdaloid nucleus were found in dorsal PV. The peptidergic zones were only loosely correlated with the distribution of different classes of projection neurons. The type 2 pattern overlapped best with neurons projecting to accumbens shell, and to a lesser extent to central amygdaloid nucleus, while the type 3 pattern overlapped best with neurons projecting to core of accumbens. This partial overlap suggests that some brainstem and hypothalamic nuclei preferentially affect different basal forebrain targets through the midline thalamus, and may allow, for example, information about stress to specifically influence accumbens shell and central amygdaloid nucleus. Nevertheless, most of the peptidergic afferents (type 1 pattern) to midline thalamus cover neurons projecting throughout the basal forebrain, which suggests that all of these neurons receive a variety of brainstem and hypothalamic inputs.

Somatovisceral projections from spinal cord and dorsal column nuclei to the thalamus in hedgehog tenrecs

In order first to overcome the difficulties in understanding the increasing amount of information available regarding the mammalian somatosensory thalamus, and then to correlate the findings among different species and integrate them into a general concept of thalamic organization, the present study investigated the spinothalamic and medial lemniscal projections in Madagascan hedgehog tenrecs (Echinops telfairi and Setifer setosus). Tracer substances were injected into the dorsal column nuclei and into spinal segments at various levels; additional injections were made into the inferior colliculus. The ascending somesthetic projections were to predominantly contralateral posterolateral target areas, and were almost mirror-like on both sides to intralaminar and medial thalamic nuclei. The densest and most extensive projections, originating mainly from the high cervical spinal cord and the dorsal column nuclei, reached the posterolateral thalamus caudal to the lateral geniculate nucleus. This region was difficult to subdivide cytoarchitecturally; nevertheless, on the basis of its labeling pattern, several subdivisions could be described and preliminary named. Some of them compared tentatively with the internal portion of the medial geniculate nucleus (GM) and the ventral posterior nuclear complex (VPC) in more differentiated mammals. The most prominent subdivision, however, located subjacent to the lateral surface of the brainstem, was shown to receive additional fibers from the inferior colliculus. This region might be considered a further subdivision of GM, VPC, a perigeniculate area, and/or a region of its own not comparable at present, with thalamic regions in other mammals. On the other hand, it may also be a remnant of the hypothetical, diffuse multimodal region from which GM and VPC have possibly evolved.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of neurons in the rat thalamic nucleus submedius to cutaneous, muscle and visceral nociceptive stimuli

The findings of recent studies have suggested that nucleus submedius (Sm) may be an important thalamic relay for nociceptive information. The aim of the present electrophysiological study was to examine in greater detail the activity and response properties of neurons in the rat Sm in order to further evaluate this hypothesis. Single unit extracellular recordings from neurons histologically verified to be in Sm were obtained in urethane/chloralose-anesthetized rats. Noxious but not innocuous mechanical stimulation elicited responses in 75% of the 204 neurons studied. Most (85%) of these neurons were excited, 10% were inhibited and a few neurons (5%) were excited by stimulation at some sites on the body and inhibited from other sites. The receptive fields were usually very large and bilateral. No marked differences were observed in the incidence, response type, or spontaneous activity of neurons located in dorsal, ventral, rostral or caudal parts of Sm. Most of these neurons (99 of 108, 92%) also responded to noxious heating and had a mean threshold of 47 degrees C. The majority of the neurons (19 of 21, 90%) also responded to subcutaneous, intramuscular or intraperitoneal injections of noxious chemicals (formalin or hypertonic saline). The responses elicited by pinching skin or squeezing muscle were frequently facilitated by the subcutaneous or intramuscular injections of formalin. Single electrical stimuli delivered to the cutaneous receptive field rarely produced responses. However, short trains (15-25 msec trains of 200 Hz, 3 msec pulses at 5-10 mA) delivered repetitively elicited responses in 90% (n = 73) of the neurons. These responses appearing after repetitive stimulation frequently resembled the 'wind-up' pattern observed in spinal cord dorsal horn. The conduction velocities of the primary afferents which elicited the Sm neuronal responses as estimated from the latency differences of responses elicited by stimulation at two points along the tail, were indicative of recruitment of A delta and C fibers. These findings provide further support for the proposed role of Sm in thalamic nociceptive mechanisms.

Serotoninergic projections from the dorsal raphe nucleus to the nucleus submedius in the rat and cat

The nucleus submedius in the medial thalamus has been known to receive spinothalamic and trigeminothalamic fibers, and to contain neurons which can be activated by noxious stimuli. These previous findings suggest that the nucleus submedius may be involved in the processing and relay of pain-related information. In the present study, we immunohistochemically observed in the rat and cat that the nucleus submedius was distributed with a considerable amount of serotoninergic fibers. After iontophoretic injection of cholera toxin B subunit into the nucleus submedius, the sequential double-antigen immunofluorescence histochemistry for retrogradely transported cholera toxin B subunit and serotonin revealed that the serotoninergic fibers to the nucleus submedius arose mainly from the dorsal raphe nucleus, and additionally from the ventrolateral and medial parts of the midbrain periaqueductal gray. The direct projections from the dorsal raphe nucleus to the nucleus submedius were confirmed by anterograde axonal tracing after iontophoretic injection of Phaseolus vulgaris-leucoagglutinin into the dorsal raphe nucleus. The disappearance of almost all serotoninergic fibers in the nucleus submedius was also observed after destruction of the dorsal raphe nucleus. The fluorescent retrograde double-labeling with Diamidino Yellow and Fast Blue further revealed that some neurons in the dorsal raphe nucleus projecting directly to the nucleus submedius sent their axon collaterals to the ventrolateral orbital region of the cerebral cortex, nucleus accumbens, amygdala, nucleus raphe magnus, caudal spinal trigeminal nucleus, or spinal cord. The possible roles of the serotoninergic projections from the dorsal raphe nucleus to the nucleus submedius in pain control and/or the olfactolimbic functions are discussed.

