Cholinergic systems in the rat brain: II. Projections to the interpeduncular nucleus

Localization of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin binding to nicotinic sites in the chicken forebrain and midbrain

We have previously localized cholinergic cell bodies and fibers within the midbrain of the chicken with choline acetyltransferase immunohistochemistry. In a continuing effort to characterize the central cholinergic system, the present study examines the distribution of various nicotinic acetylcholine receptors in the forebrain and midbrain of the chicken. The binding of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin was localized by film autoradiography in adjacent sections of the adult chicken brain, allowing a comparison of the distribution of different classes of nicotinic binding sites within the brain. Although all three ligands were often co-localized, there were areas that bound 3H-nicotine but not the 125I-neurotoxins, or vice versa. Very high densities of all three ligands were found in the hyperstriatum ventrale; the nucleus geniculatus lateralis, pars ventralis; the griseum tectale; the nucleus dorsolateralis anterior thalami; the nucleus lentiformis mesencephali, pars lateralis and pars medialis; the periventricular organ; and the stratum griseum et fibrosum superficiale, layer f of the optic tectum. The nucleus spiriformis lateralis had the highest levels of 3H-nicotine binding in the chicken brain, but it did not bind either of the two snake neurotoxins. On the other hand, high levels of both 125I-alpha-bungarotoxin and 125I-kappa-bungarotoxin binding were found in the nucleus semilunaris and the nucleus ovoidalis, but these areas contained little or no 3H-nicotine binding. No unique 125I-kappa-bungarotoxin sites, unrecognized by 125I-alpha-bungarotoxin, were identified by the low resolution autoradiography performed in this study. In general, nicotinic receptors were found in areas that have been reported to contain cholinergic cell bodies or fibers. Comparison of our results with the expression of neuronal nicotinic receptor subunits, as determined by in situ hybridization, suggests that many of the high affinity 3H-nicotine sites are localized presynaptically, as, for example, in the retinorecipient nuclei and the nucleus interpeduncularis. The lack of 125I-kappa-bungarotoxin binding in the presence of alpha-bungarotoxin indicates that the chicken brain has only very low levels of a unique kappa-bungarotoxin site. This is in marked contrast to chicken, frog, and rat autonomic ganglia, where a unique kappa-neurotoxin-sensitive receptor has been identified and shown to mediate nicotinic neurotransmission.

Postnatal development of cholinergic neurons in the rat: I. Forebrain

The postnatal development of cholinergic projection and local-circuit neurons in the rat forebrain was examined by use of choline acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) histochemistry. Although regional nuances were apparent, a general trend emerged in which cholinergic projection neurons in the basal nuclear complex (i.e., medial septal nucleus, vertical and horizontal diagonal band nuclei, magnocellular preoptic field, substantia innominata, nucleus basalis, and nucleus of the ansa lenticularis) demonstrated ChAT-like immunoreactivity earlier in postnatal development than intrinsically organized cholinergic cells in the caudate-putamen nucleus and nucleus accumbens, although this disparity was less apparent for local circuit neurons in the olfactory tubercle and Islands of Calleja complex. Ontologic gradients of enzyme expression also existed in some regions. A lateral to medial progression of ChAT and AChE appearance was observed as a function of increasing postnatal age in the nucleus accumbens and rostral caudate-putamen nucleus. By comparison, a rostrocaudal gradient of expression of ChAT-like immunoreactivity was apparent within the basal nuclear complex. Moderate to intense ChAT positivity, for example, appeared first in the medial septal nucleus. Furthermore, compared to more caudal regions, a greater proportion of AChE-positive neurons in rostral aspects of the basal forebrain expressed ChAT immunoreactivity on postnatal day 1, a difference that was no longer present by postnatal day 5. Cholinergic neurons in all forebrain regions also underwent an initial stage of progressive soma and proximal-dendrite hypertrophy, which peaked during the third postnatal week, followed by a period of cell-body and dendritic shrinkage that persisted into the fifth postnatal week when adult configurations were reached. These soma and dendritic size increases and decreases were not correlated with the magnitude of postnatal ChAT expression, which increased progressively until adult levels were attained approximately by the third to fifth weeks after birth. Expression of AChE in putative cholinergic neurons appeared to precede that of ChAT, especially in the caudate-putamen complex. Staining intensity of AChE also incremented earlier than that of ChAT.

A PHA-L analysis of ascending projections of the dorsal raphe nucleus in the rat

Ascending projections from the dorsal raphe nucleus (DR) were examined in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). The majority of labeled fibers from the DR ascended through the forebrain within the medial forebrain bundle. DR fibers were found to terminate heavily in several subcortical as well as cortical sites. The following subcortical nuclei receive dense projections from the DR: ventral regions of the midbrain central gray including the 'supraoculomotor central gray' region, the ventral tegmental area, the substantia nigra-pars compacta, midline and intralaminar nuclei of the thalamus including the posterior paraventricular, the parafascicular, reuniens, rhomboid, intermediodorsal/mediodorsal, and central medial thalamic nuclei, the central, lateral and basolateral nuclei of the amygdala, posteromedial regions of the striatum, the bed nucleus of the stria terminalis, the lateral septal nucleus, the lateral preoptic area, the substantia innominata, the magnocellular preoptic nucleus, the endopiriform nucleus, and the ventral pallidum. The following subcortical nuclei receive moderately dense projections from the DR: the median raphe nucleus, the midbrain reticular formation, the cuneiform/pedunculopontine tegmental area, the retrorubral nucleus, the supramammillary nucleus, the lateral hypothalamus, the paracentral and central lateral intralaminar nuclei of the thalamus, the globus pallidus, the medial preoptic area, the vertical and horizontal limbs of the diagonal band nuclei, the claustrum, the nucleus accumbens, and the olfactory tubercle. The piriform, insular and frontal cortices receive dense projections from the DR; the occipital, entorhinal, perirhinal, frontal orbital, anterior cingulate, and infralimbic cortices, as well as the hippocampal formation, receive moderately dense projections from the DR. Some notable differences were observed in projections from the caudal DR and the rostral DR. For example, the hippocampal formation receives moderately dense projections from the caudal DR and essentially none from the rostral DR. On the other hand, virtually all neocortical regions receive significantly denser projections from the rostral than from the caudal DR. The present results demonstrate that dorsal raphe fibers project significantly throughout widespread regions of the midbrain and forebrain.

Substance P immunoreactivity in the rat interpeduncular nucleus: synaptic interactions between substance P-positive profiles and choline acetyltransferase- or glutamate decarboxylase-immunoreactive structures

The subnuclear and synaptic distribution of substance P immunoreactivity was examined in the rat interpeduncular nucleus at the light and electron microscope level. The nucleus possessed a prominent substance P-immunoreactive axonal plexus in the lateral and dorsomedial subnuclei, and in the dorsal cap of the rostral subnucleus. The density of substance P-immunoreactive axons in the remaining subnuclear divisions was sparse to moderate. Terminals of immunoreactive axons contained spherical vesicles and formed asymmetric contacts on dendritic processes exclusively. Immunoreactive neurons, restricted to the rostral subnucleus, possessed long, sparsely branched dendrites. Unlabelled terminals containing either spherical or pleomorphic vesicles contacted substance P-immunoreactive dendritic profiles. Axodendritic and axosomatic synapses containing substance P immunoreactivity pre- and postsynaptically were not observed. Ultrastructural evidence for synaptic relationships between substance P-containing profiles and those containing either choline acetyltransferase or glutamate decarboxylase was obtained by means of double antigen immunohistochemistry. Terminals of fasciculus retroflexus axons stained for choline acetyltransferase immunoreactivity formed asymmetric synaptic contacts with substance P-immunoreactive dendritic profiles. Few substance P-positive dendrites in the rostral subnucleus received terminals possessing glutamate decarboxylase activity. Unlabelled terminals containing either spherical or pleomorphic vesicles contacted substance P- and glutamate decarboxylase-immunoreactive dendritic profiles simultaneously. Terminals possessing either substance P or glutamate decarboxylase immunoreactivity formed synaptic contacts with dendritic processes of neurons in the lateral subnucleus. Many of the neurons within this subnuclear division contained glutamate decarboxylase. This study provides direct evidence of synaptic relationships between choline acetyltransferase-immunoreactive axons and substance P-immunoreactive dendritic profiles, and between substance P-positive axons and glutamate decarboxylase-immunoreactive dendrites. These findings reveal that two types of transmitter-specific axons of the fasciculus retroflexus innervate neuronal populations of the interpeduncular nucleus stained immunohistochemically for either substance P or glutamate decarboxylase.

Cholinergic-dopaminergic interactions in cognitive performance

Both acetylcholinergic (ACh) and dopaminergic (DA) systems have been found to be crucial for the maintenance of accurate cognitive performance. In a series of studies examining those aspects of cognitive function revealed by the radial-arm maze, we have found that these two neurotransmitter systems interact in a complex fashion. Choice accuracy deficits in the radial-arm maze can be induced by blockade of either muscarinic- or nicotinic-ACh receptors. The choice accuracy deficit induced by blockade of muscarinic receptors with scopolamine can be reversed by the DA receptor blocker, haloperidol. The specific DA D1 blocker SCH 23390 also has this effect, whereas the specific D2 blocker raclopride does not, implying that it is D1 blockade that is critical for reversing the scopolamine effect. On the other hand, the choice accuracy deficit induced by nicotinic blockade with mecamylamine is potentiated by haloperidol. This effect is also seen with the D2 antagonist raclopride, but not with the D1 antagonist SCH 23390, implying that it is the D2 receptor which is important for the potentiation of the mecamylamine effect. The relevance of the D2 receptor for nicotinic actions on cognitive function is emphasized by the finding that the selective D2 agonist LY 171555 reverses the choice accuracy deficit caused by mecamylamine. Nicotinic and muscarinic blockade are synergistic in the deficit they produce. Antagonist doses subthreshold when given alone produce a pronounced impairment when given together. This latter deficit can be reversed by the D2 agonist LY 171555. These studies have outlined the complex nature of ACh-DA interactions with regard to cognitive function. Possible neural circuits for these interactions are discussed. The effectiveness of these selective DA treatments in reversing cognitive deficits due to ACh underactivation suggests a novel approach to treating cognitive dysfunction in syndromes such as Alzheimer's disease.

