Differential input from the amygdaloid body to the ventromedial hypothalamic nucleus in the rat

The lighter side of BDNF

Brain-derived neurotrophic factor (BDNF) mediates energy metabolism and feeding behavior. As a neurotrophin, BDNF promotes neuronal differentiation, survival during early development, adult neurogenesis, and neural plasticity; thus, there is the potential that BDNF could modify circuits important to eating behavior and energy expenditure. The possibility that "faulty" circuits could be remodeled by BDNF is an exciting concept for new therapies for obesity and eating disorders. In the hypothalamus, BDNF and its receptor, tropomyosin-related kinase B (TrkB), are extensively expressed in areas associated with feeding and metabolism. Hypothalamic BDNF and TrkB appear to inhibit food intake and increase energy expenditure, leading to negative energy balance. In the hippocampus, the involvement of BDNF in neural plasticity and neurogenesis is important to learning and memory, but less is known about how BDNF participates in energy homeostasis. We review current research about BDNF in specific brain locations related to energy balance, environmental, and behavioral influences on BDNF expression and the possibility that BDNF may influence energy homeostasis via its role in neurogenesis and neural plasticity.

Genetic and dietary effects on dendrites in the rat hypothalamic ventromedial nucleus

Both genetic and environmental factors contribute to individual differences in body weight regulation. The present study examined a possible role for the dendritic arbor of hypothalamic ventromedial nucleus (VMH) neurons in a model of diet-induced obesity (DIO) in male rats. Rats were screened and selectively bred for being either susceptible, i.e., exhibiting DIO, or diet resistant (DR) when exposed to a 31% fat diet. A 2x2 experimental design was used, based on these two strains of rats and exposure to rat chow versus the 31% fat diet for seven weeks. Golgi-impregnated neurons were measured for soma size and dendrite parameters, including number, length, and direction. As previously observed, each VMH neuron had a single long primary dendrite. Genetic background and diet did not affect soma size or the number of dendrites of VMH neurons. However, genetic background exerted a main effect on the length of the long primary dendrites. In particular, the long primary dendrites were approximately 12.5% shorter on the VMH neurons in the DIO rats compared with DR rats regardless of diet. This effect was isolated to the long primary dendrites extending in the dorsolateral direction, with these long primary dendrites 19% shorter for the DIO group compared with the DR group. This finding implicates the connectivity of the long primary dendrites on VMH neurons in the control of energy balance. The functional significance of these shortened dendrites and their afferents warrants further study.

V1R and V2R segregated vomeronasal pathways to the hypothalamus

The vomeronasal system is segregated from the epithelium to the bulb. Two classes of receptor neurons are apically and basally placed in the vomeronasal epithelium, express Gi2alpha and Goalpha proteins and V1R and V2R receptors and project to the anterior and posterior portions of the accessory olfactory bulb, respectively. Apart from common vomeronasal recipient structures in the amygdala, only the anterior accessory olfactory bulb projects to the bed nucleus of the stria terminalis and only the posterior accessory olfactory bulb projects to the dorsal anterior amygdala. The efferent projections from these two amygdaloid structures to the hypothalamus were investigated. These two vomeronasal subsystems mediated by V1R and V2R receptors were partially segregated, not only in amygdala, but also in the hypothalamus.

Brain-derived neurotrophic factor in the ventromedial nucleus of the hypothalamus reduces energy intake

Recent studies show that brain-derived neurotrophic factor (BDNF) decreases feeding and body weight after peripheral and ventricular administration. BDNF mRNA and protein, and its receptor TrkB, are widely distributed in the hypothalamus and other brain regions. However, there are few reports on specific brain sites of actions for BDNF. We evaluated the effect of BDNF, given into the ventromedial nucleus of the hypothalamus (VMH), on normal and deprivation- and neuropeptide Y (NPY)-induced feeding behavior and body weight. BDNF injected unilaterally or bilaterally into the VMH of food-deprived and nondeprived rats significantly decreased feeding and body weight gain within the 0- to 24-h and the 24- to 48-h postinjection intervals. Doses effectively producing inhibition of feeding behavior did not establish a conditioned taste aversion. BDNF-induced feeding inhibition was attenuated by pretreatment of the TrkB-Fc fusion protein that blocks binding between BDNF and its receptor TrkB. VMH-injected BDNF significantly decreased VMH NPY-induced feeding at 1, 2, and 4 h after injection. In summary, BDNF in the VMH significantly decreases food intake and body weight gain, by TrkB receptor-mediated actions. Furthermore, the anorectic effects of BDNF in this site appear to be mediated by NPY. These data suggest that the VMH is an important site of action for BDNF in its effects on energy metabolism.

