GABAergic synaptic transmission in projections from the basal forebrain and hippocampal formation to the amygdala: an in vivo iontophoretic study

We recorded extracellular responses from rat amygdaloid neurons in vivo after electrical stimulation of the basal forebrain and hippocampal formation. Iontophoretic application of the GABA_A receptor antagonist, bicuculline, lead to the appearance of short latency evoked bursts after stimulation of either region. This occurred whether the baseline response was inhibitory or excitatory. Bicuculline only affected an early phase of inhibition, leaving a longer latency, longer duration phase unchanged or even increased. By contrast, the GABA_B receptor antagonist, phaclofen, never produced such short latency evoked bursts. Both bicuculline and phaclofen increased the spontaneous rate of firing of amygdaloid neurons. The excitatory burst response to hippocampal formation stimulation of an amygdaloid candidate inhibitory neuron was blocked by CNQX (an antagonist of the AMPA subtype of glutamate receptor). Based on these and prior studies, it seems likely that the effects of hippocampal formation stimulation are mediated by feed-forward inhibition, in which GABAergic amygdaloid inhibitory neurons are excited by glutamatergic projections from the hippocampal formation. The effects of basal forebrain stimulation may be mediated by both feed-forward inhibition and direct, GABAergic inhibition.     

Convergence of somatosensory and baroreceptive inputs onto parabrachio-subfornical organ neurons in the rat: an electrophysiological study.

Electrophysiological characteristics were described for neurons of the parabrachial nucleus (PBN) which receive baroreceptive and somatosensory inputs in the rat. Following focal electrical stimulation in the ipsilateral caudal nucleus of the tractus solitarii (NTS), the firing rates of these neurons were increased in 94 (55.6%), and decreased in 38 (22.5%). Fifty-three (54.5%) of 97 PBN neurons tested were excited, and 11 (11.3%) inhibited in response to contralateral common peroneal nerve (CPN) stimulation. Of these neurons, 52 were found to respond to both caudal NTS and CPN stimulation. Effects of subfornical organ (SFO) stimulation were also examined in 151 of 169 parabrachial neurons. 13 (7.7%) were activated antidromically and were located in the lateral division of the PBN, while 34 (22.5%) were affected orthodromically. Stimulation of the caudal NTS resulted in both a fall in the heart rate and changes of PBN neuronal firing rates. Similar effects were elicited by activating peripheral baroreceptors by the administration of phenylephrine, an alpha-adrenergic agonist. These results strongly indicate that: (1) the lateral PBN is involved in central cardiovascular control; (2) somatosensory and baroreceptive messages may converge onto some PBN neurons; and (3) some PBN neurons may relay baroreceptive information from the caudal NTS to the SFO.     

Choline acetyltransferase and acetylcholinesterase containing projections from the basal forebrain to the amygdaloid complex of the rat

The origin of the cholinergic innervation to the amygdaloid complex was investigated with the use of acetylcholinesterase (AChE) histochemistry and choline acetyltransferase (ChAT) assay of microdissected nuclei. Visualization of AChE-positive neurones in the ventral forebrain was facilitated by pretreatment of rats with 1.5 mg/kg di-isopropyl phosphofluoridate (DFP). The AChE-positive neurones in the ventral forebrain are distributed in a continuous system from the septum through the lateral preoptic area to the entopeduncular nucleus caudally. Knife cuts or kainic acid injections (1.5 microgram/l microliter) placed in the lateral preoptic area resulted in substantial depletion of the AChE and ChAT content of the amygdala nuclei. Kainic acid injections (1.5 microgram/l microliter) in the diagonal band area or cuts through the stria terminalis dorsally did not significantly modify the AChE staining or ChAT content of the amygdala (although diagonal band injections partially depleted the hippocampus of ChAT). Knife cuts severing both the so-called ventral pathway and the stria terminalis did not produce significantly greater ChAT depletion in the amygdala than those produced by the knife cuts or kainic acid injections in the lateral preoptic area. Parasagittal knife cuts undercutting the lateral pyriform cortex also failed to modify the AChE or ChAT content of the amygdala, but they depleted the undercut cortex of both ChAT and AChE; AChE-positive material accumulated ventrally and medially to the knife cut. It is suggested that the major source of the cholinergic innervation of the amygdala is the magnocellular AChE-positive neurones in the lateral preoptic area and adjacent regions of the ventral forebrain.     

