Cells of origin of the spinohypothalamic tract in the rat

Neurons in the rat brain and spinal cord labeled after pseudorabies virus injected into the external urethral sphincter

Male Sprague-Dawley rats, with their pelvic and hypogastric nerves transected, were infected with pseudorabies virus (PRV) injected into the external urethral sphincter. Animals were sacrificed at 2, 2.5, 3, and 4 days postinfection. Spinal cord and brain tissue were sectioned and processed by immunohistochemical techniques with antisera against PRV and choline acetyl transferase (CAT). At 2 days postinfection, virus-labeled neurons were found in the ventrolateral divisions of Onuf's nucleus and in the dorsal gray commissure (DGC). At progressively later incubation times, labeled neurons were found in the intermediolateral regions, the superficial layer of the dorsal horn, and the brainstem, in particular, the pontine micturition center. PRV/CAT-positive neurons were only found in Onuf's nucleus. Preganglionic neurons in the L6-S1 intermediolateral regions were CAT positive but PRV negative, thus suggesting that they are interneurons, not sacral parasympathetic preganglionic neurons. After 4 days, virus had spread to neurons in the paraventricular, preoptic, and even cortical regions. The distribution of these PRV-labeled brain neurons strongly resembled that obtained after the injection of PRV into the urinary bladder (Nadelhaft et al. [1992] Neurosci. Lett. 143:271-274). In both cases, neurons were labeled in the DGC in the spinal cord. The data therefore suggest that neurons in the DGC may be involved in the integrated control of the bladder and the external urethral sphincter.

Estrogen receptor-immunoreactive neurons are present in the female rat lumbosacral spinal cord

Presence of an estrogen receptor is crucial for cells to respond to estrogen; thus, estrogen-responsive neurons should be identifiable by immunohistochemically staining for the estrogen receptor (ER). Even though spinal neurons are involved in sexual behaviors and innervation of genital organs, little information is available about ER-containing neurons in the spinal cord. Consequently, we have undertaken a study of ER-containing neurons in the female rat lumbosacral cord, an area involved in reproductive functions and predicted to contain estrogen-responsive neurons. In addition, since parasympathetic preganglionic neurons in the lumbosacral cord produce nitric oxide (NO), we also sought to determine if ER-immunoreactive (-IR) neurons contain the enzymes for NO production. Finally, we compared the distribution of ER-IR neurons to the presence of uterine cervix-related neurons. Uterine cervix-related neurons were identified by expression of FOS-immunoreactivity after vaginocervical mechanostimulation (VCS). The lumbosacral spinal cords were removed from intact, ovariectomized, and VCS-treated rats and sections stained by immunohistochemistry. ER-IR was present in the nuclei of neurons located predominately in the dorsal one-half of the spinal cord. Specific sites include the dorsal horn, lamina V, the sacral parasympathetic nucleus (SPN) (which contains preganglionic parasympathetic neurons) and extending into the lateral funiculus, and lamina X. Some ER-IR neurons were NADPH-d-positive and were localized in laminae V and VII. FOS-IR neurons had a distribution pattern similar to the distribution of neurons containing ER. The presence of ER neurons in these regions suggest that they are responsive to circulating estrogen.

Distinct lateral and medial projections of the spinohypothalamic tract of the rat

We recently described a direct nociceptive projection from the spinal cord to the hypothalamus in the rat. Several electrophysiological studies of this projection indicated that the axons of some spinohypothalamic tract neurons (SHT) reach the hypothalamus either by a lateral or by a medial route. The purpose of this study was to determine the origin of all SHT neurons that reach the hypothalamus through the lateral and the medial projections, and to investigate the possibility of ablating the SHT without damaging other important sensory and motor tracts by combining retrograde tracing techniques with axonal ablation. As compared with control cases, significant (P < .05) reductions in the number of labeled SHT neurons were encountered, 26% in the ipsilateral spinal cord following lesions of the medial projection, 67% in the contralateral spinal cord following lesions of the lateral projection, and 94% in both contra- and ipsilateral sides following lesions of both the medial and lateral projections. Bilateral lesions of the lateral projections had no effect on the distribution of labeled neurons in the spinal cord and dorsal column nuclei following injections of Fluoro-Gold (FG) into the thalamus, and a small unilateral lesion of the lateral projection reduced the ipsilateral labeling in the motor cortex following injections of FG into the pyramidal decussation. These findings suggest that most SHT neurons ascend through the contralateral lateral projection and that less than half continue in the medial projection to the ipsilateral side. They also suggest a site that can be lesioned without affecting other ascending sensory spinal pathways.

Differential distribution of Fos-like immunoreactivity in the spinal trigeminal nucleus after noxious and innocuous thermal and chemical stimulation of rat cornea

Corneal afferent nerves project to two spatially distinct sites within the spinal trigeminal nucleus: the subnucleus interpolaris/caudalis transition and the subnucleus caudalis/upper cervical spinal cord transition. The role of these two regions in processing corneal input is uncertain. To determine if neurons in these regions encode different features of an applied corneal stimulus, immunoreactivity for the immediate early gene protein product, Fos, was quantified in barbiturate-anesthetized rats. Intensity was varied across thermal (thermal probe 5, 35, 42, 52 degrees C; radiant heat of approximately 45 degrees C) stimuli and compared with that seen after mustard oil (5 microliters, 20%) or mineral oil application. All stimuli increased the number of Fos-positive neurons located at the ventrolateral pole of the subnucleus interpolaris/caudalis transition compared with unstimulated controls. By contrast, only 52 degrees C thermal probe and mustard oil produced an additional peak of Fos-positive neurons within the superficial laminae at the subnucleus caudalis/cervical cord transition. Further, the magnitudes of the bimodal peaks of Fos produced by 52 degrees C thermal probe and mustard oil stimuli were different quantitatively. Mustard oil caused a greater Fos response at the subnucleus interpolaris/caudalis transition than 52 degrees C thermal probe stimulation, whereas the opposite was true at the subnucleus caudalis/cervical cord transition. Double-labeling revealed that Fos immunoreactive neurons within the spinal trigeminal nucleus were restricted to regions densely labeled for calcitonin gene-related peptide. These results indicate that select features of corneal stimuli such as modality are encoded differently by neurons in the trigeminal subnucleus interpolaris/caudalis transition compared with those located in the subnucleus caudalis/cervical cord transition. It is likely that neurons in these two brainstem regions subserve different aspects of corneal sensation.