The afferent and efferent connections of the nucleus submedius in the rat

The afferent and efferent connections of the nucleus submedius (Sm) in the medial thalamus of the rat were examined. Injections of wheat-germ agglutinin conjugated horseradish peroxidase (WGA-HRP) into the Sm resulted in dense terminal labeling in the middle layers of the ipsilateral ventrolateral orbital cortex (VLO). Less dense labeling was also observed in the superficial and deep layers of VLO and in the medial part of the lateral orbital cortex (LO) and in the contralateral VLO. Retrogradely labeled neurons were observed primarily in the deep layers of VLO and the dorsal peduncular cortex (DP). Labeled neurons were also observed bilaterally, in the nucleus of the horizontal limb of the diagonal band, the lateral hypothalamus, the thalamic reticular nucleus (Rt), medial parabrachial nucleus (MPB), and the laterodorsal tegmental nucleus (LDT). Many labeled neurons were also observed in the trigeminal brain-stem complex. Injections of Fluoro-Gold (FG) into Sm resulted in a very similar distribution of retrogradely labeled neurons. Injections of WGA-HRP and FG in the orbital cortex confirmed the ipsilateral Sm projection to VLO and suggested that the middle and deep layers of VLO receive a specific ipsilateral projection from the dorsal Sm and that the superficial layers receive a projection primarily from the ventral Sm. Injections of WGA-HRP into the lateral hypothalamus, LDT, and MPB confirmed the retrograde labeling findings; the lateral hypothalamus was found to send a projection to the medial Sm, the LDT region to the ventromedial Sm and the MPB to the medial and dorsal Sm. These findings confirm and extend the results of previous studies in cat and rat indicating that Sm has a major and specific reciprocal connection with VLO. This finding, in conjunction with previous studies showing direct spinal and trigeminal inputs and the existence of nociceptive neurons in Sm and VLO, provides further support for a role of Sm in nociception.

Diencephalic projections from the superficial and deep laminae of the medullary dorsal horn in the rat

An important function of the medullary dorsal horn (MDH) is the relay of nociceptive information from the face and mouth to higher centers of the central nervous system. We studied the central projection pattern of axons arising from the MDH by examining the axonal transport of Phaseolus vulgaris-leucoagglutinin (PHA-L). Labeled axon and axon terminal distributions arising from the MDH were analyzed at the light microscopic level. After large injections of PHA-L into both superficial and deep laminae of the MDH in the rat, labeled axons were observed in the nucleus submedius of the thalamus (SUB), ventroposterior thalamic nucleus medialis (VPM), ventroposterior thalamic nucleus parvicellularis (VPPC), posterior thalamic nuclei (PO), zona incerta (ZI), lateral hypothalamic nucleus (LH), and posterior hypothalamic nucleus (PH). Restriction of PHA-L into only the superficial laminae resulted in heavy axon and varicosity labeling in the SUB, VPM, PO, and VPPC and light labeling in LH. In contrast, after injections into deep laminae, labeled axons were mainly distributed in ZI and PH; some were also in VPM and LH, and fewer still in PO and SUB. Varicosities in VPM, SUB, and PO were significantly larger than those in VPPC, ZI, LH, and PH. Varicosity density was highest in SUB and lowest in the VPPC. We concluded that there are two distinct nociceptive pathways, one originating from the superficial MDH and terminating primarily in the dorsal diencephalon and the second originating from deep laminae of the MDH and terminating primarily in the ventral diencephalon. We propose that in the rat, input from the deeper laminae is primarily involved in the motivational-affective component of pain, whereas input from the superficial MDH is related to both the sensory-discriminative and motivational-affective component of pain.

Limbic thalamus in rabbit: architecture, projections to cingulate cortex and distribution of muscarinic acetylcholine, GABAA, and opioid receptors

