Claustral and amygdaloid afferents to the head of the caudate nucleus in macaque monkeys

A single dose of horseradish peroxidase polyacrylamide gel (10%, 1.0 microliter) was injected, after callosotomy, into the head of the caudate nucleus in six macaque monkeys. In addition to the previously reported structures sending fibers to the caudate nucleus, such as the cerebral cortex, substantia innominata, thalamus, hypothalamus, substantia nigra and brainstem, labeled cells were found abundantly in the rostral portion of the ipsilateral claustrum, and fewer labeled cells were dispersed in the ipsilateral external and extreme capsules. Many labeled cells were also present in the ipsilateral insular cortex throughout its rostrocaudal extent. Moreover, labeled cells were seen ipsilaterally in the basolateral, basomedial, accessory basal, and cortical nuclei of the amygdaloid complex.     

The principal pathway from the piriform cortex to the deep amygdaloid nuclei in the cat

Pathways relaying olfactory information to the deep amygdaloid nuclei (AMYGd) were analyzed with electrophysiological techniques in anesthetized cats. Stimulation of the piriform cortex (PC) produced orthodromic spikes in some AMYGd neurons with a mean latency of 12.2 ms and antidromic responses in other neurons with a mean latency of 9.3 ms. Stimulation of the AMYGd produced antidromic spikes in some PC neurons with a mean latency of 11.5 ms. Some neurons in the entorhinal area (EA) were activated orthodromically from the PC with a mean latency of 22 ms, and a proportion of these cells was also activated antidromically from the AMYGd. Some neurons in the agranular insular cortex were activated orthodromically from the PC, but none of them responded antidromically to AMYGd stimulation. From these observations, it is suggested that olfactory information reaches the AMYGd directly from the PC or indirectly via the EA, and that the direct path conveys the major olfactory input from the PC to the AMYGd.     

Serotonergic and nonserotonergic neurons in the dorsal raphe nucleus send collateralized projections to both the vestibular nuclei and the central amygdaloid nucleus

Using a combination of double retrograde tracing and serotonin immunofluorescence staining, we examined whether individual serotonergic and nonserotonergic neurons in the dorsal raphe nucleus are sources of collateralized axonal projections to vestibular nuclei and the central amygdaloid nucleus in the rat. Following unilateral injections of Diamidino Yellow into the vestibular nuclei and Fast Blue into the central amygdaloid nucleus, it was observed that approximately one-fourth of the dorsal raphe nucleus neurons projecting to the vestibular nuclei send axon collaterals to the central amygdaloid nucleus. Immunofluorescence staining for serotonin revealed that more than half of the dorsal raphe nucleus neurons from which these collateralized projections arise contain serotonin-like immunoreactivity. These findings indicate that a subpopulation of serotonergic and nonserotonergic dorsal raphe nucleus cells may act to co-modulate processing in the vestibular nuclei and the central amygdaloid nucleus, regions implicated in the generation of emotional and affective responses to real and perceived motion.     

Corticotropin-releasing factor projections from limbic forebrain and paraventricular nucleus of the hypothalamus to the region of the ventral tegmental area

Corticotropin-releasing factor (CRF) is a peptide neurotransmitter with high numbers of cell bodies found in limbic regions of the rat brain including the oval nucleus of the bed nucleus of the stria terminalis (BNSTov) and central nucleus of the amygdala (CeA) as well as in the paraventricular nucleus of the hypothalamus (PVN). CRF systems are activated in response to acute stressors and mediate a wide variety of physiological and behavioral responses to acute stress including aversive responses and responses that support appetitive behaviors. CRF is released in the ventral tegmental area (VTA), the cell body region of the mesocorticolimbic dopaminergic neurons, in response to acute stress and plays a role in stress-activation of appetitive behavior [Wang B, Shaham Y, Zitzman D, Azari S, Wise RA, You ZB (2005) Cocaine experience establishes control of midbrain glutamate and dopamine by corticotropin-releasing factor: a role in stress-induced relapse to drug seeking. J Neurosci 25:5389-5396]. However, although it is known that the VTA region contains significant levels of CRF-immunoreactive fibers [Swanson LW, Sawchenko PE, Rivier J, Vale WW (1983) Organization of ovine corticotropin-releasing factor immunoreactive cells and fibers in the rat brain: an immunohistochemical study. Neuroendocrinology 36:165-186], the source of CRF input to the region has not been identified. We used infusions of a fluorescent retrograde tracer, fluorogold, into the VTA region, combined with fluorescent immunocytochemistry for CRF to identify sources of this input. Double-labeled cells were found in BNSTov, CeA and PVN. The percent of fluorogold-labeled cells in each region that were CRF-positive was 30.8, 28.0 and 16.7% respectively. These data point to diffusely distributed sources of CRF-containing fibers in the VTA.     

