Responses of neurons in the rat ventrolateral orbital cortex to phasic and tonic nociceptive stimulation

Projections of the insular cortex to orbitofrontal and medial prefrontal cortex: A tracing study in the rat

The dense fiber pathways that connect the insular cortex with frontal cortices are thought to provide these frontal areas with interoceptive information, crucial for their involvement in executive functions. Using anterograde neuroanatomical tracing, we mapped the detailed organization of the projections from the rat insular cortex to its targets in orbitofrontal (OFC) and medial prefrontal (mPFC) cortex. In OFC, main insular projections distribute to lateral and medial parts, avoiding ventral parts. Whereas projections from the primary gustatory cortex densely innervate dorsolateral OFC, likely corresponding to what in primates is known as the secondary gustatory cortex, these projections avoid mPFC. Instead, mPFC is targeted almost exclusively by projections from agranular fields of the insular cortex. Finally, "parietal" domains of the insular cortex project specifically to the dorsolateral OFC, and strongly innervate ventral portions of mPFC, i.e., the dorsal peduncular cortex.

Thalamic Input to Orbitofrontal Cortex Drives Brain-wide, Frequency-Dependent Inhibition Mediated by GABA and Zona Incerta

Anatomical and behavioral data suggest that the ventrolateral orbitofrontal cortex (VLO), which exhibits extensive connectivity and supports diverse sensory and cognitive processes, may exert global influence over brain activity. However, this hypothesis has never been tested directly. We applied optogenetic fMRI to drive various elements of VLO circuitry while visualizing the whole-brain response. Surprisingly, driving excitatory thalamocortical projections to VLO at low frequencies (5-10 Hz) evoked widespread, bilateral decreases in brain activity spanning multiple cortical and subcortical structures. This pattern was unique to thalamocortical projections, with direct stimulations of neither VLO nor thalamus eliciting such a response. High-frequency stimulations (25-40 Hz) of thalamocortical projections evoked dramatically different-though still far-reaching-responses, in the form of widespread ipsilateral activation. Importantly, decreases in brain activity evoked by low-frequency thalamocortical input were mediated by GABA and activity in zona incerta. These findings identify specific circuit mechanisms underlying VLO control of brain-wide neural activities.

Cerebral cortex modulation of pain

Pain is a complex experience encompassing sensory-discriminative, affective-motivational and cognitiv e-emotional components mediated by different mechanisms. Contrary to the traditional view that the cerebral cortex is not involved in pain perception, an extensive cortical network associated with pain processing has been revealed using multiple methods over the past decades. This network consistently includes, at least, the anterior cingulate cortex, the agranular insular cortex, the primary (SI) and secondary somatosensory (SII) cortices, the ventrolateral orbital cortex and the motor cortex. These cortical structures constitute the medial and lateral pain systems, the nucleus submedius-ventrolateral orbital cortex-periaqueductal gray system and motor cortex system, respectively. Multiple neurotransmitters, including opioid, glutamate, GABA and dopamine, are involved in the modulation of pain by these cortical structures. In addition, glial cells may also be involved in cortical modulation of pain and serve as one target for pain management research. This review discusses recent studies of pain modulation by these cerebral cortical structures in animals and human.

Human intracranially-recorded cortical responses evoked by painful electrical stimulation of the sural nerve

Intracranial recordings were obtained from 5 epilepsy patients to help identify the generators of the scalp somatosensory evoked potential (SEP) components that appear to be involved in orienting attention towards a potentially threatening, painful sural nerve electrical stimulus. The intracranial recording data support, for the most part, the generators suggested by our scalp SEP studies. The generators of the central negativity at 70-110 ms post-stimulus and the contralateral temporal negativity at 100-180 ms are located in the somatosensory association areas in the medial wall of the parietal cortex and in the parietal operculum and insula, respectively. The negative potential at 130-200 ms recorded from over the fronto-central scalp appears to be generated in the medial prefrontal cortex and primary somatosensory cortex foot area. The intracranial recording data suggest that the positive scalp potential at 280-320 ms, which corresponds to the pain-related P2, has multiple generators, including the anterior cingulate cortex, inferior parietal cortex, and possibly the somatosensory association areas in the medial wall of the parietal cortex. Finally, the positive scalp potential at 320-400 ms has generators in brain areas that others have shown to generate the P3a, including the dorsolateral and medial prefrontal cortices, temporal parietal junction, and the posterior hippocampus, which supports our hypothesis that this potential is a pain-evoked P3a. The putative functional roles of the brain areas generating these components and the response properties of the intracranial peaks recorded from these brain areas are in most cases consistent with the attention- and pain-related properties of their corresponding scalp SEP components. The intracranial recordings also demonstrate that the source configuration underlying the SEP components are often more complex than was suggested from the scalp studies. This complexity implies that the dipole source localization analysis of these components will at best provide only a very crude estimate of source location and activity, and that caution must be used when interpreting a change in the scalp component amplitude.

Rostral agranular insular cortex and pain areas of the central nervous system: a tract-tracing study in the rat

The rostral agranular insular cortex (RAIC) has recently been identified as a site where local changes in GABA and dopamine levels, or application of opioids, can alter nociceptive thresholds in awake animals. The connections of the cortex dorsal to the rhinal fissure that includes the RAIC have been examined previously, with emphasis on visceral and gustatory functions but not nociception. Here we examined the afferent and efferent connections of the RAIC with sites implicated in nociceptive processing. Sensory information from the thalamus reaches the RAIC via the submedius and central lateral nuclei and the parvicellular part of the ventral posterior nucleus. The RAIC has extensive reciprocal cortico-cortical connections with the orbital, infralimbic, and anterior cingulate cortices and with the contralateral RAIC. The amygdala, particularly the basal complex, and the nucleus accumbens are important targets of RAIC efferent fibers. Other connections include projections to lateral hypothalamus, dorsal raphe, periaqueductal gray matter, pericerulear region, rostroventral medulla, and parabrachial nuclei. The connectivity of the RAIC suggests it is involved in multiple aspects of pain behavior. Projections to the RAIC from medial thalamic nuclei are associated with motivational/affective components of pain. RAIC projections to mesolimbic/mesocortical ventral forebrain circuits are likely to participate in the sensorimotor integration of nociceptive processing, while its brainstem projections are most likely to contribute to descending pain inhibitory control.

