Axon collaterals of pontine taste area neurons project to the posterior ventromedial thalamic nucleus and to the gustatory neocortex

Retrograde axonal transport of fluorescent dyes was used to demonstrate collateral projections from neurons of the pontine taste area (PTA) to gustatory-responsive areas of the posterior ventromedial thalamic nucleus (VPM), and to the gustatory neocortex (GN) of the rat. Dual-labeled PTA neurons were reliably observed following application of two different fluorescent dyes to the GN and to VPM thalamus. Dye injections into the GN and into thalamic regions surrounding the VPM nucleus, the bed nucleus of stria terminalis or the infralimbic neocortex, did not result in dual-labeled cells within the PTA. This finding suggests that gustatory information may be relayed simultaneously and specifically to VPM thalamus and to the GN via collateral axons of PTA neurons.     

Direct connectivity between pontine taste areas and gustatory neocortex in rat

Horseradish peroxidase histochemistry was used to determine the course and extent of neuronal projections from the pontine taste area (PTA) to the gustatory neocortex (GN) in rat. Two distinct findings were encountered: (1) thalamocortical projections from posterior ventromedial thalamus to GN were confirmed, and (2) direct projections from cells located in the PTA to the GN were described. This novel anatomical finding supports previous suggestions that some gustatory information may be relayed to forebrain areas without making synaptic connection in the diencephalon.     

Retrograde double-labeling study of the mammillothalamic and the mammillotegmental projections in the rat

Collateral axonal branching from the medial or lateral mammillary nuclei to the anterior thalamus, Gudden's tegmental nuclei, the nucleus reticularis tegmenti pontis, and the medial pontine nucleus was studied using the fluorescent retrograde double-labeling method. One day after injection of Fast Blue into the anterior thalamic nuclei or Gudden's tegmental nuclei, Nuclear Yellow was injected into Gudden's tegmental nuclei or the nucleus reticularis tegmenti pontis and the medial pontine nucleus. Following 1 day survival, single- and double-labeled neurons were examined in the mammillary nuclei. The lateral mammillary nucleus contains neurons whose collateral fibers project to both the dorsal tegmental nucleus of Gudden and the ipsilateral or contralateral anterodorsal thalamic nucleus, to both the medial pontine nucleus and the anterodorsal thalamic nucleus, and to both the dorsal tegmental nucleus of Gudden and the medial pontine nucleus. The pars medianus and pars medialis of the medial mammillary nucleus contain neurons whose collateral fibers project to both the anteromedial thalamic nucleus and the ventral tegmental nucleus of Gudden, to both the anteromedial thalamic nucleus and the medial part of the nucleus reticularis tegmenti pontis, and to both the ventral tegmental nucleus of Gudden and the medial part of the nucleus reticularis tegmenti pontis. The dorsal half of the pars posterior of the medial mammillary nucleus contains a few neurons whose collateral fibers project to both the anteromedial thalamic nucleus and the rostral part of the ventral tegmental nucleus of Gudden, and to both the caudal part of the anteroventral thalamic nucleus and the rostral part of the ventral tegmental nucleus of Gudden, while the pars lateralis of the medial mammillary nucleus contains no double-labeled neurons and projects only to the anteroventral thalamic nucleus.     

Cerebral infarction accompanied by dysgeusia--a clinical study on the gustatory pathway in the CNS]

Dysgeusia was investigated in 11 patients with thalamic infarction and 13 patients with corona radiata infarction to locate the gustatory pathways based on the sites of the lesions. Dysgeusia was present in 4 out of 11 patients with thalamic infarction and 3 out of 13 patients with corona radiata infarction. The dysgeusia was contralateral to the lesion in all these patients. Cheiro-oral syndrome was observed as a complication in 2 patients each from both groups. The responsible lesion was located on the medial side of the ventral posterolateral (VPL) nucleus and the ventral posteromedial (VPM) nucleus in the patients with thalamic infarction who developed dysgeusia, and was located posteriorly to the corona radiata in the other group. In the patients without gustatory disturbance, on the other hand, the lesions showed no such spread. These findings suggest that the gustatory pathway ascends contralaterally in the cerebral hemisphere and that the pathway from the thalamus projects to the cerebral cortex via the posterior part of the corona radiata. It is also suggested that the pathways in the thalamus and corona radiata are very close to the sensory fibers from the mouth and hands projecting to the sensory area.     