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.

Activation of anterior lobe corticotrophs by electroacupuncture or noxious stimulation in the anaesthetized rat, as shown by colocalization of Fos protein with ACTH and beta-endorphin and increased hormone release

A marked expression of the c-fos proto-oncogene has been recently reported in cells of the anterior lobe of the pituitary gland in rats subject to electroacupuncture or noxious thermal stimulation under pentobarbital anaesthesia. The present study was undertaken to identify the activated pituitary cells. Following both kinds of stimulation, most Fos-immunoreactive anterior lobe cells showed colocalization with adrenocorticotropic hormone or beta-endorphin immunoreactivity. No c-fos expression occurred in pituitary cells immunoreactive for growth hormone, prolactin, luteinizing hormone, or thyrotropin-stimulating hormone. A marked rise of adrenocorticotropic hormone and beta-endorphin concentrations occurred in plasma. In the hypothalamus, c-fos expression was increased in the mediobasal nuclei-namely, the arcuate nucleus-and in the paraventricular nucleus, but more in the former. It is suggested that somatosensory noxious input, or the partly noxious input evoked by electroacupuncture, activate the hypothalamo-pituitary-adrenocortical axis as in common forms of stress, but with a specific activation of the mediobasal hypothalamic nuclei and no stimulation of intermediate lobe cells. Opiate release from the pituitary gland may contribute to acupuncture analgesia or the intrinsic antinociceptive reactions triggered by noxious stimulation.

Transneuronal labeling of CNS neuropeptide and monoamine neurons after pseudorabies virus injections into the stellate ganglion

The viral transneuronal labeling method was used in combination with immunohistochemical procedures to identify CNS neuropeptide and monoamine neurons that innervate the sympathetic preganglionic neurons (SPNs) which project to the stellate ganglion--the principal source of the sympathetic supply to the heart. Transneuronal labeling was found at three CNS levels: spinal cord, brainstem, and hypothalamus. In the thoracic spinal cord, apart from the pseudorabies virus (PRV)-labeled stellate SPNs, PRV-labeled neurons were localized in laminae I/II, IV, V, VII, and X as well as in the lateral spinal nucleus and lateral funiculus. In the C1-C4 spinal segments, labeled neurons were found in the lateral funiculus as well as laminae V and VII of the spinal gray matter. PRV-labeled cells were identified in lamina V and the dorsolateral funiculus of the lumbar spinal cord. Three medullary areas were consistently labeled: rostral ventromedial medulla (RVMM), rostral ventrolateral medulla (RVLM), and caudal raphe nuclei. The greatest concentration of labeling was found in the RVMM. This projection arose from adrenergic, serotonergic (5-HT), thyrotropin releasing hormone (TRH), substance P, somatostatin, enkephalin, and vasoactive intestinal peptide (VIP) immunoreactive neurons. The RVLM projection originated mainly from C1 adrenergic neurons, some of which contained immunoreactive neuropeptide Y (NPY). C3 adrenergic-NPY neurons lying near the floor of the 4th ventricle were also labeled. Enkephalin-, somatostatin- and VIP-immunoreactive RVLM neurons also contributed to this projection. 5-HT neurons of the caudal raphe nuclei (raphe pallidus, raphe obscurus, and raphe magnus) were labeled; some of these contained substance P or TRH-immunoreactivity with an occasional neuron staining for all three putative neurotransmitters. In the pons, catecholamine neurons in the A5 cell group, subcoeruleus and Kolliker-Fuse nuclei were labeled. The midbrain contained relatively few infected cells, but some were present in the Edinger-Westphal and precommissural nuclei. Forebrain labeling was concentrated in the paraventricular hypothalamic nucleus (PVN) with lesser amounts in the lateral hypothalamic area (LHA) and the perifornical region. In the PVN, oxytocin-immunoreactive neurons accounted for the greatest chemically-defined projection while corticotrophin releasing factor (CRF), vasopressin-, and angiotensin II-immunoreactive neurons provided successively lesser inputs. In the LHA, angiotensin II-immunoreactive neurons were labeled. In summary, this study provides the first detailed map of the chemically-coded CNS neurons involved in the control of the cardiosympathetic outflow.

Topographic organization of spinal and trigeminal somatosensory pathways to the rat parabrachial and Kölliker-Fuse nuclei

We examined the organization of somatosensory projections to the parabrachial (PB) and Kölliker-Fuse (KF) nuclei by employing the retrograde and anterograde axonal transport of Fluorogold and Phaseolus vulgaris-leucoagglutinin (PHA-L), respectively. Small PHA-L injections were made into different parts of the spinal trigeminal complex, including the paratrigeminal nucleus, and into different segments and laminae of the spinal dorsal horn. The subnuclear distribution of axonal labeling in the PB and KF was mapped with a camera lucida. Our results show that the somatosensory input to the PB and KF is highly organized. Neurons in the spinal trigeminal nuclei project predominantly to the KF and to the ventral portion of the external lateral PB. Neurons in the paratrigeminal nucleus project to the ventral lateral PB, the external medial PB, and to caudal aspects of the medial PB. These findings were supported by retrograde tracing experiments with Fluorogold. Spinal cord neurons located in the superficial dorsal horn (laminae I-II) of upper cervical segments project specifically to the ventral portion of the external lateral PB and, although more sparsely, to various other lateral PB nuclei. In contrast, neurons in the superficial dorsal horn of thoracic and lumbar spinal segments project mainly to the dorsal lateral and the central lateral PB. Finally, neurons in the lateral reticulated area and the lateral spinal nucleus of all spinal segments project almost exclusively to the internal lateral PB, whereas neurons in the respective nuclei of upper cervical segments also project to the KF. From our data we conclude that the somatosensory projections to the PB and KF are topographically organized. It is assumed that these pathways, which run from trigeminal and spinal neurons through the PB and KF to various forebrain, medullary, and spinal nuclei, form functionally different neural circuits that are involved in somatoautonomic processing.

