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PubMed2003

Large cholinergic nerve terminals on subsets of motoneurons and their relation to muscarinic receptor type 2.

The cholinergic C-bouton is a large nerve terminal found exclusively apposing motoneuron cell somata and proximal dendrites. The origin and function of the C-bouton is not known. An antiserum against the vesicular acetylcholine transporter was used to identify large cholinergic nerve terminals putatively of the C-type in close apposition to motoneuron cell somata. This type of nerve terminal was present in the rat spinal cord ventral horn, but only in some cranial motor nuclei. Fluoro-Gold tracing showed that subsets of spinal motoneuron cell somata were contacted by different numbers of putative C-boutons. Thus, motoneurons innervating an intrinsic foot muscle were contacted by about half the number of cholinergic terminals found on motoneurons of the predominantly fast-twitch gastrocnemius muscle. Slow-twitch soleus motoneurons showed an intermediate innervation. There was a strong correlation between the presence of putative C-boutons and muscarinic receptor 2 (m2)-like immunoreactivity (-LI) within a motor nucleus. By using confocal laser microscopy, the m2-LI appeared to be confined to the motoneuron cell membrane and strongly enriched beneath the C-type nerve terminal. Thus, our results suggested a differential distribution of large cholinergic C-boutons, depending on motoneuron type, and that the presence of this nerve terminal type is associated with m2-LI in the postsynaptic membrane.

Afferent Pathways↗
PubMed2003

Direct input from cochlear root neurons to pontine reticulospinal neurons in albino rat.

The cochlear root neurons (CRNs) are thought to mediate the auditory startle reflex (ASR) in the rat, which is widely used as a behavioral model for the investigation of the sensorimotor integration. CRNs project, among other targets, to the nucleus reticularis pontis caudalis (PnC), a major component of the ASR circuit, but little is known about the organization of this projection. Thus, we injected biotinylated dextran amine (BDA) in CRNs to study their projections with light and electron microscopy. Also, we performed double-labeling experiments, injecting BDA in the CRNs and subunit B of the cholera toxin or Fluorogold in the spinal cord to verify that CRNs project onto reticulospinal neurons. Electron microscopy of the labeled CRNs axons and terminals showed that even their most central and thinnest processes are myelinated. Most of the terminals are axodendritic, with multiple asymmetric synapses, and contain round vesicles (50 nm diameter). Double-labeling experiments demonstrated that CRN terminals are apposed to retrogradely labeled reticulospinal neurons in the contralateral nucleus reticularis PnC and bilaterally in the lateral paragigantocellular nucleus. Analyses of serial sections revealed that multiple CRNs synapse on single reticulospinal neurons in PnC, suggesting a convergence of auditory information. The morphometric features of these neurons classify them as giant neurons. This study confirms that CRNs project directly onto reticulospinal neurons and presents other anatomical features of the CRNs that contribute to a better understanding of the circuitry of the ASR in the rat.

Animals↗
PubMed2003

The suprachiasmatic nucleus balances sympathetic and parasympathetic output to peripheral organs through separate preautonomic neurons.

Opposing parasympathetic and sympathetic signals determine the autonomic output of the brain to the body and the change in balance over the sleep-wake cycle. The suprachiasmatic nucleus (SCN) organizes the activity/inactivity cycle and the behaviors that go along with it, but it is unclear how the hypothalamus, in particular the SCN, with its high daytime electrical activity, influences this differentiated autonomic balance. In a first series of experiments, we visualized hypothalamic pre-sympathetic neurons by injecting the retrograde tracer Fluoro-Gold into the thoracic sympathetic nuclei of the spinal cord. Pre-parasympathetic neurons were revealed by injection of the retrograde trans-synaptic tracer pseudorabies virus (PRV) into the liver and by sympathetic liver denervation, forcing the virus to infect via the vagus nerve only. This approach revealed separate pre-sympathetic and pre-parasympathetic neurons in the brainstem and hypothalamus. Next, selective retrograde tracing with two unique reporter PRV strains, one injected into the adrenal and the other into the sympathetic denervated liver, demonstrated that there are two separate populations of pre-sympathetic and pre-parasympathetic neurons within the paraventricular nucleus of the hypothalamus. Interestingly, this segregation persists into the SCN, where, as a result, the day-night balance in autonomic function of the organs is affected by specialized pre-sympathetic or pre-parasympathetic SCN neurons. These separate preautonomic SCN neurons provide the anatomical basis for the circadian-driven regulation of the parasympathetic and sympathetic autonomic output.