Attenuation of neuropathic manifestations by local block of the activities of the ventrolateral orbito-frontal area in the rat

Clinical and recent imaging reports demonstrate the involvement of various cerebral prefrontal areas in the processing of pain. This has received further confirmation from animal experimentation showing an alteration of the threshold of acute nociceptive reflexes by various manipulations in the orbito-frontal cortical areas. The present study investigates the possible involvement of this area in the modulation of neuropathic manifestations in awake rats. Several groups of rats were subjected to mononeuropathy following the spared nerve injury model, known to produce evident tactile and cold allodynia and heat hyperalgesia. The activity of the ventrolateral orbital areas was selectively blocked by using either chronic or acute injection of lidocaine, electrolytic lesion, or chemical lesion with kainic acid or 6-hydroxydopamine (6-OHDA). The effects of these manipulations were compared with those following lesion of the somatic sensorimotor cortical areas. Local injection of lidocaine resulted in a reversible depression of all neuropathic manifestations while electrolytic or chemical lesions elicited transient attenuation affecting mainly the heat hyperalgesia and to a lesser extent the cold allodynia. The magnitude of the observed effects with the different procedures used can be ranked as follows: 6-OHDA<lesion<electrolytic lesion<kainic acid lesion<lidocaine injection. The observed effects were transient despite the permanence of the lesions while lesion of the somatosensorimotor cortices produced sustained reduction of the neuropathic manifestations. Our results correlate well with the established connections of the ventrolateral orbital area with the thalamic nucleus subnucleus involved in the procession of thermal nociception. The transient effects reported following permanent lesions in the orbital areas may reflect its flexible role in pain modulation. This observation provides further evidence on the plasticity of the neural networks involved in the regulation of nociceptive behavior.

Electrical stimulation of thalamic Nucleus Submedius inhibits responses of spinal dorsal horn neurons to colorectal distension in the rat

In 78 halothane-anesthetized rats, we characterized the responses of single neurons in the dorsal horn of L(6)-S(1) spinal segments to a noxious visceral stimulus (colorectal balloon distension, CRD), and studied the effects of focal electrical stimulation of Nucleus Submedius (Sm) on these responses using standard extracellular microelectrode recording techniques. A total of 102 neurons were isolated on the basis of spontaneous activity. Eighty (78%) responded to CRD, of which 70% had excitatory and 30% had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 100 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 73 of the spinal dorsal horn neurons isolated. Fifty-seven (78%) of these neurons (46 CRD-responsive and 11 CRD-nonresponsive) had cutaneous receptive fields, of which 35 (61%) were small and ipsilateral, 14 (25%) were large and ipsilateral, 7 (12%) were large or small and bilateral, and 1 (2%) was small and contralateral. Sixty-one percent of these neurons responded to both noxious and innocuous cutaneous stimulation, 35% responded only to noxious stimulation, and 4% responded only to innocuous stimulation. Electrical stimulation (50-300 microA) of the contralateral Sm produced intensity-dependent attenuation of the CRD-evoked activities of most neurons (18/28 of CRD-excited and 7/12 of CRD-inhibited) tested. Sm stimulation produced facilitation of CRD responses of only one neuron (CRD-inhibited). Sm stimulation had no effects on spontaneous activity. These data indicate that Sm may be involved in the descending inhibitory modulation of visceral nociception at the spinal level.

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.

Periaqueductal gray matter projections to midline and intralaminar thalamic nuclei of the rat

The periaqueductal gray matter (PAG) projections to the intralaminar and midline thalamic nuclei were examined in rats. Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in discrete regions of the PAG, and axonal labeling was examined in the thalamus. PHA-L was also placed into the dorsal raphe nuclei or nucleus of Darkschewitsch and interstitial nucleus of Cajal as controls. In a separate group of rats, the retrograde tracer cholera toxin beta-subunit (CTb) was injected into one of the intralaminar thalamic nuclei-lateral parafascicular, medial parafascicular, central lateral (CL), paracentral (PC), or central medial nucleus-or one of the midline thalamic nuclei-paraventricular (PVT), intermediodorsal (IMD), mediodorsal, paratenial, rhomboid (Rh), reuniens (Re), or caudal ventral medial (VMc) nucleus. The distribution of CTb labeled neurons in the PAG was then mapped. All PAG regions (the four columns of the caudal two-thirds of the PAG plus rostral PAG) and the precommissural nucleus projected to the rostral PVT, IMD, and CL. The ventrolateral, lateral, and rostral PAG provided additional inputs to most of the other intralaminar and midline thalamic nuclei. PAG inputs to the VMc originated from the rostral and ventrolateral PAG areas. In addition, the lateral and rostral PAG projected to the zona incerta. No evidence was found for a PAG input to the ventroposterior lateral parvicellular, ventroposterior medial parvicellular, caudal PC, oval paracentral, and reticular thalamic nuclei. PAG --> thalamic circuits may modulate autonomic-, nociceptive-, and behavior-related forebrain circuits associated with defense and emotional responses.