Subpallial and hypothalamic areas activated following sexual and agonistic encounters in male chickens

Male sexual and agonistic behaviors are controlled by the common social behavior network, involving subpallial and hypothalamic brain areas. In order to understand how this common network generates different behavioral outcomes, induction of FOS protein was used to examine the patterns of neuronal activation in adult male chickens following interaction with a female or a male. Males were subjected to one of the following treatments: handling control, non-contact interaction with a female, contact interaction with a live female, a taxidermy female model or another male. The number of FOS-immunoreactive (FOS-ir) cells, and the area and immunostaining density of individual cells were quantified in the medial preoptic nucleus (POM), medial extended amygdala (nucleus taeniae of the amygdala, TnA, and dorsolateral and ventromedial subdivisions of the medial portion of the bed nucleus of stria terminalis, BSTM1 and BSTM2, respectively), lateral septum (SL), hypothalamic paraventricular nucleus (PVN), bed nucleus of the pallial commissure (NCPa) and ventrolateral thalamic nucleus (VLT). An increase in FOS-ir cells following appetitive sexual behavior was found in BSTM2 and NCPa. Copulation augmented FOS-ir in POM, SL, VLT, and PVN. Intermale interactions increased FOS-ir in all examined brain regions except the TnA and BSTM. Within the SL, copulatory and agonistic behavior activated spatially segregated cell groups. In the PVN, different social behaviors induced significant changes in the distribution of FOS-ir cell sizes suggesting activation of heterogeneous subpopulations of cells. Collectively, behavioral outcomes of male-female and male-male interactions are associated with a combination of common and site-specific patterns of neural activation.     

The feline oculomotor nucleus: morphological subdivisions and projection to the cerebellar cortex and nuclei

Summary The cytoarchitecture of the feline oculomotor nucleus was examined in sections stained with thionin and neutral red. Five different subdivisions (caudal central, paramedian, ventral, dorsomedial and dorsolateral divisions) can be identified on each side of the midline. This observation is discussed, and our findings are compared to previous studies of the cytoarchitecture or central muscular representation of the oculomotor nucleus in which different subgroups have been distinguished. Implants or injections of the wheat germ agglutinin-horseradish peroxidase complex have revealed that all five subdivisions project to different parts of the cerebellar cortex and nuclei. Retrogradely labelled cells were found in the oculomotor nucleus in 18 cases following deposition of tracer in the fastigial and interposed nuclei and certain regions of the anterior, posterior and flocculonodular lobes. The projection is bilateral and appears to have its main termination in flocculus. It originates from small neurons, especially from those located along the dorsal border of the oculomotor nucleus.     

Afferent connections of the parabrachial nucleus in C57BL/6J mice

Although the mouse is an experimental model with an increasing importance in various fields of neuroscience, the characteristics of its central gustatory pathways have not yet been well documented. Recent electrophysiological studies using the rat and hamster have revealed that taste processing in the brainstem gustatory relays is under the strong influence of inputs from forebrain gustatory structures. In the present study, we investigated the organization of afferent projections to the mouse parabrachial nucleus (PbN), which is located at a key site between the brainstem and gustatory, viscerosensory and autonomic centers in the forebrain. We made injections of the retrograde tracer fluorogold centered around the “waist” area of the PbN, whose neurons are known to be highly responsive to taste stimuli. Retrogradely labeled neurons were found in the infralimbic, dysgranular and agranular insular cortex as well as the claustrum; the bed nucleus of the stria terminalis and the substantia innominata; the central nucleus of the amygdala; the lateral and medial preoptic areas, the paraventricular, the dorsomedial, the ventromedial, the arcuate, and the lateral hypothalamic areas; the periaqueductal gray, the substantia nigra pars compacta, and the ventral tegmental area; the supratrigeminal nucleus, rostral and caudal nucleus of the solitary tract; the parvicellular intermediate and gigantocellular reticular nucleus; the caudal and interpolar divisions of the spinal trigeminal nucleus, dorsomedial spinal trigeminal nucleus, and the area postrema. Numbers of labeled neurons in the main components of the gustatory system including the insular cortex, bed nucleus of the stria terminalis, central nucleus of the amygdala, lateral hypothalamus, and rostral nucleus of the solitary tract were quantified. These results are basically consistent with those of the previous rat and hamster studies, but some species differences were found. Functional implications of these afferent inputs are discussed with an emphasis on their role in taste.     