Gustatory afferents to ventral forebrain

The pontine taste area (PTA) receives afferents from the gustatory zone of the nucleus of the solitary tract, and projects bilaterally to the thalamic taste area. Lesions of PTA also result in degenerating axons entering the substantia innominata in the ventral forebrain. The technique of antidromic activation has been used to demonstrate that pontine neurons which respond to gustatory stimuli send collaterals to both the thalamic taste area and substantia innominata. This establishes that, like olfactory input, gustatory information reaches the ventral telencephalon without first synapsing in the diencephalon.     

Efferent projections from temperature sensitive recording loci within the marginal zone of the nucleus caudalis of the spinal trigeminal complex in the cat

The efferent projections from nucleus caudalis of the spinal trigeminal complex in cats were studied with retrograde and anterograde axonal transport techniques combined with localization of recording sites in the thalamus and marginal zone of nucleus caudalis to innocuous skin cooling. Results showed brainstem projections from nucleus caudalis to rostral levels of the spinal trigeminal complex, to the ventral division of the principal trigeminal nucleus, the parabrachial nucleus, cranial motor nuclei 7 and 12, solitary complex, contralateral dorsal inferior olivary nucleus, portions of the lateral reticular formation, upper cervical spinal dorsal horn and, lateral cervical nucleus. Projections to the thalamus included: a dorsomedial region of VPM (bilaterally) and to the main part of VPM and PO contralaterally. Neuronal activity was recorded in the dorsomedial region of VPM to cooling the ipsilateral tongue. HRP injections in this thalamic region retrogradely labeled marginal neurons in nucleus caudalis. These results show that marginal neurons of nucleus caudalis provide a trigeminal equivalent of spinothalamic projections to the ventroposterior nucleus in cats.     

Both oral and caudal parts of the spinal trigeminal nucleus project to the somatosensory thalamus in the rat

Recent evidence has been accumulated that not only spinal trigeminal nucleus caudalis (Sp5C) neurons but also spinal trigeminal nucleus oralis (Sp5O) neurons respond to noxious stimuli. It is unknown, however, whether Sp5O neurons project to supratrigeminal structures implicated in the sensory processing of orofacial nociceptive information. This study used retrograde tracing with Fluorogold in rats to investigate and compare the projections from the Sp5O and Sp5C to two major thalamic nuclei that relay ascending somatosensory information to the primary somatic sensory cortex: the ventroposteromedial thalamic nucleus (VPM) and the posterior thalamic nuclear group (Po). Results not only confirmed the existence of contralateral projections from the Sp5C to the VPM and Po, with retrogradely labelled neurons displaying a specific distribution in laminae I, III and V, they also showed consistent and similar numbers of retrogradely labelled cell bodies in the contralateral Sp5O. In addition, a topographic distribution of VPM projections from Sp5C and Sp5O was found: neurons in the dorsomedial parts of Sp5O and Sp5C projected to the medial VPM, neurons in the ventrolateral Sp5O and Sp5C projected to the lateral VPM, and neurons in intermediate parts of Sp5O and Sp5C projected to the intermediate VPM. All together, these data suggest that not only the Sp5C, but also the Sp5O relay somatosensory orofacial information from the brainstem to the thalamus. Furthermore, trigemino-VPM pathways conserve the somatotopic distribution of primary afferents found in each subnucleus. These results thus improve our understanding of trigeminal somatosensory processing and help to direct future electrophysiological investigations.     

Serotonin receptors modulate trigeminovascular responses in ventroposteromedial nucleus of thalamus: a migraine target?