Nuclei of the thalamus that project to cingulate cortex have been implicated in responses to noxious stimuli, cholinergic and motor functions. The rabbit limbic thalamus may play an important role in these functions, but has not been studied extensively in terms of its cytoarchitecture, the topographical organization of its cortical projections, and differential transmitter regulation of its subnuclei. Therefore, the architecture, projections to cingulate cortex, and radioligand binding were investigated in the anterior, ventral, lateral, and midline nuclei of rabbit thalamus. The anterior nuclei are highly differentiated because both the dorsal and ventral nuclei have parvicellular and magnocellular divisions. Fluorescent dyes were injected into cingulate cortex to evaluate limbic thalamocortical connections. The anterior medial, submedial, and parafascicular nuclei project primarily to anterior cingulate cortex, while they have small or no projections to posterior areas. The ventral anterior and ventral lateral nuclei have a significant projection to dorsal cingulate cortex, including areas 24b and 29d. Projections of the anterior ventral nucleus are topographically organized, since medial parts of the parvicellular division project to rostral area 29, and lateral parts project to caudal area 29. The lateral nuclei and the parvicellular and magnocellular divisions of the anterior dorsal nucleus project with progressively higher densities in the rostrocaudal plane of area 29. Finally, the magnocellular division of the anterior ventral nucleus projects almost exclusively to caudal and ventral area 29, i.e., granular retrosplenial cortex. Ligand binding studies employed coverslip autoradiography and single grain counting techniques. Muscarinic receptor binding was moderate for both pirenzepine and oxotremorine-M in the parvicellular anterior ventral nucleus, while in other nuclei, there was an inverse relationship in the binding for these ligands. Most notably, the anterior dorsal nucleus, which receives no cholinergic input, had very high oxotremorine-M and low pirenzepine binding, while the anterior medial nucleus, which receives a moderate cholinergic input, had the highest pirenzepine binding and very low oxotremorine-M binding. Muscimol binding to GABAA receptors was highest in the anterior ventral nucleus, while it was at moderate levels in the anterior dorsal and lateral nuclei. The binding of Tyr-D-Ala-Gly-MePhe-Gly-ol to mu opioid receptors and 2-D-penicillamine-5-D-penicillamine-enkephalin to delta opioid receptors were both high in the parvicellular and low in the magnocellular divisions of the anterior dorsal nucleus. The magnocellular division of the anterior ventral, the lateral dorsal, and the parafascicular nuclei had high mu opioid binding, while the lateral dorsal and lateral magnocellular nuclei had low levels of delta opioid binding.(ABSTRACT TRUNCATED AT 400 WORDS)

Substance P innervation of the rat and cat thalamus. I. Distribution and relation to ascending spinal pathways

An antiserum for substance P (SP) with minimal cross-reactivity for other tachykinins was employed to map the distribution of SP-positive nerve fibers and terminals in the thalamus of cats and rats with special emphasis on the innervation by these fibers of nuclei related to the somatosensory system. In both species SP innervation is predominantly along the midline, in medial and posterior thalamic regions, and sparser in sensory relays for specific modalities. Among the most densely innervated nuclei are the parafascicular, paraventricular, rhomboid, central medial and parts of mediodorsal, lateral posterior, and ventral lateral geniculate. SP innervation of somatosensory-related nuclei is also evident in central lateral nucleus, posterior complex (PO), and in ventroposterolateral (VPL) nucleus of both cats and rats. In VPL of cats SP fibers and terminals are present along its ventral and lateral border, a paralaminar area in which spinothalamic fibers have been shown to terminate and where neurons responsive to noxious stimuli have been reported. Also in rats the SP innervation of VPL is similar to that of spinothalamic tract fibers. The SP innervation of somatosensory thalamic nuclei may be supplied, at least in part, by spinothalamic afferent as suggested by the depletion of SP after anterolateral chordotomy but not after ablation of the dorsal column nuclei. The presence of SP-positive spinothalamic neurons in the spinal cord is reported in the following paper.

The primate dorsal spinothalamic tract: evidence for a specific termination in the posterior nuclei (Po/SG) of the thalamus

The spinothalamic tract in primates and other mammals arises primarily from cells in lamina I of the dorsal horn, from lamina V cells and to a lesser extent from other laminae. Most of the neurons of lamina I respond only to noxious mechanical or thermal stimuli. Spinothalamic tract (STT) cells of lamina V tend to respond to both innocuous and noxious stimuli. Recent studies have suggested that the classical STT in the anterolateral quadrant (ALQ) contains primarily the axons of lamina V cells and that the axons of lamina I cells travel more dorsally in the dorsolateral quadrant (DLQ) to constitute the dorsal spinothalamic tract (DSTT). Using the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) injected into the spinal cord in conjunction with a contralateral anterolateral cordotomy, we have found there is a substantial projection of the DSTT to the posterior nuclei of the caudal-ventral thalamus, designated Po/SG. This projection is almost entirely abolished when the lesion includes the area of spinal cord white matter at the level of the denticulate ligament. Larger lesions that destroy the ALQ and much of the lateral column white matter, but that spare the dorsolateral column white matter in the region of the corticospinal tract, abolish all transport of WGA-HRP to the thalamus. We conclude that the spinothalamic pathway in the non-human primate encompasses a continuous fiber bundle that extends dorsally to include the region of lateral column white matter opposite the denticulate ligament and that the more dorsal aspect of this pathway projects primarily to Po/SG of thalamus.