Differential effects of fasciculus retroflexus lesions on serotonin, glutamate and gamma-aminobutyrate content and choline acetyltransferase activity in the interpeduncular nucleus

After placing bilateral electrolytic lesions in the fasciculus retroflexus (FR) of the rat, the endogenous content of serotonin, glutamate and gamma-aminobutyrate (GABA) as well as choline acetyltransferase activity (ChAT) were measured in the interpeduncular nucleus (IPN) at the 7th, 28th and 120th survival days. Confirming earlier results, an almost total depletion of ChAT was obtained in the IPN following complete FR lesions at any survival day studied. In such cases, the following changes were observed; 1) the serotonin level increased consistently and roughly doubled at the 120th survival day, suggesting heterotypic sprouting of serotonergic fibers and/or enhanced serotonin synthesis in the serotonergic neurons in the IPN, 2) the glutamate level decreased by approximately one-half, while the activity of high affinity uptake of glutamate remained unaltered, at the 7th survival day, suggesting a lowered glutamate formation coupled with lowered glucose utilization in the IPN, and 3) the GABA level decreased at a slower rate and reached one-third of the control at the 120th survival day, for which either transsynaptic degeneration of GABA neurons in the IPN or a suppressed metabolic rate in the GABA shunt following the lowered glutamate formation is a possible explanation.

Cholinergic neurons of the feline pontomesencephalon. II. Ascending anatomical projections

Immunoreactivity for choline acetyltransferase (ChAT) was analyzed in unoperated cats and in cats in which stereotaxic lesions were made in the pedunculopontine and laterodorsal tegmental nuclei. The fine reaction product revealed moderate to dense ChAT-immunoreactive fiber plexuses throughout the telencephalon, diencephalon, and midbrain. A pontomesencephalic origin of cholinergic innervation to virtually every nucleus of the diencephalon, as well as to various midbrain and basal telencephalic sites was indicated in the cats with lesions, in which the optical density of ChAT-immunoreactivity was significantly decreased as compared to controls. Pontomesencephalic lesions produced no changes, however, in the density of ChAT staining in the cerebral cortex, basolateral amygdala, or caudate nucleus. In addition to ChAT-positive terminal fiber arborizations which were widely distributed, cholinergic fibers-of-passage were traced in the unoperated and operated feline brains. The general course of ChAT fibers cut in cross-section was followed in successive transverse levels, and although pathways originating from the pedunculopontine nucleus demonstrated orientations in every direction, many demonstrated a rostral course. A particularly dense aggregate of ascending ChAT-positive fibers was localized in the dorsolateral sector of the pedunculopontine area which could be followed at more rostral levels into the central tegmental fields and the compact part of the substantia nigra. From the central tegmental fields, numerous ChAT-immunopositive fibers cut in cross-section continued to course rostrally in the intralaminar, reticular and lateroposterior nuclei of the thalamus, and a distinct bundle of ChAT fibers coursing dorsolaterally was observed medial to the optic tract ascending to the lateral geniculate. ChAT fibers with dorsolateral orientations were additionally observed in the zona incerta, ventral anterior thalamus, and ansa lenticularis on route to the reticular thalamus, the globus pallidus, and the substantia innominata. Pathways consisting of fibers traced from ChAT-containing cells in the laterodorsal tegmental nucleus could be traced to medial structures such as the periaqueductal gray, ventral tegmental area and dorsal raphe. Medially placed ChAT fibers were additionally followed through the ventral tegmental area, the midline thalamus, and the hypothalamus, up to the medial and lateral septal nuclei. The trajectories of the ascending cholinergic pathways from the pontomesencephalon are discussed in relation to locally generated electrophysiological responses in the cat.

Differential effects of atropine, procaine and dopamine in the rat ventral tegmentum on lateral hypothalamic rewarding brain stimulation

Microinjections of the muscarinic antagonist, atropine, of dopamine, or of the local anesthetic, procaine, in the ventral tegmentum elevated frequency thresholds for lateral hypothalamic self-stimulation. The largest and most robust effects were observed following atropine (30 or 60 micrograms) microinjections. The most sensitive sites for the atropine effect were near dopamine cells. In order to determine if the effects of atropine can be reversed by pretreatment with a cholinergic agonist, carbachol (1-3 micrograms) was microinjected 15 min prior to atropine. Carbachol pretreatment attenuated the frequency threshold elevation of atropine by 47-95%. Since atropine is a local anesthetic, the effects of procaine on self-stimulation thresholds were tested as well. Procaine (100 or 250 micrograms) in ventral tegmentum elevated frequency thresholds by much less than atropine. Therefore, while atropine attenuates reward primarily through blockade of muscarinic receptors, the local anesthetic effect of atropine may enhance the threshold elevation. Dopamine (1-10 micrograms) also elevated frequency thresholds, but when dopamine injections were repeated daily, the threshold elevations were attenuated. This attenuation contrasted with the robust effects of atropine, and may reflect the development of autoreceptor subsensitivity. Hence, both dopaminergic and muscarinic receptors in ventral tegmentum are involved in lateral hypothalamic brain stimulation reward.

A comparison of the subnuclear and ultrastructural distribution of acetylcholinesterase and choline acetyltransferase in the rat interpeduncular nucleus

The subnuclear and synaptic staining patterns for acetylcholinesterase (AChE) activity and choline acetyltransferase (ChAT) activity were studied in the rat interpeduncular nucleus (IPN) using histochemical and immunohistochemical methods. AChE reactivity was prominent in the neuropil of the rostral, lateral and dorsomedial subnuclei, whereas ChAT immunoreactivity was confined to axons and terminals in the rostral, intermediate and central subnuclei. AChE-positive somata were evident in all the subnuclear divisions of the IPN, and possessed reaction product in the rough endoplasmic reticulum and nuclear envelope. ChAT-positive somata were not present in the IPN. Characteristic axodendritic synapses in the rostral, intermediate and central subnuclei possessed ChAT immunoreactivity presynaptically, and AChE reactivity both pre- and postsynaptically. Other synaptic arrangements in the lateral subnucleus lacked ChAT-immunoreactive terminals, yet possessed prominent AChE reactivity. The results of the present study reveal that AChE reactivity and ChAT immunoreactivity are heterogeneously distributed among the subnuclear divisions of the rat IPN, and that AChE reactivity is present in both the cholinoceptive and noncholinoceptive subnuclei. Although neuronal colocalization of ChAT and AChE activity is not evident in the IPN, AChE-positive neurons are in receipt of putative cholinergic, as well as peptidergic, afferent inputs.

Cholinergic neurons of the pontomesencephalic tegmentum release acetylcholine in the basal nuclear complex of freely moving rats

Two major systems of cholinergic projection neurons are found within the centrum of the mammalian brain: the basal nuclear complex, projecting predominantly to the cerebral cortex, amygdala, and hippocampus, and the pontomesencephalotegmental network, innervating primarily the thalamus. Neurons comprising the latter network also project to the basal forebrain, but the functional properties of that fiber connection, if any, are unknown. In an attempt to address this issue, the extracellular concentration of acetylcholine was measured in the basal nuclear complex of freely moving rats, both singularly and in combination with lesions and pharmacologic manipulations. Acetylcholine release monitored in the presence of physostigmine sulfate in the basal forebrain was (a) calcium-dependent, (b) increased by systemic scopolamine injection, the rise persisting in the presence of quisqualate lesions of the basal nuclear complex, (c) blocked by tetrodotoxin, and (d) abolished by ablation of cell bodies in the pontomesencephalic tegmentum, which also produced a decrease of choline acetyltransferase activity in the nucleus basalis/substantia innominata region, but not by quisqualate lesions of the basal forebrain. It is concluded from these data that the calcium-dependent release of acetylcholine in the basal nuclear complex (a) is largely axonal in nature, (b) derives substantially from axons of the cholinergic pontomesencephalic tegmentum, and (c) appears to be controlled by presynaptic muscarinic receptors on axon terminals of the latter system. The pontomesencephalotegmental cholinergic complex might thus influence cortical acetylcholine release, in part at least, by means of serial-order cholinergic-cholinergic interactions in the basal nuclear complex.

A double-labeling method for AChE and fluorescent retrograde tracers

Staining for the degradative enzyme acetylcholinesterase (AChE) is an important tool in studying central cholinergic/cholinoceptive systems. AChE staining has also been useful in identifying the projections of AChE-containing neurons and codistribution of AChE with other neurotransmitters. The intensity and opacity of conventional AChE histochemical reaction products, however, pose problems for such double-labeling studies. Here, we have successfully combined a modified version (37) of the Koelle-Friedenwald AChE reaction with retrograde transport of the fluorescent tracer, Fluoro-Gold (FG). By omitting the final intensification steps of the Koelle-Friedenwald reaction, a translucent, light-stable reaction product is created. Viewed under darkfield illumination, this precipitate is of similar intensity and sensitivity to that produced by conventional AChE histochemical processing. Prior administration of an AChE-inhibitor yields preferential staining of AChE-positive neuronal somata. This nonintensified darkfield AChE (NIDA) histochemical method was compatible with visualization of retrogradely transported FG in AChE-positive neurons, allowing unambiguous identification of the projections of AChE-containing neurons.