Neuropeptide FF-containing efferent projections from the medial hypothalamus of rat: a Phaseolus vulgaris leucoagglutinin study

Neuropeptide FF (FMRFamide-like peptide, morphine-modulating peptide) is an octapeptide isolated from the bovine brain. There is evidence that neuropeptide FF participates in the modulation of nociceptive mechanisms. Neuropeptide FF acts through its own receptors which are distinct from the opiate receptors. In the rat brain neuropeptide FF is found in two major cell populations. We have studied the efferent connections of the hypothalamic neuropeptide FF-containing cell group, which is located in the medial hypothalamus between the dorsomedial, ventromedial and periventricular hypothalamic nuclei. By using an anterograde tracing method (Phaseolus vulgaris leucoagglutinin) combined with double-staining immunohistochemistry we characterized the connections of this cell group with the limbic system, certain hypothalamic nuclei, periaqueductal gray and with the solitary tract nucleus. In the limbic system, the major targets were the lateral septal nucleus, bed nucleus of stria terminalis and certain subnuclei in the amygdala. These connections suggest that neuropeptide FF may act, in addition to its well-characterized action in the sensory system, in limbic functions. Efferent connections to the periaqueductal gray suggest that neuropeptide FF may modulate the opiate mediated analgesia at this site. Good correlation between our results and receptor autoradiography support the idea that the terminal areas which our results show are target areas of the neuropeptide FF-containing system.

Spinal distribution and brainstem projection of lamina I neurons in the pigeon

Lamina I neurons of the spinal dorsal horn serve nociception both in mammals and in birds. The projection of these neurons to the brain is largely unknown in birds. Injections of retrogradely transported fluorescent tracers into various brainstem nuclei showed that these neurons, which are distributed throughout the spinal cord, heavily project to the nucleus of the solitary tract and the parabrachial area but not to the hypothalamus. Injections into the nucleus of the solitary tract revealed a group of neurons located in Lissauer's tract of thoracic segments. These results point to a functional role of spinal lamina I neurons in avian visceronociception.

Spread of a neurotropic coronavirus to spinal cord white matter via neurons and astrocytes

Mouse hepatitis virus strain JHM (MHV-JHM) causes a chronic encephalomyelitis in susceptible mice, with histological evidence of demyelination in the spinal cord. After intranasal inoculation, virus spreads retrogradely to several brain structures along neuroanatomic projections to the main olfactory bulb. In the absence of experimental intervention, mice become moribund before the spinal cord is infected. In this study, infusions of anti-MHV neutralizing monoclonal antibodies were administered to protect mice from the MHV-JHM-induced acute encephalitis and to allow survival until virus spread to the spinal cord. Under these conditions, virus was observed to enter specific layers (primarily laminae V to VII) in the gray matter of the upper spinal cord, consistent with transneuronal spread. While the brain structures which are the sources for virus spread to the spinal cord cannot be determined with certainty, the ventral reticular nucleus is likely to be important since it is consistently and extensively labeled in all mice and receives projections from subsequently infected areas of the spinal cord. After initial entry into the gray matter, virus rapidly spread to the white matter of the spinal cord. During the early stages of this process, extensive infection of astrocytes was noted, suggesting that cell-to-cell spread via these glial cells is an important part of this process. Reports from other laboratories using cultured cells strongly suggested that astrocytes serve as important regulators of oligodendrocyte function and, by extrapolation, have a major role in vivo in the processes of both demyelination and remyelination. Thus, our results not only outline the probable pathway used by MHV-JHM to infect the white matter of the spinal cord but also, with the assumption that infection of astrocytes leads to subsequent dysfunction, raise the possibility that infection of these cells contributes to the demyelinating process.

Fos expression within regions of the preoptic area, hypothalamus and brainstem during pregnancy and parturition

Vaginocervical stimulation, that occurs during mating or with the birth of pups, is believed to induce specific sexual and maternal behaviours in the rat as well as stimulating a number of neuroendocrine responses including the secretion of oxytocin, prolactin and luteinizing hormone. Since the medial preoptic area has been implicated in the induction of maternal behaviour, the expression of the immediate-early gene product Fos was compared between non-pregnant, late pregnant and parturient rats. Although no difference was detected in the number of Fos-positive neuronal profiles in the preoptic area of non-pregnant and late-pregnant rats, a large increase was observed in the medial preoptic nucleus and the anteroventral periventricular region, as well as in the hypothalamic supraoptic nucleus, of parturient rats. Double labelling for Fos and tyrosine hydroxylase immunoreactivity in the brainstem of parturient rats showed the activation of catecholaminergic neurons in both the nucleus of the tractus solitarius and in the ventrolateral medulla that may form part of the afferent pathway from the uterus and cervix to the preoptic area and hypothalamus.

Hypoglossal afferents to lamina I neurons of the cervical spinal cord projecting to the parabrachial nucleus in the cat

Attempts were made to determine the hypoglossal sensory inputs to the parabrachial nucleus (PBN) through the spinal cord. Wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) was injected into the cat hypoglossal nerve. HRP-labeled fibers, predominantly derived from the glossopharyngeal and vagal nerves, were observed to terminate in lamina I of the upper cervical spinal cord. A few fibers were also distributed to laminae IV-V and VII-VIII ipsilaterally. WGA:HRP injection into the lateral portion of the PBN also resulted in retrograde labeling in lamina I with ipsilateral predominance. Light-microscopic data raised the possibility of a relay of hypoglossal sensory information to the PBN in lamina I of the cervical spinal cord. In order to confirm the spinal relay, electron-microscopic observations were carried out on lamina I of C1 spinal cord after sectioning of the hypoglossal nerve and WGA:HRP injection into the lateral portion of the PBN on the same side in each animal. It was of particular interest that degenerated hypoglossal afferent fibers made synaptic contacts with lamina I neurons, which were retrogradely labeled with HRP.

