Ultrastructural evidence of amygdalofugal axons terminating on cholinergic cells of the rostral forebrain

Activation of dopaminergic neurons modulates ventral pallidal responses evoked by amygdala stimulation

The ventral pallidum is a basal forebrain region that is thought to integrate cognitive processes with motoric behaviors. These functions are influenced by ventral pallidal inputs, which include projections from the amygdala and the ventral tegmental area/substantia nigra zona compacta. By examining the consequences of this convergence at the neuronal level, the present study indicates that electrical activation of ventral tegmental regions releases dopamine in the ventral pallidum which subsequently modulates pallidal electrophysiological responses evoked by stimulating the amygdala. Stimulation-evoked responses were characterized for extracellular single unit recordings of spontaneously active ventral pallidal neurons from chloral hydrate anesthetized rats. Stimulation of the amygdala evoked short latency (< or = 12 ms; possibly monosynaptic) and/or long latency (> 12 ms; polysynaptic) responses in all ventral pallidal neurons tested. Fifty-nine per cent of the tested neurons responded to ventral tegmental stimulation with short latency inhibition, and these neurons were often sensitive to microiontophoretically applied dopamine. Iontophoresis of dopamine antagonists SCH23390 (a D1 antagonist) or sulpiride (a D2 antagonist) attenuated the ventral tegmental-induced inhibitions. These observations indicate that the evoked responding was the result of endogenously released dopamine, and that D1 and D2 receptors were involved in this effect. Ninety-two per cent of the ventral pallidal neurons that demonstrated short latency responses to amygdala stimulation also exhibited short latency responses to activation of the ventral tegmentum. This suggests that these inputs often converge onto the same pallidal neurons. Amygdala-evoked responses were consistently attenuated by prior stimulation of the ventral tegmentum. Similarly, microiontophoretic ejection of dopamine attenuated amygdala-evoked effects. These results indicate that dopamine modulates amygdala-evoked pallidal responses. Such modulation may contribute to the integrative functions of the ventral pallidum.

Contribution of the amygdala and nucleus accumbens to ventral pallidal responses to dopamine agonists

Neurons recorded from ventral pallidum/substantia innominata (VP) of the basal forebrain respond to dopaminergic agonists that activate either the D1 or D2 the receptor subtype. Major afferent systems to the VP originate within amygdaloid nuclei (AMN) and the nucleus accumbens (NA). Since both the AMN and the NA are dopaminoceptive, the present study sought to analyze the contribution of these afferent systems to VP responses to dopaminergic agonists. Single VP neurons were electrophysiologically recorded in vivo from chloral hydrate-anesthetized rats, and the following determinations were made. 1) Effects of pharmacologic inactivation of an afferent system were assessed by monitoring VP neurons during intracerebral microinjections of the local anesthetic procaine, administered directly into either the AMN or the NA. 2) With procaine-induced VP rate changes used to indicate an afferent influence on the recorded neuron, VP responses to apomorphine (an agonist that acts at D1 and D2 receptor subtypes), SKF38393 (a D1 agonist), or quinpirole (a D2 agonist) were determined and compared with responses in rats not receiving the procaine pretreatment. Following pharmacologic inactivation of either the AMN or the NA, approximately 80% of the VP neurons monitored demonstrated rate changes, illustrating that spontaneous neuronal firing in the Vp is dependent on tonically input systems. Following afferent cessation, responses to apomorphine and quinpirole remained intact, suggesting that the AMN or NA is not necessary for VP responding to the systemic administration of dopaminergic agonists that act at D2 receptors. In contrast, the number of neurons that responded to SKF38393 was diminished follow intra-AMN (but not intra-NA) procaine. This suggests that D1-induced VP responses are mediated, at least in part, via the AMN.