Synaptic organization of septal projections in the rat medial habenula: a wheat germ agglutinin-horseradish peroxidase and immunohistochemical study

The synaptic organization of septal inputs to the rat habenular complex of the dorsal diencephalon was examined employing the anterograde tracer wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The cellular distribution of substance P (SP) and choline acetyltransferase (ChAT) immunoreactivity was also studied at the light and electron microscopic level. Following placements of tracer within the entire septum, labeled axons were observed in the stria medullaris and in the medial and lateral subnuclei of the habenula. Following injections of tracer in the nuclei triangularis and septofimbrialis of the posterior septum, the medial subnucleus was heavily labeled, whereas the lateral subnucleus was devoid of peroxidase activity. The medial subnucleus possessed labeled myelinated axons and terminals that contained clear, spherical vesicles and formed asymmetric contacts with dendritic spines and shafts. Terminals possessing WGA-HRP activity also formed non-synaptic junctions with other labeled or unlabeled terminals. SP and ChAT immunoreactivity in normal and colchicine-treated animals was confined to dendrites and somata within the medial habenula. Terminals containing clear spherical vesicles formed asymmetric synaptic contacts with these immunoreactive somatic and dendritic profiles. Based on the combined anterograde tracing and immunohistochemical data, it is proposed that septal projections provide a direct innervation to habenular neurons that contain ChAT or SP activity. These septal inputs may play an important role in the facilitation of the ChAT- and SP-positive habenular neurons, both of which provide prominent afferent inputs to the interpeduncular nucleus. Thus, neurons of the habenula and interpeduncular nucleus are under the direct and indirect influence of septal neurons within the limbic forebrain circuit.

Developmental profiles of cholinergic activity in the habenulae and interpeduncular nucleus of the rat

Choline acetyltransferase (ChAT) was measured in the habenula and in the interpeduncular nucleus of rats from 1 to 12 weeks of age. A remarkable degree of parallelism was shown by the developmental curves in the two nuclei. In both cases the highest level of enzyme activity was reached at 3 weeks of age and was followed by some decrease towards adult values. A statistically highly significant correlation was demonstrated between ChAT levels in the two nuclei at the various developmental stages. The rise of the cholinergic marker was slightly advanced in the habenula in comparison with the interpeduncular nucleus. The present data may be useful for studies focused on neonatal synaptogenesis, plasticity and synaptic neurochemistry of this relatively simple model of brain connections.

Cholinergic systems in the rat brain: IV. Descending projections of the pontomesencephalic tegmentum

Descending projections from cholinergic neurons in the pedunculopontine and laterodorsal tegmental nuclei, collectively referred to as the pontomesencephalotegmental (PMT) cholinergic complex, were studied by use of the fluorescent retrograde tracers fluorogold, true blue, or Evans Blue in combination with choline acetyltransferase (ChAT) immunohistochemistry of acetylcholinesterase (AChE) pharmacohistochemistry. Pedunculopontine somata positive for ChAT or staining intensely for AChE were retrogradely labeled with fluorescent tracers following infusions into the motor nuclei of cranial nerves 5, 7, and 12. ChAT-positive cells in both the pedunculopontine and laterodorsal tegmental nuclei demonstrated projections to the vestibular nuclei, the spinal nucleus of the 5th cranial nerve, deep cerebellar nuclei, pontine nuclei, locus ceruleus, raphe magnus nucleus, dorsal raphe nucleus, median raphe nucleus, the medullary reticular nuclei, and the oral and caudal pontine reticular nuclei. Fluorescent tracers used in combination with AChE pharmacohistochemistry corroborated these projections and, in addition, provided evidence for cholinergic pontomesencephalic projections to the lateral reticular nucleus and inferior olive. The majority of retrogradely labeled neurons demonstrating ChAT-like immunoreactivity were found ipsilateral to the injection site, but, in all cases, tracer-containing cholinergic cells contralateral to the infused side of the brain were detected also. More retrogradely labeled cells containing ChAT were observed in the pedunculopontine tegmental than in the laterodorsal tegmental nucleus following tracer injections at all sites with the exceptions of the locus ceruleus and dorsal raphe nucleus where the converse profile was observed. None of the pedunculopontine or laterodorsal tegmental cells immunopositive for ChAT or stained intensely for AChE contained retrogradely transported tracers following dye infusions into the cerebellar cortex or cervical spinal cord. Triple-label experiments using two tracers infused into different sites in the same animal revealed that individual ChAT-immunoreactive cells in the PMT cholinergic complex projected to more than one hindbrain site in some cases and had ascending projections as well. Certain ChAT-positive somata in the pedunculopontine and laterodorsal tegmental nuclei were found in close association with several fiber tracts, including the superior cerebellar peduncle, lateral lemniscus, dorsal tegmental tract, and medial longitudinal fasciculus.

Ontogeny of cholinergic neurons in the mouse forebrain

The development of cholinergic neurons in the mouse forebrain was studied by immunocytochemistry with a monoclonal antibody to choline acetyltransferase (ChAT), the rate-limiting enzyme for acetylcholine synthesis. Since this antibody stained dividing cells in ventricular germinal zones as well as differentiating neurons, likely routes of migration could be inferred on the basis of the location of immunoreactive (IR) cells at different gestational ages. Germinal zones for cholinergic cells were observed in all ventricular zones of the forebrain with the ventral zones generating the earliest cells by gestational day 13.5 (GD13.5). On GD14, ChAT IR cells were visible in the germinal zones of the eye, olfactory ventricle, anterior horn, and dorsolateral aspect of the lateral ventricle, lateral ganglionic eminence, ventro- and dorsolateral third ventricle, and in the pineal anlage (epiphysis). ChAT IR neurons continued to develop in these and additional germinal zones on GD15, including the medial, dorsal, and dorsomedial walls of the lateral ventricle, and the medial and dorsal ganglionic eminence. On GD16, ChAT IR neurons were located in the prelimbic, pyriform, and parietal cortices and the lamina terminalis, and a cluster of IR cells was observed in the ventricular zone of the caudatopallial angle. On GD17-18, neurons in the anterior olfactory nucleus, olfactory tubercle, horizontal and vertical nucleus of the diagonal band, and medial septal nucleus stained more darkly and were multipolar, whereas immature bipolar neurons appeared to continue their migration into the hippocampus and along major fiber tracts, such as the corpus callosum, external capsule, fornix and anterior commissure. This study provides a comprehensive view of the zones of origin, probable routes of migration, and final destination of cholinergic neurons in the mouse forebrain.

Local cerebral glucose utilization in the postictal phase of amygdaloid kindled rats

Local cerebral glucose utilization was measured by means of the quantitative autoradiographic 2-[14C]deoxyglucose method during the postictal phase of various seizure stages of amygdaloid kindling in conscious rats. The partially kindled animals exhibited a partial seizure such as chewing and/or head nodding, and the fully kindled animals, a generalized tonic-clonic convulsion. The control animals were implanted with an electrode, but not electrically stimulated. Cerebral glucose utilization of the fully kindled animals was deeply depressed in the postictal phase as compared to the control, and that of the partially kindled animals was moderately decreased. The side-to-side differences of cerebral glucose utilization were observed only in the partially kindled group in which glucose utilization was more depressed on the side of stimulation. Among the structures with depressed glucose utilization, only one structure, the interpeduncular nucleus, showed a relative increase in glucose utilization during the postictal phase of the kindled groups. As the postictal phase has been considered as a period of inhibition, these results may indicate that the neural networks linking the interpeduncular nucleus play an active role in the mechanisms of termination of a seizure and postictal refractoriness.

Immunohistochemical localization of choline acetyltransferase in the chicken mesencephalon

Choline acetyltransferase, a specific marker for cholinergic neurons, has been immunohistochemically localized in the mesencephalon and in the caudal diencephalon of the chicken. A complete series of transverse sections through the mesencephalon is presented. In the diencephalon, cholinergic fibers were found in the stria medullaris, the fasciculus retroflexus, and the ventral portion of the supraoptic decussation. The nucleus triangularis and the nucleus geniculatus lateralis, pars ventralis also contained cholinergic fibers. Small cholinergic cell bodies were found in the medial habenula. In the pretectum, cholinergic fibers innervated the nucleus lentiformis mesencephali and the tectal gray. The nucleus spiriformis lateralis also contained cholinergic fibers, while most of the cell bodies in the nucleus spiriformis medialis were cholinergic. In the mesencephalon, labelled fibers were found in the nucleus intercollicularis and in all layers of the optic tectum except the stratum opticum. The highest density of tectal cholinergic fibers was in the stratum griseum et fibrosum superficiale (SGFS), layer f. Radial cells located in SGFS, layer i were also cholinergic. In the isthmic nuclei, cholinergic fibers were found in the pars magnocellularis, while the pars parvicellularis and the nucleus semilunaris contained labelled cells. The oculomotor, Edinger-Westphal, trochlear, and trigeminal motor nuclei all had cholinergic cell bodies. Cholinergic axons were present in the oculomotor and trochlear nerves. In the tegmentum, cell bodies were labelled in the nucleus mesencephalicus profundus, pars ventralis, while the nucleus interpeduncularis had dense cholinergic innervation. Our localization of cholinergic cell bodies and fibers has been compared with earlier autoradiographic and anatomical studies to help define cholinergic systems in the avian brain. For example, the results indicate that the chicken may have a cholinergic habenulointerpeduncular system similar to that reported in the rat. Establishing the cholinergic systems within the avian midbrain is important for designing future neurophysiological and pharmacological studies of cholinergic transmission in this region.