Spread of MHV-JHM from nasal cavity to white matter of spinal cord. Transneuronal movement and involvement of astrocytes

C57B1/6 mice infected intranasally with mouse hepatitis virus, strain JHM (MHV-JHM) develop hindlimb paralysis with histological evidence of demyelination several weeks after inoculation. Virus must spread from the site of inoculation, the nasal cavity, to the site of disease, the white matter of the spinal cord. It has been shown previously that after intranasal inoculation, virus enters the brain via the olfactory nerve and spreads to infect many of its neuroanatomic connections within the central nervous system (CNS). In this report, it is shown that virus infecting the spinal cord is first detected in the gray matter, with spread occurring to the white matter soon thereafter. Astrocytes are heavily infected during the process of spread from the gray to the white matter of the spinal cord. Since astrocytes are in intimate contact with neuronal synapses and are themselves connected via gap junctions, these results suggest that astrocytes may be a conduit for the spread of virus in these mice. Astrocytes provide factors for the proliferation and survival of oligodendrocytes, and widespread infection of these cells might contribute to the demyelinating process eventually observed in these mice. Additionally, since virus first appears at specific locations in the spinal cord, it should be possible to determine the source of the virus infecting the cord. While the results are not definitive, the data are most consistent with virus spreading from the ventral reticular formation to the gray matter of the cervical spinal cord.

Induction of c-fos-like protein in the spinoparabrachial tract-neurons locating within the sacral parasympathetic nucleus in the rat

Immunoreactivity against c-fos-like protein (FOS) was induced in many neuronal cell bodies within the sacral parasympathetic nucleus (SPN) in urethane-anesthetized rats by chemical irritation of the urinary bladder with formalin. More than half of these FOS-immunoreactive neurons were retrogradely labeled with Fluoro-Gold injected into the lateral parabrachial nucleus (PBN1). The results indicate that some neurons within the SPN may transmit noxious information from the urinary bladder to the PBN1.

Direct spinal pathways to the limbic system for nociceptive information

The hypothalamus is believed to play important roles in several aspects of nociception. Previously, nociceptive information was thought to reach hypothalamic neurons through indirect, multisynaptic pathways. However, we have found that thousands of neurons throughout the length of the spinal cord in rats send axons directly into the hypothalamus, and many of these axons carry nociceptive information. The axons often follow a complex course, ascending through the contralateral spinal cord, brainstem, thalamus and hypothalamus. They then cross the midline and enter the ipsilateral hypothalamus, turn posteriorly, and continue into the ipsilateral thalamus. These axons might provide nociceptive information to a variety of nuclei in the thalamus and hypothalamus bilaterally.

Catecholaminergic projections from the solitary tract nucleus to the perifornical hypothalamus

The source of adrenergic and other catecholaminergic fibers innervating the perifornical lateral hypothalamus was localized in the medulla after combination of Fluoro-Gold retrograde tracing and immunohistochemistry for either tyrosine-hydroxylase or phenylethanolamine-N-methyltransferase. Following perifornical injections, Fluoro-Gold-labeled neurons were observed mainly in regions including the noradrenergic and adrenergic cell groups. In the caudal solitary tract nucleus, two kinds of doubly labeled neurons were found: a) numerous noradrenergic neurons in the A2 group at the level of, or caudal to the area postrema; b) some adrenergic neurons in the C2 group at a level immediately rostral to the area postrema. These catecholaminergic neurons connecting the caudal solitary tract nucleus to the perifornical hypothalamus might convey feeding relevant information such as glycemic level or satiety signals.

[Work of the inner clock. Neuroanatomy of circadian systems of mammals]

Many aspects of mammalian life exhibit distinct alterations throughout the 24-h cycle. Morphological, physiological, and biochemical parameters display circadian rhythms which are thought to be generated by an endogenous pacemaker and regulated by environmental factors. The morphological substrates of the endogenous circadian system have been studied extensively during the last two decades. Although knowledge is far from complete, there is general agreement that the pathways involved consist mainly of retina, hypothalamus, spinal cord, sympathetic trunk, and pineal gland. This review characterizes the anatomical structures and tracts responsible for generation and maintenance of circadian rhythmicity and discusses functional implications of neurotransmitter involvement and the selectivity of connections.

Sexual stimulation activates c-fos within estrogen-concentrating regions of the female rat forebrain

Regions of the brain that concentrate estrogen and progesterone are thought to regulate female sexual behavior by altering gene expression and neural sensitivity to afferent stimulation. We used immunocytochemistry and in situ hybridization to examine c-fos gene expression within estrogen-concentrating regions of the forebrain following various types of sexual stimulation with or without hormone treatment. Ovariectomized rats received injections of estradiol benzoate 48 h and progesterone 4 h before testing. Control rats that had been ovariectomized at least 5 months before testing did not receive hormone treatment. Rats were then either placed into bilevel testing chambers with sexually vigorous males, received manual stimulation of the flanks, received vaginocervical stimulation with a glass rod, or were left in their home cages. Copulation with intromission and ejaculation in hormone-treated rats, or stimulation of the vaginal cervix in both hormone-treated and control rats, produced a dramatic induction of c-fos mRNA and Fos-like immunoreactivity in estrogen-concentrating regions, such as the lateral septum, medial preoptic area, bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus, ventromedial hypothalamus, lateral habenula, and medial amygdala, in addition to regions that do not readily concentrate estrogen, such as the neocortex, thalamus, and striatum. Mechanical stimulation of the flanks produced a smaller induction of Fos in these rats, whereas hormone treatment alone had no effect. These data demonstrate that afferent sensory stimulation, but not estrogen or progesterone, regulates c-fos gene expression within different estrogen-concentrating and non-concentrating regions of the female rat forebrain.