Triptans, serotonin 5-HT(1B/1D), receptor agonists, which are so effective in acute migraine, are considered to act directly on the trigeminovascular system. Using an in vivo model of trigeminovascular nociception, we report a potentially novel action for the triptans within the somatosensory thalamus. Both microiontophoretically applied and intravenous naratriptans potently and reversibly modulate nociceptive neurotransmission by trigeminovascular thalamic neurons in the ventroposteromedial nucleus (VPM) driven by stimulation of the superior sagittal sinus. Naratriptan also suppresses l-glutamate activated trigeminovascular VPM neurons. Co-ejection of naratriptan with the 5-HT(1B/1D) receptor antagonist GR127935 antagonized this effect. (S)-WAY 100135 the 5-HT(1A) receptor antagonist also partially inhibited the effect of naratriptan in the VPM when co-ejected with it. Taken together, the new data suggest a potential effect of triptans in the VPM nucleus of the thalamus acting through 5-HT(1A/1B/1D) mechanisms, and offer an entirely new direction for the development of and understanding of the effects of anti-migraine medicines.     

Origin, distribution, and morphology of galaninergic fibers in the rodent trigeminal system.

The neuropeptide galanin (Gal) is found throughout the central nervous system. Of particular interest is the fact that Gal is present within the majority of noradrenergic locus coeruleus (LC) neurons. However, very few, if any, Gal-immunoreactive fibers have been identified in many of the major efferent targets of LC, including sensory neocortex and dorsal thalamus. The goal of the present study was to examine the Gal fiber innervation of the rodent trigeminal somatosensory system and its connection to the LC. Our results show that at least two different morphological profiles of Gal-immunoreactive fibers are present within relay nuclei along the ascending trigeminal pathway. Numerous small caliber Gal-immunoreactive fibers with bouton-like swellings were noted within the barrel cortex, the ventroposterior medial (VPM) nucleus, the posterior medial (POm) nucleus, the zona incerta (ZI), the reticular nucleus (nRT) of the thalamus, and the principal (PrV) and spinal (SpV) nuclei of the trigeminal complex. Immunoreactive fibers were prevalent in, but not restricted to, layer I of the barrel cortex. Within the somatosensory thalamus, the density of Gal-immunoreactive fibers was higher in POm than in VPM. Laminae I and II of SpV and the nRT and ZI also contained dense, large-diameter Gal-immunoreactive fibers. These large-diameter Gal-immunoreactive fibers did not co-contain dopamine beta-hydroxylase (DBH). In contrast, virtually every small-caliber Gal-immunoreactive fiber colocalized with DBH. To determine whether Gal-immunoreactive fibers originated from LC, we combined immunohistochemical procedures with fluorescent tracing techniques. After retrograde tracer injections into several trigeminal relay nuclei, we observed that approximately 50% of the labeled LC neuronal population was immunoreactive for Gal. Our results suggest an extensive Gal-immunoreactive fiber innervation of the rodent trigeminal system, much of which may originate from LC neurons in the brainstem.     

Differential Foci and Synaptic Organization of the Principal and Spinal Trigeminal Projections to the Thalamus in the Rat

