Efferent connections of the dorsal cortex of the lizard Gekko gecko studied with Phaseolus vulgaris-leucoagglutinin

The efferent connections from the dorsal cortex of the lizard Gekko gecko have been studied with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. It appeared that the dorsal cortex is not a homogeneous structure as far as the efferent connections are concerned. All parts of the dorsal cortex project to the septum. All parts except the most medial project to the dorsal ventricular ridge, amygdala, nucleus periventricularis hypothalami, area lateralis hypothalami, and the anterior olfactory nucleus. The most medial part, in addition to the septal projections, is connected with the medial cortex and the contralateral medial and dorsal cortices. From the rostral part additional projections could be traced to the nucleus dorsolateralis hypothalami, nucleus ventromedialis thalami, nucleus dorsolateralis thalami, striatum, pallial thickening, medial cortex, nucleus olfactorius anterior, and the main and accessory olfactory bulbs. From the caudal part additional projections exist to the nucleus dorsomedialis thalami, nucleus accumbens, and the contralateral dorsal cortex. A system of intrinsic connections exists that can be subdivided into four subsystems, each of which subserves the interconnections within four subdivisions of the cortex: 1) the superficial medial part, 2) the deep medial part, 3) the caudal lateral and caudal intermediate parts, and 4) the rostral lateral and rostral intermediate parts. Connections between these four areas are scarce. From the present results the conclusion is drawn that the dorsal cortex of the lizard Gekko gecko has many hodological aspects in common with the ventral subiculum of mammals. The present results do not support the hypothesis that the dorsal cortex is the reptilian equivalent of the mammalian neocortex.     

Medial cortex of the lizard Gekko gecko: A hodological study with emphasis on regional specialization

There is increasing evidence that the archicortex in mammals and reptiles is not a homogeneous structure. However, little is known about the regional specialization of this cortical area in reptiles. Therefore, the efferent connections of the medial cortex of the lizard Gekko gecko were studied with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. The small-celled part of the medial cortex (Cxms) projects to various parts of the septum in a topological way: the rostral part projects to the anterior septal nucleus, whereas the caudal part projects to the lateral septal nucleus and the nucleus septi impar. In addition, Cxms projects to the large-celled part of the medial cortex (Cxml). Axons that originate from the dorsal part of Cxms terminate at the proximal parts of the apical and basal dendrites of the neurons of Cxml caudal to the injection site. In contrast, fibers originating from the ventral part terminate on more distal parts of the dendrites of neurons of Cxml rostral to the injection site. Other projections from Cxms to the dorsal cortex (Cxd) and the external amygdaloid nucleus were found. The Cxml projects bilaterally to Cxms. These projections terminate in the superficial and deep plexiform layers. In addition, projections to the cell plate of Unger, Cxd, and to the lateral septal nucleus were found. It appears, on the basis of the efferent connections, that Cxms can be divided into a rostral and caudal part, while hodological differences also exist between the dorsal and ventral parts of Cxms. The results of the present study do not suggest a subdivision of Cxml. The regional variations of the medial cortex in the lizard Gekko gecko differ from the regional variations described in other reptilian species. © 1993 Wiley-Liss, Inc.     

Afferent and efferent connections of the nucleus sphericus in the snake Thamnophis sirtalis: Convergence of olfactory and vomeronasal information in the lateral cortex and the amygdala

This paper is an account of the afferent and efferent projections of the nucleus sphericus (NS), which is the major secondary vomeronasal structure in the brain of the snake Thamnophis sirtalis. There are four major efferent pathways from the NS: 1) a bilateral projection that courses, surrounding the accessory olfactory tract, and innervates several amygdaloid nuclei (nucleus of the accessory olfactory tract, dorsolateral amygdala, external amygdala, and ventral anterior amygdala), the rostral parts of the dorsal and lateral cortices, and the accessory olfactory bulb; 2) a bilateral projection that courses through the medial forebrain bundle and innervates the olfactostriatum (rostral and ventral striatum); 3) a commissural projection that courses through the anterior commissure and innervates mainly the contralateral NS; and 4) a meager bilateral projection to the lateral hypothalamus. On the other hand, important afferent projections to the NS arise solely in the accessory olfactory bulb, the nucleus of the accessory olfactory tract, and the contralateral NS. This pattern of connections has three important implications: first, the lateral cortex probably integrates olfactory and vomeronasal information. Second, because the NS projection to the hypothalamus is meager and does not reach the ventromedial hypothalamic nucleus, vomeronasal information from the NS is not relayed directly to that nucleus, as previously reported. Finally, a structure located in the rostral and ventral telencephalon, the olfactostriatum, stands as the major tertiary vomeronasal center in the snake brain. These three conclusions change to an important extent our previous picture of how vomeronasal information is processed in the brain of reptiles. J. Comp. Neurol. 385:627–640, 1997. © 1997 Wiley-Liss, Inc.     