Fine structure of the ventral lateral nucleus (VL) of the Macaca mulatta thalamus: cell types and synaptology

Ultrastructure of the major cerebellar territory of the monkey thalamus, or VL as delineated in sagittal maps by Ilinsky and Kultas-Ilinsky (J. Comp. Neurol. 262:331-364, '87), was analyzed by using neuroanatomical tracing, immunocytochemical, and quantitative morphometric techniques. The VL nucleus contains nerve cells of two types. Multipolar neurons (PN) retrogradely labeled with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) from the precentral gyrus display a tufted branching pattern of the proximal dendrites and have a range of soma areas from 200 to 1,000 microns2 (mean 535.2 microns2, SD = 159.5). Small glutamic acid decarboxylase (GAD) immunoreactive cells (LCN) exhibit sizes from 65 to 210 microns2 (mean 122.5 microns2, SD = 32.8) and remain unlabeled after cortical injections. The two cell types can be further distinguished by ultrastructural features. Unlike PN, LCN display little perikaryal cytoplasm, a small irregularly shaped nucleolus, and synaptic vesicles in proximal dendrites. The ratio of PN to LCN is 3:1. The LCN dendrites establish synaptic contacts on PN somata and all levels of dendritic arbor either singly or as a part of complex synaptic arrangements. They are also presynaptic to other LCN dendrites. Terminals known as LR type, i.e., large boutons containing round vesicles, are the most conspicuous in the neuropil. They form asymmetric contacts on somata and proximal dendrites of PN as well as on distal dendrites of LCN. The areas of these boutons range from 0.7 to 12 microns2 and the appositional length on PN dendrites ranges from 1.1 to 14 microns. All LR boutons except the largest ones become anterogradely labeled from large WGA-HRP injections in the deep cerebellar nuclei. These boutons are also encountered as part of triads and glomeruli, but very infrequently since the latter complex synaptic arrangements are rare. The most numerous axon terminals in the neuropil are the SR type, i.e., small terminals (mean area 0.42 micron2) containing round vesicles. The SR boutons become anterogradely labeled after WGA-HRP injections in the precentral gyrus. They form distinct asymmetric contacts predominantly on distal PN and LCN dendrites; however, their domain partially overlaps that of LR boutons at intermediate levels of PN dendrites. The SR boutons are components of serial synapses with LCN dendrites which, in turn, contact somata and all levels of dendritic arbors of PN. They also participate in complex arrangements that consist of sequences of LCN dendrites, serial synapses, and occasional boutons with symmetric contacts.(ABSTRACT TRUNCATED AT 400 WORDS)

Compartmental organization of the thalamostriatal connection in the cat

The compartmental organization of the thalamostriatal connection in the cat was studied by labelling thalamic fibers in anterograde axonal transport experiments and comparing their striatal distributions with the arrangement of striosomes and matrix tissue identified by histochemical staining methods. When analyzed according to their principal compartmental targets in dorsal striatum, the thalamic deposits indicated the existence of medial and lateral divisions within the thalamostriatal projection. Nuclei of the medial division, which includes parts of the thalamic midline, projected primarily to striosomes. The lateral division, which embraces the anterior and posterior intralaminar groups, the rostral ventral tier nuclei, and parts of the posterior lateral nuclear complex, predominantly innervated matrix tissue. In the dorsal division of the nucleus accumbens, the medial system preferentially terminated in zones that stain heavily in butyrylcholinesterase and substance P preparations, but fibers from both the medial and the lateral systems largely avoided the histochemically marked compartments such as the border islands of the nucleus accumbens that are seen elsewhere in the ventral striatum. Medial division: Thalamic deposits involving the paraventricular and rhomboid nuclei of the thalamic midline elicited labelling of striosomes and, invariably, ventral extrastriosomal matrix, the nucleus accumbens, and the amygdala. This projection was topographically organized: rostral thalamic deposits elicited labelling in the medial caudate nucleus and the medial nucleus accumbens. More caudal injections produced more lateral labelling. Lateral division: The lateral division is composed of at least three projection systems distinguished by their patterns of matrix innervation. Deposits involving the anterior intralaminar nuclei and the striatally projecting cells located lateral to the stria medullaris (anterior intralaminar complex) produced an even, diffuse labelling of the matrix tissue and weak labelling of the striosomes. Injections placed in the ventroanterior, ventrolateral, and ventromedial nuclei (rostral ventral complex) elicited fibrous labelling of matrix tissue that often showed nonstriosomal inhomogeneities. Deposits involving the centromedian and parafascicular nuclei (posterior intralaminar complex) produced a highly variable pattern of matrix labelling that included both homogeneous and decidedly patchy innervations of the extrastriosomal matrix. Each of these lateral thalamostriatal systems showed a similar spatial organization, whereby dorsoventral and mediolateral thalamic axes were roughly preserved in the projection to striatum.

Subcortical projections to the centromedian and parafascicular thalamic nuclei in the cat