The bed nucleus-amygdala continuum in human and monkey

The cytoarchitecture and distributions of seven neuropeptides were examined in the the bed nucleus of the stria terminalis (BST), substantia innominata (SI), and central and medial nuclei of the amygdala of human and monkey to determine whether neurons of these regions form an anatomical continuum in primate brain. The BST and centromedial amygdala have common cyto- and chemo-architectonic characteristics, and these regions are components of a distinct neuronal complex. This neuronal continuum extends dorsally, with the stria terminalis, from the BST and merges with the amygdala; it extends ventrally from the BST through the SI to the centromedial amygdala. The cytoarchitectonics of the BST-amygdala complex are heterogeneous and compartmental. The BST is parcellated broadly into anterior, lateral, medial, ventral, supracapsular, and sublenticular divisions. The central and medial nuclei of the amygdala are also parcellated into several subdivisions. Neurons of central and medial nuclei of the amygdala are similar to neurons in the lateral and medial divisions of the BST, respectively. Neurons in the SI form cellular bridges between the BST and amygdala. The BST, SI, and amygdala share several neuropeptide transmitters, and patterns of peptide immunoreactivity parallel cytological findings. Specific chemoarchitectonic zones were delineated by perikaryal, peridendritic/perisomatic, axonal, and terminal immunoreactivities. The results of this investigation demonstrate that there is a neuronal continuity between the BST and amygdala and that the BST-amygdala complex is prominent and discretely compartmental in forebrains of human and monkey.

Organization of ascending hypothalamic projections to the rostral forebrain with special reference to the innervation of cholinergic projection neurons

Axonal projections from hypothalamic nuclei to the basal forebrain, and their relation to cholinergic projection neurons in particular, were studied in the rat by using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) in combination with choline acetyltransferase (ChAT) immunocytochemistry. Discrete iontophoretic PHA-L injections were delivered to different portions of the caudal lateral hypothalamus, as well as to various medial hypothalamic areas, including the ventromedial, dorsomedial, and paraventricular nuclei, and anterior hypothalamic and medial preoptic areas. The simultaneous detection of PHA-L-labeled fibers/terminals and ChAT-positive neurons was performed by using nickel-enhanced diaminobenzidine (DAB) and nonenhanced DAB as chromogens. Selected cases were investigated at the electron microscopic level. Ascending hypothalamic projections maintained an orderly lateromedial arrangement within the different components of the medial forebrain bundle, as well as with respect to their terminal projection fields (e.g., within the bed nucleus of the stria terminalis and lateral septal nucleus). The distribution pattern of hypothalamic inputs to cholinergic projection neurons corresponded to the topography of ascending hypothalamic axons. Axons originating from neurons in the far-lateral hypothalamus reached cholinergic neurons in a zone that extended from the dorsal part of the sublenticular substantia innominata (SI) caudolaterally, to the lateral portion of the bed nucleus of the stria terminalis rostromedially, encompassing a narrow band along the ventral part of the globus pallidus and medial portion of the internal capsule. Axons originating from cells in the medial portion of the lateral hypothalamus reached cholinergic cells primarily in more medial and ventral parts of the SI, and in the magnocellular preoptic nucleus and horizontal limb of the diagonal band nucleus (HDB). Axons from medial hypothalamic cells appeared to contact cholinergic neurons primarily in the medial part of the HDB, and in the medial septum/vertical limb of the diagonal band complex. Electron microscopic double-labeling experiments confirmed contacts between labeled terminals and cholinergic cells in the HDB and SI. Individual hypothalamic axons established synapses with both cholinergic and noncholinergic neuronal elements in the same regions. These findings have important implications for our understanding of the organization of afferents to the basal forebrain cholinergic projection system.

Single-unit activity in the dorsomedial prefrontal cortex during the expression of discriminative bradycardia in rabbits

Much recent evidence suggests that the medial prefrontal cortex (PFCm) participates in the development and/or expression of learned, primary bradycardiac adjustments in rabbits. Accordingly, the present experiments were undertaken to determine whether single-unit activity in the precentral agranular and anterior cingulate regions of the PFCm is systematically affected by differential aversive Pavlovian conditioning and, if so, whether such activity is correlated with concomitant heart rate (HR) variables. In these studies, extracellular recordings were made during a training procedure that involved two distinctive, tone conditioned stimuli (CSs); one of these (CS+) was often paired with eye-shock (unconditioned stimulus, US), whereas the other (CS-) was never so paired. Of the 100 spontaneously-active cells whose activity was so evaluated, 74 were found to be tone-responsive and could be classified as belonging to one of 5 subpopulations on the basis of their specific patterns of evoked changes in discharge; additional data suggested that these cells were also generally responsive to the eye-shock US. Regarding the effects of training, the vast majority of cells in these subpopulations showed greater evoked activity changes on CS+, as opposed to CS-, trials, irrespective of the sign (i.e. increase and/or decrease) of their responses. This finding was not confounded by differences in baseline activity preceding CS+ vs CS- trials, which was generally characterized as being both irregular and unrelated to concomitant HR variables; nor could it be attributed to unconditioned responses biases. Thus, the discriminative activity patterns of these cells appeared to reflect the differential Pavlovian contingencies in effect. Moreover, approximately half of these cells exhibited tone-evoked activity changes that were reliably correlated with concomitant HR changes on a trial-by-trial basis. Consequently, the present data indicate that training-induced changes in the CS-evoked activity of PFCm cells are significantly related to aversively conditioned bradycardia in rabbits.