Adrenal Glands↗
PubMed2003

Autonomic and motor neuron death is progressive and parallel in a lumbosacral ventral root avulsion model of cauda equina injury.

Injuries to the cauda equina of the spinal cord result in autonomic and motor neuron dysfunction. We developed a rodent lumbosacral ventral root avulsion injury model of cauda equina injury to investigate the lesion effect in the spinal cord. We studied the retrograde effects of a unilateral L5-S2 ventral root avulsion on efferent preganglionic parasympathetic neurons (PPNs) and pelvic motoneurons in the L6 and S1 segments at 1, 2, 4, and 6 weeks postoperatively in the adult male rat. We used Fluoro-Gold-prelabeling techniques, immunohistochemistry, and quantitative stereologic analysis to show an injury-induced progressive and parallel death of PPNs and motoneurons. At 6 weeks after injury, only 22% of PPNs and 16% of motoneurons remained. Furthermore, of the neurons that survived at 6 weeks, the soma volume was reduced by 25% in PPNs and 50% in motoneurons. Choline acetyltransferase (ChAT) protein was expressed in only 30% of PPNs, but 80% of motoneurons remaining at 1 week postoperatively, suggesting early differential effects between these two neuronal types. However, all remaining PPNs and motoneurons were ChAT positive at 4 weeks postoperatively. Nuclear condensation and cleaved caspase-3 were detected in axotomized PPNs and motoneurons, suggesting apoptosis as a contributing mechanism of the neural death. We conclude that lumbosacral ventral root avulsions progressively deplete autonomic and motor neurons. The findings suggest that early neuroprotection will be an important consideration in future attempts of treating acute cauda equina injuries.

Animals↗
PubMed2004

Effects of anesthetics on hypoglossal nerve discharge and c-Fos expression in brainstem hypoglossal premotor neurons.

This study examined the effects of anesthesia on the hypoglossal nerve and diaphragm activities and on c-Fos expression in brainstem hypoglossal premotor neurons (pmXII). Experiments were performed in 71 rats by using halothane inhalation, pentobarbital sodium, or mixtures of alpha-chloralose and urethane or ketamine and xylazine. First, various cardiorespiratory parameters were measured in the rats (n = 31) during both awake and anesthetized conditions. The volatile anesthetic halothane, but not the other anesthetics, was always associated with a strong phasic inspiratory activity in the hypoglossal nerve. Second, a double-immunohistochemical study was performed in awake and anesthetized rats (n = 40) to gauge the level of activity of pmXII neurons. Brainstem pmXII neurons were identified after microiontophoresis of the retrograde tracer Fluoro-Gold in the right hypoglossal motor nucleus. Patterns of c-Fos expression at different brainstem levels were compared in five groups of rats (i.e., awake or anesthetized with halothane, pentobarbital, chloralose-urethane, and ketamine-xylazine). Sections were processed for double detection of c-Fos protein and Fluoro-Gold by using the standard ABC method and a two-color peroxidase technique. Anesthesia with halothane induced the strongest c-Fos expression in a restricted pool of pmXII located in the pons at the level of the Kölliker-Fuse nucleus and the intertrigeminal region. The results demonstrated a major effect of halothane in inducing changes in hypoglossal activity and revealed a differential expression of c-Fos protein in pmXII neurons among groups of anesthetized rats. We suggest that halothane mediates changes in respiratory hypoglossal nerve discharge by altering activity of premotor neurons in the Kölliker-Fuse and intertrigeminal region.

Action Potentials↗
PubMed2004

Connection from the dorsal column nuclei to the superior colliculus in the rat: topographical organization and somatotopic specific plasticity in response to neonatal enucleation.