The thalamus is known to receive single-whisker ‘lemniscal’ inputs from the trigeminal nucleus principalis (Prv) and multiwhisker ‘paralemniscal’ inputs from the spinal trigeminal nucleus (Spv), yet the responses of cells in the thalamic ventroposteromedial nucleus (VPM) are most similar to and contingent upon inputs from PrV. This may reflect a differential termination pattern, density and/or synaptic organization of PrV and SpV projections. This hypothesis was tested in adult rats using anterograde double-labelling with fluorescent dextrans, horseradish peroxidase (HRP) and choleragenoid, referenced against parvalbumin and calbindin immunoreactivity. The results indicated that Prv's most robust thalamic projection is to the whisker-related barreloids of VPM. The SpV had robust projections to non-barreloid thalamic regions, including the VPM ‘shell’ encapsulating the barreloid area, a caudal and ventral region of VPM that lacks barreloids and PrV inputs, the posterior thalamic nucleus, nucleus submedius and zona incerta. Within the barreloid portion of VPM, SpV projections were sparse relative to those from PrV, and most terminal labelling occurred in the peripheral fringes of whisker-related patches and in inter-barreloid septae. Thus, PrV and SpV have largely complementary projection foci in the thalamus. Intra-axonal staining of a small sample of trigeminothalamic axons with whisker or guard hair receptive fields revealed highly localized and somatotopic terminal aggregates in VPM that spanned areas no larger than that of a single barreloid. In the electron microscopic component of this study, HRP transport to the barreloid region of VPM from left SpV and right PrV in the same cases revealed PrV terminals contacting dendrites with a broad range of minor axis diameters (mean ± SD: 1. 51 ± 0. 10 μm). SpV terminals were indistinguishable from those of PrV, but they had a disproportionate number of contacts on narrow dendrites (1. 27 ± 0. 07 μm, P 0. 01). PrV endings were also more likely to contact VPM somata (11. 0 ± 4. 2% of all labelled terminals) than those from SpV (3. 0 ± 1. O%, P 0. 01). Insofar as primary dendrites are thicker than distal dendrites in VPM, these data suggest a differential distribution of PrV and SpV inputs onto VPM cells that may account for their relative efficacies in dictating the responses of VPM cells to whisker stimulation. Multiwhisker receptive fields in VPM may also reflect direct transmission of convergent inputs from PrV.     

Thalamic Input to Representations of the Teeth,Tongue, and Face in Somatosensory Area 3b of Macaque Monkeys

Representations of the parts of the oral cavity and face in somatosensory area 3b of macaque monkeys were identified with microelectrode recordings and injected with different neuroanatomical tracers to reveal patterns of thalamic projections to tongue, teeth, and other representations in primary somatosensory cortex. The locations of injection sites and resulting labeled neurons were further determined by relating sections processed to reveal tracers to those processed for myeloarchitecture in the cortex and multiple architectural stains in the thalamus. The ventroposterior medial subnucleus (VPM) for touch was identified as separate from the ventroposterior medial parvicellular nucleus (VPMpc) for taste by differential expression of several types of proteins. Our results revealed somatotopically matched projections from VPM to the part of 3b representing intra‐oral structures and the face. Retrogradely labeled cells resulting from injections in area 3b were also found in other thalamic nuclei including: anterior pulvinar (Pa), ventroposterior inferior (VPI), ventroposterior superior (VPS), ventroposterior lateral (VPL), ventral lateral (VL), center median (CM), central lateral (CL), and medial dorsal (MD). None of our injections, including those into the representation of the tongue, labeled neurons in VPMpc, the thalamic taste nucleus. Thus, area 3b does not appear to be involved in processing taste information from the thalamus. This result stands in contrast to those reported for New World monkeys. J. Comp. Neurol. 521:3954–3971, 2013. © 2013 Wiley Periodicals, Inc.     

Intratrigeminal and thalamic projections of nucleus caudalis in the squirrel monkey (Saimiri sciureus): a degeneration and autoradiographic study.