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.     

Association and commissural fiber systems of the olfactory cortex of the rat II. Systems originating in the olfactory peduncle

The structure and connections of areas within the olfactory peduncle (anterior olfactory nucleus and tenia tecta) have been examined. The anterior olfactory nucleus has been divided into external, lateral, dorsal, medial, and ventro-posterior parts. In spite of the term nucleus which is applied to these areas, all of them contain pyramidal-type cells with apical and basal dendrites oriented normal to the surface, and are essentially cortical in organization. Experiments utilizing retrograde and anterograde axonal transport of horseradish peroxidase (HRP) have demonstrated that each of these parts of the anterior olfactory nucleus possesses a unique pattern of afferent and efferent connections with other olfactory areas. All subdivisions have projections to both the ipsilateral and contralateral sides, although the ipsilateral projection of the pars externa (to the olfactory bulb) is extremely light. Interestingly, crossed projections are in each case directed predominantly to areas adjacent to the homotopic areas. Two primary subdivisions may also be distinguished in the tenia tecta: a dorsal part composed largely of tightly packed neurons which closely resemble the granule cells of the dentate gyrus (bushy apical but no basal dendrites) and a ventral part which contains predominantly pyramidal-type cells. The connections of these two parts are also very different. The ventral tenia tecta receives substantial projections from the olfactory bulb, pars lateralis of the anterior olfactory nucleus, piriform cortex and lateral entorhinal area. It gives off a heavy return projection to the pars lateralis and lighter projections to the olfactory bulb, piriform cortex and olfactory tubercle. The dorsal tenia tecta receives a heavy projection from the piriform cortex, but none from the olfactory bulb. A few cells in the dorsal tenia tecta are retrogradely labeled from HRP injections into the medial aspect of the olfactory peduncle (involving the ventral tenia tecta and adjacent areas), but none are labeled from the other olfactory areas that have been injected. An area on the dorsal aspect of the olfactory peduncle that differs significantly from the anterior olfactory nucleus, tenia tecta and piriform cortex in terms of its connections and cytoarchitecture has been termed the dorsal peduncular cortex. The most striking feature of this area is its very heavy reciprocal connection with the entorhinal cortex, although it is also reciprocally connected with the olfactory bulb and piriform cortex and projects to the olfactory tubercle. Cells in layer I of the medial and ventral aspects of the olfactory peduncle have been retrogradely labeled from HRP injections into the olfactory tubercle and lateral hypothalamic area. These cells overlie the ventral tenia tecta, medial part of the anterior piriform cortex and pars ventro-posterior and pars lateralis of the anterior olfactory nucleus, but do not appear to be distributed in relation to the cytoarchitectonic boundaries. Possible functional roles of the areas within the olfactory peduncle have been discussed.     

Amygdaloid projections to subcortical structures within the basal forebrain and brainstem in the rat and cat

The efferent fiber connections of the nuclei of the amygdaloid complex with subcortical structures in the basal telencephalon, hypothalamus, midbrain, and pons have been studied in the rat and cat, using the autoradiographic method for tracing axonal connections. The cortical and thalamic projections of these nuclei have been described in previous papers (Krettek and Price, ′77b,c). Although the subcortical connections of the amygdaloid nuclei are widespread within the basal forebrain and brain stem, the projections of each nucleus have been found to be well defined, and distinct from those of the other amygdaloid nuclei. The basolateral amygdaloid nucleus projects heavily to the lateral division of the bed nucleus of the stria terminalis (BNST), to the caudal part of the substantia innominata, and to the ventral part of the corpus striatum (nucleus accumbens and ventral putamen) and the olfactory tubercle; it projects more lightly to the lateral hypothalamus. The central nucleus also projects to the lateral division of the BNST and the lateral hypothalamus, but in addition it sends fibers to the lateral part of the substantia nigra and the marginal nucleus of the brachium conjunctivum. The basomedial nucleus has projections to the ventral striatum and olfactory tubercle which are similar to those of the basolateral nucleus, but it also projects to the core of the ventromedial hypothalamic nucleus and the premammillary nucleus, and to a central zone of the BNST which overlaps the medial and lateral divisions. The medial nucleus also projects to the core of the ventromedial nucleus and the premammillary nucleus, but sends fibers to the medial division of the BNST and does not project to the ventral striatum. The posterior cortical nucleus projects to the premammillary nucleus and to the medial division of the BNST, but a projection from this nucleus to the ventromedial nucleus has not been demonstrated. Projections to the “shell” of the ventromedial nucleus have been found only from the ventral part of the subiculum and from a structure at the junction of the amygdala and the hippocampal formation, which has been termed the amygdalo-hippocampal area (AHA). The AHA also sends fibers to the medial part of the BNST and the premammillary nucleus. Virtually no subcortical projections outside the amygdala itself have been demonstrated from the lateral nucleus, or from the olfactory cortical areas around the amygdala (the anterior cortical nucleus, the periamygdaloid cortex, and the posterior prepiriform cortex). However, portions of the endopiriform nucleus deep to the prepiriform cortex project to the ventral putamen, and to the lateral hypothalamus.     