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.

Measuring cholinergic sensitivity: II. Arecoline effects on metabolic activity in pontine regions of rat brain

Brain imaging studies may help to localize areas particularly sensitive to cholinergic agonists. A study in rats using the 2-deoxyglucose method for studying changes in brain metabolic activity was performed. Among regions in the pons/medulla, arecoline, 0.05 mg/kg ip, increased glucose utilization in the dorsal raphe, median raphe, and basilar pontine nuclei without producing behavioral or cardiovascular effects. These brain areas, along with the hippocampus, may be pertinent to studies of muscarinic supersensitivity in humans.

The pontomesencephalotegmental cholinergic system does not degenerate in Alzheimer's disease

The pedunculopontine and laterodorsal tegmental nuclei comprising the pontomesencephalotegmental (PMT) cholinergic system were examined for the presence of neuropathology and loss of presumed cholinergic cells in patients diagnosed with Alzheimer's disease-senile dementia of the Alzheimer type (AD-SDAT), Parkinsonian dementia, and multi-infarct dementia. Although neurofibrillary changes and plaque-like entities were observed in both nuclei in AD-SDAT and Parkinson's disease dementia, these pathologic indices were not seen consistently in control individuals or in those with multi-infarct dementia, and in none of the diagnostic categories was loss of neuronal somata found in the PMT cholinergic complex. Because appreciable degeneration of cholinergic neurons does occur in the basal forebrain in AD-SDAT, it is concluded that cholinergic phenotype alone is not a sufficient condition indicating predilection for neuronal loss in that dementing illness.

Glutamate decarboxylase immunoreactivity in the rat interpeduncular nucleus: a light and electron microscope investigation

The distribution of immunohistochemically demonstrable glutamate decarboxylase, the synthetic enzyme for GABA, was examined in the rat interpeduncular nucleus at the light and electron microscope levels. Immunoreactive perikarya were distributed in a characteristic pattern among the subnuclear divisions. The rostral, ventral and caudal portions of the nucleus possessed numerous immunoreactive perikarya, while few immunoreactive somata were observed in the subnuclei of the dorsal aspect. A dense field of immunostained axons and terminals was also present throughout. Ultrastructural examination of glutamate decarboxylase immunoreactivity revealed numerous labelled somata, dendritic processes, axons and boutons. Axodendritic and axosomatic synapses with immunoreactive postsynaptic profiles were numerous throughout those subnuclei with large numbers of immunoreactive somata. Immunostained terminals in contact with both immunoreactive and non-immunoreactive somatic and dendritic profiles were also present. An abundance of immunostained terminals was observed in the subnuclei that possessed a sparse population of immunoreactive somata. Immunoreactive myelinated axons of unknown origin were also present. This investigation demonstrates that the rat interpeduncular nucleus possesses a large population of glutamate decarboxylase-immunoreactive neurons coextensive with a plexus of immunostained axons and terminals. The results suggest that the immunoreactive neurons give rise to axons which contribute to an intrinsic circuit interconnecting the different subnuclear divisions. These immunoreactive neurons are in receipt of non-immunoreactive afferent inputs of variable morphology, as well as projections from intrinsic immunoreactive neurons.

Characterization and quantitative autoradiographic distribution of [3H]acetylcholine muscarinic receptors in mammalian brain. Apparent labelling of an M2-like receptor sub-type

[3H]Acetylcholine receptor binding characteristics (under muscarinic conditions) have been investigated using membrane binding assays and in vitro receptor autoradiography. In rat, guinea-pig and monkey brain membrane preparations, [3H]acetylcholine binds with high affinity (25-50 nM) to an apparently single class of sites which is differentially distributed across brain regions. The ligand selectivity pattern reveals that the potency of (-)quinuclidinyl benzylate is greater than (greater than) atropine greater than scopolamine greater than oxotremorine greater than carbamylcholine greater than pirenzepine greater than methylcarbamyl-choline = nicotine in competing for [3H]acetylcholine binding sites, indicating that [3H]acetylcholine selectively binds to muscarinic sites under these incubation conditions. Moreover, the low potency of pirenzepine suggests that [3H]acetylcholine does not label a significant proportion of the M1 receptor sub-type but most likely binds to putative M2-like receptor sites. This hypothesis is also supported by the autoradiographic distribution of [3H]acetylcholine binding sites in all species studied here. High densities of [3H]acetylcholine binding sites are seen in various nuclei of the medulla and pons, certain thalamic nuclei, medial septum, laminae III, V and VI of the cortex and just above the pyramidal cell layer of the hippocampus. Such localization is much different from that seen with the non-selective antagonist [3H]quinuclidinyl benzylate and the selective M1 receptor ligand [3H]pirenzepine, although it resembles that of the selective M2 receptor antagonist [3H]AF-DX 116. Thus, [3H]acetylcholine apparently mostly binds with high affinity mainly to non-M1 muscarinic receptor types in mammalian brain tissues. Moreover, the ligand selectivity pattern and in vitro receptor autoradiographic data suggest that at low concentrations (10-20 nM) most of [3H]actylcholine labelled sites are of the M2-like receptor class.

Cholinergic antagonists in ventral tegmentum elevate thresholds for lateral hypothalamic and brainstem self-stimulation

Frequency thresholds for lateral hypothalamic self-stimulation are elevated following microinjections of atropine into ventral tegmentum (73). Many self-stimulation sites in brainstem are situated near cholinergic cell groups and axons, and ventral tegmentum receives cholinergic afferents terminals. To test the hypothesis that ventral tegmental muscarinic receptors are involved in lateral hypothalamic and brainstem self-stimulation, stimulating electrodes were placed in lateral hypothalamus and dorsal tegmentum near the midbrain-pons border, and cannulae were implanted in ventral tegmentum. Microgram injections of muscarinic antagonists, atropine or scopolamine, or a choline uptake blocker, hemicholinium-3, elevated frequency thresholds for both self-stimulation sites in a dose-dependent and time-dependent fashion. In addition, summation and collision between the two self-stimulation sites was tested using paired-pulse methods (53). Summation ranged from 31 to 87% (i.e., 24 to 47% reductions in frequency threshold were observed at long intrapair intervals), but no collision-like effects were observed at short intrapair intervals. The ventral tegmentum is a likely site for the convergence of dorsal tegmental and lateral hypothalamic self-stimulation pathways.

Organization of GABA and GABA-transaminase containing neurons in the gustatory zone of the nucleus of the solitary tract

Previous cytoarchitectural and electron micrographic studies have indicated that the gustatory zone of the nucleus of the solitary tract (NST) may contain local circuit neurons. It is known that neurons of the caudal "visceroceptive" NST contain GABA, glutamic acid decarboxylase (EC 4.1.1.15), and GABA-transaminase (GABA-T; 4-aminobutyrate: 2-oxoglutarate aminotransferase; EC 2.6.1.19). The present study was conducted to determine whether or not neurons in the gustatory zone of the NST of rat contain GABA and the principle degradative enzyme of GABA, GABA-T. Transganglionic transport of horseradish peroxidase (HRP) was used to identify chorda tympani (CT) nerve terminal fields. Immunohistochemical studies were combined with transport experiments to evaluate the organization of GABA immunoreactive neurons in CT terminal fields. Results show that GABA immunoreactive neurons and puncta are located within CT terminal fields. These neurons evince small ovoid morphologies resembling Golgi interneurons, and comprise an average of 18% of total neurons in CT terminal fields. Independent histochemical studies reveal that approximately 82% of GABA immunoreactive neurons within CT terminal fields exhibit GABA-T activity. Retrograde transport of HRP was used in additional studies to evaluate whether or not axons of putative GABAergic neurons project to the second-order central gustatory relay located in the caudal parabrachial nucleus (PBNc), to the caudal NST, or to regions surrounding the rostral or caudal NST. Combined studies indicate that GABA immunoreactive neurons in the gustatory NST do not project axons to the PBNc, to the caudal NST, or to regions adjacent to the rostral or caudal NST.(ABSTRACT TRUNCATED AT 250 WORDS)

Basal forebrain inputs to the interpeduncular nucleus in the rat studied with the Phaseolus vulgaris-leucoagglutinin tracing method

The connections between the basal forebrain and the interpeduncular nucleus (IP) were studied in the rat using the anterograde transport of Phaseolus vulgaris-leucoagglutinin (PHA-L). PHA-L was injected in the septum-diagonal band complex, the preoptic area, the substantia innominata, the globus pallidus, and the ventral pallidum. In a number of cases sections of the IP were double-immunostained for PHA-L and serotonin. Only following PHA-L injections in the medial septal nucleus and the nucleus of the vertical limb of the diagonal band, were substantial terminations found throughout all subnuclei of IP. Following injections in the medial and lateral preoptic area labeling was confined to the caudal part of IP. This finding suggests that the area in the basal forebrain that contributes to these projections is smaller than has been indicated by previous retrograde tracing studies. Labeled fibers reach the IP predominantly via the medial forebrain bundle. Only a very small number of fibers reaches the ventral mesencephalon, and possibly the IP, via the stria medullaris and the fasciculus retroflexus. The highest density of terminations was seen in the apical subnucleus. The apical subnucleus contains serotonin-immunoreactive neurons, in the direct vicinity of which varicosities on the labeled fibers were seen. This finding suggests innervation of the serotonergic neurons by fibers from the medial septum and preoptic area.