Cholinergic projections from the basal forebrain to the basolateral amygdaloid complex: a combined retrograde fluorescent and immunohistochemical study

We have examined the location of cholinergic and non-cholinergic neurons that project to the rat basolateral amygdaloid nucleus by using choline acetyltransferase (ChAT) immunohistochemistry in combination with retrograde fluorescent tracing on the same tissue section. Since many tracer-and ChAT-positive neurons were identified in basal forebrain areas, including the ventral pallidum, we also stained many of the sections for glutamate decarboxylase, a suitable marker for the delineation of pallidal areas. Cholinergic neurons projecting to the basolateral amygdaloid nucleus were observed in a continuous territory stretching from the dorsal part of ventral pallidum, through sublenticular substantia innominata to ventral parts of globus pallidus and peripallidal areas. Non-cholinergic neurons projecting to the basolateral amygdaloid nucleus were found intermixed within the same structures and constitute approximately 25% of the amygdalopetal projection neurons in these ventral forebrain structures. Since amygdalopetal cholinergic neurons were demonstrated in areas generally recognized as giving rise to cholinergic projections to cerebral cortex, several retrograde double-labeling experiments with two different fluorescent tracers were performed for the purpose of detecting the possible existence of collateral projections. The results obtained showed that the cholinergic basal forebrain neurons in general project to only one forebrain region, and, furthermore, that the cholinergic system consists of partially overlapping subsets of neurons that project to various neocortical and allocortical areas and to the amygdaloid body.     

Perirhinal cortex projections to the amygdaloid complex and hippocampal formation in the rat

The differential efferent projections of the perirhinal cortex were traced by using anterograde and retrograde tracing techniques. The dorsal bank cortex (area 36) projected lightly to the lateral entorhinal cortex and more strongly to the lateral, posterolateral cortical, and posterior basomedial amygdaloid nuclei and amygdalostriatal transition zone. The ventral bank (dorsolateral entorhinal cortex) projected to the lateral entorhinal cortex, dorsal subiculum, and subfield CA1 and mainly targeted the basolateral amygdaloid nucleus. Corticocortical projections from the dorsal and ventral banks targeted different cortical areas. The fundus of the rhinal sulcus (area 35) projected to both lateral and medial entorhinal cortices, ventral subiculum, lateral and basolateral nuclei, and amygdalostriatal transition zone. Corticocortical projections targeted areas projected to by both dorsal and ventral banks and also by second somatosensory area, first temporal cortical area, and striate cortex. Neurons projecting to the lateral nucleus were distributed in all layers of the dorsal bank, wheras those projecting to CA1 and subiculum were found in superfical layers (mostly layer III) of the ventral bank. Projections to the basolateral nucleus arose from superfical layers (mostly layer II) of the fundus and deep layers of the ventral bank. Furthermore, projections to the amygdala mostly arose from rostral levels, whereas hippocampal projections primarily originated caudally. The rat perirhinal cortex is heterogeneous in its efferent connectivity, and distinct projections arise from the dorsal and ventral banks and fundus of the rhinal sulcus. The widespread cortical connectivity of the fundus suggests that only this part of the perirhinal cortex is similar to area 35 of the primate brain.     