Opposing influence of basolateral amygdala and footshock stimulation on neurons of the central amygdala

BACKGROUND

The basolateral complex (BLA) and the central nucleus of the amygdala (CeA) are believed to mediate the expression of affective responses. After affective learning, conditioned stimulus-related information is thought to be conveyed from the BLA to the CeA; the medial CeA (Cem), in turn, projects to hypothalamic and brainstem structures involved with induction of affective responses. Although the conditioned stimulus and unconditioned stimulus both evoke affective responses, the precise response often differs. It is unknown whether this difference is represented by distinct activity patterns of single Cem neurons. Furthermore, the nature of the interaction between the BLA and Cem is unknown.

METHODS

Using in vivo extracellular and intracellular recordings, we examined how the BLA affects the Cem and compared this with effects induced by footshock (unconditioned stimulus) in the same neurons.

RESULTS

Our results demonstrate that, contrary to conventional views, BLA stimulation primarily inhibits Cem neurons by a polysynaptic circuit, and show that single Cem neurons respond to both BLA input and footshock in an opposite manner.

CONCLUSIONS

These results demonstrate the predominantly inhibitory nature of the BLA-Cem interaction. These data further demonstrate the distinct cellular events that might lead to differential modulation of conditioned and unconditioned affective responses.

Estrogen regulation of cell proliferation and distribution of estrogen receptor-alpha in the brains of adult female prairie and meadow voles

Adult female prairie (Microtus ochrogaster) and meadow (M. pennsylvanicus) voles were compared to examine neural cell proliferation and the effects of estrogen manipulation on cell proliferation in the amygdala, ventromedial hypothalamus (VMH), and dentate gyrus of the hippocampus (DG). Unlike prior studies, our study focused on the amygdala and VMH, because they are involved in social behaviors and may underlie behavioral differences between the species. Meadow voles had a higher density of cells labeled with the cell proliferation marker 5-bromo-2'-deoxyuridine (BrdU) in the amygdala and DG than did prairie voles. Treatment with estradiol benzoate (EB) for 3 days increased the density of BrdU-labeled cells in the amygdala, particularly in the posterior cortical (pCorA) and medial (pMeA) nuclei, in meadow, but not prairie, voles. Furthermore, the majority of the BrdU-labeled cells in the pCorA and pMeA displayed either a neuronal or a glial progenitor phenotype, but no species or treatment differences were found in the percentage of neuronal or glial progenitor cells. To understand better estrogen's effects on adult neurogenesis, we also examined estrogen receptor-alpha (ERalpha) distribution. Meadow voles had more ERalpha-labeled cells in the pCorA and VMH, but not in the pMeA or DG, than did prairie voles. In addition, more than one-half of the BrdU-labeled cells in the amygdala of both species coexpressed ERalpha labeling. Together, these data indicate that estrogen alters cell proliferation in a species- and region-specific manner, and some of these effects may lie in the specific localization of estrogen receptors in the adult vole brain.

Fear and power-dominance drive motivation: neural representations and pathways mediating sensory and mnemonic inputs, and outputs to premotor structures

Based on the available literature on activation of brain structures by fear- and anger-inducing stimuli, on the effects of electrical and chemical stimulation and lesions of candidate structures, and on connectional data, we propose that both the fear and power-dominance drives are represented in four distinct locations: the medial hypothalamus, lateral/dorsolateral periaqueductal gray, midline thalamic nuclei, and medial prefrontal cortex. The hypothalamic fear representation is located in the dorsomedial and posterior hypothalamic nuclei, the midbrain representation in the caudal part of the lateral/dorsolateral periaqueductal gray, the thalamic representation primarily in parts of the paraventricular and reuniens thalamic nuclei, and the cortical representation in prelimbic cortex. The hypothalamic power-dominance representation is located in the anterior hypothalamic nucleus, dorsomedial aspect of the ventromedial nucleus, and in adjacent parts of the medial preoptic area. The corresponding midbrain representation occurs in rostral part of the lateral/dorsolateral periaqueductal gray, and the thalamic representation in parts of the paraventricular, parataenial, and reuniens thalamic nuclei. We discuss sensory/mnemonic inputs to these representations, and outputs to premotor structures in the medulla, caudate-putamen, and cortex, and their differential contributions to involuntary, learned sequential, and voluntary motor acts. We examine potential contributions of neuronal activities in these representations to the subjective awareness of fear and anger.

