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.     

Convergence of olfactory and vomeronasal projections in the rat basal telencephalon

Olfactory and vomeronasal projections have been traditionally viewed as terminating in contiguous non-overlapping areas of the basal telencephalon. Original reports, however, described areas such as the anterior medial amygdala where both chemosensory afferents appeared to overlap. We addressed this issue by injecting dextran amines in the main or accessory olfactory bulbs of rats and the results were analyzed with light and electron microscopes. Simultaneous injections of different fluorescent dextran amines in the main and accessory olfactory bulbs were performed and the results were analyzed using confocal microscopy. Similar experiments with dextran amines in the olfactory bulbs plus FluoroGold in the bed nucleus of the stria terminalis indicate that neurons projecting through the stria terminalis could be integrating olfactory and vomeronasal inputs. Retrograde tracing experiments using FluoroGold or dextran amines confirm that areas of the rostral basal telencephalon receive inputs from both the main and accessory olfactory bulbs. While both inputs clearly converge in areas classically considered olfactory-recipient (nucleus of the lateral olfactory tract, anterior cortical amygdaloid nucleus, and cortex-amygdala transition zone) or vomeronasal-recipient (ventral anterior amygdala, bed nucleus of the accessory olfactory tract, and anteroventral medial amygdaloid nucleus), segregation is virtually complete at posterior levels such as the posteromedial and posterolateral cortical amygdalae. This provides evidence that areas so far considered receiving a single chemosensory modality are likely sites for convergent direct olfactory and vomeronasal inputs. Therefore, areas of the basal telencephalon should be reclassified as olfactory, vomeronasal, or mixed chemosensory structures, which could facilitate understanding of olfactory-vomeronasal interactions in functional studies.     

Central Connections of the Ovine Olfactory Bulb Formation Identified Using Wheat Germ Agglutinin-Conjugated Horseradish Peroxidase

Pheromonal stimuli elicit rapid behavioral and reproductive endocrine changes in the ewe. The neural pathways responsible for these effects in sheep are unknown, in part, because the olfactory bulb projections have not been examined in this species. Using the anterograde and retrograde neuronal tracer, wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP), we describe the afferent and efferent olfactory bulb connections of the Suffolk ewe. Injections of WGA-HRP limited to the main olfactory bulb resulted in retrograde labeling of cells in numerous telencephalic, diencephalic, and metencephalic regions. Terminal labeling was limited to layer Ia of ipsilateral cortical structures extending rostrally from the anterior olfactory nucleus (AON), piriform cortex, anterior-, and posterolateral-cortical amygdaloid nuclei to lateral entorhinal cortex caudally. Injections involving the accessory olfactory bulb and AON produced additional labeling of cells within the bed nucleus of the stria terminalis (BNST), medial nucleus of the amygdala, and a few cells in the posteromedial cortical nucleus of the amygdala. Terminal labeling included a small dorsomedial quadrant of BNST and also extended to the far lateral portions of the supraoptic nucleus. A clearly defined accessory olfactory tract and nucleus was not evident, perhaps due to limitations in the sensitivity of the method. With this possible exception, the afferent and efferent olfactory connections in the sheep appear similar to those reported for other species.     

Topography of projections from amygdala to bed nuclei of the stria terminalis

A collection of 125 PHAL experiments in the rat has been analyzed to characterize the organization of projections from each amygdalar cell group (except the nucleus of the lateral olfactory tract) to the bed nuclei of the stria terminalis, which surround the crossing of the anterior commissure. The results suggest three organizing principles of these connections. First, the central nucleus, and certain other amygdalar cell groups associated with the main olfactory system, innervate preferentially various parts of the lateral and medial halves of the bed nuclear anterior division, and these projections travel via both the stria terminalis and ansa peduncularis (ventral pathway). Second, in contrast, the medial nucleus, and the rest of the amygdalar cell groups associated with the accessory and main olfactory systems innervate preferentially the posterior division, and the medial half of the anterior division, of the bed nuclei. And third, the lateral and anterior basolateral nuclei of the amygdala (associated with the frontotemporal association cortical system) do not project significantly to the bed nuclei. For comparison, inputs to the bed nuclei from the ventral subiculum, infralimbic area, and endopiriform nucleus are also described. The functional significance of these projections is discussed with reference to what is known about the output of the bed nuclei.     