Prefrontal cortical projections to the cholinergic neurons in the basal forebrain

The prefrontal cortex (PFC) projections to the basal forebrain cholinergic cell groups in the medial septum (MS), vertical and horizontal limbs of the diagonal band of Broca (VDB and HDB), and the magnocellular basal nucleus (MBN) in the rat were investigated by anterograde transport of Phaseolus vulgaris leuco-agglutinin (PHA-L) combined with acetylcholinesterase (AChE) histochemistry or choline acetyltransferase (ChAT) immunocytochemistry. The experiments revealed rich PHA-L-labeled projections to discrete parts of the basal forebrain cholinergic system (BFChS) essentially originating from all prefrontal areas investigated. The PFC afferents to the BFChS display a topographic organization, such that medial prefrontal areas project to the MS, VDB, and the medial part of the HDB, whereas the orbital and agranular insular areas predominantly innervate the HDB and MBN, respectively. Since the recurrent BFChS projection to the prefrontal cortex is arranged according to a similar topography, the relationship between the BFChS and the prefrontal cortex is characterized by reciprocal connections. Furthermore, tracer injections in the PFC resulted in anterograde labeling of numerous "en passant" and terminal boutons apposing perikarya and proximal dendrites of neurons in the basal forebrain, which were stained for the cholinergic marker enzymes. These results indicate that prefrontal cortical afferents make direct synaptic contacts upon the cholinergic neurons in the basal forebrain, although further analysis at the electron microscopic level will be needed to provide conclusive evidence.

The contribution of basal forebrain to limbic-motor integration and the mediation of motivation to action

The contribution of hippocampal glutamatergic and VTA dopaminergic inputs to the nucleus accumbens and the role of accumbens--ventral and subpallidal GABAergic pathway in integrating the limbic signals into motor responses via pedunculopontine nucleus were examined with electrophysiological and behavioural techniques. Stimulation of hippocampal input to the accumbens activates GABAergic output to the subpallidal area which leads to suppression of spontaneous firing of subpallidal neurons, while activation of dopamine receptors in the accumbens suppresses GABAergic output to subpallidal area and thus increases the firing of picrotoxin-sensitive ventral pallidal neurons. However, both treatments induced hypermotility suggesting the functional heterogeneity of the ventral and subpallidal areas in "limbic-motor integration". Furthermore, both hippocampal output signals and dopaminergic input to the accumbens descend via ventral and subpallidal areas serially to the pedunculopontine nucleus, the region of the mesencephalic locomotor region. In addition, a parallel ascending pathway from the subpallidal area to the mediodorsal nucleus, and subsequently to the medial prefrontal cortex, probably mediates behaviour, e.g. food hoarding, that requires higher cognitive processing.

Hypothalamic axons terminate on forebrain cholinergic neurons: an ultrastructural double-labeling study using PHA-L tracing and ChAT immunocytochemistry

A correlated light and electron microscopic study combining choline acetyltransferase immunocytochemistry with anterograde tracing of Phaseolus vulgaris leucoagglutinin revealed that axons of lateral hypothalamic neurons terminate on forebrain cholinergic projection cells. This finding is discussed in relation to direct and indirect corticopetal systems.