In order to test the hypothesis that thalamic efferents of trigeminal nucleus caudalis (NC) are the cranial analogue of the spinothalamic system, lesion and autoradiographic studies were carried out in the squirrel monkey, and the terminal projection fields in thalamus were noted. Results showed that NC, including lateral reticular formation (LRF), projects to contralateral VPM, the VPM-VPL border and medial VPL, and a region dorsal to ventroposterior nucleus (VP) proper which contains cells larger than those in VPM yet which stain as darkly as VPL neurons; this latter zone of termination may be homologous with VPL_o (Vim) in other species, which is that area receiving lemniscal and cerebellar afferents (Mehler, '71; Walsh and Ebner, '73; Boivie, '74). In addition, a small projection is noted in an area intercalated between dorsomedial MG, limitans nucleus and posterior VP which closely agrees with the medial division of Posterior nucleus (Po) described in rhesus and squirrel monkey (Burton and Jones, '76). No terminations were observed in the gustatory nucleus medial to VPM. Bilateral, terminal projection fields were observed in posterior mediodorsal nucleus (MD), and a paralaminar area (PL) which lies in the ventrolateral strip of MD and is particularly prominent in primates; other bilateral fields were noted in CL, particularly the more medial segment of the nucleus. A sparse projection was noted in contralateral CM. Ipsilateral, intratrigeminal connections between NC and main sensory nucleus (MSV) also were observed. We conclude that, in the squirrel monkey, NC efferents, probably including LRF, may be considered analagous to the spinothalamic system by virtue of terminations in older medial and newer ventroposterior thalamus. Terminations in posterior MD may be specific to Primates. Moreover, projections to an area just dorsal to VP proper in squirrel monkey may be included within the broader definition of a neo-spinothalamic area as reflected in spinothalamic tract projections to the ventrolateral complex in cat (Boivie, '71b; Jones and Burton, '74). The small NC projection to a part of Po is consistent with spinothalamic terminations to a “posterior” thalamic area in other primates (Mehler, '69), and with the suggestion that medial Po transmits pain information (Burton and Jones, '76).     

The lateral cervical nucleus in the cat: anatomic organization of cervicothalamic neurons.

The morphology of the lateral cervical nucleus (LCN) and the organization of the cervicothalamic projection neurons were studied in cats which had received thalamic injections of horseradish peroxidase (HRP). The boundaries of the LCN were defined following very large thalamic (HRP injections. Roughly 92-97% of LCN cells project contralaterally to thalamus; an additional 1.5% project ipsillaterally. Computer-assisted measurements of perikaryal areas demonstrated that there are two sizes of LCN cells, large (175-900 micrometer 2) and small (less than 175 micrometer 2); the small cells are localized in the medial third of the LCN. LCN cells which are not labeled after large thalamic HRP injections are predomininantly small, medially-located neurons. Small HRP injections into physiologically identified regions of ventroposterior thalamus demonstrated that cervicothalamic neurons are organized in a topography consistent with that observed physiologically in the LCN (Craig and Tapper, '78). Dorsolateral LCN cells are retrogradely labeled from nucleus ventroposterolateralis, pars lateralis (VPL1), ventromedial LCN cells are labeled from pars medialis (VPL m), and a few medial cells are labeled from nucleus ventroposteromedialis (VPM). A few cells in the medial portion of the LCN are also labeled from each part of ventroposterior thalamus. Some interspersion was observed even in the cases with the most well-restricted labeling. We conclude that the LCN maintains a basic somatotographic organization with an inherent variability, certain aspects of which are consistently demonstrable both physiologically and anatomically. Evidence was also obtained suggestive of a rostrocaudal inversion in the cervicothalamic projection. The cervicothalamic projection, the differentiation of the medial LCN subpopulation, and the possible redefinition of the LCN are discussed in light of these results.     

Gustatory pathways in the bullhead catfish. II. Facial lobe connections.

The second order projections of the gustatory system in catfish were examined using both retrograde (HRP) and anterograde (degeneration and autoradiographic) hodological methods. Golgi-stained material was used to demonstrate the different cell types in the primary gustatory sensory area, the facial lobe. Efferents from the facial lobe gather into ascending and descending secondary gustatory tracts. The descending tract terminates largely in the medial funicular nucleus and the commissural nucleus of Cajal in the region of the obex. A small portion of the descending tract continues caudally to terminate in the dorsal horn of the spinal cord. The ascending secondary gustatory tract terminates mostly in the ipsilateral superior secondary gustatory nucleus in the isthmic region. A small portion of the ascending tract continues rostrally to terminate in the posterior thalamic nucleus and in the region of the nucleus lobo-bulbaris. Sparser contralateral projections are also seen in the posterior thalamus and isthmic gustatory regions. Three cell types can be discerned in the facial lobe: small, medium and large. The small cells are intrinsic neurons, the medium cells project to the isthmic gustatory nucleus, and the large cells send fibers to the other terminal areas described above, as summarized in figure 19.     