Depletion of cholinergic habenulo-interpeduncular neurons by selectively timed methylazoxymethanol acetate (MAM) treatment during pregnancy

Methylazoxymethanol acetate (MAM) was injected to female rats at the beginning of the 17th day of gestation. Resulting offspring showed a remarkable decrease in the size of the medial habenula while the interpeduncular nucleus, whose neurons are generated before the time of MAM treatment, appeared anatomically unaffected. Choline acetyltransferase was significantly reduced in the habenulae and in the interpeduncular nucleus suggesting that MAM treatment had depleted a portion of the cholinergic neurons of the medial habenula which project to the interpeduncular nucleus. Aromatic amino acid decarboxylase significantly increased in the interpeduncular nucleus, a likely effect of monoaminergic hyperinnervation in response to partial cholinergic deprivation. MAM strategy can be usefully adopted for the study of general aspects of brain development when connected nuclei showing no overlapping in neuronal generation times are involved.

Topographical and ultrastructural investigation of the habenulo-interpeduncular pathway in the rat: a wheat germ agglutinin-horseradish peroxidase anterograde study

The topographical and ultrastructural organization of the habenular projection to the interpeduncular nucleus (IPN) of the rat was examined employing the anterogradely transported tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and the chromogen tetramethylbenzidine (TMB). Unilateral placements of WGA-HRP in the habenular complex resulted in heavy terminal labelling in the rostral, central, and intermediate subnuclei bilaterally, and in the lateral subnuclei ipsilaterally. The apical subnucleus possessed only a sparse amount of label. Placements confined to the medial habenula (mH) produced similar results to those observed when the entire habenula was filled, suggesting that the afferent contribution made by the lateral habenula (lH) to the IPN is small. Unilateral placements of WGA-HRP in the dorsal portion of the mH resulted in heavy, predominantly ipsilateral labelling in the lateral subnucleus and the dorsal cap of the rostral subnucleus. In the lateral subnucleus labelled habenular terminals consistently contacted single dendritic processes shared by one or more other boutons, possibly of nonhabenular origin. Labelled habenular terminals in the rostral subnucleus normally contacted one or two dendrites. Labelled terminals in both subnuclei possessed clear, spherical vesicles and a variable number of dense-core vesicles. Unilateral placements of WGA-HRP in the ventral portion of the mH resulted in heavy labelling in the rostral half of the rostral subnucleus with a slight ipsilateral predominance, and in the central and intermediate subnuclei bilaterally. Terminal labelling was observed in crest and S synapses in the intermediate and central subnuclei respectively. Crest synapses, which consist of two parallel habenular terminals contacting an attenuated dendritic process, normally possessed label in only one of the two boutons. In the central subnucleus labelled horizontal axons formed several en passant S synapses with dendritic processes of small and medium diameter. These synaptic specializations of habenular axons contained numerous clear, spherical vesicles. This study demonstrates that a major topographically organized projection to the IPN originates from two distinct subpopulations of habenular neurons which comprise a dorsal sector and a ventral sector of the mH. Ultrastructural examination demonstrated that axons originating from neurons in the ventral and dorsal mH form characteristic contacts in the various IPN subnuclei.

Afferents to the rat red nucleus studied by means of D-[3H]aspartate, [3H]choline and non-selective tracers

Following injection of horseradish peroxidase-labeled wheat germ agglutinin or of rhodamine-labeled microspheres as non-selective tracers into the rat red nucleus, the origins of the corticorubral and cerebellorubral pathways, as well as a considerable number of other brain structures including dorsal raphé nucleus, zona incerta and several hypothalamic nuclei showed retrogradely labeled perikarya. Labeling patterns obtained with horseradish peroxidase-labeled wheat germ agglutinin compared well with those observed following application of rhodamine-labeled microspheres which produced injection sites restricted to the small nucleus. In these latter cases, counterstaining with phosphine allowed a better definition of anatomical structures. After D-[3H]aspartate application, retrogradely labeled perikarya were observed in cerebral cortex (layer V), zona incerta, dorsal raphé nucleus and in several other structures also labeled by non-selective tracers. Following application of [3H]choline and using an improved autoradiographic method, perikaryal labeling was massive within nucleus interpositus, while it was absent in dorsal raphé nucleus, cerebral cortex and zona incerta. Retrograde tracing experiments with D-[3H]aspartate and [3H]choline revealed that these transmitter related compounds are selective markers for two subsets of afferents to the red nucleus. The transmitter specificity of the selective labeling with [3H]choline in the cerebellorubral pathway is supported only in part by the results obtained with other methods. The selective labeling with D-[3H]aspartate in the corticorubral pathway, on the other hand, is consistent with its transmitter specificity.

Analysis of the rat interpeduncular subnuclei by immunocytochemical double-staining for enkephalin and substance P, with some reference to the coexistence of both peptides

The rat interpeduncular nucleus (IPN) was immunocytochemically double-stained for enkephalin (ENK) and substance P (SP) on the same sections. On the basis of both peptidergic distribution patterns and topographic relationship, the IPN was divided into nine subnuclei and one cap: the rostral subnucleus (IP-R), the central subnucleus (IP-C), the rostral-lateral subnucleus (IP-RL), the main lateral subnucleus (IP-L), the caudal-lateral subnucleus (IP-CL), the dorsal-lateral subnucleus (IP-DL), the dorsal-medial subnucleus (IP-DM), the apical subnucleus (IP-A), the intermediate subnucleus (IP-I), and the dorsal cap (IP-Cap). As the descriptions of the IP-RL, IP-L, and IP-CL were inconsistent with previous reports, they were reevaluated; the IP-RL was proposed as the region situated in the lateral portion at rostral levels and characterized by the lack of ENK and SP immunoreactive structures, the IP-L as the region situated throughout the rostrocaudal extent in the lateral portion of the IPN and containing the highest density of SP immunoreactive fibers but no ENK immunoreactive fibers, and the IP-CL as the region situated just laterocaudal to the IP-L in the caudal pole of the IPN and containing ENK immunoreactive cells and fibers but no SP immunoreactive structures. Our results also showed that some cells in the IP-R have both ENK and SP immunoreactivity. This coexistence was observed in some small spherical cells of the IP-R, but rarely in larger oval-shaped cells, which occasionally showed only ENK immunoreactivity. In addition, paired ENK immunoreactive fiber bundles entering the IP-R were found to run just rostral to the paired SP immunoreactive columns, both of which composed parts of the interpedunculotegmental tract. A three-dimensional model representing the subnuclear organization of the IPN was proposed on the basis of the present results.

Contrasting properties of medial septal neurons projecting to hippocampus or interpeduncular nucleus in the rat

The properties of neurons of the medial septal nucleus and of the nucleus of the diagonal band of Broca which project to hippocampus or to interpeduncular nucleus were compared in rats anesthetized with urethane. Neurons projecting to the interpeduncular nucleus had a slower conduction velocity and a lower spontaneous discharge rate. In contrast, their responses to various putative neurotransmitters (glutamate, GABA, acetylcholine) were similar. In a few cases, neurons projecting to both structures (i.e., with branched axons) were observed. Both septohippocampal and septointerpeduncular pathways are known to be partly cholinergic. Our results show that they originate from two independent populations of medial septal-nucleus of the diagonal band of Broca neurons with different physiologic properties.

Serotonergic and nonserotonergic projections from the rat interpeduncular nucleus to the septum, hippocampal formation and raphe: a combined immunocytochemical and fluorescent retrograde labelling study of neurons in the apical subnucleus

This study examined the subnuclear distribution and transmitter content of neurons in the interpeduncular nucleus (IPN) that projected to the septum, dorsal hippocampal formation, and/or raphe. Following the injection of fast blue into the medial septum/diagonal band nucleus and rhodamine-conjugated microspheres into the dorsal hippocampal formation (or vice versa), retrogradely-labelled cells were found throughout the apical subnucleus of the IPN. Incubation of these sections with 5-hydroxytryptamine antiserum indicated that a small number of fast blue- or rhodamine-positive cells also contained serotonin. Occasional apical cells contained both fast blue and rhodamine, indicating a dual projection via collaterals to both the septum and hippocampus. Injection of either dye into the raphe also retrogradely labelled cells in the apical subnucleus, none of which contained serotonin. These results suggest that the IPN may function to integrate the activity within subcortical limbic nuclei via widespread serotonergic and non-serotonergic projections.