Hypothalamic involvement in the regulation of maternal behaviour in the rat: inhibitory roles for the ventromedial hypothalamus and the dorsal/anterior hypothalamic areas

The present report examines the possible involvement of the ventromedial hypothalamus (VMH), the dorsal hypothalamus (DH), and the anterior hypothalamic area (AHA) in the regulation of maternal behaviour in the female rat. In a series of experiments it was found that either infusions of saline or lowering cannulas into the VMH stimulated a rapid onset of maternal behaviour in progesterone plus oestrogen-primed, nulliparous rats. The stimulatory effect of cannula lowering into the VMH on maternal behaviour was shown to be steroid-dependent. Next, the involvement of cell bodies located in the DH/AHA in maternal behaviour was examined after bilateral lesions of these regions with the neurotoxin, N-methyl-D-aspartic acid (NMA). NMA lesions of the DH/AHA stimulated a rapid onset of maternal behaviour in oestrogen-treated, nulliparous rats, while NMA lesions in non-steroid-treated animals or vehicle infusions in steroid or non-steroid-treated rats failed to induce a rapid onset of behaviour. In a final study the effects of NMA lesions of the VMH were evaluated. As in AHA lesioned rats, NMA lesions of the VMH stimulated a fast onset of maternal behaviour in steroid-primed females. These findings indicate that the VMH as well as the DH/AHA exert chronic steroid-dependent inhibitory influences on the induction of maternal behaviour.

Organization of projections from the basomedial nucleus of the amygdala: a PHAL study in the rat

The organization of axonal projections from the basomedial nucleus of the amygdala (BMA) was examined with the Phaseolus vulgaris leucoagglutinin (PHAL) method in adult male rats. The anterior and posterior parts of the BMA, recognized on cytoarchitectonic grounds, display very different projection patterns. Within the amygdala, the anterior basomedial nucleus (BMAa) heavily innervates the central, medial, and anterior cortical nuclei. In contrast, the posterior basomedial nucleus (BMAp) sends a dense projection to the lateral nucleus, and to restricted parts of the central and medial nuclei. Extra-amygdalar projections from the BMA are divided into ascending and descending components. The former end in the cerebral cortex, striatum, and septum. The BMAa mainly innervates olfactory (piriform, transitional) and insular areas, whereas the BMAp also innervates inferior temporal (perirhinal, ectorhinal) and medial prefrontal (infralimbic, prelimbic) areas and the hippocampal formation. Within the striatum, the BMAa densely innervates the striatal fundus, whereas the nucleus accumbens receives a heavy input from the BMAp. Both parts of the BMA send massive projections to distinct regions of the bed nuclei of the stria terminalis. Descending projections from the BMA end primarily in the hypothalamus. The BMAa sends a major input to the lateral hypothalamic area, whereas the BMAp innervates the ventromedial nucleus particularly heavily. Injections were also placed in the anterior cortical nucleus (COAa), a cell group superficially adjacent to the BMAa. PHAL-labeled axons from this cell group mainly ascend into the amygdala and olfactory areas, and descend into the thalamus and lateral hypothalamic area. Based on connections, the COAa and BMAa are part of the same functional system. The results suggest that cytoarchitectonically distinct anterior and posterior parts of the BMA are also hodologically distinct and form parts of distinct anatomical circuits probably involved in mediating different behaviors (for example, feeding and social behaviors vs. emotion-related learning, respectively).