Cortical and thalamic connections of the representations of the teeth and tongue in somatosensory cortex of new world monkeys

Connections of representations of the teeth and tongue in primary somatosensory cortex (area 3b) and adjoining cortex were revealed in owl, squirrel, and marmoset monkeys with injections of fluorescent tracers. Injection sites were identified by microelectrode recordings from neurons responsive to touch on the teeth or tongue. Patterns of cortical label were related to myeloarchitecture in sections cut parallel to the surface of flattened cortex, and to coronal sections of the thalamus processed for cytochrome oxidase (CO). Cortical sections revealed a caudorostral series of myelin dense ovals (O1-O4) in area 3b that represent the periodontal receptors of the contralateral teeth, the contralateral tongue, the ipsilateral teeth, and the ipsilateral tongue. The ventroposterior medial subnucleus, VPM, and the ventroposterior medial parvicellular nucleus for taste, VPMpc, were identified in the thalamic sections. Injections placed in the O1 oval representing teeth labeled neurons in VPM, while injections in O2 representing the tongue labeled neurons in both VPMpc and VPM. These injections also labeled adjacent part of areas 3a and 1, and locations in the lateral sulcus and frontal lobe. Callosally, connections of the ovals were most dense with corresponding ovals. Injections in the area 1 representation of the tongue labeled neurons in VPMpc and VPM, and ipsilateral area 3b ovals, area 3a, opercular cortex, and cortex in the lateral sulcus. Contralaterally, labeled neurons were mostly in area 1. The results implicate portions of areas 3b, 3a, and 1 in the processing of tactile information from the teeth and tongue, and possibly taste information from the tongue.     

Single thalamic dopaminergic neurons project to both the neocortex and spinal cord

Cells in the rat subparafascicular thalamic nucleus (Spf) belonging to the diencephalic A11 cell group, were immunohistochemically stained with antibodies against tyrosine hydroxylase (TH) and dopamine itself. Employing a combination of retrograde fluorescent double-labeling and TH immunofluorescence techniques, we revealed the existence of dopaminergic Spf cells, giving rise to collateral projections to the neocortex and spinal cord.     

Widespread dopaminergic projections of the subparafascicular thalamic nucleus in the rat

The subparafascicular thalamic nucleus (Spf) contains a substantial number of dopaminergic neurons. The present study was designed to investigate in the rat whether or not Spf neurons projecting to a variety of central nervous system structures are dopaminergic. The following eight structures were tested for projection sites of Spf dopamine neurons: the neocortex, olfactory tubercle, nucleus accumbens, striatum. globus pallidus. amygdala, inferior olive, and spinal cord. By using a combination of fluorescent retrograde axonal tracing and immunofluorescence histochemistry for tyrosine hydroxylase, it has been revealed that the Spf provides a dopaminergic innervation, in varying degree, to each of these structures: the neocortex and spinal cord were the largest targets for dopaminergic projections from the Spf. The Spf was also found to contain significant numbers of dopaminergic neurons projecting to the olfactory tubercle and amygdala. In contrast, dopaminergic projections of Spf neurons to the nucleus accumbens, striatum, globus pallidus, and inferior olive were only minor. Furthermore, a series of fluorescent retrograde double-labeling experiments have indicated that individual Spf neurons are poorly collateralized to more than one of the eight terminal fields examined; the Spf neurons descending to the spinal cord relatively more frequently send axon collaterals ascending to the telencephalic structures, including the neocortex, olfactory tubercle, nucleus accumbens, striatum, and amygdala. The present results suggest that the Spf constitutes a major origin of widespread dopaminergic projections arising from the thalamus.     

Topographic organization and neurochemical identity of dorsal raphe neurons that project to the trigeminal somatosensory pathway in the rat.