Light and electron microscopic studies of calcitonin gene-related peptide-like immunoreactive terminals in the central nucleus of the amygdala and the bed nucleus of the stria terminalis of the rat

Summary Light and electron microscopic analysis of calcitonin gene-related peptide (CGRP)-like immunoreactive (LI) terminals in the bed nucleus of the stria terminalis (BST) and the central nucleus of the amygdala (Ce) was carried out using the peroxidase-antiperoxidase method. CGRP-LI fibers were densely distributed in the dorsal subdivision of the lateral BST (BSTL) and the lateral and lateral capsular subdivisions of the Ce, where the CGRP-LI terminals formed symmetrical and asymmetrical axo-dendritic, and symmetrical axosomatic synapses. One of the most characteristic features of the CGRP-LI terminals was the presence of large, long boutons, each of which surrounded a cell soma and made many synaptic contacts. These findings suggest that CGRP exerts a significant influence on neurons in the BSTL and Ce.     

Projections from the central nucleus of the amygdala to the nucleus pontis oralis in the rat: An anterograde labeling study

The present study was designed to elucidate the neuronal projections from the amygdala to the nucleus pontis oralis (NPO). We propose that glutamatergic cells in the central nucleus of the amygdala (CNA) activate neurons in the NPO, which is the critical brainstem site that is responsible for the generation and maintenance of active (REM) sleep. Phaseolus vulgaris-leucoagglutinin (PHA-L), an anterograde transported neuronal tracer, was iontophoresed into the CNA of adult male Sprague-Dawley rats. After a survival time of 7-8 days, the animals were perfused with a fixative and brain tissue was prepared for histological analysis. Sections of the NPO and CNA, which were immunostained with an antibody against PHA-L, were examined with light microscopy. In addition, in order to identify the phenotype of PHA-L-labeled fibers and terminals in the NPO, a double immunohistochemical technique was employed with antibodies against PHA-L and the vesicular glutamate transporter type 2 (VGluT2). Numerous PHA-L-labeled axons and terminals were found in the NPO ipsilateral to the injection site in the CNA. Within the NPO, the majority of labeled fibers were located in the dorsolateral portion of the caudal part of the nucleus. Double-labeling immunostaining studies revealed that PHA-L-labeled axons and terminals in the NPO were glutamatergic. The present demonstration of direct, excitatory (glutamatergic) projections from the CNA to the NPO provide an anatomical basis for the amygdalar control of active sleep.     

Reorganization of the vestibulothalamic projections in lesions to the interpositus nucleus of the cerebellum and the vestibular nucleus of Deiters

The potential for plastic reorganization in the vestibulothalamic system was studied in adult cats. Preliminary (three months) lesioning of the contralateral nucleus interpositus of the cerebellum or the lateral vestibular nucleus of Deiters led to reorganization of vestibulothalamic projections with formation of ipsilateral projections to the ventrolateral nucleus of the thalamus from the nuclei of the vestibular complex, along with changes in the normal representation of the contralateral projections of the vestibular complex to this thalamic nucleus. The distribution and morphological composition of cells in the vestibular nuclei forming the new projections to the ventrolateral nucleus of the thalamus were studied. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 93, No. 11, pp. 1275–1284, November, 2007.     

Axons from non-cochlear sources in the anteroventral cochlear nucleus of the cat. A study with the rapid Golgi method.

Six groups of non-cochlear axons which project to the anteroventral cochlear nucleus of the cat can be identified in rapid Golgi preparations. The axons in three of these groups enter the anteroventral cochlear nucleus from its medial border, most of the fibers coming from the trapezoid body. Group I axons terminate in the anterior part of the anterior division of the anteroventral cochlear nucleus. Group II axons terminate in a portion of the small cell cap and in part of the posteroventral cochlear nucleus; they supply some endings to the dorsal part of the posterior division of the anteroventral nucleus as well. Group III axons end diffusely throughout the anterior division but not in the posterior division. Two groups of axons travel from caudal parts of the cochlear nucleus to the anteroventral part within the small cell cap. Group IV axons end in the dorsal part of the posterior division. Group V axons terminate in the dorsal part of the anterior division. Group VI axons course through the granule cell layer and form endings there but not in the anteroventral cochlear nucleus proper. The axons of each group form characteristic patterns of terminal branches, which give the different parts of the anteroventral cochlear nucleus a distinctive appearance in rapid Golgi preparations.Each subdivision of the anteroventral cochlear nucleus receives cochlear input. However, the present findings demonstrate differential non-cochlear inputs to the various subdivisions, implying that non-cochlear influences on the activity of the neurons may not be the same throughout the nucleus. Moreover, each subdivision contains several types of neurons and the non-cochlear inputs may project to all or to only some of these cell types. Thus, the arrangements of the non-primary inputs to the neurons of the cochlear nuclear complex introduce another level of complexity to its synaptic organization.     