Brainstem afferents to the magnocellular basal forebrain studied by axonal transport, immunohistochemistry, and electrophysiology in the rat

Brainstem afferents to the magnocellular basal forebrain were studied by using tract tracing, immunohistochemistry and extracellular recordings in the rat. WGA-HRP injections into the horizontal limb of the diagonal band (HDB) and the magnocellular preoptic area (MgPA) retrogradely labelled many neurons in the pedunculopontine and laterodorsal tegmental nuclei, dorsal raphe nucleus, and ventral tegmental area. Areas with moderate numbers of retrogradely labelled neurons included the median raphe nucleus, and area lateral to the medial longitudinal fasciculus in the pons, the locus ceruleus, and the medial parabrachial nucleus. A few labelled neurons were seen in the substantia nigra pars compacta, mesencephalic and pontine reticular formation, a midline area in the pontine central gray, lateral parabrachial nucleus, raphe magnus, prepositus hypoglossal nucleus, nucleus of the solitary tract, and ventrolateral medulla. A similar but not identical distribution of labelled neurons was seen following WGA-HRP injections into the nucleus basalis magnocellularis. The possible neurotransmitter content of some of these afferents to the HDB/MgPA was examined by combining retrograde Fluoro-Gold labelling and immunofluorescence. In the mesopontine tegmentum, many retrogradely labelled neurons were immunoreactive for choline acetyltransferase. In the dorsal raphe nucleus, some retrogradely labelled neurons were positive for serotonin and some for tyrosine hydroxylase (TH); however, the majority of retrogradely labelled neurons in this region were not immunoreactive for either marker. The ventral tegmental area, substantia nigra pars compacta, and locus ceruleus contained retrogradely labelled neurons which were also immunoreactive for TH. Of the retrogradely labelled neurons occasionally observed in the nucleus of the solitary tract, prepositus hypoglossal nucleus, and ventrolateral medulla, some were immunoreactive for either TH or phenylethanolamine-N-methyltransferase. To characterize functionally some of these brainstem afferents, extracellular recordings were made from antidromically identified cortically projecting neurons, mostly located in the HDB and MgPA. In agreement with most previous studies, about half (48%) of these neurons were spontaneously active. Electrical stimulation in the vicinity of the pedunculopontine tegmental and dorsal raphe nuclei elicited either excitatory or inhibitory responses in 21% (13/62) of the cortically projecting neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

[3H]vesamicol binding in brain: autoradiographic distribution, pharmacology, and effects of cholinergic lesions

An autoradiographic analysis of high-affinity binding sites for the vesicular acetylcholine transport blocker [3H]vesamicol (2-(4-phenylpiperidino) cyclohexanol; AH 5183) was conducted in rat brain. [3H]Vesamicol binding was displaced 52-99% by DPPN [( 2,3,4,8]-decahydro-3-(4-phenyl-1-piperidinyl)-2-napthalenol) (IC50 = 14 nM) and by ketanserin (500 nM), haloperidol (43 nM), and vesamicol analogs, but not by drugs selective for adenosine, adrenergic, amino acid, calcium channel, monoaminergic, opioid, PCP, sigma, or several other receptor classes. [3H]Vesamicol binding was most concentrated in the interpeduncular nucleus and fifth and seventh cranial nerve nuclei. Moderate binding was found in the lateral caudate-putamen, medial nucleus accumbens, olfactory tubercle, vertical and horizontal diagonal bands of Broca, and basolateral amygdala. The distribution of [3H]vesamicol binding was similar to distributions of acetylcholine (r = 0.88), acetylcholine esterase (r = 0.97), choline acetyltransferase (ChAT) (r = 0.97), and [3H]hemicholinium-3 binding sites (r = 0.95-0.99). Lower correlations were obtained between [3H]vesamicol and muscarinic receptor densities (r = 0.50-0.70). Few exceptions to the match between binding and cholinergic neuronal markers were found, e.g., the molecular layer of the cerebellum and the thalamus. Lesions of cholinergic neuronal projections to the neocortex or hippocampus reduced [3H]vesamicol binding in each of these regions, but to a lesser extent than reductions in ChAT. [3H]Vesamicol binding sites appear to be anatomically associated with brain cholinergic neurons, a locus that is consistent with the control by this site of vesicular acetylcholine uptake.

Projections between the interpeduncular nucleus and basal forebrain in the rat as demonstrated by the anterograde and retrograde transport of WGA-HRP

The distribution of anterogradely-labeled fibers and retrogradely-labeled cell bodies in the interpeduncular nucleus (IPN) was mapped after injections of wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into various structures of the basal forebrain in adult rats. WGA-HRP injections into the medial septum/vertical limb of the diagonal band nucleus resulted in: 1) dense anterograde labeling in the rostral, intermediate, and central subnuclei; and 2) retrograde labeling in the apical and central subnuclei. Injections into the lateral septum produced: 1) no anterograde labeling in the IPN; and 2) retrograde labeling which was dense in the apical subnucleus, moderate in the central and lateral subnuclei, and light in the intermediate subnucleus. Injections into the horizontal limb of the diagonal band nucleus resulted in: 1) anterograde labeling which was most pronounced in the central, rostral, and intermediate subnuclei; and 2) retrograde labeling which was strongest in the apical, central, and lateral subnuclei. After injections into the substantia innominata-magnocellular preoptic nucleus, there was: 1) dense anterograde labeling in the rostral and central subnuclei and moderate anterograde labeling in the intermediate subnucleus; and 2) essentially no retrograde cell labeling in the IPN. These findings demonstrate that the IPN receives inputs from, and projects to specific portions of the basal forebrain. The rostral and central subnuclei are the primary targets of inputs from the basal forebrain to the IPN, and the apical subnucleus is the primary source of IPN projections to the basal forebrain.

Immunohistochemical localization of a neuronal nicotinic acetylcholine receptor in mammalian brain

A monoclonal antibody generated against purified acetylcholine receptor from Torpedo electric organ was used to immunohistochemically localize a neuronal nicotinic acetylcholine receptor. Regions of the rat brain stained with this antibody paralleled those areas of the brain exhibiting [3H]nicotine binding sites and corresponded to areas in which mRNAs encoding for alpha subunits of the neuronal nicotinic acetylcholine receptor are present. Thus, the anteroventral thalamus, cortex, hippocampus, medial habenula, interpeduncular nucleus, and substantia nigra/ventral tegmental area exhibited significant immunoreactivity. Neurons of the medial habenula and substantia nigra were densely stained, and processes were prominently delineated. Furthermore, in the projection areas of the medial habenula (interpeduncular nucleus and median raphe) axons were strongly immunoreactive and were distributed to distinct subdivisions of the target sites. The present data suggest that there are several discrete neuronal systems in which nicotinic acetylcholine receptors have functional importance. These immunohistochemical studies delineate at the single-cell level the localization within the mammalian central nervous system of certain nicotinic acetylcholine receptors.

Descending projections of the basal forebrain in the rat demonstrated by the anterograde neural tracer Phaseolus vulgaris leucoagglutinin (PHA-L)

Descending pathways from the mediobasal forebrain were studied in the rat by injecting anterograde axonal tracer Phaseolus vulgaris leucoagglutinin into the substantia innominata and diagonal band of Broca. From both areas, positive fibers which varied in density were observed in the mediodorsal and ventral parts of the ventroposterior and ventromedial thalamic nuclei, the lateral habenula, the stria medullaris, the lateral hypothalamus and the ventral tegmental area. This descending complex appeared predominantly course through the medial forebrain bundle from which positive fibers ramified into the fasciculus thalamicus to distribute in the thalamic nuclei. A minor descending pathway through the stria medullaris was also noted which terminated in the lateral habenula and the mediodorsal thalamic nucleus. An obvious difference in terminal distribution in the medial habenula, mediodorsal thalamic nucleus and pons could be observed following substantia innominata or diagonal band injection.

Choline acetyltransferase immunocytochemistry of Edinger-Westphal and ciliary ganglion afferent neurons in the cat

The distribution of cholinergic neurons in the region of the cat Edinger-Westphal nucleus (EW) was determined by immunocytochemical localization of the acetylcholine-synthesizing enzyme choline acetyltransferase (ChAT). Neurons containing ChAT-like immunoreactivity (ChAT-LI) were densely distributed within EW, the anteromedian nucleus (AM), and the oculomotor nucleus (III), and were also present in immediately adjacent regions of the periaqueductal gray and ventral tegmental region. The majority of labelled neurons in EW and AM showed a markedly lower intensity of ChAT-LI than the labelled neurons in III and adjacent regions. To determine the relationship of cells with ChAT-LI to the distribution of ciliary ganglion afferent neurons, a double labelling immunocytochemistry/retrograde transport technique was also used. These experiments showed that many of the cells located outside of III that stained intensely for ChAT-LI project to ciliary ganglion. Very few ciliary ganglion afferent neurons were found in EW or AM itself; instead, the distribution of lightly labelled ChAT-LI-positive neurons in EW and AM more closely matched the known distribution of peptide-containing cells that have descending, central projections.

Acetylcholine in the interpeduncular nucleus of the rat: normal distribution and effects of deafferentation

We studied the cholinergic projection to the interpeduncular nucleus (IPN) by examining localization of choline acetyltransferase (ChAT) in the habenula, fasciculus retroflexus (FR) and among the subnuclei of the IPN of the rat, using and antibody raised against ChAT. ChAT-containing neurons were present in the ventral portion of the medial habenula, ChAT-stained axons were present in the FR and ChAT-stained axons and terminals were present in the rostral, central and intermediate subnuclei of the IPN. No ChAT staining was seen in the lateral or dorsal subnuclei. The pattern of ChAT localization was thus complementary to the pattern of the habenular substance P projection to the IPN. Lesions of the FR eliminated all ChAT from the IPN while lesions of the stria medullaris produced a modest decrease. Unilateral FR lesions indicated that the FR projection to the central and rostral subnuclei is largely bilateral and symmetrical and that to the intermediate subnuclei is largely ipsilateral. We found no evidence of lesion-induced plasticity, i.e. replacement of ChAT immunoreactivity, by surviving FR axons in these adult brains.