Collateral projections from the rat hippocampal formation to the lateral and medial prefrontal cortex

Projections from the hippocampal formation to the medial prefrontal cortex are well known. In this report we used two retrogradely transported tracers to show that a small but significant subpopulation of pyramidal neurons in area CA1 and subiculum of the hippocampal formation projects to the lateral prefrontal cortex. About half of these neurons also possess collateral projections to the medial prefrontal cortex. The neurons projecting only to the lateral PFC are found in the intermediate hippocampal formation and in the most ventral part of the temporal subdivision. On the other hand, most of the neurons projecting to the medial prefrontal cortex only are present in the temporal and ventral intermediate hippocampal formation, and their number decreases in the dorsal intermediate subdivision. The distribution of neurons having collateral projections is comparable to that of neurons projecting to the medial prefrontal cortex only. In view of proposed functional differences between the septal one-third and the temporal two-third of the hippocampal formation, it is of interest that the neurons projecting to the prefrontal cortex are only present in the temporal two-thirds. Hippocampus 7:397–402, 1997. © 1997 Wiley-Liss, Inc.     

Efferents from medial basal forebrain and hypothalamus in the rat. I. An autoradiographic study of the medial preoptic area.

Efferent projections from the medial and periventricular preoptic area, bed nucleus of the stria terminalis and nuclei of the diagonal band were traced using tritiated amino acid autoradiography in albino rats. Medial and periventricular preoptic area efferents were not restricted to short-axon projections. Ascending projections from the medial preoptic area (mPOA) were traced through the diagonal band into the septum. Descending mPOA axons coursed in the medial parts of the medial forebrain bundle. Projections to most hypothalamic nuclei, including the arcuate nucleus and median eminence, were observed. In the midbrain, mPOA efferents were distributed in the central grey, raphe nuclei, ventral tegmental area and reticular formation. Projections from the mPOA were also observed to the amygdala through the stria terminalis, to the lateral habenula through the stria medullaris, and to the periventricular thalamus. Axons of the most medial and periventricular preoptic area (pvPOA) neurons had a distribution similar to more lateral mPOA neurons but their longest-axoned projections were weaker. The pvPOA did not send axons through the stria medullaris but did project more heavily than the more lateral mPOA to the arcuate nucleus and median eminence. Projections from the bed nucleus of the stria terminalis (nST) were in most respects similar to those from the medial preoptic area, with the major addition of a projection to the accessory olfactory bulb. The nuclei of the diagonal band of Broca (nDBB) gave a different pattern of projections than mPOA or nST, projecting, for instance, to the medial septum and hippocampus. Descending nDBB efferents ran in the ventral portion of the medial forebrain bundle. Among hypothalamic cell groups, only the medial mammillary nuclei received nDBB projections. nDBB efferents also distributed in the medial and lateral habenular nuclei and the mediodorsal thalamic nucleus.     

Ultrastructural morphology of projections from the medial geniculate nucleus and its adjacent region to the basal ganglia.

The efferent projections of the medial geniculate nucleus (MG) and its adjacent nuclei to the basal ganglia were studied in the rat by the antero- and retrograde tracing methods. Injections of wheat germ agglutinin conjugated to horseradish peroxidase into the caudal parts of the striatum and globus pallidus produced retrograde neuronal labeling in the medial division of the MG (MGm) and its adjacent structures including the suprageniculate, posterior intralaminar and peripeduncular nuclei, and substantia nigra pars lateralis. Injections of [3H]leucine into the MG and its surroundings resulted in anterograde labeling not only in the striatum but also in the globus pallidus. The resulting labeling was distributed exclusively in the caudal parts of these two nuclei. The electron microscopic autoradiography showed preferential radiolabeling of terminals and myelinated axons in both the globus pallidus and striatum. Labeled terminals in the pallidum mostly made symmetrical synapses on somata and major dendrites, while labeled terminals in the striatum established asymmetrical synapses on dendritic spines. These morphological differences in the synapses of the efferent systems originating from the MGm and its surrounding region suggest functional/chemical differentiations at their target sites in the basal ganglia.     

The localization of nerve growth factor-like immunoreactivity in the adult rat basal forebrain and hippocampal formation.