Somatosensory stimuli from the body to deep and intermediate strata of the superior colliculus (SC) are relayed from the dorsal column nuclei (DCN), gracile (GrN) and cuneate (CuN). Electrophysiological studies have shown that the somatosensory representation in SC is arranged into a map-like pattern. However, there is a lack of studies confirming a morphological correlate of such an organization. On the other hand, after neonatal enucleation in rodents, somatosensory inputs ascend from their normal termination territory in intermediate and deep collicular strata to invade the more dorsally located visual strata. However, the origin of these reactive afferents has not been specified. By using anterograde (biotinylated dextran amine 10,000; BDA) and retrograde (Fluoro-Gold; FG) tracers, we studied separately the connection from GrN and CuN to the intact and neonatally deafferented SC. GrN-collicular afferents were found to terminate mainly within the periphery of the caudomedial SC quadrant, whereas CuN-collicular fibers innervated primarily the lateral part of the rostrolateral and caudolateral collicular quadrants, in a way consistent with previously described functional data. Retrograde tracing experiments using FG injected in SC confirmed this topographical arrangement. Injections of BDA in GrN or CuN of neonatally enucleated rats showed that reactive fibers reaching superficial strata are only those CuN-collicular fibers innervating the caudolateral SC quadrant, where the forelimb is represented. The present results provide an anatomical substrate for the known somatotopic organization of tactile representation in SC and further reinforce the previous proposal that the plastic reorganization of DCN-collicular afferents following neonatal enucleation constitutes a functional compensatory response.

Animals↗
PubMed2001

Projections from the amygdalo-piriform transition area to the amygdaloid complex: a PHA-l study in rat.

The amygdalo-piriform transition area is a poorly defined region in the temporal lobe that is heavily connected with the olfactory system. As part of an ongoing project aimed at understanding the neuronal pathways that provide sensory information to the amygdala, we investigated the cytoarchitectonic and chemoarchitectonic features of the amygdalo-piriform transition area and its connections to the amygdaloid complex in 13 rats by using the anterograde tracer, Phaseolus vulgaris-leucoagglutinin. Our analysis indicates that the amygdalo-piriform transition area has medial (rostral and caudal portions) and lateral parts. The rostromedial part projects heavily to the intermediate and lateral divisions of the central nucleus, whereas the caudomedial part projects mainly to the medial division. The lateral part of the amygdalo-piriform transition area projects heavily to the capsular and lateral divisions of the central nucleus. Electron microscopic analysis revealed that the projection to the lateral division of the central nucleus forms asymmetric contacts with the spines and shafts of postsynaptic neurons and, therefore, is assumed to be excitatory. The amygdalo-piriform transition area also projects moderately to other amygdaloid nuclei, including the parvicellular division of the basal nucleus, the anterior cortical nucleus, and the nucleus of the lateral olfactory tract. The lateral and medial parts of the amygdalo-piriform transition area also project to the distal temporal CA1 and distal temporal subiculum, respectively. Unlike the adjacent entorhinal cortex, the amygdalo-piriform transition area does not project to the dentate gyrus. These data suggest that the amygdalo-piriform transition area is a region that influences both emotional and memory processing in parallel by means of pathways to the amygdala and the hippocampus, respectively.

Amygdala↗
PubMed2001

Neurokinin-1 receptor-immunoreactive neurons of the ventral respiratory group in the rat.

The rostral end of the ventral respiratory group (VRG) contains neurons that are intensely neurokinin-1 receptor (NK1R) immunoreactive (ir). It has been theorized that some of these cells might be critical to respiratory rhythmogenesis (Gray et al. [1999] Science 286:1566-1568). In the present study we determined what major transmitter these NK1R-ir cells make and whether they are bulbospinal or propriomedullary. NK1R-ir neurons were found in the VRG between Bregma levels -11.7 and -13.6 mm. The highest concentration was found between Bregma -12.3 and -13.0 mm. This region overlaps with the pre-Bötzinger complex (pre-BötC) as it was found to contain many pre-inspiratory neurons, few E2-expiratory neurons, and no I-incremental neurons. VRG NK1R-ir neurons contain neither tyrosine hydroxylase (TH) nor choline acetyl-transferase (ChAT) immunoreactivity, although dual-labeled neurons were found elsewhere within the rostral medulla. GAD67 mRNA was commonly detected in the ventrolateral medulla (VLM) but rarely in the NK1R-ir neurons of the pre-BötC region (6 % of somatic profiles). GlyT2 mRNA was commonly found in the pre-BötC region but rarely within NK1R-ir neurons (1.3 %). Up to 40% of VRG NK1R-ir neurons were retrogradely labeled by Fluoro-Gold (FG) injected in the contralateral pre-BötC region. Some NK1R-ir VRG neurons located caudal to Bregma -12.6 mm were retrogradely labeled by FG injected in the spinal cord (C4-C5, T2-T4). In sum, NK1R immunoreactivity is present in many types of ventral medullary neurons. Within the VRG proper, NK1R-ir neurons are concentrated in an area that overlaps with the pre-BötC. Within this limited region of the VRG, NK1R-ir neurons are neither cholinergic nor catecholaminergic, and very few are gamma-aminobutyric acid (GABA)ergic or glycinergic. The data suggest that most NK1R-ir neurons of the pre-BötC region are excitatory. Furthermore, the more rostral NK1R-ir cells are propriomedullary, whereas some of the caudal ones project to the spinal cord.