The origins of cholinergic and other subcortical afferents to the thalamus in the rat

The origins of the cholinergic and other afferents of several thalamic nuclei were investigated in the rat by using the retrograde transport of wheat germ agglutinin conjugated-horseradish peroxidase in combination with the immunohistochemical localization of choline acetyltransferase immunoreactivity. Small injections placed into the reticular, ventral, laterodorsal, lateroposterior, posterior, mediodorsal, geniculate, and intralaminar nuclei resulted in several distinct patterns of retrograde labelling. As expected, the appropriate specific sensory and motor-related subcortical structures were retrogradely labelled after injections into the principal thalamic nuclei. In addition, other basal forebrain and brainstem structures were also labelled, with their distribution dependent on the site of injection. A large percentage of these latter projections was cholinergic. In the brainstem, the cholinergic pedunculopontine tegmental nucleus was retrogradely labelled after all thalamic injections, suggesting that it provides a widespread innervation to the thalamus. Neurons of the cholinergic laterodorsal tegmental nucleus were retrogradely labelled after injections into the anterior, laterodorsal, central medial, and mediodorsal nuclei, suggesting that it provides a projection to limbic components of the thalamus. Significant basal forebrain labelling occurred only with injections into the reticular and mediodorsal nuclei. Only injections into the reticular nucleus resulted in retrograde labelling of the cholinergic neurons in the nucleus basalis of Meynert. The results provide evidence for an organized system of thalamic afferents arising from cholinergic and noncholinergic structures in the brainstem and basal forebrain. The brainstem structures, especially the cholinergic pedunculopontine tegmental nucleus, appear to project directly to principal thalamic nuclei, thereby providing a possible anatomical substrate for mediating the well-known facilitory effects of brainstem stimulation upon thalamocortical transmission.

Pedunculopontine tegmental nucleus of the rat: cytoarchitecture, cytochemistry, and some extrapyramidal connections of the mesopontine tegmentum

The pedunculopontine tegmental nucleus (PPTn) was originally defined on cytoarchitectonic grounds in humans. We have employed cytoarchitectonic, cytochemical, and connectional criteria to define a homologous cell group in the rat. A detailed cytoarchitectonic delineation of the mesopontine tegmentum, including the PPTn, was performed employing tissue stained for Nissl substance. Choline acetyltransferase (ChAT) immunostained tissue was then analyzed in order to investigate the relationship of cholinergic perikarya, dendritic arborizations, and axonal trajectories within this cytoarchitectonic scheme. To confirm some of our cytoarchitectonic delineations, the relationships between neuronal elements staining for ChAT and tyrosine hydroxylase were investigated on tissue stained immunohistochemically for the simultaneous demonstration of these two enzymes. The PPTn consists of large, multipolar neurons, all of which stain immunohistochemically for ChAT. It is present within cross-sections that also include the A-6 through A-9 catecholamine cell groups and is traversed by catecholaminergic axons within the dorsal tegmental bundle and central tegmental tract. The dendrites of PPTn neurons respect several nuclear boundaries and are oriented perpendicularly to several well-defined fiber tracts. Cholinergic axons ascend from the mesopontine tegmentum through the dorsal tegmental bundle and a more lateral dorsal ascending pathway. A portion of the latter terminates within the lateral geniculate nucleus. It has been widely believed that the PPTn is reciprocally connected with several extrapyramidal structures, including the globus pallidus and substantia nigra pars reticulata. Therefore, the relationships of pallidotegmental and nigrotegmental pathways to the PPTn were investigated employing the anterograde autoradiographic methodology. The reciprocity of tegmental connections with the substantia nigra and entopeduncular nucleus was investigated employing combined WGA-HRP injections and ChAT immunohistochemistry. The pallido- and nigrotegmental terminal fields did not coincide with the PPTn, but, rather, were located just medial and dorsomedial to it (the midbrain extrapyramidal area). The midbrain extrapyramidal area, but not the PPTn, was reciprocally connected with the substantia nigra and entopeduncular nucleus. We discuss these results in light of other cytoarchitectonic, cytochemical, connectional, and physiologic studies of the functional anatomy of the mesopontine tegmentum.

Cholinergic projections in the telencephalo-habenulo-interpeduncular system of the goldfish

Experiments were performed in order to see whether acetylcholine is a neurotransmitter in the telencephalo-habenulo-interpeduncular system of the goldfish. After telencephalic ablation choline acetyltransferase decreased by 53% in the habenular nuclei but was unchanged in the interpeduncular nucleus. After combined telencephalic and habenular lesions, the enzyme level dropped by 95% in the interpeduncular nucleus. These results suggest the existence of a telencephalo-habenular and a habenulo-interpeduncular cholinergic projection in the goldfish, as previously demonstrated in mammals. Acetylcholinesterase histochemistry additionally demonstrates some similarities between the habenulae and the interpeduncular nucleus in the goldfish and in mammals.

Ventral tegmental (A10) system: neurobiology. 1. Anatomy and connectivity

The VTA contains the A10 group of DA containing neurons. These neurons have been grouped into nuclei to be found on the floor of the midbrain tegmentum--Npn, Nif, Npbp and Nln rostralis and caudalis. The VTA is traversed by many blood vessels and nerve fibers. Close to its poorly defined borders are found DA (A8, A9, A11) and 5-HT containing neurons (B8). Efferent projections of the VTA can be divided into 5 subsystems. The mesorhombencephalic projects to other monoaminergic nuclei, the cerebellum and a fine projection descends to other tegmental nuclei as far as the inferior olive. Fibers to the spinal cord have not been demonstrated. The mesodiencephalic path projects to several thalamic and hypothalamic nuclei and possibly the median eminence. Functionally important examples are the anterior hypothalamic-preoptic area, N. medialis dorsalis and reuniens thalami. These two subsystems are largely non-dopaminergic. A minor mesostriatal projection is overshadowed by the large mesolimbic projection to the accumbens, tuberculum olfactorium, septum lateralis and n. interstitialis stria terminalis. There are also mesolimbic connections with several amygdaloid nuclei (especially centralis and basolateralis), the olfactory nuclei and entorhinal cortex. A minor projection to the hippocampus has been detected. The mesocortical pathway projects to sensory (e.g. visual), motor, limbic (e.g. retrosplenial) and polysensory association cortices (e.g. prefrontal). Prefrontal, orbitofrontal (insular) and cingulate cortices receive the most marked innervation from the VTA. A more widespread presence of DA in other cortices of rodents becomes progressively more evident in carnivores and primates. Most but not all projections are unilateral. Some neurons project to more than one area in mesodiencephalic, limbic and cortical systems. The majority of these fibers ascend in the MFB. Most areas receiving a projection from the VTA (DA or non-DA) project back to the VTA. The septohippocampal complex in particular and the limbic system in general provide quantitatively much less feedback than other areas. The role of the VTA as a mediator of dialogue with the frontostriatal and limbic/extrapyramidal system is discussed under the theme of circuit systems. The large convergence of afferents to certain VTA projection areas (prefrontal, entorhinal cortices, lateral septum, central amygdala, habenula and accumbens) is discussed under the theme of convergence systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Topography of cholinergic and substance P pathways in the habenulo-interpeduncular system of the rat. An immunocytochemical and microchemical approach

The topography of cholinergic and substance P containing habenulo-interpeduncular projections has been studied in the rat. The research has been carried out by combining choline acetyltransferase and substance P immunohistochemistry to experimental lesions and biochemical assays in microdissected brain areas. In addition, computer-assisted image analysis has been performed in order to obtain quantification of immunohistochemical data. The results show that cholinergic and substance P containing neurons have a different localization in the medial habenula and project to essentially different areas of the interpeduncular nucleus. Cholinergic neurons are crowded in the ventral two-thirds of the medial habenula while substance P containing cells are exclusively localized in the dorsal part of the nucleus. In most parts of the interpeduncular nucleus, choline acetyltransferase and substance P containing fibres and terminals are similarly segregated and no overlapping is apparent except for the rostralmost and the caudalmost ends of the nucleus. Cholinergic activity is largely concentrated in the central core of the nucleus, while substance P is preferentially localized in the peripheral subnuclei of the interpeduncular nucleus. In addition, both substance P and choline acetyltransferase levels show peculiar regional variations along the rostrocaudal axis of the interpeduncular nucleus. The results of experimental lesions demonstrate that the substance P projection carried by each fasciculus retroflexus is prevailingly ipsilateral in the rostral part of the interpeduncular nucleus and becomes progressively bilateral as far as more caudal regions of the nucleus are reached. By contrast, the cholinergic projections carried by each fasciculus retroflexus intermingle more rapidly and only show a slight ipsilateral dominance in the interpeduncular nucleus. The results of the study are discussed with reference to previous anatomical and neurochemical data which, in several instances, had given rise to discrepant interpretations.

The immunohistochemical localization of choline acetyltransferase in the cat brain

The distribution of neurons displaying choline acetyltransferase (ChAT) immunoreactivity was examined in the feline brain using a monoclonal antibody. Groups of ChAT-immunoreactive neurons were detected that have not been identified previously in the cat or in any other species. These included small, weakly stained cells found in the lateral hypothalamus, distinct from the magnocellular rostral column cholinergic neurons. Other small, lightly stained cells were also detected in the parabrachial nuclei, distinct from the caudal cholinergic column. Many small ChAT-positive cells were also found in the superficial layers of the superior colliculus. Other ChAT-immunoreactive neurons previously detected in rodent and primate, but not in cat, were observed in the present study. These included a dense cluster of cells in the medial habenula, together with outlying cells in the lateral habenula. Essentially all of the cells in the parabigeminal nucleus were found to be ChAT-positive. Additional ChAT-positive neurons were detected in the periolivary portion of the superior olivary complex, and scattered in the medullary reticular formation. In addition to these new observations, many of the cholinergic cell groups that have been previously identified in the cat as well as in rodent and primate brain such as motoneurons, striatal interneurons, the magnocellular rostral cholinergic column in the basal forebrain and the caudal cholinergic column in the midbrain and pontine tegmentum were confirmed. Together, these observations suggest that the feline central cholinergic system may be much more extensive than previous studies have indicated.

Distribution of choline acetyltransferase-immunoreactive neurons in the brain of a cyprinid teleost (Phoxinus phoxinus L.)