Opposite changes in the expression of enkephalin in the amygdala and hypothalamus after lesions of the bed nucleus of the stria terminalis in the rat

The expression of enkephalin in neurons of the rat forebrain was studied by in situ hybridization and immunohistochemistry after unilateral injections of ibotenic acid into the bed nucleus of the stria terminalis. Initially, we observed that the destruction of nerve cell bodies in this nucleus resulted in a prominent bilateral increase in the number of neuronal perikarya immunoreactive for [Met]enkephalin in the lateral/basolateral amygdaloid complex-especially in the anterior division of the latter nucleus-as compared with NaCl-injected rats. In a separate set of experiments, this effect was associated with a significant (two times) enhancement of the number of nerve cell bodies containing preproenkephalin A messenger RNAs in the same amygdaloid nucleus ipsilateral to the injection, as compared with controls. In the hypothalamus of both experimental and control rats, the nerve cell bodies immunoreactive for [Met]enkephalin were few since the animals were not pretreated with colchicine, and the effects of the lesion were difficult to appreciate. However, using in situ hybridization, numerous nerve cell bodies containing preproenkephalin A messenger RNAs were detected bilaterally in the perifornical area, the paraventricular (parvocellular division) and the ventromedial nuclei of the hypothalamus. In the latter nucleus, the lesion of the bed nucleus of the stria terminalis resulted in a strong decrease (about two times) in the number of labelled cell bodies as compared with the controls, whereas no significant changes were found bilaterally in the paraventricular nucleus. In agreement with some data of the literature, our results indicate that the bed nucleus of the stria terminalis plays an important role in the regulation of neuropeptide genes expression in certain regions of the limbic system. Such a role is often exerted by nerve fibres afferents to the nerve cell bodies considered. However, from numerous neuroanatomical data of the literature, it appears more probable that the induction or inhibition of the expression of enkephalin in presynaptic neurons is due to the disappearance of their postsynaptic target in the bed nucleus of the stria terminalis.

Organization of projections from the medial nucleus of the amygdala: a PHAL study in the rat

The organization of axonal projections from the four recognized parts of the medial amygdalar nucleus (MEA) were characterized with the Phaesolus vulgaris leucoagglutinin (PHAL) method in male rats. The results indicate that the MEA consists of two major divisions, ventral and dorsal, and that the former may also consist of rostral and caudal regions. As a whole, the MEA generates centrifugal projections to several parts of the accessory and main olfactory sensory pathways, and projections to a) several parts of the intrahippocampal circuit (ventrally); b) the ventral striatum, ventral pallidum, and bed nuclei of the stria terminalis (BST) in the basal telencephaon; c) many parts of the hypothalamus; d) midline and medial parts of the thalamus; and e) the periaqueductal gray, ventral tegmental area, and midbrain raphé. The dorsal division of the MEA (the posterodorsal part) is characterized by projections to the principal nucleus of the BST, and to the anteroventral periventricular, medial, and central parts of the medial preoptic, and ventral premammillary hypothalamic nuclei. These hypothalamic nuclei project heavily to neuroendocrine and autonomic-related parts of the hypothalamic periventricular zone. The ventral division of the MEA (the anterodorsal, anteroventral, and posteroventral parts) is characterized by dense projections to the transverse and interfascicular nuclei of the BST, and to the lateral part of the medial preoptic, anterior hypothalamic, and ventromedial hypothalamic nuclei. However, dorsal regions of the ventral division provide rather dense inputs to the medial preoptic region and capsule of the ventromedial nucleus, whereas ventral regions of the ventral division preferentially innervate the anterior hypothalamic, dorsomedial, and ventral parts of the ventromedial nuclei. Functional evidence suggests that circuits associated with dorsal regions of the ventral division may deal with reproductive behavior, whereas circuits associated with ventral regions of the ventral division may deal preferentially with agonistic behavior.