Acetylcholine↗
PubMed2001

Local differences in vagal afferent innervation of the rat esophagus are reflected by neurochemical differences at the level of the sensory ganglia and by different brainstem projections.

The objective of the present study was to characterize further the vagal afferent fibers in the rat esophagus, particularly those in its uppermost part, their cell bodies in vagal sensory ganglia, and their central projections. We applied immunohistochemistry for calretinin, calbindin, and calcitonin gene-related peptide (CGRP); retrograde tracing with FluoroGold; and transganglionic tracing with wheat germ agglutinin-horseradish peroxidase in combination with neurectomies. Vagal terminal structures in the muscularis propria of the whole esophagus consisted of calretinin-immunoreactive intraganglionic laminar endings that were linked to cervical vagal and recurrent laryngeal nerve pathways. The mucosa of the uppermost esophagus was innervated by a very dense net of longitudinally arranged, calretinin-positive fibers that were depleted by section of the superior laryngeal nerve. Distal to this area, the mucosa was virtually devoid of calretinin-immunoreactive vagal afferents. Calretinin-positive mucosal fibers in the upper cervical esophagus were classified into four types. One type, the finger-like endings, was sometimes immunoreactive also for CGRP. About one-third of cell bodies in vagal sensory ganglia retrogradely labeled from the upper cervical esophagus expressed CGRP, whereas two-thirds coexpressed calretinin and calbindin but not CGRP. In addition to the central subnucleus of the nucleus of the solitary tract, vagal afferents from the upper cervical esophagus also projected heavily to the interstitial subnucleus. This additional projection was attributed to mucosal afferents traveling through the superior laryngeal nerve. The present study provides a possible morphological basis for bronchopulmonary and aversive reflexes elicited upon stimulation of the esophagus.

Animals↗
PubMed2001

Number, origins, and chemical types of rat pallidostriatal projection neurons.

The dorsal globus pallidus (GP) receives major inputs from the dorsal neostriatum (Str), the subthalamic nucleus (STN) and the dorsal thalamus. The GP projects to multiple basal ganglia nuclei. One of the GP projection sites is the Str. The pallidostriatal projection has been considered minor. However, several recent studies have suggested that this projection is heavier than previously thought and that it might play a significant role in controlling the activity of the Str. To reveal more details of this projection, we examined the number of GP neurons that participated in the projection, their origins in the GP and their immunoreactivity for the calcium binding protein parvalbumin (PV), by using a combination of Fluoro-Gold (FG) retrograde labeling and immunohistochemical methods. Immunostaining for the calcium binding protein calbindin-28K (CaBP) was used to identify the CaBP-poor sensorimotor and CaBP-rich associative Str regions and the corresponding CaBP-poor middle, CaBP-rich border, and the caudomedial GP regions. The CaBP-poor dorsolateral Str region occupies a small portion of the Str, whereas the CaBP-poor middle GP region occupies a large portion of the GP. The immunostaining for neuron-specific nuclear protein (NeuN) was used to visualize neurons that were immunonegative for FG or PV. Cell counts revealed that the middle GP region contained a higher density of neurons and also a higher percentage of PV-positive neurons than the border and caudomedial regions of the GP. These observations suggested that the GP is involved more in sensorimotor function than associative function. Approximately 40% of neurons in the CaBP-poor middle GP region project to the CaBP-poor part of the dorsolateral Str. Approximately 30% of the neurons in both the CaBP-rich border and the caudomedial GP regions project to the CaBP-rich Str region. More than 40% of the pallidostriatal neurons in CaBP-poor middle GP region are PV-positive, whereas most of those in CaBP-rich GP regions are PV-negative. It was estimated from the cell count data that most of the PV-negative neurons in all three regions of the GP project to the Str. The results indicate that the sensorimotor and associative territories of the Str have reciprocal projections between corresponding territories of the GP. The involvement of a large number of GP neurons suggested that the pallidostriatal projection should be taken into account in the analysis of functional roles of the basal ganglia.