Organization and synaptic interconnections of GABAergic and cholinergic elements in the rat amygdaloid nuclei: single- and double-immunolabeling studies

The aim of this study was to describe the localization of cholinergic and GABAergic neurons and terminals in the amygdaloid nuclei of the rat. Double immunolabeling was performed to study cholinergic-GABAergic synaptic interconnections. Cholinergic elements were labeled by using a monoclonal antibody to choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme. Antibodies against glutamate decarboxylase (GAD), the GABA- synthesizing enzyme, were employed to identify GABAergic perikarya and terminals. The tissue sites of the antibody bindings were detected by using either Sternberger's peroxidase-antiperoxidase (PAP) method or a biotinylated secondary antibody and avidinated ferritin. These two contrasting immunolabels allowed us to study GABAergic-cholinergic interconnections at the electron microscopic level. Our study revealed a characteristic distribution of GABAergic and cholinergic elements in the various amygdaloid nuclei: 1) Large, ChAT-immunopositive cells with heavily labeled dendrites were observed in the anterior amygdaloid area and in the lateral and medial zones of the central nucleus. These cells seem to constitute the intraamygdaloid extension of the magnocellular basal nucleus. Their dendrites invaded other amygdaloid nuclei, in particular the intercalated nuclei, the lateral olfactory tract nucleus, and the central zone of the central nucleus. These ChAT-immunoreactive dendrites formed synaptic contacts with GAD-positive terminals. GABAergic terminals probably thus exert an inhibitory amygdaloid influence onto cholinergic neurons of the magnocellular basal nucleus. 2) Two amygdaloid nuclei-the basal dorsal nucleus and the lateral olfactory tract nucleus-contained a dense network of ChAT-immunoreactive fibers and terminals, but they also contained numerous GAD-positive perikarya. Double-immunolabeling experiments revealed cholinergic terminals forming synaptic contacts on GAD-immunopositive cell bodies, dendritic shafts, and spines. 3) The central and medial nucleus seem to be the main target of GABAergic fibers to the amygdala. Both nuclei contained a dense plexus of GAD-immunoreactive terminals that may arise, at least in part, from the GABAergic neurons in the basal dorsal nucleus. Inhibition of the centromedial "excitatory" region through intraamygdaloid GABAergic connections may reduce excitatory amygdaloid influence onto hypothalamus and brainstem.

Afferent connections of the forebrain cholinergic projection neurons, with special reference to monoaminergic and peptidergic fibers

Earlier light microscopic data on afferent connections to the cholinergic forebrain neurons are reconsidered in the light of EM cross-identification of neurons and synapses by combinations of tracer and immunocytochemical techniques. Such studies suggest that brainstem monoaminergic afferents terminate on cholinergic forebrain neurons, and may modulate the activity of choline acetyltransferase levels in the postsynaptic neurons. A monosynaptic relationship between cholinergic forebrain neurons and neuropeptide Y and somatostatin containing axons is also supported by studies using double immunolabeling techniques at the EM level. These peptidergic afferents originate in part from locally arborizing neurons. Based upon the new data a circuit model for basal forebrain cholinergic neurons is proposed.

Neural associations of the substantia innominata in the rat: afferent connections

The afferent connections of the substantia innominata (SI) in the rat were determined employing the anterograde axonal transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), in combination with histochemical procedures to characterize the neuropil of the SI and identify cholinergic cells. Both neurochemical and connectional data establish that the SI is organized into a dorsal and a ventral division. Each of these divisions is strongly affiliated with a different region of the amygdala, and, together with its amygdalar affiliate, forms part of one of two largely distinct constellations of interconnected forebrain and brainstem cell groups. The dorsal SI receives selective innervation from the lateral part of the bed nucleus of the stria terminalis, the central and basolateral nuclei of the amygdala, the fundus of the striatum, distinctive perifornical and caudolateral zones of the lateral hypothalamus, and caudal brainstem structures including the dorsal raphe nucleus, parabrachial nucleus, and nucleus of the solitary tract. Projections preferentially directed to the ventral SI arise from the medial part of the bed nucleus of the stria terminalis, the rostral two-thirds of the medial nucleus of the amygdala, a large region of the rat amygdala that lies ventral to the central nucleus, the medial preoptic area, anterior hypothalamus, medialmost lateral hypothalamus, and the ventromedial hypothalamus. Both SI divisions appear to receive afferents from the dorsomedial and posterior hypothalamus, supramammillary region, ventral tegmental area, and the peripeduncular area of the midbrain. Projections to the SI whose selectivity was not determined originate from medial prefrontal, insular, perirhinal, and entorhinal cortex and from midline thalamic nuclei. Findings from both PHA-L and WGA-HRP experiments additionally indicate that cell groups preferentially innervating a single SI division maintain numerous projections to one another, thus forming a tightly linked assembly of structures. In the rat, cholinergic neurons that are scattered throughout the SI and in parts of the globus pallidus make up a cell population equivalent to the primate basal nucleus of Meynert (Mesulam et al.: Neuroscience 10:1185-1201, '83). PHA-L-filled axons, labelled from lectin deposits in the dorsal raphe nucleus, peripeduncular area, ventral tegmental area, or caudomedial hypothalamus were occasionally seen to approach individual cholinergic neurons int he SI, and to contact the surface of such cells with axonal varicosities (putative synaptic boutons.(ABSTRACT TRUNCATED AT 400 WORDS)