The primary objective of this study is to identify the totality of input to the centromedian and parafascicular (CM-Pf) thalamic nuclear complex. The subcortical projections upon the CM-Pf complex were studied in the cat with three different retrograde tracers. The tracers used were unconjugated horseradish peroxidase (HRP), horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), and rhodamine-labeled fluorescent latex microspheres (RFM). Numerous subcortical structures or substructures contained labeled neurons with all three tracing techniques. These labeled structures included the central nucleus of the amygdala; the entopeduncular nucleus; the globus pallidus; the reticular and ventral lateral geniculate nuclei of the thalamus; parts of the hypothalamus including the dorsal, lateral, and posterior hypothalamic areas and the ventromedial and parvicellular nuclei; the zona incerta and fields of Forel; parts of the substantia nigra including the pars reticularis and pars lateralis, and the retrorubral area; the pretectum; the intermediate and deep layers of the superior colliculus; the periaqueductal gray; the dorsal nucleus of the raphe; portions of the reticular formation, including the mesencephalic, pontis oralis, pontis caudalis, gigantocellularis, ventralis, and lateralis reticular nuclei; the nucleus cuneiformis; the marginal nucleus of the brachium conjunctivum; the locus coeruleus; portions of the trigeminal complex, including the principal sensory and spinal nuclei; portions of the vestibular complex, including the lateral division of the superior nucleus and the medial nucleus; deep cerebellar nuclei, including the medial and lateral cerebellar nuclei; and lamina VII of the cervical spinal cord. Moreover, the WGA-HRP and rhodamine methods (known to be more sensitive than the HRP method) revealed several afferent sources not shown by HRP: the anterior hypothalamic area, ventral tegmental area, lateral division of the superior vestibular nucleus, nucleus interpositus, and the nucleus praepositus hypoglossi. Also, the rhodamine method revealed labeled neurons in laminae V and VI of the cervical spinal cord.

Responsiveness of reorganized primary somatosensory (SI) cortex after local inactivation of normal SI cortex in chronic spinal cats

The cortical map of adult cats that sustained spinal cord transection at T12 when they were 2 weeks old is characterized by a clear duplication of the representation of the forelimb, rostral trunk, and neck. The novel representation is located in the cortical region that is, in nonoperated animals, normally devoted to the hindlimb representation. We have investigated the possibility that the reactivation of the deprived hindlimb cortex may be mediated by corticocortical projections from normal to reorganized cortex. The primary somatosensory (SI) cortex was initially mapped to determine the boundaries of the normal and reorganized cortical representations. Somatotopically corresponding regions in both normal and reorganized cortex representing the trunk, the web space, or the shoulder were more precisely mapped. Inactivation of normal cortex was achieved by the nanoinjection of a solution of lidocaine hydrochloride stained with Chicago sky blue. Two major findings are described. First, inactivation of a circumscribed region of normal cortex representing a given receptive field (RF) failed to reduce or inhibit the responsiveness of a somatotopically corresponding RF represented in reorganized cortex. Therefore, it is unlikely that intracortical connections between normal and reorganized cortex could account for the reorganizational processes observed in cats that sustained spinal cord transection at 2 weeks of age. Second, the chemical blockade of normal cortex provoked an increase of the responsiveness and of the size of the peripheral RFs represented in reorganized cortex. This finding suggests that there are corticocortical connections (possibly topographically organized) between normal and reorganized cortex, and that these connections are inhibitory.

Expression of c-fos-like protein as a marker for neuronal activity following noxious stimulation in the rat

C-fos is a proto-oncogene that is expressed within some neurons following depolarization. The protein product, c-fos protein, can be identified by immunohistochemical techniques. Therefore, c-fos expression might be used as a marker for neuronal activity throughout the neuraxis following peripheral stimulation. This study has analyzed patterns of c-fos expression in both control and anesthetized animals and in anesthetized rats subjected to various forms of peripheral stimulation. Labeled cells were counted in the spinal cord, brainstem, hypothalamus, and thalamus. Little c-fos immunoreactivity was found in control animals. Prolonged inhalational anesthesia increased the number of labeled cells at several brainstem sites. Noxious stimulation of anesthetized rats induced c-fos within the neuraxis in patterns consistent with data obtained from electrophysiological studies and in additional locations for which few direct electrophysiological data are available, such as the ventrolateral medulla, the posterior hypothalamic nucleus, and the reuniens and paraventricular thalamic nuclei. Gentle mechanical stimulation was ineffective in inducing c-fos-like protein. The data suggest that c-fos can be used as a transynaptic marker for neuronal activity following noxious stimulation. However, c-fos is expressed only in some kinds of neurons following peripheral stimulation, and it therefore may be an incomplete marker for nociresponsive activity. In addition, at least a few neurons express c-fos protein in the absence of noxious stimulation. Experiments analyzing c-fos expression must be designed with care, as both extraneous stimuli and anesthetic depth influence the results.

Collaterals of primate spinothalamic tract neurons to the periaqueductal gray

Collateral projections are an important feature of the organization of ascending projections from the spinal cord to the brain. Primate spinothalamic tract (STT) neurons with collaterals to the periaqueductal gray (PAG) were studied by means of a fluorescent double-labeling method. Granular Blue and rhodamine-labeled latex microspheres were placed in the ventral posterior lateral (VPL) nucleus of the thalamus and the periaqueductal gray, respectively. Single and double labeled neurons were studied in the upper cervical cord, cervical enlargement, thoracic cord, lumbar enlargement, and sacral segments. The laminar distribution of double labeled neurons was similar to that of spinomesencephalic tract (SMT) neurons. Most double labeled (STT-SMT) neurons were located in contralateral laminae I, V, VII, and X. Relatively more lamina I STT-SMT neurons were found in the cervical enlargement and more lamina V STT-SMT neurons in the lumbar enlargement. The density of STT-SMT neurons in the upper cervical segments and cervical enlargement was almost equal. The density of STT-SMT neurons in the lumbar enlargement was 40% of that in the cervical enlargement. The thoracic and sacral segments had the lowest density of STT-SMT neurons, about 10% of that in the cervical enlargement. STT-SMT neurons constituted 14.7% of SMT neurons and 6% of STT neurons in the cervical enlargement and 15.3% of SMT neurons and 2.9% of STT neurons in the lumbar enlargement. The branch points of eight STT-SMT axons were studied electrophysiologically. The average percentage of conduction time spent in the parent axon was more than 85% for an antidromic action potential from the VPL nucleus and 91% from the PAG. Branch points of STT-SMT axons were calculated to be 9-13 mm caudal to the PAG, in the pons or rostral medulla.