Animals↗
PubMed2001

The deep mesencephalic nucleus as an output center of basal ganglia: morphological and electrophysiological similarities with the substantia nigra.

The deep mesencephalic nucleus (DMN) is a large midbrain reticular region between the superior colliculus, the substantia nigra compacta, the periaqueductal gray, and the medial geniculate body. Although some data suggest that it is involved in nociception and visceral control, its functions remain unclear. In the present study, by using morphological (combination of anterograde and retrograde tracers with immunocytochemistry and in situ hibrydization) and electrophysiological (firing activity and transynaptic response to striatal stimulation) methods, we show that a subpopulation of DMN cells shares many morphological and electrophysiological characteristics with those of the substantia nigra reticulata (SNR). These similarities include the following: 1) firing rate, firing pattern, and conduction velocity; 2) expression of GAD65, GAD67, and PV; 3) excitatory and inhibitory inputs from the striatum; and 4) projections to the ventral thalamus, superior colliculus, and pedunculopontine tegmental nucleus. Some differences were also found. In comparison with SN, DMN cells and striatal afferents are more sparsely distributed and they show conspicuous contralateral projections to the thalamus and superior colliculus. This suggests that, similarly to the SNR, the DMN acts as an output center of basal ganglia and probably facilitates the inter-hemispheric regulation of these centers.

Action Potentials↗
PubMed2001

Distribution of gonadal steroid receptor-containing neurons in the preoptic-periaqueductal gray-brainstem pathway: a potential circuit for the initiation of male sexual behavior.

The present study used anterograde and retrograde tract tracing techniques to examine the organization of the medial preoptic-periaqueductal gray-nucleus paragigantocellularis pathway in the male rat. The location of neurons containing estrogen (alpha subtype; ER alpha) and androgen receptors (AR) were also examined. We report here that injection of the anterograde tracer biotinylated dextran amine (BDA) into the medial preoptic (MPO) produced dense labeling within the periaqueductal gray (PAG); anterogradely labeled fibers terminated in close juxtaposition to neurons retrogradely labeled from the nucleus paragigantocellularis (nPGi). Dual immunostaining for Fluoro-Gold (FG) and ER alpha or FG and AR showed that over one-third of MPO efferents to the PAG contain receptors for either estrogen or androgen. In addition, approximately 50% of PAG neurons retrogradely labeled from the nPGi were immunoreactive for either ER alpha or AR. These results are the first to establish an MPO-->PAG-->nPGi circuit and further indicate that gonadal steroids can influence neuronal synaptic activity within these sites. We reported previously that nPGi reticulospinal neurons terminate preferentially within the motoneuronal pools of the lumbosacral spinal cord that innervate the pelvic viscera. Together, we propose that the MPO-->PAG-->nPGi circuit forms the final common pathway whereby MPO neural output results in the initiation and maintenance of male copulatory reflexes.

Animals↗
PubMed2001

Identification of NPY-induced c-Fos expression in hypothalamic neurones projecting to the dorsal vagal complex and the lower thoracic spinal cord.

Neuropeptide Y exerts profound effects on body weight and glucose homeostasis. We have investigated the effect of centrally administered neuropeptide Y on the activity of descending neurones of the hypothalamic paraventricular nucleus by combining retrograde tract tracing with c-Fos immunocytochemistry. Male rats were injected with True Blue into the dorsal vagal complex and with FluoroGold into the intermediolateral column of the lower thoracic spinal cord. One week after the last surgical procedure, animals were injected centrally with an orexigenic dose of neuropeptide Y (5 microg) and sacrificed 60 to 240 minutes following this injection. Temporal analysis of NPY-induced c-Fos expression showed a peak at 90 minutes, which was nearly returned to basal levels between 120 and 240 minutes. Expression of c-Fos was prominent in several of the subnuclei of the paraventricular nucleus and in the adjacent perifornical nucleus. Neurones projecting to the spinal cord were prominent in the dorsal, lateral, and ventral portion of the medial parvicellular subnuclei of the PVN. About 15% of IML projecting neurones of the medial parvicellular subnucleus were Fos-positive, whereas less than 5% of IML projecting neurones from other subnuclei were Fos-positive. Hardly any PVN neurones projecting to the dorsal vagal complex were concomitantly Fos-positive. A considerably larger (>10%) proportion of perifornical neurones projecting to the nucleus of the solitary tract were c-Fos-immunopositive. In conclusion, NPY induces c-Fos in paraventricular neurones projecting to intermediolateral column of the spinal cord and in neurones of the perifornical nucleus projecting to the dorsal vagal complex.