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.

Microinjections of glutamate within trigeminal subnucleus interpolaris alters adrenal and autonomic function in the cat

The influence of rostral portions of the trigeminal sensory complex on adrenal and autonomic function was assessed by microinjections of L-glutamate (500 or 5 mM, 100 nl) directed at subnucleus interpolaris (Vi) or at the nucleus principalis/subnucleus oralis level (Vp/Vo) in chloralose-anesthetized cats. Microinjections of glutamate (500 mM) within Vi evoked prompt (by +1 min) dose-related increases in the adrenal secretion of epinephrine (+11.4 +/- 2.5 ng/min, P < 0.001), adrenal blood flow (+0.19 +/- 0.06 ml/min, P < 0.05), mean arterial pressure (+6.6 +/- 3.0 mmHg, P < 0.025) and heart rate (+8.0 +/- 2.7 beats/min, P < 0.01, n = 16). Microinjections of lower doses of L-glutamate (5 mM, n = 7) within Vi had no effect. Microinjections of 500 mM glutamate within VP/Vo (n = 15) or within the spinal trigeminal tract (n = 13) had no consistent effect on adrenal or autonomic function. Plasma concentrations of ACTH were not altered significantly by glutamate regardless of dose or of the site of injection. The results suggest that local release of glutamate within Vi, but not within Vp/Vo, influences adrenal and autonomic function. Together with previous results obtained after injections of glutamate within subnucleus caudalis, these data indicate that glutaminergic input to both Vi and to more caudal portions of the spinal trigeminal nucleus contribute to the control of autonomic function such as that which often accompanies trigeminal nociception.

Highly polysialylated NCAM expression in the developing and adult rat spinal cord

The expression of a highly polysialylated neural cell adhesion molecule (NCAM-H) appeared in motor neurons, presumptive commissural neurons and floor plate at embryonic day 12, and then spread throughout the spinal cord during late embryonic and early postnatal stages. In the adult stage, the expression almost disappeared, but remained in the superficial laminae of the dorsal horn, the lateral spinal nucleus and the area around the central canal. These results suggest that the NCAM-H expression of the spinal cord is involved in the developmental events and possibly in the processing system of somatic and/or visceral information during the adult stage.

Development of commissural neurons in the embryonic rat spinal cord

Little is known about the development of the various populations of interneurons in the mammalian spinal cord. We have utilized the lipid-soluble tracer DiI in fixed tissue to study the migration and dendritic arborization of spinal neurons with axons in the ventral commissure in embryonic rats. Crystals of DiI were placed in various locations in the thoracic spinal cord in order to label commissural neurons within the dorsal horn, intermediate zone, and ventral horn at E13.5, E15, E17, and E19. Seven different groups of commissural interneurons are present in the spinal cord by E13.5. Migration is relatively simple with groups occupying a position along the dorsoventral axis roughly corresponding to their position of origin along the neuroepithelium. By E15, commissural cells are near their final locations and exhibit characteristic morphology. One striking feature is the tendency of cells with similar morphology to cluster in distinct groups. By E19, at least 18 different types of commissural interneurons can be identified on morphological grounds. Although the situation is complex, some generalities about dendritic morphology are apparent. Commissural neurons located in the dorsal horn are small and have highly branched dendrites oriented along the dorsoventral axis. In more ventral regions, commissural neurons are larger and possess dendritic arbors oriented obliquely or parallel to the mediolateral axis with long dendrites extending toward the lateral and ventral funiculi. The number of primary dendrites of most groups is set by E15 and dendritic growth occurs in the transverse plane by lengthening and branching of these primary processes. This study demonstrates that a large number of classes of commissural interneurons can be recognized on the basis of characteristic morphologies and locations within the dorsal horn, intermediate zone and ventral horn of the embryonic rat spinal cord. This finding is consistent with the fact that commissural neurons project to many different targets and mediate a variety of different functions. The demonstration that dendritic arbors of spinal interneurons with characteristic morphologies can be conveniently labelled with DiI should prove useful in future studies on the development of specific circuits in the mammalian spinal cord.

Expression of c-fos protein in lumbosacral spinal cord in response to vaginocervical stimulation in rats

The pattern of vaginocervical stimulation-evoked expression of the proto-oncogene c-fos in lumbar 5-sacral 1 segments of the spinal cord of ovariectomized adult rats was mapped using immunocytochemistry. A calibrated force of mechanostimulation was applied to the vaginal cervix of experimental animals and to the perineum of control animals while they were gently restrained. The number of cells expressing c-fos was significantly greater in the experimental than the control animals in laminae I, IV, V-VI and X. The implications of the present findings for elucidating the spinal pathways mediating the various behavioral, neuroendocrine and autonomic effects of vaginocervical stimulation (VS) are discussed.

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.

NADPH diaphorase in the spinal cord of rats

To identify spinal neurons that may synthesize nitric oxide, cells and fibers histochemically stained for NADPH diaphorase (a nitric oxide synthase) were studied in the spinal cord of rats. The histochemical reaction gave an image similar to the best Golgi impregnations, staining cells down to their finest processes. Transverse, horizontal, and parasagittal 50 and 100 microns sections were used to follow dendritic and axonal arborizations of stained neurons. Major cell groups were identified in the superficial dorsal horn and around the central canal (at all spinal levels), and in the intermediolateral cell column (at thoracic and sacral levels). Scattered positive cells were also found in deeper dorsal horn, ventral horn, and white matter. In some cases, axons of cells in the dorsal horn could be traced into the white matter; many of these cells resembled neurons projecting to various supraspinal targets. Stained cells in the intermediolateral column, which sent their axons into the ventral root, were presumed to be preganglionic autonomic neurons. Dense plexes of fibers were stained in laminae I and II and in the intermediolateral column. A large number of NADPH diaphorase-positive neurons in the spinal cord appear to be involved in visceral regulation. Fibers of the intermediolateral system had a special relationship with vasculature, suggesting that nitric oxide may help to couple neural activity with regional blood flow in the spinal cord. The abundance of NADPH diaphorase-positive neurons and fibers in the superficial dorsal horn suggests that nitric oxide may also be involved in spinal sensory processing.