Connections between area 3b of the somatosensory cortex and subdivisions of the ventroposterior nuclear complex and the anterior pulvinar nucleus in squirrel monkeys

The goal of this study was to determine whether somatosensory thalamic nuclei other than the ventroposterior nucleus proper (VP) have connections with area 3b of the postcentral cortex in squirrel monkeys. Small injections of the anatomical tracers wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) or 3H-proline were placed in electrophysiologically identified representations of body parts. The results indicate that, besides the well-established somatotopically organized connections with VP, area 3b has connections with three other nuclei of the somatosensory thalamus: the ventroposterior superior nucleus (VPS ["shell" of VP]), the ventroposterior inferior nucleus (VPI), and the anterior pulvinar nucleus (Pa). Injections confined to area 3b or involving adjacent parts of area 3a or area 1 indicate that connections between VPS, VPI, and Pa and the postcentral cortex are somatotopically organized. In VPS, connections related to the hand were found medially, and connections related to the foot were lateral. In VPI, connections with the cortical representations of the mouth, hand, and foot were successively more lateral. In Pa, connections related to the mouth, hand, and foot were successively more ventral, lateral, and caudal, and the trunk region was caudomedial. The findings suggest that VPI contains a representation of all parts of the body, including the face. The connections of Pa with the primary somatosensory cortex, area 3b, the location of Pa relative to the ventroposterior nucleus, and the high degree of topographic order in the connections of Pa with the postcentral cortex suggest that Pa is an integral part of the somatosensory thalamus in monkeys and is homologous to the medial nucleus of the posterior group (Pom) in other mammals. Overall, the results contribute to the growing evidence that individual somatosensory cortical areas in monkeys receive inputs from multiple thalamic sources, and that a single thalamic nucleus has several cortical targets.

Nociceptive responses in medial thalamus of the normal and arthritic rat

Recordings were obtained from 773 neurons located in the medial thalamus of rats. 23 of the 46 rats studied had been rendered arthritic by prior inoculation with Freund's adjuvant. 262 of the neurons could be activated by peripheral stimulation. In all cases but one, only stimuli considered to be nociceptive were effective in producing responses. Most of the responses were excitatory. The majority of the responsive neurons were located in the submedius (SM), mediodorsal (MD), centrolateral, paracentral, ventromedial (VM) nuclei and medial parts of the ventrolateral (VL) nucleus. A few nociceptive neurons were also recorded in anteromedial (AM), reuniens and a few other nearby regions of thalamus. Most neurons could be activated by stimuli applied bilaterally and frequently to large regions of the body. In almost all cases the responses were maintained for the entire duration of the 15 sec stimuli used and in some cases continued after cessation of the stimuli. No marked differences in incidence of responsive neurons were found between the normal and arthritic rats or between different regions. There were also no marked differences in the spontaneous rates, magnitudes of responses, or incidence of after-discharges of neurons in the various regions of medial thalamus. These findings indicate the existence of neurons responding to nociceptive stimuli in MD, AM, VM, and VL in addition to the intralaminar nuclei and SM and suggest that all these regions may be involved in mediating various aspects of nociception.

Distribution and synaptic contacts of the cortical terminals arising from neurons in the rat ventromedial thalamic nucleus

Injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin within the ventromedial thalamic nucleus resulted in many filled fibres in the frontal areas of rat cerebral cortex. The fibres were restricted to the upper part of layer I except in a small area of motor cortex where terminals were also found in deeper layers. Terminals were also seen in the striatum, in parts of the mesencephalic reticular formation and occasionally in the contralateral ventromedial nucleus. There is some topographical order in the projection with medial and dorsal areas well represented in medial cortex while lateral parts of ventromedial nucleus are more directly related to the cortical area that receives the ventrolateral thalamic nucleus projection. Electron microscopic examination showed the terminals in layer I of cortex making synaptic contact with dendritic spines and small dendritic profiles that showed a very dense postsynaptic specialization. Neurons in the ventromedial nucleus could be antidromically driven from electrode positions along strips of cortex which could not be easily related to any known organizational pattern in the cortex. Thalamic neurons responding antidromically to only one stimulation site were more common when the stimulation was within motor cortical areas, suggesting that in this region a more restricted pattern of termination is the rule.