Inhibitory effects of glutamate-induced activation of thalamic nucleus submedius are mediated by ventrolateral orbital cortex and periaqueductal gray in rats

This study found that in lightly-anesthetized rats a unilateral micro-injection of glutamate (200 mm, 0.5 µl) into the thalamic nucleus submedius (Sm) markedly depressed the radiant heat-evoked tail flick (TF) reflex. After injection, the mean TFL increased 25.6+/-6.5% (n=24) of the baseline at 5 min, up to a peak value (48.4+/-7.2%) at 20 min, and recovered to the baseline level at 60 min. This inhibitory effect was dose-related and repeatable over a time interval of 1.0-1.5 h in the same animal. Furthermore, micro-injections of gamma-aminobutyric acid (GABA) (100 mm) into the ipsilateral ventrolateral orbital cortex (VLO) (0.7 µl), or bilaterally into the lateral or ventrolateral parts of the periaqueductal gray (PAG) (0.5 µl on each side), eliminated the Sm-evoked inhibition. After GABA was injected into VLO or PAG, the Sm applications of glutamate failed to produce any significant changes in TFL, with the TFL changes being similar to the saline control (p>0.05). These results confirmed our previous findings that electrical stimulation of Sm depressed the rat TF reflex and that this inhibitory effect was blocked by electrolytic lesion of the VLO or PAG. Therefore, the present study provides further support for the hypothesis that Sm plays an important role in modulation of nociception, and that its effects are mediated by the VLO-PAG pathway, leading to activation of the brainstem descending inhibitory system and depression of the nociceptive inputs at the spinal cord level. Copyright 1998 European Federation of Chapters of the International Association for the Study of Pain.

A comparative fMRI study of cortical representations for thermal painful, vibrotactile, and motor performance tasks

Cortical activity due to a thermal painful stimulus applied to the right hand was studied in the middle third of the contralateral brain and compared to activations for vibrotactile and motor tasks using the same body part, in nine normal subjects. Cortical activity was demonstrated utilizing multislice echo-planar functional magnetic resonance imaging (fMRI) and a surface coil. The cortical activity was analyzed based upon individual subject activity maps and on group-averaged activity maps. The results show significant differences in activations across the three tasks and the cortical areas studied. The study indicates that fMRI enables examination of cortical networks subserving pain perception at an anatomical detail not available with other brain imaging techniques and shows that this cortical network underlying pain perception shares components with the networks underlying touch perception and motor execution. However, the thermal pain perception network also has components that are unique to this perception. The uniquely activated areas were in the secondary somatosensory region, insula, and posterior cingulate cortex. The posterior cingulate cortex activity was in a region that, in the monkey, receives nociceptive inputs from posterior thalamic medial and lateral nuclei that in turn are targets for spinothalamic terminations. Discrete subdivisions of the primary somatosensory and motor cortical areas were also activated in the thermal pain task, showing region-dependent differences in the extent of overlap with the other two tasks. Within the primary motor cortex, a hand region was preferentially active in the task in which the stimulus was painful heat. In the primary somatosensory cortex most activity in the painful heat task was localized to area 1, where the motor and vibratory task activities were also coincident. The study also indicates that the functional connectivity across multiple cortical regions reorganizes dynamically with each task.

Electrically-evoked inhibitory effects of the nucleus submedius on the jaw-opening reflex are mediated by ventrolateral orbital cortex and periaqueductal gray matter in the rat

In previous studies we have shown that electrical stimulation of the nucleus submedius inhibits the rat radiant heat-induced tail flick reflex, and that this antinociceptive effect is mediated by the ventrolateral orbital cortex and periaqueductal gray. The aim of the present study was to examine whether electrical stimulation of the nucleus submedius could inhibit the rat jaw-opening reflex, and to determine whether electrolytic lesions of the ventrolateral orbital cortex or the periaqueductal gray could attenuate the nucleus submedius-evoked inhibition. Experiments were performed on pentobarbital-anesthetized rats. The jaw-opening reflex elicited by electrical stimulation of the tooth pulp or the facial skin was monitored by recording the evoked digastric electromyogram. Conditioning stimulation was delivered unilaterally to the nucleus submedius 90 ms prior to each test stimulus to the tooth pulp. After that, electrolytic lesions were made in ventrolateral orbital cortex or periaqueductal gray, and the effect of nucleus submedius stimulation on the jaw-opening reflex was re-examined. Unilateral electrical stimulation of nucleus submedius was found to significantly depress the jaw-opening reflex (mean threshold of 28.0+/-1.4 microA, n = 48), and the magnitude of inhibition increased linearly when the stimulus intensity was increased from 20 to 70 microA, resulting in depression of the digastric electromyogram amplitude from 18.4+/-5.4% to 74.0+/-4.9% of the control (P < 0.01, n = 37). The onset of inhibition occured 60 ms after the beginning of nucleus submedius stimulation and lasted about 100 ms, as determined by varying the conditioning-test time interval. Furthermore, ipsilateral lesions of the ventrolateral orbital cortex or bilateral lesions of the lateral or ventrolateral parts of periaqueductal gray eliminated the nucleus submedius-evoked inhibition of the jaw-opening reflex. These data suggest that the nucleus submedius plays an important role in modulation of orofacial nociception, and provide further support for a hypothesis that the antinociceptive effect of nucleus submedius stimulation is mediated by ventrolateral orbital cortex and activation of a descending inhibitory system in the periaqueductal gray.

Dopamine reuptake inhibition in the rostral agranular insular cortex produces antinociception

We provide evidence for an antinociceptive effect of dopamine in the rat cerebral cortex that is mediated through descending nociceptive inhibition of spinal neurons. Injection of the dopamine reuptake inhibitor GBR-12935 in the rostral agranular insular cortex (RAIC), a cortical area that receives a dense dopaminergic projection and is involved in descending antinociception (Burkey et al.,1996), resulted in dose-dependent inhibition of formalin-induced nociceptive behavior, without any alteration of motor function. Injection of the dopamine reuptake inhibitor in the surrounding cortical areas had no effect on nociceptive behaviors. GBR-12935 also produced a reduction in noxious stimulus-induced c-fos expression in nociceptive areas of the spinal dorsal horn, suggesting that dopamine in the RAIC acts in part through descending antinociception. Electrophysiological recording from single wide dynamic range-type spinal dorsal horn neurons confirmed the descending nociceptive inhibitory effect. GBR-12935 in the RAIC significantly reduced neuronal responses evoked by noxious thermal stimulation of the skin, an effect that was reversed by local administration of the selective D1 receptor antagonist SCH-23390. Finally, administration of SCH-23390 alone in the RAIC decreased paw withdrawal latencies from noxious heat, suggesting that dopamine acts tonically in the cortex to inhibit nociception.