Spinal distribution of extracellular field potentials generated by electrical stimulation of pudendal and perineal afferents in the cat

Electrical stimulation of sensory pudendal and superficial perineal nerves evokes focal synaptic potentials produced by activation of spinal neurons in the lumbosacral gray matter in chloralose anesthetized or decerebrate cats. The field potentials evoked by sensory pudendal nerve stimulation were located in medial parts of laminae V and VI, and lamina X in the S1 to S3 spinal segments. The superficial perineal cutaneous field potentials partially overlapped with those produced by the pudendal nerve, but in general were localized more laterally in laminae V and VI. The central latencies of the earliest portion of the field potentials evoked by either sensory pudendal or superficial perineal nerves were less than 0.9 ms suggesting that monosynaptic activation of neurons contributed to the potentials.

The segmental distribution of afferent fibers from the vaginal cervix and hypogastric nerve in rats

Injections of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) into the walls of the vagina and cervix (vaginocervical injections) of rats resulted in labeling of dorsal root ganglia (DRG) cells located at T11-L4 and L6-S2. In a second group of animals, exposure of the hypogastric nerve to HRP-WGA resulted in a similar bimodal distribution of labeled cells as compared to vaginocervical injections. In a third group, unilateral hypogastric nerve transection prior to injection of HRP-WGA into the vaginocervical walls resulted in a significant reduction in DRG cells labeled at T13, L1, L2, L6 and S1. Bilateral transection of the hypogastric nerves prior to vaginocervical injections eliminated labeled DRG cells at thoracolumbar levels but not at L6 and S1. Bilateral pelvic neurectomy reduced, but did not eliminate labeled DRG cells at L6 and S1 following vaginocervical injections. These results indicate that the hypogastric nerve constitutes a major sensory pathway from the vaginocervical walls to thoracic, lumbar and sacral levels of the spinal cord. The hypogastric nerve may subserve the transmission of noxious input from the vaginocervical walls as well as the activation of ascending spinal pathways involved in neuroendocrine reflexes during parturition.

Microinjections of norepinephrine within the superficial laminae of trigeminal subnucleus caudalis evoke increases in plasma adrenocorticotropin in the rat

To determine if local release of norepinephrine within the medullary dorsal horn influences autonomic responses often associated with nociception, microinjections of norepinephrine or of specific adrenergic receptor agonists were directed at the trigeminal subnucleus caudalis (Vc) in pentobarbital-anesthetized rats. Norepinephrine (20 nmol, 100 nl) evoked a significant increase (+ 233.8 +/- 89.5 pg/ml, P less than 0.01) in plasma concentrations of adrenocorticotropin (ACTH) after injections within the superficial laminae (I-II) of Vc, whereas mean arterial pressure or heart rate were not affected. Methoxamine (20 nmol), an alpha 1-adrenoceptor agonist, injections into laminae I-II also increased plasma ACTH (+ 90.6 +/- 32 pg/ml, P less than 0.025) without affecting arterial pressure or heart rate. Norepinephrine injections into the deeper laminae (III-V) of Vc caused a variable increase in plasma ACTH (+ 203.5 +/- 146.5 pg/ml, P less than 0.01) that was not mimicked by injections of methoxamine. Microinjections of alpha 2-(clonidine) or beta-(isoproterenol) adrenergic receptor agonists into Vc had no effect on plasma ACTH regardless of the laminar site of injection. The results suggest that norepinephrine acts within Vc to alter selected autonomic responses often associated with nociception. The involvement of an alpha 1-adrenergic receptor subtype within the superficial laminae of the medullary dorsal horn suggests a neural mechanism for norepinephrine-evoked increase in plasma ACTH that is distinct from the well known alpha 2-adrenergic receptor-mediated antinociceptive effects of norepinephrine.

Central neural pathway mediating splanchnic osmosensation

To determine the central neural pathway which carries splanchnic osmosensory information to vasopressin (AVP) neurons in the hypothalamus, bilateral electrolytic lesions were placed in the ascending catecholaminergic fiber bundle, the locus coeruleus (LC), the locus subcoeruleus (subLC), the lateral parabrachial nucleus (LPB), the caudal periaqueductal gray (PAG) and the median preoptic nucleus (MPO). Six and seven days later, plasma AVP levels, plasma osmolality, mean arterial pressure and heart rate were measured following gastric infusion of hypertonic (598 mosm/kg; 2 ml/4 min) or isotonic (290 mosm/kg) saline in conscious rats with indwelling tail artery catheters and nasogastric tubes. The most effective pontine lesions, which were located in the ventral locus subcoeruleus (vsubLC) approximately 1.0 mm below the LC, decreased the AVP response to hypertonic gastric infusion by 59.7% (P less than 0.05) as compared to sham-lesioned controls. In addition, unilateral vsubLC lesions dramatically reduced the catecholamine innervation of the ipsilateral paraventricular nucleus (PVN), as qualitatively determined with dopamine beta-hydroxylase immunocytochemistry, suggesting that a pathway ascending with catecholaminergic fibers was disrupted. Lesions of the MPO were also very effective, decreasing the AVP response to hypertonic saline infusion by 60.3% (P less than 0.05), suggesting that the MPO is an integral relay center in this pathway. On the other hand, LC, LPB and PAG lesions were ineffective. Systemic plasma osmolality or cardiovascular factors did not mediate the AVP response. These results demonstrate, for the first time, that splanchnic osmotic information is transmitted to the hypothalamus via pathways within the ascending catecholaminergic fiber bundles, the MPO is a relay center where peripheral and central osmotic information may be integrated, and the LC, LPB, and PAG are not part of the splanchnic osmotic pathway.