The primary goals of this study were to: 1) examine the distribution of neurons within the dorsal raphe (DR) nucleus that project to cortical and subcortical sites along the trigeminal somatosensory pathway in rat; 2) determine the extent to which different regions within this ascending sensory system receive collateral projections from the same DR neuron; and 3) identify the putative transmitters contained within these DR projection neurons. Long-Evans hooded rats received pressure injections of various combinations of retrograde fluorescent tracers; into the whisker-related regions of the primary somatosensory cortex (barrel field cortex [BC]), ventral posterior medial thalamus (VPM), and principal nucleus of the trigeminal complex (PrV). The distribution of retrogradely labeled neurons within the DR was examined by fluorescence microscopy. The major finding was that cortically projecting neurons were located within the midline regions of the rostral portion of the DR, whereas cells projecting to subcortical trigeminal somatosensory structures were distributed bilaterally in the lateral wing regions of the DR as well as in the midline portions of the nucleus. Single neurons that send axon collaterals to multiple cortical and subcortical trigeminal somatosensory targets were observed in the dorsomedian and ventromedian regions of the DR. DR neurons that projected to cortical and subcortical sites contained serotonin but not tyrosine hydroxylase, the marker enzyme for catecholamine transmitters. Taken together, these findings provide further evidence of neurochemical specificity and functional anatomical organization within the DR efferent projection system.     

Comparison of numbers of interneurons in three thalamic nuclei of normal and epileptic rats

The inhibitory sources in the thalamic nuclei are local interneurons and neurons of the thalamic reticular nucleus. Studies of models of absence epilepsy have shown that the seizures are associated with an excess of inhibitory neurotransmission in the thalamus. In the present study, we used light-microscopic gamma-aminobutyric acid (GABA) immunocytochemistry to quantify the interneurons in the lateral geniculate (LGN), ventral posteromedial (VPM), and ventral posterolateral (VPL) thalamic nuclei, and compared the values from normal Wistar rats and genetic absence epilepsy rats from Strasbourg (GAERS). We found that in both Wistar rats and GAERS, the proportion of interneurons was significantly higher in the LGN than in the VPM and VPL. In the LGN of Wistar rats, 16.4% of the neurons were interneurons and in the GAERS, the value was 15.1%. In the VPM, the proportion of interneurons was 4.2% in Wistar and 14.9% in GAERS; in the VPL the values were 3.7% for Wistar and 11.1% for the GAERS. There was no significant difference between Wistar rats and the GAERS regarding the counts of interneurons in the LGN, whereas the VPM and VPL showed significantly higher counts in GAERS. Comparison of the mean areas of both relay cells and interneuronal profiles showed no significant differences between Wistar rats and GAERS. These findings show that in the VPL and the VPM there are relatively more GABAergic interneurons in GAERS than in Wistar rats. This may represent a compensatory response of the thalamocortical circuitry to the absence seizures or may be related to the production of absence seizures.     

Thalamic processing of vibrissal information in the rat. I. Afferent input to the medial ventral posterior and posterior nuclei.

Retrograde tracing with true blue (TB) and diamidino yellow (DY) and anterograde tracing with either wheatgerm agglutinin-conjugated horseradish peroxidase (WGA-HRP) or Phaseolus vulgaris leucoagglutinin (PHA-L) were employed to investigate the projections from trigeminal nucleus principalis (PrV) and trigeminal subnucleus interpolaris (SpI) to their targets in the medial ventral posterior (VPM) and posterior (POm) nuclei of the thalamus. Many more cells in both PrV and SpI were labeled by tracer injections into VPM than into POm. Only a very small number of double-labeled neurons were observed in either PrV or SpI. However, a significantly higher percentage of SpI cells projected to POm or to both POm and VPM than was the case for PrV. Anterograde tracing with WGA-HRP showed that the projections from both PrV and SpI to VPM were much denser than those from the same nuclei to POm. Small injections of PHA-L into either PrV or SpI produced a focus of fairly dense labeling in VPM and much more diffuse terminal labeling in POm. These anatomical data provide evidence for two separate trigeminothalamic pathways, one originating from PrV and the second originating from SpI. Both of these pathways converge and diverge at the thalamic level. That is, information from the PrV pathway and from the SpI pathway are both provided to VPM in a morphologically restricted fashion and to POm in a morphologically widespread fashion.