Locomotor hyperactivity in the rat after infusion of muscimol and [D-Ala2]Met-enkephalin into the nucleus basalis magnocellularis. Possible interaction with cortical cholinergic projections

Locomotor activity in the rat was studied after infusion of GABAergic and enkephalinergic agonists into the nucleus basalis magnocellularis (NBM) of the forebrain. The experiments were designed to find out whether pharmacological blockade of cholinergic neurons in the NBM had similar behavioral effects to those observed after lesion of the same structure. Three experiments were carried out. In the first experiment, infusion of the GABAergic agonist muscimol (50 ng) into the NBM led to a marked locomotor hyperactivity. In the second experiment, it was shown that muscimol-induced locomotor response was reduced by pretreatment with the GABAergic antagonist picrotoxin (3 mg/kg). Further, locomotor hyperactivity was also observed after injection of the indirect GABA agonist, ethanolamine-o-sulfate (50 micrograms) into the NBM. The third experiment was designed to investigate the relationship between the blockade of NBM cholinergic neurons and the development of locomotor hyperactivity. The locomotor hyperactivity produced by the cholinergic antagonist scopolamine (0.4 mg/kg) was increased two-fold after infusion of 10 ng muscimol into the NBM. This dose of muscimol on its own had no effect on locomotor behavior. Similar enhancement of the locomotor response to that found with GABAergic agonists was observed after infusion of [D-Ala2]Met-enkephalinamide (2.5 micrograms) into the NBM. This enkephalin mediated locomotor response was blocked by the opiate antagonist naloxone (2 mg/kg). Pharmacological manipulations of the afferent inputs to the NBM could therefore be of value in studies on the behavioral role of cholinergic neurons in the NBM.

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)

Light microscopic evidence for a substance P-containing innervation of the human nucleus basalis of Meynert

A two color histochemical/immunohistochemical method was used to demonstrate substance P and acetylcholinesterase in sections of the human nucleus basalis of Meynert (nbM). Substance P-immunoreactive terminal-like structures were found to make contact with magnocellular, acetylcholinesterase-positive perikarya and primary dendrites throughout all subdivisions of the nbM. This apparent innervation of nbM neurons was in most cases a relatively sparse one, but a small percentage of these neurons appeared to be recipients of a very heavy innervation which covered their perikarya and primary dendrites.

Morphology of cortically projecting basal forebrain neurons in the rat as revealed by intracellular iontophoresis of horseradish peroxidase

The intracellular horseradish peroxidase technique was employed to study the morphology of basal forebrain neurons that were identified as cortically projecting by antidromic invasion from the cerebral cortex. Four neurons were examined in detail; they were located at different rostrocaudal levels within the basal forebrain. Their somata were large, 30-50 microns in longest dimension, and gave rise to three to eight primary dendrites, which ramified into third- to fifth-order dendrites. The longest observed dendrite in each neuron terminated at a distance of 600-900 microns from the soma. The sizes of soma and dendritic field of the two most rostrally located cells were smaller than those of the other two cells located more caudally. Dendritic spines were seen in all four cortically projecting basal forebrain neurons. Spines had shafts of variable lengths, and usually had spherical or elongated heads. The density of spines varied among the four neurons; one neuron, a type II cortically projecting basal forebrain neurons as defined physiologically by Reiner et al., had a much greater number of dendritic spines than the other three neurons, which were type I neurons. No somatic spines were observed. Presumptive axons were identified in three of the four cortically projecting basal forebrain neurons. These axons originated from either the soma or a primary dendrite, and two of them gave off local collaterals, which displayed occasional bouton-like swellings. The above observations confirm and extend previous findings that cortically projecting neurons in the basal forebrain are large multipolar cells, and provide evidence to support the conclusion that these cells, although somewhat variable in size, generally have extensive dendrites which display frequent spines.