CNS pattern of metabolic activity during tonic pain: evidence for modulation by beta-endorphin

CNS correlates of acute prolonged pain, and the effects of partial blockade of the central beta-endorphin system, were investigated by the quantitative 2-deoxyglucose technique in unanaesthetized, freely moving rats. Experiments were performed during the second, tonic phase of the behavioural response to a prolonged chemical noxious stimulus (s.c. injection of dilute formalin into a forepaw), or after minor tissue injury (s.c. saline injection). During formalin-induced pain, local glucose utilization rates in the CNS were bilaterally increased in the grey matter of the cervical spinal cord, in spinal white matter tracts and in several supraspinal structures, including portions of the medullary reticular formation, locus coeruleus, lateral parabrachial region, anterior pretectal nucleus, the medial, lateral and posterior thalamic regions, basal ganglia, and the parietal, cingulate, frontal, insular and orbital cortical areas. Pretreatment with anti-beta-endorphin antibodies, injected i.c.v., led to increased metabolism in the tegmental nuclei, locus coeruleus, hypothalamic and thalamic structures, putamen, nucleus accumbens, diagonal band nuclei and dentate gyrus, and in portions of the parietal, cingulate, insular, frontal and orbital cortex. In formalin-injected rats, pretreated with anti-beta-endorphin, behavioural changes indicative of hyperalgesia (increased licking response) were found, which were paralleled by a significant enhancement of functional activity in the anterior pretectal nucleus and in thalamo-cortical systems. A positive correlation was found between the duration of the licking response and metabolic activity of several forebrain regions. These results provide a map of the CNS pattern of metabolic activity during tonic somatic pain, and demonstrate a modulatory role for beta-endorphin in central networks that process somatosensory inputs.

Posttraumatic obsessive-compulsive disorder: a three-factor model

This paper presents a three-factor causal model of obsessive-compulsive disorder (OCD), which posits that exposure to long-term traumatic stress generates an inordinate degree of anxiety during the psychological development of the premorbid OCD child. In response to these conditions the child evolves a distinct cognitive style characterized by exaggerated threat appraisal and magical beliefs, and experiences alterations in brain metabolism. An entire functional brain system (a basal ganglia-orbitofrontal circuit) enters into a state of enhanced responsiveness following exposure to protracted threat. Over time the threshold for stimulation is dramatically lowered, resulting in a hypersensitivity to cues that signify potential harm. Individuals adapt to this hypersensitivity through a variety of strategies, which constitute OCD.

Inhibitory effects of electrical stimulation of ventrolateral orbital cortex on the rat jaw-opening reflex

In previous studies, we have shown that electrically or chemically evoked activation of the ventrolateral orbital cortex (VLO) depresses the rat tail-flick (TF) reflex, and this antinociceptive effect is mediated by the periaqueductal gray (PAG). The aim of the present study was to examine whether electrical stimulation of the VLO could inhibit the rat jaw-opening reflex (JOR), and to determine whether electrolytic lesions of the PAG could attenuate this VLO-evoked inhibition. Unilateral electrical stimulation of the VLO significantly depressed the JOR elicited by tooth pulp or facial skin stimuli, with a mean threshold of 30.5+/-2.3 microA (n=22). Increasing stimulation intensities from 30 to 80 microA resulted in greater reduction of the dEMG amplitude from 22.9+/-5.0% to 69.7+/-3.7% of the baseline value (P<0.01, n=22). The inhibitory effect appeared 50 ms after the beginning of VLO stimulation and lasted about 150 ms, as determined by varying the conditioning-test (C-T) time interval. Unilateral lateral or ventrolateral lesions of the PAG produced only a small attenuation of the VLO-evoked inhibition of the JOR, but bilateral lesions eliminated this inhibition. These findings suggest that the VLO plays an important role in modulation of orofacial nociceptive inputs, and provide further support for the hypothesis that the antinociceptive effect of VLO is mediated by PAG leading to activation of a brainstem descending inhibitory system and depression of nociceptive inputs at the trigeminal level. The role played by VLO in pain modulation is discussed in association with the proposed endogenous analgesic system consisting of medullary cord-Sm-VLO-PAG-medullary cord.

The effect of morphine on responses of ventrolateral orbital cortex (VLO) neurons to colorectal distension in the rat

In 49 halothane-anesthetized rats, we characterized the responses of single neurons in the ventrolateral orbital cortex (VLO) 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 and four neurons were isolated on the basis of spontaneous activity. Fifty-seven (55%) responded to CRD, of which 32% had excitatory and 68% had inhibitory responses. Neurons showed tendencies toward graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 80 or 100 mmHg, respectively. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 80 of the VLO neurons isolated. Thirty-three (41%) of these neurons (21 CRD-responsive and 12 CRD-nonresponsive) had cutaneous receptive fields, of which 79% were large and bilateral, 18% were small and bilateral, 3% were small and ipsilateral. Ninety-four percent of these neurons responded only to noxious cutaneous stimulation, 6% responded to both noxious and innocuous stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.0625, 0.125 and 0.25 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 VLO neurons in visceral nociception.