An anatomical investigation of the corticopontaine projection in the primate (Macaca fascicularis and Saimiri sciureus)--II. The projection from frontal and parental association areas.

In continuation of a previous study on the corticopontine projection from sensorimotor areas of the cerebral cortex in primates, the projection from frontal and parietal association areas has been investigated with degeneration and autoradiographic tracing techniques. Since the connexions are ipsilateral the projection from peri-Rolandic areas was also studied on the contralateral side of the same animals as a control. It was found that association areas have only a modest contribution to the corticopontine system as compared to the massive projection from sensorimotor areas. Of those association areas studied in the present experiments, area 5 of the parietal cortex and the premotor cortex contributed most corticopontine fibres. The prefrontal cortex and area 7 were found to have only very weak connexions.It is evident, especially from autoradiographic studies, that the corticopontine system has an intricate organization, with one particular cortical area projecting to multiple small target zones in the pontine nuclei interdigitating with target zones or ‘patches’ receiving fibres from other cortical areas. The distribution of these zones may be rather widespread, especially those receiving afferents from the motor cortex, thus providing ample opportunities for integration of motor command signals with sensory, mainly somatosensory, signals. The fact that the pontine nuclei receive chiefly signals from primary cortical areas, rather than from high-order association areas, may be taken to indicate that the cortico-ponto-cerebellar system subserves rapid corrections of movements and ‘triggered actions’, a proposition which is also supported by behavioural studies in monkeys subjected to reversible cooling of the dentate nucleus.     

Involvement of oxytocin and its receptor in nociceptive modulation in the central nucleus of amygdala of rats

Studies have demonstrated that oxytocin plays important roles in pain modulation in the central nervous system. Oxytocin-ergic neurons are found in paraventricular nucleus and supraoptic nucleus of the hypothalamus. The oxytocin-ergic neurons send fibers from hypothalamus to amygdala and high density of oxytocin receptors are found in the central nucleus of amygdala (CeA). The present study was performed to investigate the influences of oxytocin and its receptors on nociceptive responses in the CeA of rats. Intra-CeA injection of 0.1, 0.5 or 1 nmol of oxytocin induced dose-dependent increases in the handpaw withdrawal latency induced by noxious thermal and mechanical stimulation in rats. The oxytocin-induced anti-nociception could be blocked by the selective oxytocin antagonist 1-deamino-2-d-Tyr-(Oet)-4-Thr-8-Orn-oxytocin. The present study demonstrated that oxytocin and its receptors are involved in nociceptive modulation in the CeA of rats.     

Cortical and subcortical distribution of ionotropic purinergic receptor subunit type 1 (P2X(1)R) immunoreactive neurons in the rat forebrain

Ionotropic purinergic receptors (P2XR) are ATP-gated cationic channels composed of seven known subunits (P2X(1-7)R) and involved in different functions in neural tissue. Although their presence has been demonstrated in the brain, few studies have investigated their expression pattern. In particular, ionotropic purinergic receptor subunit type 1 (P2X(1)R) has been observed in the cerebellum and in brainstem nuclei. The present study investigates the P2X(1)R expression pattern in the rat forebrain using immunohistochemistry. The specificity of the immunolabeling has been verified by Western blotting and in situ hybridization methods. P2X(1)R immunoreactivity was specifically localized in neurons, dendrites and axons throughout the forebrain. Characteristic differences in the distribution of P2X(1)R were observed in different cortical areas. In prefrontal, cingulate and perirhinal cortices, very intense labeling was present in neuronal bodies. In frontal, parietal, temporal and occipital cortices, immunostaining was lighter and mainly found in dendrites and axons. The hippocampal formation was intensely labeled. Labeling was present almost exclusively in dendrites and axons and never in neuronal bodies. The diencephalon was devoid of P2X(1)R positive neurons or fibers except for the medial habenular nucleus, which showed very intense P2X(1)R immunostaining. Furthermore, two subcortical regions, namely, the nucleus centralis of the amygdala and the bed nucleus of the stria terminalis, showed intense P2X(1)R neuronal labeling. Present data indicate that P2X(1)R are prevalent in forebrain areas involved in the integration of cognitive, limbic and autonomic functions.     