Cells of origin of spinothalamic tract projections to the medial and lateral thalamus in the cat

The double fluorescent retrograde labeling method was used to examine the distribution of spinothalamic tract (STT) cells that project to the medial and lateral thalamus in the cat. Injections of one fluorescent tracer (Fast Blue or Diamidino Yellow) were made throughout the lateral thalamus and injections of the other tracer were made in the medial thalamus at sites extrapolated from recording track coordinates. Survival times were successively extended (up to 5 weeks) in order to maximize labeling in both the cervical and lumbosacral spinal cord. On average, over 2,000 labeled contralateral STT cells were counted in serial sections from segments C5-7 and L5-S2. Numerical variability of the order of a factor of two was attributable to inherent differences between individual animals. The total number of cells labeled with fluorescent tracers was comparable to the number labeled with horseradish peroxidase in control cases, although there were significant differences between the laminar distributions of labeling produced by the two methods. Injections made anterior to the thalamus to control for labeling due to leakage or passing fibers did not produce substantial spinal labeling. The laminar distribution of fluorescent dye-labeled STT cells was consistent; about half (47%) were located in lamina I, 8% were in lamina V, 5% in lamina VI, 20% in lamina VII, and 20% in lamina VIII. The proportions of STT cells in laminae I and V were higher in cervical segments (57% and 12%, respectively) than in lumbosacral segments (38% and 6%). The dominant contribution of lamina I cells to the STT thus revealed by the fluorescent tracers is striking. The proportions of STT cells labeled from the medial and the lateral thalamus varied with segmental and laminar location and with injection placement. The majority (62%) of STT cells in most cases projected only to the medial thalamus, 25% projected only to the lateral thalamus, and 13% projected to both. The STT cell populations in laminae I, VII, and VIII each displayed this common projection pattern. In contrast, cells in laminae V and VI projected predominantly to the lateral thalamus. Twice as many STT cells in lamina I (19%) projected to both the medial and the lateral thalamus as from other laminae. A greater proportion of laminae V-VIII STT cells in segments L5-6 projected to the lateral thalamus, and in S1-2, more projected to the medial thalamus.(ABSTRACT TRUNCATED AT 400 WORDS)

Primate spinothalamic pathways: I. A quantitative study of the cells of origin of the spinothalamic pathway

In six monkeys spinothalamic (STT) cells were retrogradely labeled by injecting 2% wheat germ agglutinin-conjugated horseradish peroxidase into the somatosensory thalamus. Following a 5-day survival period, the animals were perfused and the tissue was removed and processed with the tetramethyl benzidine technique. In all animals there were HRP-labeled STT cells in all segments of the spinal cord. In one old world monkey, the injection included most of the thalamus and resulted in 18.235 estimated total number of STT cells. Of this total, 35% were located in the upper cervical segments (C1-C3), 18% were located in C4-C8, 19% were in the thoracic spinal cord with most found in T1-T3; 6% were in L1-L3, 13% were in L4-L7, and 7% were in the coccygeal segments. Of the total labeled STT cells, 17% were found in the spinal cord ipsilateral to the thalamic injections; 53% of these cells were located in C1-C3 primarily in lamina VIII. The percentage of label found in the contralateral lower cervical region laminae I-III (43-50%), IV-VI (33-48%), and VII-X (8-17%) was similar among three animals with similar thalamic injections. The distributions of the shapes of the labeled STT cells were similar for each lamina between the lower cervical and lower lumbar regions. The mean diameter of the labeled STT cells varied with spinal cord segment and lamina. The lamina I STT cells were the smallest. In the cervical spinal cord, lamina VIII STT cells had the largest diameters, while in the lumbar region laminae IV-VI had the largest STT cells.

Primate spinothalamic pathways: II. The cells of origin of the dorsolateral and ventral spinothalamic pathways

The cells of origin of the dorsolateral (DSTT) and the ventral (VSTT) spinothalamic tracts were studied in 11 monkeys. The spinothalamic tract cells were retrogradely labeled by horseradish peroxidase (HRP) injected in the thalamus. All animals also received a midthoracic spinal cord lesion on the side ipsilateral to the thalamic injections. The distribution of labeled cells found in these animals throughout the cervical segments was similar to animals with no spinal cord lesions. Five animals had ventral quadrant lesions to demonstrate the cells of origin of the DSTT. In macaques with complete ventral quadrant lesions, more than 80% of the HRP label in the contralateral L4-L7 segments was located in lamina I, while in squirrel monkeys, the label in the contralateral lower lumbar region was distributed between laminae I-III and IV-VI. Few labeled cells were found in laminae VII-X. Six animals received dorsolateral funiculus lesions to demonstrate the cells of origin of the VSTT. In animals with adequate lesions, 84-99% of the contralateral HRP label in L4-L7 was located in laminae IV-X. Macaques had a larger percentage of labeled cells located in lamina I than squirrel monkeys. The results indicate the existence of two spinothalamic pathways in the primate. The DSTT was calculated to compose about one fourth of the total spinothalamic population.