Noxious and innocuous cold discrimination in humans: evidence for separate afferent channels

The present study evaluated the ability of humans to discriminate temperature decreases in the noxious and innocuous cold range. Two groups of five subjects detected changes in cold stimuli applied to the maxillary face. For five subjects, adapting temperatures of 22 degrees, 16 degrees, 6 degrees and 0 degrees C were used, and thresholds for detecting temperature decreases were determined using an adaptive psychophysical paradigm. Visual analogue scale (VAS) ratings of cold and pain sensation were also recorded at 5-min intervals throughout each session. A second group of five subjects performed a similar detection task, but in this case using classical psychophysical techniques (method of constant stimuli) and adapting temperatures of 22 degrees, 16 degrees, 10 degrees and 6 degrees C. These subjects described the quality of the detected change in sensation, in addition to rating overall cold and pain sensation throughout the session. Detection thresholds were 0.27 degrees, 0.48 degrees, 4.8 degrees, 8.0 degrees and >10.0 degrees C for baselines of 22 degrees, 16 degrees, 10 degrees, 6 degrees and 0 degrees C, respectively, indicating that discrimination was better in the innocuous cool (22 degrees and 16 degrees C) than in the noxious and near-noxious cold (10-0 degrees C) range (P < 0.05). Tonic adapting temperatures of 22 degrees and 16 degrees C were always rated as cool but not painful, whereas adapting temperatures of 10 degrees and 6 degrees were sometimes and 0 degrees C usually rated as painful. Phasic temperature decreases from 22 degrees and 16 degrees C always produced cooling sensations, whereas decreases from baselines of 10 degrees and 6 degrees C produced primarily sensations of painful and non-painful prickle. These data suggest that different afferent channels mediate cool and noxious cold perception and add support to the hypothesis that noxious cold sensation is mediated by subdermal nociceptors.

Fine structural organization of spinothalamic and trigeminothalamic lamina I terminations in the nucleus submedius of the cat

We examined lamina I trigemino- and spinothalamic tract (TSTT) terminals labeled with Phaseolus vulgaris leucoagglutinin in the nucleus submedius (Sm), a nociceptive relay in the cat's thalamus. Volume-rendered (three-dimensional) reconstructions of ten lamina I TSTT terminals identified with light and electron microscopy were built from serial ultrathin sections by computer, which enabled the overall structures of the terminal complexes to be characterized in detail. Two fundamentally different terminations were observed: compact clusters of numerous boutons, which predominate in the dense focus of a lamina I terminal field in the Sm, and boutons-of-passage, which are present throughout the terminal field and predominate in its periphery. Reconstructions of cluster terminations reveal that all boutons of each cluster make synaptic contact with protrusions and branch points on a single dendrite and involve presynaptic dendrites (PSDs) in triadic arrangements, providing a basis for the secure relay of sensory information. In contrast, reconstructions show that boutons-of-passage are generally characterized by simple contacts with PSDs, indicating an ascending inhibitory lamina I influence. These different synaptic arrangements are consistent with physiological evidence indicating that the morphologically distinct nociceptive-specific and thermoreceptive-(cold)-specific lamina I TSTT neurons terminate differently within the Sm. Thus, a suitable structural substrate exists in the cat's Sm for the inhibitory effect of cold on nociception, a behavioral and physiological phenomenon of fundamental significance. We conclude that the Sm is more than a simple relay for nociception, and that it may be an integrative comparator of ascending modality-selective information that arrives from neurons in lamina I.

Reciprocal and topographic connections between the piriform and prefrontal cortices in the rat: a tracing study using the B subunit of the cholera toxin

In the present study, the reciprocal connections between the piriform cortex and the prefrontal areas are described on the basis of experiments using the anterograde and the retrograde transport of the cholera toxin B subunit (CTb). Following CTb injections placed in the anterior part of the piriform cortex, retrogradely labeled cells and anterogradely labeled fibers were mainly found in the ventrolateral and lateral orbital areas as well as in the anterior part of the agranular insular cortex. Following injections placed in the posterior part of the piriform cortex, the CTb labeling was primarily observed in the infralimbic area and the posterior part of the agranular insular cortex. Thus, we described a topographical organization of the direct reciprocal connections between the anterior and the posterior parts of the piriform cortex parts and some prefrontal areas. This could support a differential modulation of the olfactory processing along the rostrocaudal dimension of the piriform cortex.

Ventrolateral orbital cortex and periaqueductal gray stimulation-induced effects on on- and off-cells in the rostral ventromedial medulla in the rat

On- and off-cells of the rostral ventromedial medulla are thought to be involved in bulbospinal inhibition of ascending nociceptive information. Experiments were carried out in lightly anaesthetized rats to assess the effects of prefrontal cortex stimulation on the responses of neurons in the rostral ventromedial medulla. For comparison purposes, effects of periaqueductal gray stimulation were also investigated. Single unit activity was recorded in the rostral ventromedial medulla and on-, off- and neutral-cells were identified based on the tail nocifensor reflex to noxious heat. Short (0.1-1 s) and long (10-15 s) trains of bipolar electrical stimulation (100-300 Hz) were delivered to the ventrolateral orbital cortex of the rat forebrain and the periaqueductal gray. Short-train stimulation of the periaqueductal gray (including dorsolateral, ventrolateral and the dorsal raphé regions) excited 58% (25 of 43) of on-cells and 44% (seven of 16) of off-cells in the rostral ventromedial medulla. Long trains blocked the noxious stimulus-evoked pause of all seven off-cells tested and blocked the excitatory response of two, and enhanced one of three on-cells. Such stimulation also inhibited or abolished the tail-flick reflex at currents below 100 microA. Glutamate microinjections into the periaqueductal gray inhibited the noxious-evoked response of two off- and two on-cells and increased the tail-flick latency. Short-train stimulation of the ventrolateral orbital cortex (100-400 microA) excited eight of 25 on-cells and inhibited the ongoing activity of 10 of 14 off-cells. Long-train ventrolateral orbital cortex stimulation (5-15 s, 100-200 microA, 200-300 Hz) enhanced the noxious evoked responses of 10 of 11 on-cells, prolonged the noxious heat-evoked pause of all of four off-cells and decreased the tail-flick latency (pronociception). The results of this study support the proposed role of on- and off-cells in descending inhibition of nociception from the periaqueductal gray and implicate the ventrolateral orbital cortex in the control of this pathway.