The distribution of putative cholinergic neurons in the brain of a cyprinid teleost was investigated by immunocytochemistry, with well-characterized polyclonal antibodies to porcine choline acetyltransferase (ChAT), correlated with acetylcholinesterase (AChE) histochemistry. AChE-positive neurons were more numerous than ChAT-immunoreactive (ChAT-IR) neurons. Regions with ChAT-IR neurons generally also contained AChE-positive ones, but regions with AChE-positive neurons often did not contain (or contained only small numbers of) ChAT-IR neurons. ChAT-IR neurons were located in the brainstem cranial nerve motor nuclei, in the brainstem reticular formation, in the nucleus lateralis valvulae and an adjacent subnucleus "a," in the nucleus isthmi, and in the stratum griseum periventriculare of the tectum opticum. All neurons in these areas were AChE positive. ChAT-IR neurons were also observed within the boundaries of the nucleus sensibilis nervi trigemini and the n. descendens nervi trigemini. The periventricular hypothalamus and the paraventricular organ, the pineal organ, and (possibly) the nucleus suprachiasmaticus also contained ChAT-IR neurons. In these areas, AChE activity was either low or located mainly in neurons other than the ChAT-IR ones. A small population of ChAT-IR neurons was observed in area ventralis telencephali pars lateralis. This was the only telencephalic ChAT-IR cell group. Furthermore, some previously unrecognized cell groups were observed. A small number of ChAT-IR neurons, located on the dorsal aspect of the fasciculus longitudinalis medialis (caudal to n. raphe dorsalis), emitted axons that passed caudally along the raphe midline and innervated some of the large reticular neurons. Another group of ChAT-IR neurons was observed caudal to the thalamic nucleus centralis posterior and was tentatively designed n. tractus rotundus, on the basis of the neuronal morphology. The almost Golgilike staining of some of the ChAT-IR cell groups permitted the identification of their efferent connections and the areas covered by their dendrites.

Characterization and autoradiographic distribution of hemicholinium-3 high-affinity choline uptake sites in mammalian brain

[3H]hemicholinium-3 (HC-3) binding characteristics have been investigated using membrane binding assays and in vitro receptor autoradiography. In rat brain membrane preparations, [3H]HC-3 binds with high affinity to an apparent single class of sites. [3H]HC-3 binding is Na+-dependent. The ligand selectivity pattern strongly suggests that [3H]HC-3 selectivity labels the high affinity choline uptake (HACU) in brain membranes (HC-3 greater than choline greater than carbamylcholine greater than acetylcholine). This hypothesis is also supported by quantitative autoradiographic data which demonstrate that the discrete distribution of [3H]HC-3 binding sites correlates very well with the known distribution of other cholinergic markers such as choline acetyltransferase (ChAT), acetylcholinesterase (AChE), HACU, and [3H]AH-5183 (blocker of the vesicular transport of acetylcholine). For example, high densities of labelling are observed for these different markers in the interpeduncular nucleus, anteroventral nucleus of the thalamus, striatum, basolateral nucleus of the amygdala, and an exquisite laminar distribution in the hippocampus. Similar autoradiographic distributions of [3H]HC-3 binding sites are observed in other mammalian species such as guinea pig and monkey. Finally, 7-day unilateral kainic acid lesions of the nucleus basalis magnocellularis (nbm) decrease cortical [3H]HC-3 binding and ChAT activity, although not to a similar extent. In summary, these results demonstrate that [3H]HC-3 is a selective ligand of the HACU in mammalian brain. Thus, it is now possible to characterize precisely various structural components of the cholinergic synapses using markers such as [3H]HC-3, ChAT, HACU, [3H]AH-5183, and selective muscarinic and nicotinic receptor radioligands.

A cholinergic projection to the rat superior colliculus demonstrated by retrograde transport of horseradish peroxidase and choline acetyltransferase immunohistochemistry

Acetylcholinesterase (AChE) has been localized by histochemistry in the superior colliculus and in the tegmentum of the caudal midbrain and rostral pons of the rat. The pattern of AChE localization in the superior colliculus was characterized by homogeneous staining in the superficial layers and patchlike staining in the intermediate gray layer. In the tegmentum, AChE was localized in the pedunculopontine nucleus (PPN), beginning rostrally at the caudal pole of the substantia nigra and extending caudally to the level of the parabrachial nuclei, and in the lateral dorsal tegmental nucleus (LDTN) of the central gray. The localization of AChE in these nuclei overlapped the distribution of neurons stained by immunohistochemistry using an antibody to choline acetyltransferase (ChAT), the synthesizing enzyme of the neurotransmitter acetylcholine. Other neighboring areas that were stained with AChE, but that did not contain ChAT-immunoreactive neurons, included the microcellular tegmental nucleus and the ventral tegmental nucleus. Neurons in the PPN and LDTN were determined to be potential sources of the cholinergic projection to the intermediate gray layer of the rat superior colliculus by double labelling with retrograde transport of horseradish peroxidase (HRP) combined with the immunohistochemical localization of ChAT. Three populations of neurons were identified. A predominantly ipsilateral ChAT-immunoreactive population was located in the pars compacta subdivision of PPN (PPNpc). Retrograde HRP-labelled neurons in the pars dissipata subdivision of the PPN (PPNpd), located ventral to the superior cerebellar peduncle (SCP) at the level of the inferior colliculus, composed a second population that was predominantly contralateral but was not ChAT immunoreactive. A third population of retrogradely labelled neurons was predominantly ipsilateral and ChAT immunoreactive and was located in the LDTN. These findings compared favorably with the full extent of the projection from this tegmental region revealed by retrograde transport of HRP from the superior colliculus when more compatible fixation and chromogen procedures were used. The results suggest that the PPN and the LDTN are two sources of the cholinergic input to the superior colliculus. Since the PPN also has extensive efferent, and afferent, connections with basal-ganglia-related structures, this cholinergic excitatory input to the superior colliculus, like the GABA-ergic inhibitory input from the substantia nigra pars reticulata, may provide the basis for an additional influence of the basal ganglia on visuomotor behavior.

Organization of atriopeptin-like immunoreactive neurons in the central nervous system of the rat

The atrial natriuretic peptide, atriopeptin, is a circulating hormone that plays an important role in the regulation of fluid and electrolyte homeostasis. Several recent studies have shown that atriopeptin-like immunoreactivity is present within the central nervous system as well as peripheral tissues. In the present report, we describe in detail the organization of atriopeptin-like immunoreactive (APir) perikarya and fibers in the central nervous system of the rat. The most prominent collection of APir perikarya was found in the hypothalamus, adjacent to the anteroventral tip of the third ventricle. Additional groups of APir perikarya were observed along the wall of the third ventricle and in the paraventricular and arcuate nuclei. Separate, smaller groups with distinctive morphology were seen in the lateral hypothalamic area, in the supra-mammillary, medial, and lateral mammillary nuclei, medial habenular nucleus, bed nucleus of the stria terminalis, and the central nucleus of the amygdala. In the pons and brain-stem, APir neurons were observed in the pedunculopontine and laterodorsal tegmental nuclei, as well as in the ventral tegmental area, Barrington's nucleus, the parabrachial nucleus, and the nucleus of the solitary tract. The densest terminal fields of APir fibers were found in the paraventricular nucleus of the hypothalamus, the bed nucleus of the stria terminalis, the median eminence, and the interpeduncular nucleus. The presence of atriopeptin immunoreactivity within the central nervous system suggests that atriopeptin may function as a central neuromediator. Potential functions of this candidate neuromediator deduced from its anatomical distribution are discussed, including the possibility that atriopeptin may function as both a central neuromediator and a systemic hormone in the regulation of the cardiovascular system.

3H-nicotine- and 125I-alpha-bungarotoxin-labeled nicotinic receptors in the interpeduncular nucleus of rats. II. Effects of habenular deafferentation

The cholinergic innervation of the interpeduncular nucleus (IPN) is wholly extrinsic and is greatly attenuated by bilateral habenular destruction. We describe changes in the labeling of putative nicotinic receptors within this nucleus at 3, 5, or 11 days after bilateral habenular lesions. Adjacent tissue sections of the rat IPN were utilized for 3H-nicotine and 125I-alpha-bungarotoxin (125I-BTX) receptor autoradiography. Compared to sham-operated controls, habenular destruction significantly reduced autoradiographic 3H-nicotine labeling in rostral (-25%), intermediate (-13%), and lateral subnuclei (-36%). Labeling in the central subnucleus was unchanged. Loss of labeling was maximal at the shortest survival time (3 days) and did not change thereafter. In order to establish whether this loss was due to a reduction in the number or the affinity of 3H-nicotine-binding sites, a membrane assay was performed on microdissected IPN tissue from rats that had received surgery 3 days previously. Bilateral habenular lesions produced a 35% reduction of high-affinity 3H-nicotine-binding sites, with no change in binding affinity. Bilateral habenular lesions reduced 125I-BTX labeling in the intermediate subnuclei, and a slight increase occurred in the rostral subnucleus. In the lateral subnuclei, 125I-BTX labeling was significantly reduced (27%) at 3 days but not at later survival times. In view of the known synaptic morphology of the habenulointerpeduncular tract, it is concluded that a subpopulation of 3H-nicotine binding sites within the IPN is located on afferent axons and/or terminals. This subpopulation, located within rostral, intermediate, and lateral subnuclei, may correspond to presynaptic nicotinic cholinergic receptors. Sites that bind 125I-BTX may include a presynaptic subpopulation located in the lateral and possibly the intermediate subnuclei.