Efferent connections of the hypothalamic "aggression area" in the rat

The efferent connections of the hypothalamic area of the rat, where attack behaviour can be elicited by electrical stimulation, were studied using iontophoretic injections of Phaseolus vulgaris-leucoagglutinin. Specificity for the hypothalamic "attack area" was investigated by comparison with efferents of hypothalamic sites outside the attack area. The hypothalamic attack area consists of the intermediate hypothalamic area and the ventrolateral pole of the ventromedial hypothalamic nucleus. Fibres from the hypothalamic attack area, as well as fibres from several other hypothalamic sites, form diffuse fibre "streams" running rostrally or caudally. Many varicosities that are found on the fibres suggest, that these fibres are capable of influencing many brain sites along their way. Projection sites were found throughout the brain. In the comparison between attack area efferents and controls, many overlapping brain sites were found. Hypothalamic efferents preferentially originating in the largest part of the attack area, i.e. the intermediate hypothalamic area, were found in the mediodorsal and parataenial thalamic nuclei. Within the septum, a spatial organization of hypothalamic innervation was found. Fibres from the attack area formed specialized "pericellular baskets" in the dorsolateral aspect of the intermediate part of the lateral septal nucleus. Fibres from other hypothalamic sites were found in other septal areas and did not form these septal baskets. Within the mesencephalic central gray, fibres from the attack area were found specifically in the dorsal part and dorsal aspect of the lateral part of the central gray. Physiological and pharmacological studies have shown that several brain sites are involved in different aspects of aggressive behaviour. Some of these areas, as for instance the dorsomedial thalamic nucleus, septum and central gray, are innervated by efferents from the hypothalamic attack area, whereas other sites, like ventral premammillary nucleus and ventral tegmental area, are not. It is concluded from the present findings, that a number of brain sites, that are known to be involved in agonistic behaviour, receive hypothalamic information preferentially from the hypothalamic attack area through diffusely arranged varicose fibres. The function of each connection in the regulation of specific behaviours remains to be further investigated.

Differential projections of the anterior and posterior regions of the medial amygdaloid nucleus in the Syrian hamster

The medial nucleus of the amygdala is important for the neural control of reproductive behavior in the adult male Syrian hamster. Two types of signals are essential for this behavior, chemosensory stimuli and gonadal steroids; these signals appear to be received in different parts of the medial nucleus. The anterior region receives input from olfactory and vomeronasal systems, both of which are required for this behavior, whereas the posterior region receives gonadal hormone inputs. Behavioral studies have also suggested a functional differentiation of these two areas; electrolytic lesions of the anterior, but not the posterior, part eliminates normal sexual behavior. In this study, the efferent projections of the anterior and posterior divisions of the medial nucleus of the amygdala in the Syrian hamster were analyzed throughout the forebrain after injections of the anterograde neuronal tracer, Phaseolus vulgaris-leucoagglutinin. Neurons of the anterior, but not the posterior, medial nucleus, were found to project to numerous olfactory bulb projection areas and to the ventral striatopallidal complex. Within areas of the chemosensory circuitry that control reproductive behavior, the anterior region of the medial nucleus projects to the intermediate part of the posterior bed nucleus of the stria terminalis and the lateral part of the medial preoptic area, whereas the posterior region of the medial nucleus projects to the medial parts of these areas. Differences in targets were also observed in the ventromedial nucleus of the hypothalamus where the anterior region projects to the core while the posterior part projects to the shell of this nucleus. Furthermore, reciprocal projections between the anterior and posterior regions of the medial nucleus were observed. Taken together, these studies support the hypothesis that the anterior and posterior regions of the medial amygdaloid nucleus provide substantially different contributions to the control of reproductive behaviors.

The human amygdaloid complex: a cytologic and histochemical atlas using Nissl, myelin, acetylcholinesterase and nicotinamide adenine dinucleotide phosphate diaphorase staining

We examined the distribution of acetylcholinesterase and nicotinamide adenine dinucleotide phosphate diaphorase enzyme activity in the human amygdala using histochemical techniques. Both methods revealed compartments of higher or lower enzyme activity, in cells or neuropil, which corresponded to the nuclear subdivisions of the amygdala as defined with classical Nissl and myelin methods. The boundaries between the histochemical compartments were usually so sharp that the identification of these nuclear subdivisions was enhanced. There was also variation of staining intensity within many of the nuclear subdivisions, such as the lateral and central nuclei, anterior amygdaloid area and the intercalated groups. This histochemical difference corresponded to more subtle differences in Nissl and myelin staining patterns, and suggests further structural subdivisions of potential functional significance. We present a revised scheme of anatomical parcellation of the human amygdala based upon serial analysis with all four techniques. Our expectation is that this will allow the delineation of a clearer homology between the cytoarchitectonic subdivisions of the human amygdala and those of experimental animals.