Inhibitory effects of electrical stimulation of thalamic nucleus submedius area on the rat tail flick reflex

This study in lightly anesthetized rats found that unilateral electrical stimulation delivered to the ventral part of the thalamic nucleus submedius (Sm), the thalamic reuniens nucleus (Re) and the hypothalamic dorsal area (DA) markedly depressed the TF reflex, and this inhibitory effect increased following increasing stimulation intensity. Stimulation in the dorsal part of Sm did not produce any or only slight depression of the TF reflex. Furthermore, an ipsilateral electrolytic lesion of the ventrolateral orbital cortex (VLO) eliminated the unilateral Sm-evoked inhibition, but not the inhibition elicited by Re and DA and contralateral Sm stimulation. Finally, after bilateral electrolytic lesions of the ventrolateral periaqueductal gray (PAG) the DA and Re and contralateral Sm-evoked inhibitions were also eliminated. The results suggest that the Sm plays an important role in modulation of nociceptive inputs, and this role of Sm is mediated by the VLO and leads to activation of the PAG descending inhibitory system and depression of the nociceptive inputs at the spinal cord level.

Pain-related cerebral potentials in patients with frontal or parietal lobe lesions

The present study investigated the processing of painful electrical stimuli in patients with unilateral frontal or parietal lobe damage and matched control subjects. Patients with frontal lesions showed increased pain thresholds when the stimuli were administered contralateral to the lesion. While the peak-to-peak amplitudes of the N150/P250 components of the somatosensory potentials increased linearly with stimulus intensity in the control subjects, the responses in the frontal group did not change significantly between stimulation at pain and tolerance threshold. There was no evidence for altered pain processing in patients with parietal lobe lesions. The findings of the present study support the hypothesis of an involvement of the frontal cortex in pain perception in humans.

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.

Responses of neurons in ventrolateral orbital cortex to noxious visceral stimulation in the rat

In pentobarbital-anesthetized rats, responses of single neurons in ventrolateral orbital cortex (VLO) to noxious visceral (colorectal distension, CRD) and cutaneous stimulation were recorded. Of 71 neurons identified on the basis of spontaneous activity, 44 responded to CRD. CRD caused inhibition of neuronal activity in 38, facilitation of activity in four and 'mixed' responses in two of these cells. Cutaneous receptive fields were identified in 31 CRD-responsive and 10 CRD-non-responsive neurons. Cutaneous receptive fields were large and bilateral. 25 CRD-responsive cells responded only to noxious cutaneous stimulation, six had wide dynamic range responses. Six CRD-non-responsive cells responded only to noxious stimuli, four had wide dynamic range responses. No VLO neuron responded only to innocuous stimuli. These data are consistent with involvement of VLO in visceral nociception, possibly in non-discriminative aspects of nociception.

Evaluation of pain perception after anterior capsulotomy: a case report

The medial prefrontal cortex has been implicated in pain perception by recent anatomical, physiological, and functional imaging data demonstrating that frontal and anterior cingulate cortices receive inputs related to nociception; neurosurgical case reports suggest that lesions involving these areas may specifically reduce the affective or emotional component of chronic intractable pain. We examined this hypothesis more closely by assessing psychophysical ratings of (1) warmth, pain intensity, and unpleasantness evoked by phasic thermal stimuli, (2) tolerance to tonic cold stimuli, and (3) perceived intensity of visual stimuli, both before and after neurosurgical lesions of the fiber tracts connecting the frontal lobes to subcortical structures. A 22-year-old male, with no history of chronic pain, underwent psychophysical testing 3 days before, 5 days after, and 6 months after receiving bilateral lesions of the anterior internal capsule (aIC), performed as treatment for obsessive-compulsive disorder. In each session, the patient rated the intensity and unpleasantness of 5-sec cutaneous heat stimuli (39-47 degrees C); pain tolerance was measured by means of a cold-pressor test (hand immersion in 1 degrees C water). The patient was able to differentially rate the intensities of heat stimuli during both pre- and postsurgical testing sessions (p < 0.001). However, he rated heat stimuli as less intense 5 days after surgery than during presurgical testing (p < 0.001), with significant decreases in both pain intensity (p < 0.005) and unpleasantness (p < 0.05). Likewise, the patient described the cold-water immersion as less painful following surgery, although his tolerance times were substantially shorter than those of the presurgical evaluation. Ratings of visual stimulus intensity did not differ across the pre- and postsurgical testing periods, suggesting that changes in pain perception were not related to attentional or cognitive deficits. Magnetic resonance imaging 5 days following surgery revealed bilateral lesions and edema centered in the aIC, with some edema in the left frontal lobe. Those 6 months later showed substantially smaller lesions involving less than half of the aIC and no edema; pain ratings and cold-water tolerance measured at that time indicated a substantial return toward the patient's presurgical values. These data suggest that blocking subcortical input to the anterior cingulate and frontal cortices reduces both the perceived intensity and the unpleasantness of noxious stimuli; reduced cold tolerance times--in the face of decreased pain perception--may reflect a disinhibition of cortical control on spinal reflexes.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of the basal ganglia in nociception and pain