Primate spinothalamic pathways: III. Thalamic terminations of the dorsolateral and ventral spinothalamic pathways

The termination sites of the dorsolateral (DSTT) and ventral (VSTT) spinothalamic pathways were determined by using anterograde transport of horseradish peroxidase from the lumbar spinal cord in primates. One animal had no spinal cord lesion, while of two other animals, one received a midthoracic dorsolateral funiculus lesion, and the other received a midthoracic ventral quadrant lesion contralateral to the injection. The thalamic label in the animal with no spinal cord lesion was much less than the label in the two animals with spinal lesions. Moreover, in the animals with spinal lesions, HRP-labeled cells were found within the thalamus. Therefore, the remaining six animals received ipsilateral hemisections and bilateral dorsal column lesions, irrespective of the contralateral lesions. The thalamic label in the animals without contralateral lesions were assumed to represent the total spinothalamic input to the diencephalon. In these animals, label was located mainly in suprageniculate and pulvinar oralis, caudal and oral divisions of ventral posterior lateral nucleus, the lateral half of ventral posterior inferior nucleus, and zona incerta, while in the medial thalamus label was primarily in two distinct bands in medial dorsal nucleus and in the posterior dorsal portion of central lateral nucleus. Scattered lighter labeling was found in other thalamic nuclei. The pattern of terminal labeling observed in the ventral posterior lateral region was arranged in patches, while elsewhere in the thalamus a more uniform labeling pattern was observed. The thalamic label in animals with contralateral ventral quadrant lesions represented the terminations of the DSTT, while the label in animals with contralateral dorsolateral funiculus lesions represented VSTT terminations. The labeling pattern was similar between these two groups. However, there were small differences between them. These results indicate that DSTT and VSTT terminations largely overlap and innervate the lateral and medial thalamamus.

Induction of c-fos-like protein within the lumbar spinal cord and thalamus of the rat following peripheral stimulation

Noxious stimulation induces c-fos-like protein within neurons of the spinal cord and thalamus in patterns that suggest c-fos induction may serve as a marker for activity within nociresponsive pathways. Similar patterns of immunoreactivity were not seen following gentle mechanical stimulation or in control animals. Within the thalamus, noxious stimulation induces immunoreactivity not only in traditionally expected locations, but also within the paraventricular, submedial and reuniens nuclei.

Retinotopic organization within the lateral posterior complex of the cat

Electrophysiological mapping methods were employed to systematically study the retinotopic organization within the cat's lateral posterior complex (LP). Visual responses were recorded in all the major subdivisions of the LP as well as in several adjoining cell groups. Specifically, separate representations of the visual field were identified for pulvinar, zones LP1-c, LP1-r, LPi, and LPm. Partial representations of the visual field were also evident in the geniculate wing, subdivisions of the lateral posterior shell, the inferior division of the posterior nuclear group, the suprageniculate nucleus, and the central lateral nucleus. Sufficient mapping observations were made to define the internal organization of major visual representations. Additionally, there was a very close correspondence between the mapping observations when they were compared with the cytoarchitectural criteria for recognizing functional cell groups (Updyke: J. Comp. Neurol. 219:143-181, '83).

Spino-diencephalic relays through the parabrachial nucleus in the cat

Previous studies have shown that the spinal input to the parabrachial nucleus (PBN) in the cat is limited to certain portions of its lateral division 8,21,45. The purpose of the present study was to determine some of the output targets of PBN neurons located within this spinal terminal domain by means of single, double and triple light microscopic labeling strategies. Combinations of tracers included the retrograde transport of tritiated wheat germ agglutinin, wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) and Fluoro-Gold from the hypothalamus, amygdala or thalamus/zona incerta together with either the anterograde transport of WGA-HRP from the spinal cord or the degeneration of spinal terminals following spinal lesions. The results (summarized in Fig. 10) showed that the spinal terminal domain contains separable populations of neurons projecting to the thalamus/zona incerta and hypothalamus. Only a limited number of amygdala-projecting neurons was located in this domain. Evidence from several laboratories supports the conclusion that these potential spino-diencephalic relays are involved somehow in nociception. More information is needed, however, regarding differences in the response properties of these separable populations of spinal-recipient neurons before more specific hypotheses concerning the precise nature of their nociceptive functions can be formulated.

The pretectum as a site for relaying dorsal column input to thalamic VL neurons

Intracellular recording techniques were used to study dorsal column input to 122 feline ventral thalamus (VL) relay neurons, before (61 cells) and after (61 cells) lesioning the pretectum. Prior to the lesions, 75% (46/61) of the neurons responded with short and longer latency postsynaptic potentials to dorsal column stimulation. Latencies of the postsynaptic potentials ranged from 4 (short) to 20 ms (long). After the lesions, only long latency responses were encountered, and those responses were seen in only 16% (10/61) of the cells. These data indicate that the pretectum may play an important role in mediating dorsal column information to VL, ultimately influencing cerebellar commands to the motor cortex.

Medial, intralaminar, and lateral terminations of lumbar spinothalamic tract neurons: a fluorescent double-label study

A dorsolateral spinothalamic tract (DSTT), consisting primarily of lamina I neurons, was confirmed in the cat lumbar spinal cord by the use of thalamic injections of fluorescent dyes combined with selective thoracic spinal cord lesions. In addition, collateralization of spinothalamic tract (STT) terminations to medial, lateral, and intralaminar thalamic regions was investigated by injections of two different fluorescent dyes into pairs of these regions. The results of this study indicate that less than 15% of cat lumbar STT neurons collateralize to more than one of the thalamic regions evaluated. Lumbar lamina I cells project to the lateral and to the medial thalamus (13% collateralize to these two regions) and have only a scant projection to the intralaminar thalamus. Lumbar laminae IV-VI STT cells are very few in cat and demonstrate almost no collateralization to multiple thalamic areas. Neurons of laminae VII-X project equally to the three thalamic regions evaluated, and approximately 10-14% of cells from this laminar group collateralize to any two of the thalamic sites evaluated.