Descending projections to the rat sacrocaudal spinal cord

Descending supraspinal and propriospinal neurons projecting to the female rat sacrocaudal spinal cord, the portion of the spinal cord that innervates the tail, were identified following injection of Fluoro-Gold into the S1-Ca2 spinal cord segments. This study attempted to determine anatomical substrates for propriospinal and supraspinal control of the tail. Propriospinal neurons were identified throughout laminae V-VIII and X at all levels of the spinal cord. The greatest density of labeling was in the lumbar enlargement, followed by the cervical enlargement, with least in the thoracic spinal cord. Within a given cord level, labeling was greatest within the intermediate zone. In addition, other prominent spinal cord collections included neurons in 1) lamina V of the lumbar enlargement, 2) dorsal lamina X of the cervical enlargement, and 3) the lateral spinal nucleus within the cervical enlargement. Supraspinal cells were identified within raphe nuclei, reticular formation nuclei, dorsal column nuclei, vestibular nuclei, noradrenergic groups, the red nucleus, the periaqueductal gray, the hypothalamus, and the motor cortex. These data indicate that there are significant descending projections to the sacrocaudal spinal cord, with distributions similar to those of other cord levels. Functionally, important supraspinal and propriospinal influences on tail, pelvic viscera and limbs, such as with locomotion, balance, defense, micturition, defecation, and sexual functions, may be mediated by these connections.

Functional properties of spinomesencephalic tract (SMT) cells in the upper cervical spinal cord of the cat

Response and receptive field properties were evaluated for 62 spinomesencephalic tract cells in the upper cervical spinal cord (C1-C3) of cats anesthetized with sodium pentobarbital and alpha-chloralose. Recordings were made from cells in laminae I-VIII and X contralateral to antidromic stimulating electrodes positioned in the rostral, caudal and intercollicular region of the midbrain. The mean antidromic threshold for all cells was 185 +/- 132 microA, and conduction velocities ranged from 2.3 to 38.6 m/sec. Twelve cells were backfired from both midbrain and diencephalic stimulation sites. Receptive fields ranged from simple, i.e., ipsilateral forelimb or face, to complex, i.e., excitatory and/or inhibitory responses from large portions of the body. Peripheral receptive fields included muscles, joints, cornea, dura, forelimbs, hind limbs, tail, and/or testicles. Five functional classes of cells were observed: (a) wide dynamic range (14 cells); (b) high threshold (2 cells): (c) low threshold (4 cells); (d) deep/tap (11 cells); and (e) non-responsive (31 cells). Eight cells were evaluated for responses to different doses (5-150 micrograms) of intravenous (i.v.) serotonin. Two of the 8 cells exhibited excitatory effects, whereas 2 cells classified as deep/tap and 4 cells classified as non-responsive were not affected. The results of this study have shown the upper cervical component of the spinomesencephalic tract is made up of a heterogenous population of cells involved in the integration of varied inputs from large portions of the body. It is proposed that the large population of SMT cells in the upper cervical spinal cord may be involved in pain mechanisms, especially those related to the affective consequences of acute and chronic pain.

The cells of origin of the spinohypothalamic tract in cats

Various cutaneous and visceral stimuli alter the discharge rates of neurons in the hypothalamus. Changes in the activity of hypothalamic neurons are thought to play important roles in eliciting neuroendocrine, autonomic, and affective responses to somatosensory and visceral stimuli. Information from peripheral structures has been considered generally to reach the hypothalamus via multisynaptic ascending pathways. Recently, a direct projection from the spinal cord to the hypothalamus was demonstrated in rats. The goal of this study was to determine whether a similar projection exists in cats. Either wheat germ agglutinin conjugated to horseradish peroxidase, a mixture of this tracer and the B subunit of cholera toxin conjugated to horseradish peroxidase, or fast blue was injected into the hypothalamus of cats. Injections were centered in the hypothalamus in seven cats and did not spread to the thalamus, zona incerta or midbrain. After these injections, retrogradely labeled neurons were observed bilaterally in each of the 17 spinal segments that were examined. A total of approximately 400-500 labeled neurons was observed in alternate sections through these segments in the most effective cases. Roughly 70% of the labeled neurons were located contralaterally. Labeled neurons were found predominantly in the deep dorsal horn, the intermediate zone/ventral horn and in the area surrounding the central canal. A few were also noted in the superficial dorsal horn. The first and second sacral segments contained the largest numbers of retrogradely labeled neurons in the spinal cord. The number of spinohypothalamic tract neurons observed in this study in cats was roughly an order of magnitude smaller than that previously reported for rats. This finding suggested either that the spinohypothalamic tract is relatively small in cats or that our tracing techniques did not label many spinohypothalamic tract neurons in cats. To test the sensitivity of one of our tracing techniques, control injections of wheat germ agglutinin conjugated to horseradish peroxidase that filled the ventrobasal thalamus were made in two cats. In both cases, thousands of spinal cord neurons were labeled. In summary, our results indicate that a spinohypothalamic tract exists in cats. However, our findings also suggest that the total number of spinohypothalamic tract neurons in cats may be an order of magnitude smaller than it is in rats.