Segregated pathways to the vomeronasal amygdala: differential projections from the anterior and posterior divisions of the accessory olfactory bulb

Apically and basally located receptor neurons in the vomeronasal sensory epithelium express G(i2 alpha)- and G(o alpha)-proteins, V1R and V2R vomeronasal receptors, project to the anterior and posterior accessory olfactory bulb and respond to different stimuli, respectively. The extent to which secondary projections from the two portions of the accessory olfactory bulb are convergent in the vomeronasal amygdala is controversial. This issue is addressed by using anterograde and retrograde tract-tracing methods in rats including electron microscopy. Injections of dextran-amines, Fluoro Gold, cholera toxin-B subunit and Fast Blue were delivered to the anterior and posterior accessory olfactory bulb, bed nucleus of the stria terminalis, dorsal anterior amygdala and bed nucleus of the accessory olfactory tract/anteroventral medial amygdaloid nucleus. We have demonstrated that, apart from common vomeronasal-recipient areas, only the anterior accessory olfactory bulb projects to the bed nucleus of the stria terminalis, medial division, posteromedial part, and only the posterior accessory olfactory bulb projects to the dorsal anterior amygdala and deep cell layers of the bed nucleus of the accessory olfactory tract and the anteroventral medial amygdaloid nucleus. These results provide evidence that, excluding areas of convergence, the V1R and V2R vomeronasal pathways project to specific areas of the amygdala. These two vomeronasal subsystems are therefore anatomically and functionally separated in the telencephalon.     

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.     

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). © 1996 Wiley-Liss, Inc.     

Rat central amygdaloid nucleus projections to the bed nucleus of the stria terminalis.

The projections from the central amygdaloid nucleus (Ce) to different subdivisions of the bed nucleus of the stria terminalis (BNST) were investigated using retrograde transport of fluorescent dyes. Iontophoretic injections of either Fast Blue (FB) or bisbenzimide (BB) were applied to the anterior medial, posterior medial, anterior lateral and posterior lateral parts of the bed nucleus of the stria terminalis. The anterior medial BNST receives projections from caudal part of medial Ce (CeM). The posterior medial BNST receives projections specifically from the intermediate subdivision of Ce, though in some cases projections from the ventral subdivision (CeV) of Ce were seen. The anterior lateral BNST receives projections primarily from the caudal lateral Ce (CeL) as well as middle and caudal part of CeM. The posterior lateral BNST receives projection from rostral CeL as well as the CeV and lateral capsular Ce. In general, the results indicate that the major subdivisions of the BNST receive projections from Ce subdivisions having similar connections with diencephalic or brainstem cell groups. Additional evidence is presented suggesting that Ce-BNST projections are part of an extensive system of intrinsic connections linking similar groups of neurons in both the Ce and BNST as well as within Ce.     

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.     

Afferent connections to the amygdaloid complex of the rat and cat: II. Afferents from the hypothalamus and the basal telencephalon