Synaptic connections of cholecystokinin-immunoreactive neurons and terminals in the rat fascia dentata: a combined light and electron microscopic study

We report here on the fine structure and synaptic connections of neurons and axon terminals in the rat fascia dentata displaying immunoreactivity to antibodies against cholecystokinin octapeptide (CCK). In the fascia dentata and hilar region, CCK-immunoreactivity was confined to nonpyramidal neurons that were similar in appearance to basket cells known to use gamma-aminobutyric acid (GABA) as neurotransmitter. These neurons exhibited dense accumulations of endoplasmic reticulum and infolded nuclei, and established asymmetric and symmetric synaptic contacts with presynaptic terminals. Among those terminals that formed asymmetric synaptic contacts, giant mossy fiber boutons arising from granule cell axons were identified. Cholecystokinin-immunoreactive terminals established symmetric synaptic contacts on the cell bodies and dendrites of granule cells. Similar contacts were formed on nonimmunoreactive hilar neurons. Some of these hilar cells were identified as commissural neurons by retrograde filling with horseradish peroxidase (HRP) following injection of the tracer into the contralateral fascia dentata. Synaptic contacts were rarely observed between immunolabeled pre- and postsynaptic elements. The results are discussed with regard to inhibitory processes in the fascia dentata since other studies have shown that CCK is coexistent with GABA in hippocampal nonpyramidal neurons.

Substance P-containing terminals in synaptic contact with cholinergic neurons in the neostriatum and basal forebrain: a double immunocytochemical study in the rat

Antibodies against substance P and choline acetyltransferase (ChAT) have been used in a sequential double-immunocytochemical ultrastructural study of the rat forebrain. The peroxidase-anti-peroxidase procedure was used for both antigens, however, two different substrates for the peroxidase reactions were used. The substance P-immunoreactive sites were first localized using 3,3'-diaminobenzidine as the substrate, then the ChAT-immunoreactive sites were localized using benzidine dihydrochloride. The reaction product formed by the two substrates was distinguishable in both the light and electron microscopes. Using this procedure, the cell bodies and proximal dendrites of identified cholinergic neurons in the neostriatum were found to receive symmetrical synaptic input from substance P-immunoreactive boutons. A similar pattern of substance P-immunoreactive synaptic input was observed onto magnocellular basal forebrain cholinergic neurons in the ventral pallidum and ventromedial globus pallidus. In both the striatum and basal forebrain substance P-immunoreactive boutons were also seen in contact with structures that did not display ChAT immunoreactivity.

GABAergic input to cholinergic forebrain neurons: an ultrastructural study using retrograde tracing of HRP and double immunolabeling

Amygdalopetal cholinergic neurons in the ventral pallidum were identified by combining choline acetyltransferase (ChAT) immunohistochemistry with retrograde tracing of horseradish peroxidase (HRP) following injections of the tracer in the basolateral amygdaloid nucleus. Although ChAT-positive terminals were identified in the ventral pallidum, they were never seen in contact with either immunonegative or ChAT-positive amygdalopetal neurons. In material, in which immunostaining against glutamic acid decarboxylase (GAD), the synthesizing enzyme for GABA was combined with retrograde tracing of HRP from the basolateral amygdaloid nucleus, GAD-positive terminals were seen to contact immunonegative amygdalopetal neurons. In addition, when sections of the rostral forebrain were processed, first to preserve and identify the transported HRP, and then were sequentially tested for both ChAT and GAD immunohistochemistry with the immunoperoxidase reaction for both tissue antigens, GAD-immunopositive terminals were seen to make synaptic contacts with cholinergic amygdalopetal neurons. These results suggest that amygdalopetal, presumably cholinergic, neurons receive GAD-positive terminals. In separate experiments using immunoperoxidase for ChAT and ferritin-avidin for GAD labeling, we confirmed the presence of GAD-containing terminals on cholinergic neurons. In addition, cholinergic terminals were seen in synaptic contact with GAD-positive cell bodies. These morphological studies suggest that direct GABAergic-cholinergic and cholinergic-GABAergic interactions take place in the rostral forebrain.