Animals↗
PubMed2001

Characterization of the central nervous system innervation of the rat spleen using viral transneuronal tracing.

Splenic immune function is modulated by sympathetic innervation, which in turn is controlled by inputs from supraspinal regions. In the present study, the characterization of central circuits involved in the control of splenic function was accomplished by injecting pseudorabies virus (PRV), a retrograde transynaptic tracer, into the spleen and conducting a temporal analysis of the progression of the infection from 60 hours to 110 hours postinoculation. In addition, central noradrenergic cell groups involved in splenic innervation were characterized by dual immunohistochemical detection of dopamine-beta-hydroxylase and PRV. Infection in the CNS first appeared in the spinal cord. Splenic sympathetic preganglionic neurons, identified in rats injected with Fluoro-Gold i.p. prior to PRV inoculation of the spleen, were located in T(3)-T(12) bilaterally; numerous infected interneurons were also found in the thoracic spinal cord (T(1)-T(13)). Infected neurons in the brain were first observed in the A5 region, ventromedial medulla, rostral ventrolateral medulla, paraventricular hypothalamic nucleus, Barrington's nucleus, and caudal raphe. At intermediate survival times, the number of infected cells increased in previously infected areas, and infected neurons also appeared in lateral hypothalamus, A7 region, locus coeruleus, subcoeruleus region, nucleus of the solitary tract, and C3 cell group. At longer postinoculation intervals, infected neurons were found in additional hypothalamic areas, Edinger-Westphal nucleus, periaqueductal gray, pedunculopontine tegmental nucleus, caudal ventrolateral medulla, and area postrema. These results demonstrate that the sympathetic outflow to the spleen is controlled by a complex multisynaptic pathway that involves several brainstem and forebrain nuclei.

Animals↗
PubMed2001

Anatomical and functional connections among cell groups in the gerbil brain that are activated with ejaculation.

Based on Fos expression, four areas of the gerbil brain are activated with ejaculation, i.e., the posterodorsal preoptic nucleus (PdPN), the lateral part of the posterodorsal medial amygdala (MeApd), the medial cell group of the sexually dimorphic preoptic area (medial SDA), and the parvicellular part of the subparafascicular thalamus (SPFp). The SPFp and medial SDA also express Fos earlier in the context of mating. To study connections among these areas, we injected one with FluoroGold and assessed the colocalization of FluoroGold and mating-induced Fos in the others. To determine if any of these areas activates the others, we lesioned one unilaterally and measured mating-induced Fos ipsilaterally and contralaterally in the others. Half of the SPFp cells projecting to the medial SDA, PdPN, and MeApd were activated with mating. SPFp lesions also decreased Fos expression in those areas. However, those areas do not project to the SPFp or affect its Fos expression with mating. Projections from the lateral MeApd to the medial SDA and PdPN, and from the medial SDA to the lateral MeApd, were also activated with mating, but lesions in these areas did not affect Fos expression in the others. Because 32-50% of the mating-activated cells in the SPFp participated in each SPFp projection identified, projections may have been identified for all of the mating-activated cells in the SPFp. In contrast, most of the mating-activated cells in the lateral MeApd, PdPN, and medial SDA do not participate in any projection studied, suggesting that they are either interneurons or project elsewhere.

Amygdala↗
PubMed2001

Commissurally projecting inhibitory interneurons of the rat hippocampal dentate gyrus: a colocalization study of neuronal markers and the retrograde tracer Fluoro-gold.