The role of nerve growth factor (NGF) as a target derived neurotrophic agent for specific cell populations in the peripheral nervous system has been well documented and much evidence suggests that NGF may serve a similar neurotrophic role in the CNS supporting the cholinergic neurons of the basal forebrain. Previous attempts to localize NGF by immunocytochemical methods, however, have not yielded evidence confirming the regional distribution expected based upon reported levels of extractable NGF. In the present study, affinity purified polyclonal antibodies to beta-NGF and a modified immunohistochemical protocol were used to demonstrate specific NGF-like immunoreactivity in the adult rat hippocampal formation and basal forebrain. In the hippocampal formation, NGF-like immunoreactivity was localized primarily within the hilus of the dentate gyrus and within stratum lucidum of the CA3 and CA2 hippocampal subfields. Staining appeared to be associated with cell processes and was similar to the reported distribution of mossy fibers suggesting that granule cells may either serve as a primary source of hippocampal NGF or that mossy fibers selectively accumulate NGF produced by other cell populations. In the basal forebrain, NGF-like immunoreactivity was localized within neuronal cell bodies of the medial septum, diagonal band, and nucleus basalis of Meynert and was further demonstrated to colocalize exclusively with LNGF-R positive neurons. These findings demonstrate the presence of an NGF-like antigen in association with cholinergic neurons of the basal forebrain and strongly support the hypothesis that NGF may serve as an endogenous trophic factor for this adult neuronal population.     

Efferents from medial basal forebrain and hypothalamus in the rat. II. An autoradiographic study of the anterior hypothalamus.

Using tritiated amino acid autoradiography, the efferent projections of the anterior hypothalamic area (AHA) were studied in albino rats. Axons from AHA neurons were not confined to local projections in the hypothalamus. Ascending AHA axons ran through the preoptic region, joined the diagonal band and distributed in the lateral septum. Descending AHA efferents within the hypothalamus coursed in a bundle ventromedial to the fornix. Projections were observed to the dorsomedial, ventromedial, arcuate and dorsal premammillary nuclei, and to the median eminence. Sweeping dorsomedially in the posterior hypothalamus, some AHA axons distributed in the central grey. AHA axons staying ventral projected to the supramammillary region, ventral tegmental area, raphe nuclei and midbrain reticular formation. Other AHA efferents distributed to the periventricular thalamus, to the medial amygdala via the stria terminalis or supraoptic commissure, and to the lateral habenula through the stria medullaris. For comparison with the AHA, efferent projections from the paraventricular nucleus (PVN) and from the ventromedial nucleus and adjacent basal hypothalamus (VMR) were studied. Projections from PVN neurons were not restricted to the median eminence and neurohypophysis. PVN efferents also distributed to many of the same regions as did those of the AHA but had somewhat different fiber trajectories and longer descending projections. VMR efferents were more widespread than those of the AHA, with projections extending into the lateral zona incerta and pontine reticular formation. Projections from the AHA were distinct from those of the medial preoptic area (mPOA). For example, while AHA axons descended in a bundle ventromedial to the fornix, mPOA axons ran in the medial forebrain bundle. Such anatomical differences may underlie experimentally demonstrated functional differences between the mPOA and AHA, for instance, in mediation of male and female sex behaviors.     

Extrinsic projections from area CA1 of the rat hippocampus: olfactory, cortical, subcortical, and bilateral hippocampal formation projections.

Hippocampal area CA1 provides the major cortical output of the hippocampus, but only its projections to the subiculum and lateral septal nucleus are well characterized. The present study reexamines these extrinsic projections by using anterograde and retrograde tracing techniques. Injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in the septal one-third of CA1 label axons and terminals in subicular, postsubicular, retrosplenial, perirhinal, and entorhinal cortices, lateral septal nucleus, and diagonal band of Broca. The septal CA1 injections also label terminal fields in contralateral CA1, and in contralateral subicular, postsubicular, perirhinal, and entorhinal cortices. Injections into the splenial one-third of CA1 label axons and terminals in subiculum, postsubiculum, ventral area infraradiata, and lateral septal nucleus, but they do not label axons and terminals on the contralateral side of the brain. Injections in the temporal one-third of CA1 label axons and terminals in subicular, parasubicular, entorhinal, and infraradiata cortices, anterior olfactory nucleus, olfactory bulb, lateral septal nucleus, nucleus accumbens, amygdala, and hypothalamus. The temporal CA1 injections label no axons on the contralateral side of the brain. These data demonstrate that CA1 has more widespread projections than previously appreciated, and they provide the first clear evidence that CA1 projects to the contralateral cortex and to the ipsilateral olfactory bulb, amygdala, and hypothalamus. The results also demonstrate a heterogeneity in the efferent projections originating in different septotemporal levels of CA1.     