The involvement of the basal ganglia in motor functions has been well studied. Recent neurophysiological, clinical and behavioral experiments indicate that the basal ganglia also process non-noxious and noxious somatosensory information. However, the functional significance of somatosensory information processing within the basal ganglia is not well understood. This review explores the role of the striatum, globus pallidus and substantia nigra in nociceptive sensorimotor integration and suggests several roles of these basal ganglia structures in nociception and pain. Electrophysiological experiments have detailed the non-nociceptive and nociceptive response properties of basal ganglia neurons. Most studies agree that some neurons within the basal ganglia encode stimulus intensity. However, these neurons do not appear to encode stimulus location since the receptive fields of these cells are large. Many basal ganglia neurons responsive to somatosensory stimulation are activated exclusively or differentially by noxious stimulation. Indirect techniques used to measure neuronal activity (i.e., positron emission tomography and 2-deoxyglucose methods) also indicate that the basal ganglia are activated differentially by noxious stimulation. Neuroanatomical experiments suggest several pathways by which nociceptive information may reach the basal ganglia. Neuroanatomical studies have also indicated that the basal ganglia are rich in many different neuroactive chemicals that may be involved in the modulation of nociceptive information. Microinjection of opiates, dopamine and gamma-aminobutyric acid (GABA) into the basal ganglia have varied effects on pain behavior. Administration of these neurochemicals into the basal ganglia affects supraspinal pain behaviors more consistently than spinal reflexive behaviors. The reduction of pain behavior following electrical stimulation of the substantia nigra and caudate nucleus provides additional evidence for a role of the basal ganglia in pain modulation. Some patients with basal ganglia disease (e.g., Parkinson's disease, Huntington's disease) have alterations in pain sensation in addition to motor abnormalities. Frequently, these patients have intermittent pain that is difficult to localize. Collectively, these data suggest that the basal ganglia may be involved in the (1) sensory-discriminative dimension of pain, (2) affective dimension of pain, (3) cognitive dimension of pain, (4) modulation of nociceptive information and (5) sensory gating of nociceptive information to higher motor areas. Further experiments that correlate neuronal discharge activity with stimulus intensity and escape behavior in operantly conditioned animals are necessary to fully understand how the basal ganglia are involved in nociceptive sensorimotor integration.

SEP topographies elicited by innocuous and noxious sural nerve stimulation. III. Dipole source localization analysis

The dipole source localization method was used to determine which of the brain areas known to be involved in somatosensation are the best candidate generators of the somatosensory evoked potential evoked by sural nerve stimulation. The ipsilateral central negativity and contralateral frontal positivity which occurred between 58 and 90 msec post stimulus (stable period 1) were best represented by a single source located in the primary somatosensory cortex (SI). The symmetrical central negativity and bilateral frontal positivity which occurred between 92 and 120 msec post stimulus (stable period 2) was best represented by 3 sources. One of these sources was located in SI and the other 2 were located bilaterally in either the frontal operculum or near the second somatosensory cortex (SII). The widespread negativity whose minimum was located in the contralateral fronto-temporal region and which occurred between 135 and 157 msec post stimulus (stable period 3) was also best represented by 3 sources. Two of these sources may be located bilaterally in the hippocampus. We cannot, however, eliminate the possibility that multiple sources in the cortex overlying the hippocampus (e.g., SII and frontal cortex) are responsible for these potentials. At innocuous stimulus levels the third source for stable period 3 was located near the vertex, possibly involving the supplementary motor cortex, whereas at noxious levels this source appears to be located in the cingulate cortex. We were unable to achieve any convincing source localization for the widespread positivity which occurred between 178 and 339 msec post stimulus (stable periods 4-6). Available evidence suggests that more sources were active during this interval than the three we could reliably test under these conditions.

SEP topographies elicited by innocuous and noxious sural nerve stimulation. II. Effects of stimulus intensity on topographic pattern and amplitude

The effects of innocuous and noxious sural nerve stimulation on the SEP scalp topography were examined in 15 human subjects. This analysis focused on the 6 stable periods (i.e., consecutive time points where the topography did not change) that were identified in the companion paper (Dowman 1994). Stable period 1 (SP1: 58-90 msec post stimulus), SP4 (178-222 msec) and SP5 (223-277 msec) showed amplitude-stimulus intensity relationships that are similar to those of neurons involved in the sensory-discriminative aspects of innocuous somatosensation. The SP1 topographic pattern showed little or no change across the innocuous and noxious stimulus levels, which together with the amplitude data suggests that SP1 is largely generated by neurons involved in innocuous somatosensation. The SP4 topographic pattern did not change appreciably across the innocuous and noxious stimulus levels, but its amplitude decreased with increasing noxious stimulation. These data suggest that SP4 is generated by neurons involved in innocuous somatosensation and that noxious inputs inhibit these cells. There were differences in the SP5 topographic patterns evoked at the innocuous and the noxious stimulus levels, which suggest SP5 also receives a contribution from neurons involved in noxious somatosensation. SP3 (135-157 msec) and SP6 (282-339 msec) are probably generated by neurons involved in noxious somatosensation. The topographic patterns of both were different at innocuous and noxious levels. SP3's amplitude-stimulus intensity function suggests that it is generated by neurons that respond to noxious inputs in a non-graded fashion. The amplitude and offset latency of SP6 increased with increasing noxious stimulation, which suggests that SP6 is generated by neurons that respond to noxious inputs in a graded fashion.

The effect of thalamic nucleus submedius lesions on nociceptive responding in rats

Previous studies have shown that the thalamic nucleus submedius (SM) contains nociceptive neurons and is interconnected with spinal, brain-stem and cortical regions associated with nociception. The present study was performed to examine the role of the SM in nociceptive-related behaviors. The effect of SM lesions on nociceptive responding in rats was assessed using both the radiant-heat tail-flick (TF) and the tail-shock 'pain-induced' vocalization (PIV) tests. The results of Exp. 1 indicated that the intensity of electrical shock required for vocalization responses was significantly decreased following SM lesions. No changes in vocalization responses were present in the sham-lesion group. In contrast, both the sham- and SM-lesion groups exhibited a significant post-lesion increase in TF latencies. A second experiment was performed to determine whether the effects of SM lesion on the tail flick may have been masked by conditioned antinociception associated with noxious electrical stimulation of the tail to produce PIV. The results indicated that there was no post-lesion change in TF latencies in either the SM- or sham-lesion group when the antecedent PIV test was omitted. The results suggest that the SM may play a role in supraspinally mediated inhibition of nociceptive input but not in spinally mediated responses to noxious stimuli.