The projections from the basal telencephalon and hypothalamus to each nucleus of the amygdaloid complex of the rat, and to the central amygdala of the cat, were investigated by the use of retrograde transport of horseradish peroxidase (HRP). The enzyme was injected stereotaxically by microiontophoresis, using three different approaches. The ventral pallidum (Heimer, '78) and ventral part of the globus pallidus were found to project to the lateral and basolateral nuclei of the amygdala. The substantia innominata projects diffusely to the entire amygdaloid complex, except to the lateral nucleus and the caudal part of the medial nucleus. The anterior amygdaloid area shows a similar projection field, the only difference being that this structure does not project to any parts of the medial nucleus. The dorsal subdivision of the nucleus of the lateral olfactory tract sends fibers to the ipsilateral as well as the contralateral basolateral nucleus, and possibly to the ipsilateral basomedial and cortical amygdala. The ventral subdivision of the nucleus of the lateral olfactory tract was massively labeled after an injection in the ipsilateral central nucleus, but this injection affected the commissural component of the stria terminalis. The nucleus of the horizontal limb of the diagonal band of Broca connects with the medial, central, and anterior cortical nuclei, whereas the bed nucleus of stria terminalis and medial preoptic area are related to the medial nucleus predominantly. The lateral preoptic area is only weakly labeled after intra-amygdaloid HRP injections. The hypothalamo-amygdaloid projections terminate preponderantly in the medial part of the amygdaloid complex. Thus, axons from neurons in the area dorsal and medial to the paraventricular nucleus of the hypothalamus distribute to the medial nucleus and intra-amygdaloid part of the bed nucleus of stria terminalis. Most of the amygdalopetal fibers from the ventromedial, ventral premammillary, and arcuate nuclei of the hypothalamus end in the medial nucleus, but some extend into the central nucleus. A few fibers from the ventromedial nucleus of the hypothalamus reach the basolateral nucleus. The lateral hypothalamic area projects heavily to the central nucleus, and more sparsely to the medial and basolateral nuclei. The dorsal hypothalamic area and supramammillary nucleus show restricted projections to the central and basolateral nuclei, respectively. There are only a modest number of crossed hypothalamo-amygdaloid fibers. Most of these originate in the ventromedial nucleus of the hypothalamus and terminate in the contralateral medial nucleus. The projections from the basal telencephalon and hypothalamus to the central nucleus of the amygdala of the cat are similar to the corresponding projections in the rat.     

Olfactory relationships of the telencephalon and diencephalon in the rabbit. I. An autoradiographic study of the efferent connections of the main and accessory olfactory bulbs.

The efferent connections of the main and accessory olfactory bulbs in the female albino rabbit have been studied using the autoradiographic method for tracing axonal pathways. Following unilateral injections of 3H-proline or 3H-leucine into the main olfactory bulb, radioactively labeled material transported intraaxonally by axoplasmic flow in an anterograde direction from soma to axon terminal is present ipsilaterally in the superficial half of the plexiform layer (IA) of: the entire circumference of the olfactory peduncle, the tenia tecta, the full mediolateral extent of the olfactory tubercle, the entire length of the prepyriform cortex, a transition area between the prepyriform cortex and the horizontal limb of the nucleus of the diagonal band, the nucleus of the lateral olfactory tract, the anterior cortical and posterolateral cortical amygdaloid nuclei (periamygdaloid areas 1, rostral half of 2, 5 of Rose, '31), and the ventrolateral entorhinal cortex (entorhinal areas 1, 2, 4, 5, 7 of Rose, '31). No subcortical or contralateral projection of main bulb efferents was found. After a unilateral injection of 3H-leucine into the accessory olfactory bulb, transported material could be followed caudally along the dorsal surface of the ipsilateral lateral olfactory tract. This heavily labeled projection is distinct from the unlabeled lateral olfactory tract and has been termed the accessory olfactory tract. Beginning at the level of the caudal third of the olfactory tubercle and extending caudally to the nucleus of the lateral olfactory tract is a group of small neurons intimately associated with the accessory olfactory tract. This cell group is referred to as the bed nucleus of the accessory olfactory tract. Projection sites of the accessory bulb include the bed nucleus of the accessory olfactory tract and layer IA of the medial nucleus and the posteromedial cortical nucleus of the amygdala (periamygdaloid areas 3, 4, PAM, caudal half of 2, 6 of Rose, '31). An additional accessory bulb efferent projection was found to enter the stria terminalis at the level of the medial amygdaloid nucleus and could be traced to a posterior segment of the bed nucleus of the stria terminalis. The autoradiographic findings indicate that the accessory olfactory bulb connects with portions of the amygdala that do not receive afferent input from the main olfactory bulb and provide evidence for the existence of two distinct and separate olfactory systems.     