Subfornical organ neurons with efferent projections to the hypothalamic paraventricular nucleus: an electrophysiological study in the rat

Seventeen neurons in the subfornical organ (SFO) were antidromically activated by electrical stimulation of the paraventricular nucleus (PVN) in the rat. The activity of all identified SFO neurons was excited by microiontophoretically (MIPh) applied angiotensin II (AII) and the effect of AII was blocked by MIPh-applied saralasin (Sar), an AII antagonist, but not by atropine (Atr), a muscarinic antagonist. In these identified SFO neurons, 9 were also excited and 8 were not affected by MIPh-applied acetylcholine (ACh) and the effect of ACh was attenuated by not only MIPh-applied Atr but also by Sar. These results suggest that there are specific AII- and both AII- and ACh-sensitive types of SFO neurons with efferent projections to the PVN.     

N-methylaspartate: an effective tool for lesioning basal forebrain cholinergic neurons of the rat

The ability of the excitotoxin, N-methyl-D,L-aspartic acid (NMA), to destroy basal forebrain cholinergic (BFC) neurons was evaluated. NMA (100 nmol) was directly injected into the peripallidum, a region containing a proportionately large number of cortically-projecting BFC neurons. Cholineacetyltransferase (ChAT) activity 10 days later was markedly and significantly reduced (up to 62%) in the cortex ipsilateral to the lesion. NMA induced a focal lesion affecting BFC neurons without damaging axons of passage or causing lesions distant from the site of injection. ChAT immunohistochemistry (IHC) was used to directly demonstrate loss of ChAT-positive neurons from the lesion site. This loss persisted at all survival times examined, from 2 days to 7.5 months post-injection.     

Connections of hypothalamic paraventricular neurons with the dorsal medial thalamus and neurohypophysis: an electrophysiological study in the rat

Connections between the hypothalamic paraventricular nucleus (PVN) and thalmic paraventricular nucleus (PVT) were examined using electrophysiological methods. Efferent projections of adjacent PVN cells were defined on the basis of antidromic activation from either PVT (n= 12) or neurophyphoseal (n= 38) stimulation; antidromic activation from both sites (n= 3) suggested that some PVN cells project both to the PVT and to the neurohypophysis. PVT stimulation evoked only weak orthodromic responses from 21% of PVN neurohypophyseal neurons, whereas short latency, high probability orthodromic responses were observed from 43% of PVN non-neurosecretory neurons. These data indicate reciprocal PVN-PVT connections and suggest that PVT afferents preferentially innervate non-neurosecretory PVN cells.     

Intrinsic and extrinsic connections of the rat central extended amygdala: an in vivo electrophysiological study of the central amygdaloid nucleus

Anatomical studies have shown that the central amygdaloid nucleus (CeA) is reciprocally connected with the lateral bed nucleus of the stria terminalis (BSTL), both structures being major components of the central extended amygdala. The CeA also receives projections from the insular cortex (InsCx) and the paraventricular thalamic nucleus (PVT). Extracellular unit activity was recorded from neurons in the lateral CeA (CeL) in urethane anaesthetized rats and their responses were studied after electrical stimulation of the BSTL, InsCx and PVT. The spontaneous activity of CeL neurons was low (1.69 spikes/s) and 40% of recorded cells were silent. The iontophoretic application of the GABA_A antagonist, bicuculline, increased the firing rate of 20% of neurons. The BSTL stimulation induced an antidromic response in 33% of the tested cells. Orthodromic responses were obtained from 83% (BSTL stimulation), 70% (InsCx stimulation) and 85% (PVT stimulation) of tested cells, some of which responded to both BSTL and InsCx or PVT stimulations. Orthodromic responses mostly consisted in 1–3 orthodromic spikes followed by an inhibition. During iontophoretic application of bicuculline, stimulation induced additional short latency orthodromic spikes, even in cells that were previously unresponsive. However, the duration of the inhibition was never reduced. These results indicate that GABAergic neurotransmission may play a dominant role in both spontaneous and evoked electrical activities in the CeL, probably mediated by local circuit cells involved in a feed-forward inhibition. This organization, along with the reciprocal connections between the CeL and the BSTL, is considered in the context of the extended amygdala.     