The common organization of the amygdaloid complex in tetrapods: new concepts based on developmental, hodological and neurochemical data in anuran amphibians

Research over the last few years has demonstrated that the amygdaloid complex in amniotes shares basic developmental, hodological and neurochemical features. Furthermore, homolog territories of all main amygdaloid subdivisions have been recognized among amniotes, primarily highlighted by the common expression patterns for numerous developmental genes. With the achievement of new technical approaches, the study of the precise neuroanatomy of the telencephalon of the anuran amphibians has been possible, revealing that most of the structures present in amniotes are recognizable in these anamniotes. Thus, recent investigations have yielded enough results to support the notion that the organization of the anuran amygdaloid complex includes subdivisions with origin in ventral pallial and subpallial territories, a strong relationship with the vomeronasal and olfactory systems, abundant intra-amygdaloid connections, a main output center involved in the autonomic system, profuse amygdaloid fiber systems, and distinct chemoarchitecture. When all these new data about the development, connectivity and neurochemistry of the amygdaloid complex in anurans are taken into account, it becomes patent that a basic organization pattern is shared by both amniotic and anamniotic tetrapods.     

Exposure to an open-field arena increases c-Fos expression in a subpopulation of neurons in the dorsal raphe nucleus, including neurons projecting to the basolateral amygdaloid complex

Serotonergic systems in the dorsal raphe nucleus are thought to play an important role in the regulation of anxiety states. To investigate responses of neurons in the dorsal raphe nucleus to a mild anxiety-related stimulus, we exposed rats to an open-field, under low-light or high-light conditions. Treatment effects on c-Fos expression in serotonergic and non-serotonergic cells in the midbrain raphe nuclei were determined 2 h following open-field exposure or home cage control (CO) conditions. Rats tested under both light conditions responded with increases in c-Fos expression in serotonergic neurons within subdivisions of the midbrain raphe nuclei compared with CO rats. However, the total numbers of serotonergic neurons involved were small suggesting that exposure to the open-field may affect a subpopulation of serotonergic neurons. To determine if exposure to the open-field activates a subset of neurons in the midbrain raphe complex that projects to forebrain circuits regulating anxiety states, we used cholera toxin B subunit (CTb) as a retrograde tracer to identify neurons projecting to the basolateral amygdaloid complex (BL) in combination with c-Fos immunostaining to identify cells that responded to open-field exposure. Rats received a unilateral injection of CTb into the BL. Seven to 11 days following CTb injection rats were either, 1) exposed to an open-field in low-light conditions, 2) briefly handled or 3) left undisturbed in home cages. Dual immunostaining for c-Fos and CTb revealed an increase in the percentage of c-Fos-immunoreactive BL-projecting neurons in open-field-exposed rats compared with handled and control rats. Dual immunostaining for tryptophan hydroxylase and CTb revealed that a majority (65%) of BL-projecting neurons were serotonergic, leaving open the possibility that activated neurons were serotonergic, non-serotonergic, or both. These data are consistent with the hypothesis that exposure to anxiogenic stimuli activates a subset of neurons in the midbrain raphe complex projecting to amygdala anxiety circuits.     

Influence of ascending noradrenergic fibers on the neurotensin-like immunoreactive perikarya and evidence of direct projection of ascending neurotensin-like immunoreactive fibers in the rat central nucleus of the amygdala

The influence of ascending noradrenergic neuronal input on the neurotensin (NT)-like immunoreactive neuronal perikarya located in the dorsal part of the central nucleus of the amygdala (CNA) was examined using fluorescence histochemistry and peroxidase-antiperoxidase (PAP) immunocytochemistry. Unilateral hemitransection of the ascending noradrenergic pathway by injection of 6-hydroxydopamine into the caudal mesencephalon just rostral to the locus coeruleus caused a marked depletion of immunoreactivity in NT-like immunoreactive neuronal perikarya in the CNA. Ascending noradrenergic neuronal input, therefore, is considered to facilitate production of NT-like immunoreactive substances in neuronal perikarya and to influence on the functional role of the amygdaloid complex. In addition, we obtained evidence of unilateral direct ascending projections of NT-like immunoreactive neurons into the CNA since the disappearance of NT-like immunoreactive processes occurred mainly in the ventral part of the CNA after surgical hemitransection of the ascending neuronal pathway that interrupts the ascending NT-like immunoreactive pathway arising from the neurons in the brain stem.     