Responses of neurons in the ventrolateral orbital cortex to noxious cutaneous stimulation in a rat model of peripheral mononeuropathy

The responses of ventrolateral orbital cortex neurons to noxious cold pressor were compared in rats with loose ligatures tied around their sciatic nerve with those in rats in which the sciatic nerve was exposed but not ligated. In ligated rats more cells responded to cold pressor and their average afterdischarges were longer. There were no differences in the background firing rate or the magnitude of response to the cold pressor between the two groups.

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.

Identification of periaqueductal gray and dorsal raphe nucleus neurons projecting to both the trigeminal sensory complex and forebrain structures: a fluorescent retrograde double-labeling study in the rat

The midbrain periaqueductal gray (PAG) including the dorsal raphe nucleus (DR) has been known to contain serotoninergic neurons projecting to many brain regions. Employing fluorescent retrograde double labeling combined with immunofluorescence histochemistry for serotonin (5-HT), we examined in the rat whether or not single PAG/DR neurons with 5-HT send their axons to both the trigeminal sensory complex and forebrain regions. Stereotaxic injections of Diamidino Yellow (DY) and Fast Blue (FB) were performed unilaterally; DY was injected into the caudal spinal trigeminal nucleus or principal sensory trigeminal nucleus, and FB into the ventrolateral orbital cortex, nucleus accumbens or amygdala. A small percentage of PAG/DR neurons were doubly labeled with DY and FB, and the majority of them showed 5-HT-like immunoreactivity (5-HT-LI). Most of these 5-HT-LI PAG/DR neurons that were indicated to send their axons simultaneously to both the trigeminal sensory complex and forebrain regions were distributed in the ventrolateral PAG subdivision and ventral aspects of the medial PAG subdivision at the middle and caudal PAG levels, bilaterally with a predominant distribution on the side ipsilateral to the injections. This indicates a possible role of these PAG/DR neurons in the limbic or affective-motivational aspect of the pain-related neural system.

Retrograde labeling of neurons in the spinal cord that project directly to the amygdala or the orbital cortex in the rat

The amygdala and orbital cortex are thought to play an important role in the regulation of autonomic functions, hormonal secretion, and behavioral expression in response to sensory stimulation. The responsiveness of neurons in these regions to stimulation of cutaneous and visceral organs indicates that sensory information reaches the amygdala and orbital cortex. In the past, a large number of studies have thoroughly documented multiple neural pathways by which sensory information can reach these regions via relay nuclei in the brainstem and diencephalon. Recent studies reported that the amygdala and orbital cortex also receive direct input from the spinal cord. The aim of this study was to determine the magnitude and the origin of these projections in the rat. Injections of the retrograde tracer Fluoro-Gold (FG), restricted to the amygdala, labeled several hundred neurons bilaterally (60% contralateral) throughout the length of the spinal cord. More than 60% of labeled neurons were found in the lateral reticulated area of the deep dorsal horn and the gray matter surrounding the central canal. Many neurons were also found in the lateral spinal nucleus. Labeled neurons were concentrated in upper lumbar and upper cervical segments. Injections of Fluoro-Gold that were centered in the orbital cortex labeled only a small number of neurons (73% contralateral) within the spinal cord. Most labeled neurons were found in the lateral reticulated area. Neurons located in the intermediate zone and the gray matter surrounding the central canal were found mainly in upper lumbar and upper cervical segments. These findings, together with the anterograde tracing observations, provide evidence for direct projections of spinal cord neurons to the amygdala and orbital cortex. Their laminar distribution in the spinal cord and the involvement of the amygdala and orbital cortex in limbic functions suggest that these pathways may play a role in neuronal circuits that enable somatosensory information, including pain, to affect autonomic, endocrine and behavioral functions.

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.

Retrograde tracing of projections between the nucleus submedius, the ventrolateral orbital cortex, and the midbrain in the rat

The fluorescent tracers fluoro-gold and 1,1'-dioctadecyl-3,3,3,3-tetramethyl indocarbocyanine perchlorate were used as retrograde markers to examine reciprocal connections between the rat nucleus submedius and the ventrolateral orbital cortex. In addition, midbrain projections to each of these regions were examined. In the prefrontal cortex, we found that input from the nucleus submedius terminates rostrally within the lateral and ventral areas of the ventrolateral orbital cortex. Conversely, the cortical input to the nucleus submedius originates from the medial and dorsal parts of the ventrolateral orbital cortex. Our data also demonstrated that neurons from the ventrolateral periaqueductal gray and the raphe nuclei project to the midline nuclei of the thalamus, including a small projection to the nucleus submedius. We further determined that regions within the ventrolateral periaqueductal gray and raphe nuclei project to the ventrolateral orbital cortex, and that these regions overlap with those that project to the nucleus submedius. These findings suggest that the nucleus submedius might be part of a neural circuit involved in the activation of endogenous analgesia.

Electron microscopic identification of lamina I axon terminations in the nucleus submedius of the cat thalamus

Ascending lamina I axons were labeled with Phaseolus vulgaris leucoagglutinin and the synaptic connections of their terminals in nucleus submedius (Sm) were studied in the electron microscope. The terminals were large, contained rounded synaptic vesicles, and were involved in complex synaptic aggregations with pre- and postsynaptic dendrites. It was observed that clustered large boutons from a single axon could contact a single dendritic shaft. These observations support a sensory role for lamina I input to Sm.