Brainstem projecting neurons in the rat basal forebrain: neurochemical, topographical, and physiological distinctions from cortically projecting cholinergic neurons

Magnocellular regions of the basal forebrain contain cholinergic neurons that project to the cerebral cortex. Neurons in the same basal forebrain regions innervate the brainstem. The present study investigated whether these brainstem projecting neurons are cholinergic, project also to the cortex, and share similar physiological properties as cortically projecting neurons. Data with retrograde tracing from various regions of the pons, medulla, and cortex combined with choline acetyltransferase immunofluorescence indicated that: 1) brainstem projecting neurons are usually segregated from cortically projecting and/or cholinergic neurons in the basal forebrain, 2) virtually no brainstem projecting neurons in the basal forebrain are cholinergic, and 3) only rarely do basal forebrain neurons have axon collaterals that project to both cortex and brainstem. Extracellular recordings from basal forebrain neurons confirmed the paucity of axonal collateralization and the topographic segregation between cortically and brainstem projecting basal forebrain neurons, and, in addition, showed that brainstem projecting neurons have a slower mean conduction velocity than cortically projecting neurons. These observations suggest that basal forebrain neurons projecting to the brainstem (pons, medulla) and the cortex represent separate cell populations in terms of projections, neurotransmitter content, distribution, and physiological properties.     

Morphology of neurons in the basal forebrain nuclei of the rat: a Golgi study

The neuronal cell types and their morphology in the nucleus basalis (NB), in the horizontal and vertical limbs of the diagonal band of Broca (NHL and NVL), and in the medial septal nucleus (MSN) were examined in Golgi-impregnated material. Cells appeared as multipolar or oligopolar and displayed a variable dendritic morphology; their somata varied considerably both in shape and size. The dendrites of most cells were restricted within nuclear boundaries, although occasionally neurons located near boundaries, particularly cells in NHL, extended dendritic arbors into neighboring areas. Axons were rarely seen, but when they were found they were generally not impregnated beyond the initial segment and displayed no apparent preferential direction. Three types of cells common to each of the 4 nuclear groups could be identified on the basis of soma shape and dendritic form. The first type included large multipolar neurons with triangular or polygonal perikarya and typically 3-5 dendrites emerging from the poles of each cell. These cells were especially numerous in NB, NHL, and NVL, but were much less frequent in MSN. The second type comprised medium-sized cells with round or oval somata and a small number, usually 2-3, of dendrites. They constituted a large percentage of the cell population in MSN, but were also encountered in NHL and NVL as well as in NB. The third type consisted of cells with fusiform or spindle-shaped somata with usually single dendrites emanating from each pole of the cell. A rare but distinct type of spindle-shaped neuron with dendrites bearing a rich complement of long and thin appendages was observed mainly in the ventral border of NHL. The present observations suggest that although the proportions and sizes of the 3 types of neurons vary between the 4 nuclei, neurons throughout the basal forebrain share common morphological characteristics.     

Synaptic organization of projections from basal forebrain structures to the mediodorsal thalamic nucleus of the rat.