Studies of the role of the central nucleus of the amygdala in controlling cardiovascular functions

Studies on cats anesthetized with a mixture of chloralose and Nembutal addressed the effects of high-frequency stimulation (100 impulses/sec) of the central nucleus of the amygdala on bioelectrical activity in two postganglionic sympathetic nerves—the inferior cardiac nerve and the vertebral branch of the stellate ganglion, which innervate the coronary vessels and the vessels of the anterior thorax respectively. The central nucleus of the amygdala was found to have differential, selective effects, in most experiments producing increases in the amplitude of integrated activity in the inferior cardiac nerve and decreases in the amplitude of biopotentials in the vertebral nerve. In a few experiments, a second type of modulation of the activities of these two postganglionic nerves was seen, with selective inhibition of activity in the inferior cardiac nerve and an accompanying increase in activity in the vertebral nerve. Stimulation of the central nucleus of the amygdala induced significant increases in systemic arterial blood pressure. The role of the central nucleus of the amygdala in the development of experimental neurogenic hypertension was studied in a series of chronic experiments on rats; these established that rats subjected to bilateral electrolytic lesioning of the central nucleus of the amygdala prevented the development of neurogenic hypertension induced by daily imposition of stress for four weeks for induction of operant aversive conditioned reflexes, which was not the case in control rats. The role of the central nucleus of the amygdala in the regulation of vascular tone is discussed. Translated from Rossiiskii Fiziologischeskii Zhurnal imeni I. M. Sechenova, Vol 84, No. 12, pp. 1370–1376, December, 1998.     

Electron-autoradiographic analysis of projections of somatosensory cortical areas I and II in the posterior ventral nucleus of the thalamus

Differences in the organization of corticofugal fibers arising from somatosensory cortical areas I (S_I) and II (S_II) were detected by electron-microscopic autoradiography in the posterior ventral nucleus (NVP) of the thalamus. The distribution of corticofugal fibers from the corresponding zones of the two somatosensory cortical areas within NVP differs. Endings of both types of fibers form synaptic contacts chiefly with distal dendrites of relay cells of NVP and much less frequently with dendrites of Golgi type II interneurons. No direct convergence of fibers arising from the two somatosensory areas on single cells of NVP was observed.P. K. Anokhin Institute of Normal Physiology, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR A. M. Chernukh.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 83, No. 5, pp. 604–606, May, 1977.     

Direct projections from the central amygdaloid nucleus to the globus pallidus and substantia nigra in the cat.

Employing both anterograde and retrograde axonal tracing, we investigated direct projections from the central amygdaloid nucleus to the basal ganglia in the cat. The anterograde axonal tracing of Phaseolus vulgaris-leucoagglutinin revealed that projection fibers from the central amygdaloid nucleus to the basal ganglia ended in the globus pallidus (the feline homolog to the external segment of the globus pallidus of primates) and substantia nigra. The amygdalopallidal fibers terminated chiefly in the medial most part of the globus pallidus at its caudal level. The amygdalonigral fibers terminated densely in the substantia nigra pars lateralis, and moderately in the dorsolateral part of the substantia nigra pars reticulata; none of them were found to end in the substantia nigra pars compacta. Both of the amygdalopallidal and amygdalonigral projections were ipsilateral. These neuronal connections were confirmed by retrograde axonal tracing of cholera toxin B subunit in the second set of the experiments: The cells of origin of the amygdalopallidal and amygdalonigral projections were located predominantly in the lateral part of the central amygdaloid nucleus, and additionally in the intercalated cell islands of the amygdala. Most of them were of small bipolar or multipolar type. The cells projecting to the globus pallidus were preferentially distributed at the rostral levels of the central nucleus and intercalated cell islands of the amygdaloid complex, while those projecting to the substantia nigra were mainly located at the caudal levels of these amygdaloid subdivisions. In the third set of the experiments, sequential double-antigen immunofluorescence histochemistry for transported cholera toxin B subunit and horseradish peroxidase showed that some single neurons in the lateral part of the central amygdaloid nucleus, particularly at its middle level, issued axon collaterals to both the globus pallidus and substantia nigra pars lateralis. The results of the present study indicate that the central amygdaloid nucleus sends projection fibers to the globus pallidus and substantia nigra possibly to exert a limbic influence upon forebrain motor mechanisms.