Adjacent laminar terminations of two centrifugal afferent pathways to the accessory olfactory bulb in the mouse

Anterograde and retrograde axonal tracing methods have been combined with transection of the stria terminalis to investigate the centrifugal afferent connections of the accessory olfactory bulb in the mouse.Injection of tritiated proline into the postero-medial cortical amygdaloid nucleus (C₃) gives rise to anterograde autoradiographic labelling of a pathway terminating in the internal granular layer of the accessory olfactory bulb (AOB). Transection of the ipsilateral stria terminalis completely abolishes labelling of this pathway. Injections further rostral, in the bed nucleus of the accessory olfactory tract (bnAOT) and medial amygdaloid nucleus (M), give rise to labelling of a second ipsilateral afferent pathway to the AOB which terminates in the internal plexiform layer (IPL) and is unaffected by strial transection.Injections of wheat germ lectin-HRP conjugate into the AOB confirm that it receives afferents from the ipsilateral bnAOT, M and C₃, and from a few cells in the contralateral C₃. Transection of the ipsilateral stria terminalis prevents retrograde labelling of any cells in the ipsilateral C₃, but does not affect labelling of cells in M or bnAOT (or contralateral C₃). The conjugate is also transported anterogradely in this system, labelling the efferent projections of the AOB to bnAOT, M and C₃.It is concluded that the AOB receives at least two sets of ipsilateral afferents: one set from C₃, via the stria terminalis, terminating in the internal granular layer, and a second set from M and/or bnAOT terminating in the IPL and probably running in the accessory olfactory tract.     

Tyrosine hydroxylase neurons in the male hamster chemosensory pathway contain androgen receptors and are influenced by gonadal hormones

Chemosensory and hormonal signals, both of which are essential for mating in the male Syrian hamster, are relayed through a distinct forebrain circuit. Immunocytochemistry for tyrosine hydroxylase, a catecholamine biosynthetic enzyme, previously revealed immunoreactive neurons in the anterior and posterior medial amygdaloid nucleus, one of the nuclei within this pathway. In addition, dopamine-immunoreactive neurons were located in the posterior, but not hte anterior, medial amygdala. In the present study, tyrosine hydroxylase-immunostained neurons were also observed in other areas of the chemosensory pathway, including the posteromedial bed nucleus of the stria terminalis and the posterior, lateral part of the medial preoptic area, while dopamine immunostaining was only seen in the posteromedial bed nucleus of the stria terminalis. The colocalization of tyrosine hydroxylase and androgen receptors was examined in these four tyrosine hydroxylase cell groups by a double immunoperoxidase technique. The percentage of tyrosine hydroxylase-immunolabeled neurons that were also androgen receptor-immunoreactive was highest in the posterior medial amygdaloid nucleus (74%) and the bed nucleus of the stria terminalis (79%). Fewer tyrosine hydroxylase-immunostained neurons in the anterior medial amygdala (33%) and the medial preoptic area (4%) contained androgen receptors. Surprisingly, castration resulted in a significant decrease in the number of tyrosine hydroxylase-immunoreactive neurons only in the anterior medial amygdaloid nucleus, and this effect was transient. Six weeks after castratio, the anterior medial amygdala contained 61% fewer tyrosine hydroxylase-immunolabeled neurons, but 12 weeks after gonadectomy, immunostaining returned to intact values. The number of immunostained neurons in testosterone-replaced, castrated hamsters was not significantly different from that of intact or castrated animals at any time. The results of this study indicate that a substantial number of tyrosine hydroxylase-immunostained neurons in the chemosensory pathway are influenced by androgens; the majority of these neurons in the posterior medial amygdala and the posteromedial bed nucleus of the stria terminalis produce androgen receptors, and tyrosine hydroxylase immunoreactivity is altered by castration in the anterior medial amygdala. © 1993 Wiley-Liss, Inc.     