Direct autoradiographic determination of M1 and M2 muscarinic acetylcholine receptor distribution in the rat brain: relation to cholinergic nuclei and projections

The autoradiographic distributions of receptors with high affinity for [3H]oxotremorine-M (the M2 receptor) and [3H]pirenzepine (the M1 receptor) were studied in the rat brain. M1 receptors were seen in highest density only in telencephalic structures: cerebral cortex (layers I-II), hippocampus, dentate gyrus, medial and basolateral amygdala, nucleus accumbens and caudate/putamen. M2 receptors were detected throughout the brain, with highest levels observed in cerebral cortical layers III and V, forebrain cholinergic nuclei, caudate/putamen, various thalamic areas, inferior and superior colliculus, interpeduncular and pontine nuclei, brainstem cholinergic nuclei and cervical spinal cord regions. M2 receptors were found to be good markers for cholinergic cell groups and the majority of cholinergic projection areas, whereas M1 receptors were only found in a large sub-group of telencephalic cholinergic projection areas, and the pattern of distribution of receptors in these areas differed from that of M2 receptors. Scatchard analysis of [3H]oxotremorine-M binding to inferior collicular slices revealed one site with a dissociation constant (Kd) of 1.9 nM and a receptor density (Bmax) of 1.4 pmol/mg protein. Our data support the hypothesis that M1 and M2 receptors are physically distinct sub-types of the muscarinic acetylcholine receptor.

Ultrastructural characterization of choline acetyltransferase-containing neurons in the basal forebrain of rat: evidence for a cholinergic innervation of intracerebral blood vessels

The ultrastructural morphology and vascular associations of cholinergic neurons in the horizontal limb of the nucleus of the diagonal band of Broca (nDBBhl) and amygdala of rat were determined by the immunocytochemical localization of choline acetyltransferase (ChAT), the acetylcholine biosynthetic enzyme. Within the nDBBhl peroxidase reaction product was distributed throughout the cytoplasm of selectively labeled neuronal perikarya and dendrites. Labeled perikarya were characterized by an oval cell body (7-10 microns X 17-26 microns in diameter) in which was located a large nucleus and often a prominent nucleolus. Dendrites were by far the most numerous immuno-labeled profiles in the nDBBhl. The labeled dendrites had a cross-sectional diameter of 0.4-4.6 microns and contained numerous mitochondria and microtubules. Approximately 10% of all immunolabeled dendrites received synaptic contacts from unlabeled presynaptic boutons. In contrast to the relatively large number of ChAT-labeled dendrites within the nDBBhl, ChAT-positive axons were less frequently observed and immunolabeled axon terminals were never detected. The labeled axons had an outside diameter of 0.4-1.4 micron and were myelinated. The absence or relative paucity of immunolabeled terminals in the nDBBhl indicates that most if not all of the cholinergic perikarya within this nucleus are efferent projection neurons. The nDBB is known to have widespread projections to many areas of the neocortex, hippocampus, and amygdala. In the present study we examined the amygdala and observed many ChAT-labeled axon boutons. The immunolabeled varicosities contained numerous agranular vesicles and although ChAT-positive terminals were in direct contact with unlabeled neuronal elements within the amygdala, few if any synaptic densities were detected in a single plane of section. With respect to the vasculature, immunolabeled perikarya and dendrites within the nDBBhl and axon terminals in the amygdala were often in direct apposition to blood vessels. In many instances the labeled profile was observed lying directly on the basal lamina of a capillary endothelial cell. In no instance, however, were membrane densities observed. The presence of cholinergic neuronal elements contacting the vessel wall provides morphologic evidence suggesting that the neurogenic control of cerebral vasculature is in part mediated via a cholinergic mechanism.

Efferent connections of the prelimbic (area 32) and the infralimbic (area 25) cortices: an anterograde tracing study in the cat