Projections of the intermediate subdivision of the central amygdaloid nucleus to the bed nucleus of the stria terminalis and medial diencephalon

The forebrain projections of the intermediate subdivision of the central amygdaloid nucleus (CI) were studied using the horseradish peroxidase (HRP) retrograde transport technique. Injections of wheatgerm agglutinin-conjugated HRP into the bed nucleus of the stria terminalis and medial thalamus resulted in moderate retrograde labeling in CI while injections into the medial hypothalamus produced very dense cell labeling in CI. The dorsal hypothalamic area appears to be the main target of the efferents to the hypothalamus. Although previous histochemical studies have suggested that CI may be part of the lateral subdivision of the central nucleus (CL), the projection of CI to the medial diencephalon clearly distinguishes CI from CL.

Neuropeptide Y localization in the rat amygdaloid complex

Neuropeptide Y (NPY)-, avian pancreatic polypeptide (APP)-, and molluscan cardioexcitatory peptide (FMRF)-like immunoreactivity in the amygdaloid complex of the rat was investigated immunohistochemically. The distribution of each of these peptides within the amygdala is identical and cross-blocking studies indicate that all three antisera recognize the NPY antigen. Morphologically distinct populations of NPY immunoreactive neurons are differentially distributed in the medial amygdaloid nucleus and at the base of the stria terminalis. Dense plexuses of immunoreactive axons are present in the medial third of the central nucleus and in the dorsal half of the medial nucleus, with light to moderate fiber plexuses present in the lateral and basolateral nuclei and scattered axons present throughout the remainder of the amygdala. The distribution and appearance of NPY immunoreactive plexuses in the amygdala is similar to that described previously for noradrenergic axons arising from brainstem cell groups (Fallon, Koziell, and Moore: J. Comp. Neurol. 180:509-532, '78). However, injections of the noradrenergic neurotoxin 6-hydroxydopamine into the amygdala result in a complete loss of dopamine-beta-hydroxylase (DBH) immunoreactivity in the amygdala and surrounding cortex but leave much of the NPY immunoreactive plexus intact. Similarly, lesions of the locus coeruleus deplete DBH immunoreactivity, leaving NPY-like immunoreactivity in the amygdala unaffected. These results indicate that much of the NPY immunoreactive plexus observed in the amygdala does not arise from brainstem sources in which NPY and noradrenaline are colocalized. Lesions of the stria terminalis or medial nucleus have no observable effect on the density or distribution of NPY immunoreactive terminal fields in the basal forebrain and hypothalamus, suggesting that immunoreactive neurons in the amygdaloid complex do not contribute significantly to this innervation.

Nigroamygdaloid dopamine neurons: nigral modulation of their activity

In order to study the mechanisms regulating the dopaminergic nigroamygdaloid cells, the release of dopamine was observed in the central nucleus of the amygdaloid complex. Halothane anesthetized rats were implanted, according to the experiment, with one or two push-pull cannulae in the central nuclei of the amygdala (ACE), the substantia nigra (SN) and/or the caudate nucleus (CN). Canulae were supplied with artificial cerebrospinal fluid (CSF) containing tritiated tyrosine, and labeled dopamine [3H]DA was evaluated in successive superfusate fractions. Electrical stimulation of the medial forebrain bundle with an implanted bipolar electrode induced an increase of the [3H]DA release in the ipsi- and contralateral ACE. Electrical stimulation of the SN produced only a very delayed effect in the ipsilateral ACE but an immediate and large increase of [3H]DA release in the contralateral structure. Superfusion of unlabeled DA and alpha-methyl-p-tyrosine in the SN remained ineffective on the [3H]DA release in the ipsilateral ACE. In this structure the release of [3H]DA was, however, decreased by nigral superfusion with gamma-amino-butyric acid (GABA). D-(+)-Amphetamine (1 microM), when superfused in the CN, induced a large enhancement of the [3H]DA release in the ipsilateral ACE simultaneously with the local increase of [3H]DA release. The results presented here are in agreement with the previous studies concerning the anatomical organization of the dopaminergic nigroamygdaloid pathway. The DA cell bodies located in the SN appear insensitive to a local action of DA, perhaps due to a lack of autoreceptors. They are, however, powerfully inhibited by GABA and the relation observed between the [3H]DA release in the CN and ACE support the hypothesis that the SN can act as a relay between the extrapyramidal and limbic systems.