Neural substrates for processing chemosensory information in snakes

Snakes interact with their chemical environment through their olfactory and vomeronasal systems. The present report summarizes advances on neural substrates for processing chemosensory information. First, the efferent and centrifugal afferent connections of the main and accessory olfactory bulbs were reinvestigated. Second, the afferent and efferent connections of the nucleus sphericus, the main target of the accessory olfactory bulb, were characterized. The nucleus sphericus gives rise to a very small projection to the hypothalamus, but it does project to other telencephalic structures where olfactory and vomeronasal information could converge. Third, the intra-amygdaloid circuitry and the amygdalo-hypothalamic projections were described. The medial amygdala, for instance, receives both vomeronasal and olfactory inputs and projects to the hypothalamus, namely, to the lateral posterior hypothalamic nucleus. Fourth, because the lateral posterior hypothalamic nucleus projects to the hypoglossal nucleus, the motor center controlling the tongue musculature, this projection could constitute a pathway for chemosensory information to influence tongue-flicking behavior. In summary, vomeronasal information is mostly relayed to the hypothalamus not via the nucleus sphericus but through other telencephalic structures. Convergence of olfactory and vomeronasal information appears to occur at different levels in the telencephalon. A neural substrate for the chemosensory control of tongue-flicking behavior is provided.     

Efferent projections of the infralimbic (area 25) region of the medial prefrontal cortex in the rat: an anterograde tracer PHA-L study

The efferent projections of the infralimbic region (IL) of the medial prefrontal cortex of the rat were examined by using the anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L). Major targets of the IL were found to include the agranular insular cortex, olfactory tubercle, perirhinal cortex, the whole amygdaloid complex, caudate putamen, accumbens nucleus, bed nucleus of the stria terminalis, midline thalamic nuclei, the lateral preoptic nucleus, paraventricular nucleus, supramammillary nucleus, medial mammillary nucleus, dorsal and posterior areas of the hypothalamus, ventral tegmental area, central gray, interpeduncular nucleus, dorsal raphe, lateral parabrachial nucleus and locus coeruleus. Previously unreported projections of the IL to the anterior olfactory nucleus, piriform cortex, anterior hypothalamic area and lateroanterior hypothalamic nucleus were observed. The density of labeled terminals was especially high in the agranular insular cortex, olfactory tubercle, medial division of the mediodorsal nucleus of the thalamus, dorsal hypothalamic area and the lateral division of the central amygdaloid nucleus. Several physiological and pharmacological studies have suggested that the IL functions as the 'visceral motor' cortex, involved in autonomic integration with behavioral and emotional events. The present investigation is the first comprehensive study of the IL efferent projections to support this concept.     

Afferent and efferent connections of the olfactory bulbs in the lizard Podarcis hispanica.

The connections of the olfactory bulbs of Podarcis hispanica were studied by tract-tracing of injected horseradish peroxidase. Restricted injections into the main olfactory bulb (MOB) resulted in bilateral terminallike labeling in the medial part of the anterior olfactory nucleus (AON) and in the rostral septum, lateral cortex, nucleus of the lateral olfactory tract, and ventrolateral amygdaloid nucleus. Bilateral retrograde labeling was found in the rostral lateral cortex and in the medial and dorsolateral AON. Ipsilaterally the dorsal cortex, nucleus of the diagonal band, lateral preoptic area, and dorsolateral amygdala showed labeled cell bodies. Retrogradely labeled cells were also found in the midbrain raphe nucleus. Results from injections into the rostral lateral cortex and lateral olfactory tract indicate that the mitral cells are the origin of the centripetal projections of the MOB. Injections in the accessory olfactory bulb (AOB) produced ipsilateral terminallike labeling of the ventral AON, bed nucleus of the accessory olfactory tract, central and ventromedial amygdaloid nuclei, medial part of the bed nucleus of the stria terminalis, and nucleus sphericus. Retrograde labeling of neurons was observed ipsilaterally in the bed nucleus of the accessory olfactory tract and stria terminalis, in the central amygdaloid nucleus, dorsal cortex, and nucleus of the diagonal band. Bilateral labeling of somata was found in the ventral AON, the nucleus sphericus (hilus), and in the mesencephalic raphe nucleus and locus coeruleus. Injections into the dorsal amygdala showed that the mitral neurons are the cells of origin of the AOB centripetal projections. Reciprocal connections are present between AOB and MOB. To our knowledge, this is the first study to address the afferent connections of the olfactory bulbs in a reptile. On the basis of the available data, a discussion is provided of the similarities and differences between the reptilian and mammalian olfactory systems, as well as of the possible functional role of the main olfactory connections in reptiles.     