Central neuronal pathways containing FLFQPQRFamide-like (morphine-modulating) peptides in the rat brain

Octapeptide FLFQPQRFamide (FMRFamide-like peptide; morphine-modulating peptide), isolated from bovine brain, has some opiate analgesia modulating effects. Octapeptide FLFQPQRFamide-like immunoreactivity is found in high concentrations in the posterior pituitary, hypothalamus, pons-medulla, and dorsal spinal cord. Octapeptide FLFQPQRFamide-immunoreactive neurons of the brain are localized in the medial hypothalamus and in the nucleus of the solitary tract. High densities of octapeptide FLFQPQRFamide-immunoreactive nerve terminals are found in the median eminence, lateral parabrachial nucleus, and nucleus of the solitary tract. By using the retrograde tract tracing method combined with immunohistochemistry, we studied the central pathways interconnecting the octapeptide FLFQPQRFamide-immunoreactive structures. The octapeptide FLFQPQRFamide-immunoreactive neurons of the hypothalamus sent projections bilaterally to the nucleus of the solitary tract. The octapeptide FLFQPQRFamide-immunoreactive neurons of the nucleus of the solitary tract projected to the contralateral side of the same nucleus, to the lateral parabrachial nuclei bilaterally, and to the ipsilateral periambigual region. The results give neuroanatomical evidence of interacerebral pathways containing recently identified FLFQPQRFamide-like peptides, which may belong to a larger family of peptides. These neuroanatomical findings support the previous pharmacological studies, suggesting that the mammalian FMRFamide-like peptides may, in addition to modulatory effects on nociceptive mechanisms, participate in the regulation of blood pressure, feeding behaviour and endocrine functions.

Spinal distribution and collateral projections of rat spinomesencephalic tract cells

The distribution of cells belonging to the rat spinomesencephalic tract was studied by means of the retrograde transport of fluorescent dyes. Bilateral midbrain injections of cytoplasmic and nuclear tracers were made in order to evaluate the location of ipsilateral, contralateral, or bilaterally projecting cells. Spinal neurons with ascending projections to midbrain and descending propriospinal projections were identified by midbrain and spinal injections of different cytoplasmic labels. The locations of spinomesencephalic tract cells included seven regions of the spinal gray matter: marginal zone, lateral neck of the dorsal horn, nucleus proprius, the region around the central canal, the lateral cervical and spinal nuclei and the ventral horn. Cells projecting to the ipsilateral or contralateral midbrain had similar distributions and were frequently found in clusters with overlapping dendritic fields. Approximately 75% of spinomesencephalic cells projected to the contralateral midbrain. The largest contribution to the spinomesencephalic tract cell population was found in cervical cord segments 1-4. Cells with bilateral projections accounted for nearly 2% of all labeled cells, whereas 5% had both ascending and descending projections. Spinomesencephalic cells were found to have varying dendritic fields and morphology, e.g. fusiform, pyramidal, round/oval, and multipolar. The results of the present study lend further support to the view that the spinomesencephalic tract is a multi-component pathway with varied origins and projection targets.

Evidence that Fluoro-Gold can be transported avidly through fibers of passage

Small iontophoretic injections of the retrograde tracer Fluoro-Gold were restricted to the dorsal columns in the cervical enlargement of 6 rats. Large numbers of neurons were labeled in the lumbosacral dorsal horn in each rat. In the most effective case, more than 1800 neurons were labeled in alternate sections through nine examined segments. Many neurons were also labeled in lumbosacral dorsal root ganglia of all cases. This study, in contrast to previous reports, indicates that Fluoro-Gold can be transported avidly by axons passing through, but not terminating in, injection sites.

The cells of origin of the spinothalamic tract of the rat: a quantitative reexamination

We quantitatively reinvestigated the cells of origin of the spinothalamic tract (STT) of the rat. Injections of Fluoro-Gold that filled the thalamus on one side labeled large numbers of neurons throughout the length of the spinal cord. In 3 cases, we estimated the total number of STT neurons by counting labeled neurons in 18 of the 34 total segments, applying correction factors to these counts, and estimating the numbers of labeled neurons in the 16 remaining unexamined segments. The accuracy of these estimates was tested in two animals in which labeled neurons were counted in all 34 spinal segments. In both cases, the estimated totals of STT neurons differed from the counted totals by less than 5%. In the most effective case, we estimated that more than 9500 STT neurons were labeled. This study indicates that the number of STT neurons in rats is larger than previously reported and suggests that the STT may play an important role in nociception in rats, as it does in primates including humans.

Neurons in the sacral parasympathetic nucleus that project to the hypothalamus do not also project through the pelvic nerve--a double labeling study combining Fluoro-gold and cholera toxin B in the rat

We recently reported that neurons in the sacral parasympathetic nucleus (SPN) project directly to the hypothalamus. In the present study, we examined the possibility that individual neurons in SPN send both an axon into the pelvic nerve and an ascending projection to the hypothalamus. We used a new double-labeling technique in which two sensitive retrograde tracers (Fluoro-gold and cholera toxin subunit B immunocytochemically stained with rhodamine-labeled antibodies) were combined. The effectiveness of this combination for singly and doubly labeling neurons was established in experiments in which both tracers were injected into overlapping areas of the tongue or ventrobasal thalamus. These injections doubly labeled large numbers of neurons in the hypoglossal or dorsal column nuclei, respectively. In studies of the projections of neurons in the SPN, injection of one tracer into the hypothalamus and the other into the pelvic nerve and/or pelvic ganglion singly labeled many neurons (more than 3300 in the 7 examined cases). However, no SPN neurons were doubly labeled. These findings indicate that the SPN in the rat consists of at least two distinct groups of cells, parasympathetic preganglionic neurons and neurons that project to the hypothalamus.

Retrograde labeling of neurons in spinal cord that project directly to nucleus accumbens or the septal nuclei in the rat

Somatosensory information has been thought to ascend from the spinal cord to limbic areas of the telencephalon through indirect, multisynaptic pathways. We now report that injections of Fluoro-gold into either the nucleus accumbens or the septal nuclei labeled hundreds of neurons in the spinal cord. These and our recent anterograde tracing experiments indicate that some spinal cord neurons project directly to the telencephalon and suggest that nucleus accumbens and septal nuclei process somatosensory information.