Efferent projections of the basolateral amygdala in the opossum, Didelphis virginiana

The autoradiographic anterograde axonal transport technique was used to study efferent projections of the opossum basolateral amygdala. All nuclei of the basolateral amygdala send topographically organized fibers to the bed nucleus of the stria terminalis (BST) via the stria terminalis (ST). Injections into rostrolateral portions of the basal nuclei label fibers that surround the commissural bundle of the ST, cross the midline by passing along the outer aspect of the anterior commissure, and terminate primarily in the contralateral BST, anterior subdivision of the basolateral nucleus (BLa), ventral putamen, and olfactory cortex. Each of the basal nuclei project ipsilaterally to the anterior amygdaloid area, substantia innominata and topographically to the ventral part of the striatum and adjacent olfactory tubercle. The posterior subdivision of the basolateral nucleus (BLp), but not the basomedial nucleus (BM), projects to the ventromedial hypothalamic nucleus. BLa and BLp have projections to the nucleus of the lateral olfactory tract and also send fibers to the central nucleus, as does the lateral nucleus (L). The lateral nucleus also has a strong projection to BM and both nuclei project to the amygdalo-hippocampal area. BLa and BLp send axons to the ventral subiculum and ventral lateral entorhinal area whereas L projects only to the latter area. The lateral nucleus and BLp project to the perirhinal cortex and the posterior agranular insular area. The BLa sends efferents to the anterior agranular insular area. Rostrally this projection is continuous with a projection to the entire frontal cortex located rostral and medial to the orbital sulcus. All of the nuclei of the basolateral amygdala project to areas on the medial wall of the frontal lobe that appear to correspond to the prelimbic and infralimbic areas of other mammals. Despite the great phylogenetic distance separating the opossum from placental mammals, the projections of the opossum basolateral amygdala are very similar to those seen in other mammals. The unique frontal projections of the opossum BLa to the dorsolateral prefrontal cortex appear to be related to the distinctive organization of the mediodorsal thalamic nucleus and prefrontal cortex in this species.

The amygdala and male reproductive functions: I. Anatomical and endocrine bases

The present review is mainly concerned with the role of the amygdala (AMY) in male sexual behaviour and presents the characteristics of neuroanatomical and neuroendocrine organization of the cortico-medial amygdala as compared to the medial preoptic area (MPOA), in relation to sexual behaviour and gonadotropin secretion. It stresses the part played by AMY in processing sensory information which afterwards reaches the hypothalamus (HYP). Some data on the different function of steroids and luteinizing hormone releasing hormone (LHRH) systems within AMY and MPOA are provided. The role of AMY in the positive feedback effect on luteinizing hormone (LH) secretion and the involvement of steroid and opiate interaction as its mechanism are suggested. AMY is proposed to integrate external and internal sensory information granting the optimal conditions for sexual performance and possibly for regulation of LH release according to the behavioural context. The steroid content of AMY may influence these processes.

Topographic organization of projections from the amygdala to the hypothalamus of the rat

Afferent fibers from the amygdala to subdivisions of lateral, ventromedial and dorsomedial hypothalamic nuclei were investigated in rat by retrograde transport of horseradish peroxidase. Small (intranuclear size) peroxidase deposits were placed in hypothalamic nuclei by iontophoresis of a tracer solution containing poly-L-alpha-ornithine which greatly limited diffusion. The medial, central and amygdalo-hippocampal nuclei of the amygdala were found to be the major donors of amygdaloid afferent fibers to the hypothalamus, but there was also substantial labeling of somata in cortical, basomedial, basolateral and lateral amygdaloid nuclei and the intra-amygdaloid bed nucleus of the stria terminalis. No fibers projected from the posterior cortical nucleus of the amygdala to the hypothalamus. Most amygdaloid projections to the lateral hypothalamic area originated in the anterior half of the amygdala, while projections to the ventromedial hypothalamic nucleus arose along the entire length of the amygdala except the posterior cortical nucleus. The amygdalo-hippocampal area projects to the medial hypothalamus. Other amygdaloid nuclei project to both the medial and lateral hypothalamic nuclei. These topographic organizations of amygdaloid afferent fibers to various subdivisions of the hypothalamic nuclei are discussed and compared with other anatomical studies on these connections.