Connections of the posterior nucleus of the amygdala.

The connections of a relatively homogeneous band of neurons in the caudal amygdala have been examined with anterograde and retrograde axonal tracing methods in the rat. This region, called here the posterior nucleus of the amygdala (PA), corresponds in part to an area that has been referred to as the cortico-amygdaloid transition area, posterior part of the medial nucleus of the amygdala, amygdalo-hippocampal transition area, and posteromedial basal nucleus. Experiments with fluorogold and phaseolus vulgaris leucoagglutinin (PHAL) indicate that the major neuronal input to the PA arises in the ventral premammillary nucleus, and that substantial projections also arise in olfactory-related areas such as the medial nucleus of the amygdala, bed nucleus of the accessory olfactory tract, and posterior cortical nucleus of the amygdala, as well as in the ventral subiculum and adjacent parts of hippocampal field CA1. Other seemingly minor inputs, including cholinergic fibers from the substantia innominata, dopaminergic fibers from the ventral tegmental area, and serotoninergic fibers from the dorsal nucleus of the raphe, were also identified. The efferent projections of the PA as determined with the PHAL method appear to follow five major routes: 1) a relatively small group of laterally directed fibers innervates the dorsal endopiriform nucleus, and a few of these fibers reach cortical area TR and the lateral entorhinal area; 2) another small group of fibers courses medially to innervate the ventral subiculum and adjacent parts of field CA1; 3) many fibers course ventrally to innervate the outer molecular layer of the medial part of the posterior cortical nucleus of the amygdala; 4) a moderate group of fibers courses rostrally to innervate primarily the posterodorsal part of the medial nucleus of the amygdala, although some fibers continue on to end less densely in rostral parts of the medial nucleus of the amygdala before leaving the amygdala through the ansa peduncularis; and 5) the major output of the PA courses through the stria terminalis. One branch of this pathway massively innervates the principal nucleus of the bed nuclei of the stria terminalis before entering the medial hypothalamus, where it ends massively in the anteroventral periventricular and medial preoptic nuclei, ventrolateral part of the ventromedial nucleus and adjacent parts of the basal lateral hypothalamic area, and ventral premammillary nucleus. The other branch sends fibers to the ventral lateral septal nucleus, nucleus accumbens, olfactory tubercle, and infralimbic area of the prefrontal cortex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Bidirectional connections of the medial amygdaloid nucleus in the Syrian hamster brain: Simultaneous anterograde and retrograde tract tracing

In the male Syrian hamster, mating is dependent on chemosensory and hormonal stimuli, and interruption of either input prevents copulation. The medial amygdaloid nucleus (Me) is a key nodal point in the neural circuitry controlling male sexual behavior because it relays both odor and steroid cues. Me is comprised of two major subdivisions, anterior (MeA) and posterior (MeP), which have distinct, although overlapping efferent projections. The present study investigated the afferents and efferents of MeA and MeP by using combined anterograde and retrograde tract tracing. Phaseolus vulgaris–leucoagglutinin and cholera toxin B were injected by iontophoresis through a single glass micropipette and detected by immunohistochemistry. MeA has widespread connections with olfactory structures, whereas MeP is heavily interconnected with steroid-responsive brain regions. The efferent projections of MeA and MeP were similar to those reported previously for the rat and hamster. In particular, MeP projects to the posteromedial subdivision of the bed nucleus of the stria terminalis (BNST) and to the medial preoptic nucleus, whereas MeA projects to adjacent subnuclei in BNST and the preoptic area. MeA and MeP also have distinct patterns of afferent input. Furthermore, the combination of anterograde and retrograde tract tracers shows that MeA and MeP are each bidirectionally connected with each other and with limbic nuclei. These results demonstrate that subnuclei of Me are interconnected with limbic structures in hamster brain. These connections may contribute to chemosensory and hormonal integration to control male sexual behavior. J. Comp. Neurol. 399:189–209, 1998. © 1998 Wiley-Liss, Inc.     