The projections from the caudal part of the medial frontal cortex, encompassing the prelimbic area (PL) and the infralimbic area (IL) (Brodmann's areas 32 and 25, respectively), were studied in the cat with the anterograde autoradiographic tracing technique. The results indicate that the projection fields of IL, in contrast to those of PL, are restricted almost exclusively to limbic structures. Whereas the major thalamic projections from PL reach the mediodorsal, anteromedial, and ventromedial nuclei, the medial part of the lateral posterior nucleus, and the parataenial and reticular nuclei, and weak projections from this area are directed to the nucleus reuniens and other midline nuclei, the nucleus reuniens is the major thalamic termination field of fibers arising from IL. Cortical areas that are reached by fibers originating in PL and, to a lesser degree, also in IL, include more rostral prefrontal areas (areas 8, 6, and 12), the agranular insular, and the rostral perirhinal cortices. In contrast, cortical areas that are more strongly related to IL include the cingulate, retrosplenial, caudal entorhinal, and perirhinal cortices and the subiculum of the hippocampal formation. Another prominent output of PL concerns projections to an extensive medial part of the caudate nucleus and the ventral striatum, whereas fibers from IL only distribute most ventrally in the striatum. In the amygdaloid complex, fibers from PL were found to reach the basolateral, basomedial, and central nuclei, and fibers from IL to distribute to the medial and central nuclei. PL furthermore projects to the claustrum and the endopiriform nucleus. Other structures in the basal forebrain, including the medial septum, the nuclei of the diagonal band, the preoptic area, and the lateral and dorsal hypothalamus are densely innervated by IL and only sparsely by PL. With respect to more caudal parts of the brainstem, projections from PL and IL appeared to be essentially similar. They reach the ventral tegmental area, the periaqueductal gray, the parabrachial nucleus, and in cases of PL injections were followed as far caudally as the pons.

The afferent connections of the substantia innominata in the monkey, Macaca fascicularis

The afferent connections of the substantia innominata and the magnocellular nuclei within it (the nucleus of the horizontal limb of the diagonal band, NHDB, and the nucleus basalis of Meynert, NBM) have been studied with anterograde and retrograde axonal tracing techniques. Prominent inputs arise in the amygdaloid complex, restricted areas of the cerebral cortex, parts of the thalamus and hypothalamus, and nuclei of the lower brainstem. Autoradiographic tracing experiments indicate that the amygdaloid fibers are distributed throughout the NHDB and the NBM, and to a lesser extent to the ventral pallidum. Relatively few fibers innervate the more medially located nucleus of the vertical limb of the diagonal band (NVDB) and the medial septal nucleus. Visualization of the amygdalofugal fibers with the tracer PHA-L (Phaseolus vulgaris leuco-agglutinin) shows that they have varicosities resembling boutons en passant along their length in the substantia innominata. Retrograde tracing experiments using WGA-HRP indicate that the cells of origin of the projection from the amygdala are concentrated in the parvicellular basal nucleus, the caudal part of the magnocellular basal nucleus, the magnocellular accessory basal nucleus, and the central nucleus. Relatively few fibers to the substantia innominata arise in the rostrodorsal part of the magnocellular basal nucleus, or in the lateral or parvicellular accessory basal nuclei. Cortical cells projecting to the substantia innominata were retrogradely labeled in the orbitofrontal cortex (including areas 11-14 and 25), the rostral insula (especially the agranular area), the rostroventral temporal cortex (including areas 35, 36, and parts of TG and TE), and the piriform and entorhinal cortices. The projections from the orbital and rostral temporal cortex were confirmed with anterograde tracers. Projections to the substantia innominata were not found from the more lateral, dorsal or caudal parts of the cerebral cortex, although fibers from temporal area TA may pass through the dendritic field of the most caudal cells of the NBM. Diencephalic cells projecting to the substantia innominata are distributed diffusely throughout the preoptic area and hypothalamus, with higher concentration in the lateral preoptic area and in the pre-, supra-, and tubero-mammillary nuclei. Cells are also found in the midline thalamic nuclei and in the region between the peripeduncular and subparafascicular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Simultaneous ultrastructural demonstration of retrogradely transported horseradish peroxidase and choline acetyltransferase immunoreactivity

In order to study the synaptic connections of neurons identified by their projection target and neurotransmitter content, we have adapted a method of combining retrograde tracing of horseradish peroxidase (HRP) and immunocytochemistry at the electron microscopic level. HRP was injected into the rat amygdala. Sections from the rostral forebrain were processed according to the 3,3'-diaminobenzidine/glucose oxidase reaction followed by choline acetyltransferase (ChAT) localization. Neurons in the ventral pallidum which contained both the diffuse immunoperoxidase reaction product (ChAT) and large electron dense bodies characteristic of retrogradely transported HRP were defined as double labeled, i.e. cholinergic neurons that project to the amygdaloid body.