The synaptic organization of the mediodorsal thalamic nucleus (MD) in the rat was studied with the electron microscope, and correlated with the termination of afferent fibers labeled with wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Presynaptic axon terminals were classified into four categories in MD on the basis of the size, synaptic vesicle morphology, and synaptic membrane specializations: 1) small axon terminals with round synaptic vesicles (SR), which made asymmetrical synaptic contacts predominantly with small dendritic shafts; 2) large axon terminals with round vesicles (LR), which established asymmetrical synaptic junctions mainly with large dendritic shafts; 3) small to medium axon terminals with pleomorphic vesicles (SMP), which formed symmetrical synaptic contacts with somata and small-diameter dendrites; 4) large axon terminals with pleomorphic vesicles (LP), which made symmetrical synaptic contacts with large dendritic shafts. Synaptic glomeruli were also identified in MD that contained either LR or LP terminals as the central presynaptic components. No presynaptic dendrites were identified. In order to identify terminals arising from different sources, injections of WGA-HRP were made into cortical and subcortical structures known to project to MD, including the prefrontal cortex, piriform cortex, amygdala, ventral pallidum and thalamic reticular nucleus. Axons from the amygdala formed LR terminals, while those from the prefrontal and insular cortex ended exclusively in SR terminals. Fibers labeled from the piriform cortex formed both LR and SR endings. Based on their morphology, all of these are presumed to be excitatory. In contrast, the axons from the ventral pallidum ended as LP terminals, and those from the thalamic reticular nucleus formed SMP terminals. Both are presumed to be inhibitory. At least some terminals from these sources have also been identified as GABAergic, based on double labeling with anterogradely transported WGA-HRP and glutamic acid decarboxylase (GAD) immunocytochemistry.     

Medullary expiratory neurons in the decerebrate rat: an intracellular study.

Intracellular recordings and labelings with horseradish peroxidase (HRP) of expiratory (E) neurons were performed in decerebrate, paralyzed, and ventilated rats. A total of 37 neurons were recorded, from which 4 cells and 1 axon were labeled. They were located in two regions of the ventrolateral medulla. One was in the rostral portion of the nucleus ambiguus just caudal to the facial nucleus, and the other in the nucleus retroambiguus at the level of the caudal medulla. These expiratory neurons had rhythmical changes in membrane potential similar to those reported in cat, i.e., a depolarization in the intervals between phrenic bursts which evolved in an augmenting (E-aug, n = 15), or bell-shaped or 'plateau' (E-all, n = 22) pattern until a rapid hyperpolarization at the start of inspiration. Both types were hyperpolarized during inspiration by chloride-dependent, inhibitory postsynaptic potentials (IPSPs) which were demonstrated in 17 neurons (10 E-aug and 7 E-all) from which reversal was obtained. Such IPSPs also existed during post-inspiration (stage I of expiration) in 4 of the 10 augmenting E neurons. They were identified by antidromic stimulation or HRP labeling, or both, as bulbospinal neurons (n = 2), cranial motoneurons (n = 4), or not antidromically activated (NAA) neurons (n = 31). All the identified bulbospinal neurons and the motoneurons exhibited an E-all pattern. The expiratory neurons of the caudal medulla had various projections as demonstrated with HRP labeling: one bulbospinal neuron with ipsilateral axon giving off intramedullary collaterals, and NAA neurons with rostral medullary projections or with axons crossing the midline.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electrophysiology of rat thalamo-cortical relay neurons: an in vivo intracellular recordingf and labeling study

Membrane properties and responses to frontal cortical stimulation were studied on electrophysiologically and morphologically identified thalamo-cortical neurons in anesthetized rats. Those neurons generated membrane responses resembling the low-threshold Ca-spike, g_K(Ca) and I_A that have been previously demonstrated in in vitro studies of thalamic neurons. Stimulation of the frontal cortex evoked a sequence of responses; antidromic spike, initial depolarization, long duration hyperpolarization and a short period of depolarization. The initial depolarization was considered to be a monosynaptic excitatory postsynaptic potential (EPSP) which overlapped with an inhibitory postsynaptic potential (IPSP). Major constituents of the long duration hyperpolarization were considered to be short duration IPSPs and long duration disfacilitations of cortical inputs.