Improved methods for detecting neuronal markers and the retrograde tracer Fluoro-Gold (FG) were used to identify commissurally projecting neurons of the rat hippocampus. In addition to the dentate hilar mossy cells and CA3 pyramidal cells shown previously to transport retrograde tracers after injection into the dorsal hippocampus, FG-positive interneurons of the dentate granule cell layer and hilus were detected in numbers greater than previously reported. FG labeling of interneurons was variable among animals, but was as high as 96% of hilar somatostatin-positive interneurons, 84% of parvalbumin-positive cells of the granule cell layer and hilus combined, and 33% of hilar calretinin-positive cells. By comparison, interneurons of the dentate molecular layer and all hippocampal subregions were conspicuously FG-negative. Whereas hilar mossy cells and CA3 pyramidal cells were FG-labeled throughout the longitudinal axis, FG-positive interneurons exhibited a relatively homotopic distribution. "Control" injections of FG into the neocortex, septum, and ventral hippocampus demonstrated that the homotopic labeling of dentate interneurons was injection site-specific, and that the CA1-CA3 interneurons unlabeled by contralateral hippocampal FG injection were nonetheless able to transport FG from the septum. These data suggest a hippocampal organizing principle according to which virtually all commissurally projecting hippocampal neurons share the property of being monosynaptic targets of dentate granule cells. Because granule cells innervate their exclusively ipsilateral target cells in a highly lamellar pattern, these results suggest that focal granule cell excitation may result in commissural inhibition of the corresponding "twin" granule cell lamella, thereby lateralizing and amplifying the influence of the initiating discharge.

Animals↗
PubMed2001

Striatal and central extended amygdala parts of the interstitial nucleus of the posterior limb of the anterior commissure: evidence from tract-tracing techniques in the rat.

The interstitial nucleus of the posterior limb of the anterior commissure (IPAC) lies at the junction of the striatopallidal system and the lateral bed nucleus of the stria terminalis-central amygdaloid nucleus continuum (i.e., the central extended amygdala; EAc). Its efferent connections were investigated in the rat with anterograde (Phaseolus vulgaris leucoagglutinin) and retrograde (Fluoro-Gold and cholera toxin B subunit) tracers and compared with those of the central amygdaloid nucleus. Our anterograde tracing experiments reveal that the projections of the medial IPAC largely reciprocate its afferent connections (Shammah-Lagnado et al. [1999] Neuroscience 94:1097-1123) and are very similar to those of the medial part of the central amygdaloid nucleus. The lateral IPAC, on the other hand, innervates the pallidal complex, substantia nigra and retrorubral field. Local connections are found within medial IPAC and within lateral IPAC, but the two divisions are not interconnected. Our retrograde tracing experiments confirm that IPAC projections to EAc components, parabrachial area, and nucleus of the solitary tract originate chiefly from the medial division, whereas both medial and lateral divisions innervate the retrorubral field. Moreover, in sections processed for choline acetyltransferase, the strong projections from caudal IPACm to the posterior basolateral amygdaloid nucleus and the amygdalopiriform transition area were found to arise chiefly from cholinergic cells. Overall, our results suggest that the medial IPAC is intimately related to the EAc, whereas the lateral IPAC represents a striatal territory.

Amygdala↗
PubMed2004

Glutamatergic neuronal projections from the marginal layer of the rostral ventral medulla to the respiratory centers in rats.

The marginal layer (ML) that lines the ventral surface of the medulla oblongata (VMS) contains neurons thought to contribute to central chemoreception, the process by which systemic hypercapnia activates respiration. The transmitters and connectivity of ML neurons are poorly known. The present study focuses on a group of nonserotonergic ML neurons, often located in close proximity to the entry point of penetrating blood vessels. These neurons (approximately 300/brain) contain vesicular glutamate transporter2 (VGLUT2) mRNA and are thus probably glutamatergic. They cluster below the caudal half of the facial motor nucleus, lateral to the serotonergic cells of the ML. The projections of serotonergic and nonserotonergic ML neurons were investigated by retrograde labeling with Fluoro-Gold. ML VGLUT2 mRNA-expressing neurons lack spinal projections and innervate the dorsolateral pons and the ipsilateral ventral respiratory column (VRC), most particularly, the region of the pre-Bötzinger complex and rVRG. The latter two regions receive a very small input from ML serotonergic neurons which, instead, heavily innervate the spinal cord. In conclusion, a small region of the VMS marginal layer contains glutamatergic neurons that innervate the main respiratory centers of the medulla oblongata and pons. These glutamatergic neurons are located in a chemosensitive region of the ML and their projections are consistent with a role in central chemoreception. The serotonergic neurons of the ML, though known to be activated by CO(2), probably do not contribute to central chemoreception, given that they innervate sympathetic efferents and project at best very lightly to the VRC.

Animals↗