Forebrain projections from cholecystokininlike-immunoreactive neurons in the rat midbrain

The purpose of the present study was to analyze the distribution of cholecystokininlike-immunoreactive (CCK-I) neurons within the rat ventral mesencephalon which project to several forebrain areas. The peroxidase-antiperoxidase immunocytochemical technique was used to examine the anatomical localization of CCK-I within the ventral midbrain and in the following forebrain regions: caudate-putamen, nucleus accumbens, olfactory tubercle, bed nucleus of the stria terminalis, septum, amygdala, and prefrontal, anterior cingulate, and piriform cortices. CCK-I perikarya were distributed throughout the substantia nigra, ventral tegmental area, and several midline raphe nuclei to a greater extent than previously reported, particularly in the substantia nigra pars compacta. Terminallike immunoreactivity for CCK was observed in all of the above forebrain sites. In addition, infrequent CCK-I cell bodies were localized in the caudate-putamen, nucleus accumbens, olfactory tubercle, septum, and bed nucleus of the stria terminalis. To analyze forebrain projections of the ventral midbrain CCK-I neurons, indirect immunofluorescence was combined with fluorescence retrograde tracing. CCK-I neurons of the substantia nigra and/or ventral tegmental area were found to project, to varying extents, to all of the above CCK-I forebrain terminal fields. The nucleus accumbens, olfactory tubercle, and septal and prefrontal cortical projections arose primarily from CCK-I perikarya in the ventral tegmental area whereas the projections to the caudate-putamen and anterior cingulate cortex arose predominantly from immunoreactive neurons in the substantia nigra pars compacta. The amygdala received innervation mainly from CCK-I cell bodies located in the substantia nigra pars lateralis. CCK-I afferents to the bed nucleus of the stria terminalis and piriform cortex originated from perikarya distributed approximately equally across the ventral tegmental area and substantia nigra pars compacta. The general topography of CCK-I forebrain innervation observed in this study is similar to that previously reported for the ascending dopaminergic projections from ventral mesencephalic neurons. CCK-I neurons of the midline raphe nuclei were found to provide relatively minor afferents to the caudate-putamen, bed nucleus of the stria terminalis, septum, and prefrontal cortex and more substantial projections to the amygdala. The results of this study demonstrate that CCK-I neurons of the ventral midbrain supply a much broader innervation of forebrain regions than previously appreciated.(ABSTRACT TRUNCATED AT 400 WORDS)     

Amygdala projections to the lateral bed nucleus of the stria terminalis in the macaque: Comparison with ventral striatal afferents

The lateral bed nucleus of the stria terminalis (BSTL) is involved in mediating anxiety‐related behaviors to sustained aversive stimuli. The BSTL forms part of the central extended amygdala, a continuum composed of the BSTL, the amygdala central nucleus, and cell columns running between the two. The central subdivision (BSTLcn) and the juxtacapsular subdivision (BSTLJ) are two BSTL regions that lie above the anterior commissure, near the ventral striatum. The amygdala, a heterogeneous structure that encodes emotional salience, projects to both the BSTL and ventral striatum. We placed small injections of retrograde tracers into the BSTL, focusing on the BSTLcn and BSTLJ, and analyzed the distribution of labeled cells in amygdala subregions. We compared this to the pattern of labeled cells following injections into the ventral striatum. All retrograde results were confirmed by anterograde studies. We found that the BSTLcn receives stronger amygdala inputs relative to the BSTLJ. Furthermore, the BSTLcn is defined by inputs from the corticoamygdaloid transition area and central nucleus, while the BSTLJ receives inputs mainly from the magnocellular accessory basal and basal nucleus. In the ventral striatum, the dorsomedial shell receives inputs that are similar, but not identical, to inputs to the BSTLcn. In contrast, amygdala projections to the ventral shell/core are similar to projections to the BSTLJ. These findings indicate that the BSTLcn and BSTLJ receive distinct amygdala afferent inputs and that the dorsomedial shell is a transition zone with the BSTLcn, while the ventral shell/core are transition zones with the BSTLJ. J. Comp. J. Comp. Neurol. 521:3191–3216, 2013. © 2013 Wiley Periodicals, Inc.