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.     

The topographic organization of associational fibers of the olfactory system in the rat, including centrifugal fibers to the olfactory bulb

This study analyzed the topographic organization of the associational fibers within the olfactory cortex of the rat, by using the autoradiographic method. Small injections of 3H-leucine were placed in all of the subdivisions of the olfactory cortex, to label selectively the fibers arising in each area. Intracortical fibers were identified from all of the olfactory cortical areas except the olfactory tubercle and were classified into two major systems (the layer Ib system and the layer II-deep Ib system) on the basis of their laminar pattern of termination (see Luskin and Price, '83). The layer Ib fiber system arises in the anterior olfactory nucleus, piriform cortex, and lateral entorhinal area, and is broadly organized in relation to the lateral olfactory tract. Cortical areas deep to or near the lateral olfactory tract are preferentially interconnected with areas near the tract, while parts of the cortex lateral and caudal to the lateral olfactory tract are most heavily interconnected with areas lateral, caudal, and medial to the tract. Commissural projections from the anterior olfactory nucleus and the anterior piriform cortex match some (but not all) components of the ipsilateral layer Ib fiber system. The layer II-deep Ib fiber system arises in three small areas--the ventral tenia tecta, the dorsal peduncular cortex, and the periamygdaloid cortex. The fibers from the ventral tenia tecta terminate in layer II of the anterior olfactory nucleus and are topographically organized. The fibers from the dorsal peduncular cortex and the periamygdaloid cortex are more widely distributed, especially in the lateral and caudal parts of the cortex. Two other intracortical projections do not fit into either of these fiber systems. The nucleus of the lateral olfactory tract projects bilaterally to the islands of Calleja and the medial edge of the anterior piriform cortex. The anterior cortical nucleus projects to many parts of the olfactory cortex, but the fibers end in both superficial and deep parts of layer I (layer Ia and Ib). There are projections from several of the olfactory cortical areas to the cortical areas surrounding the olfactory cortex. Virtually all of the olfactory areas also project to the ventral and dorsal endopiriform nuclei deep to the piriform cortex and/or to the polymorph zone deep to the olfactory tubercle. In addition, projections have been demonstrated to the deep amygdaloid nuclei, especially from the more ventromedial and caudal parts of the olfactory cortex.     

Efferents and centrifugal afferents of the main and accessory olfactory bulbs in the hamster

The efferents and centrifugal afferents of the hamster olfactory bulbs were studied using orthograde and retrograde tracing techniques. Following injections of tritiated amino acids which were restricted to the main olfactory bulb (MOB), autoradiographic grains were observed ipsilaterally over layer IA of the entire anterior olfactory nucleus (AON), the ventral portion of the hippocampal rudiment (HR), the entire prepyriform cortex and olfactory tubercle, the anterior and posterolateral cortical amygdaloid nuclei and the lateral entorhinal cortex. An ipsilateral projection to the nucleus of the lateral olfactory tract (nLOT) was also indicated. No subcortical or contralateral projections were observed. Amino acid injections into the accessory olfactory bulb (AOB) revealed ipsilateral projections to the superficial plexiform layer of the medial and posteromedial cortical amygdaloid nuclei and to the bed nucleus of the accessory olfactory tract (nAOT) and the bed nucleus of the stria terminalis (nST). Following injections of HRP which were restricted to the MOB, contralateral HRP-positive neurons were found predominantly in pars externa and to a lesser extent in the other subdivisions of the AON. Centrifugal projections to the MOB were identified ipsilaterally from the entire AON, the ventral portion of the HR, the anterior portion of the prepyriform cortex, and the nLOT. No labelled neurons were found in the olfactory tubercle, the anterior and posterolateral cortical amygdaloid nuclei or the entorhinal cortex. Centrifugal projections to the MOB were also identified from subcortical structures of the ipsilateral basal forebrain and from midline structures of the midbrain. Labelling occurred in the fusiform neurons of the diagonal band near the medial base of the forebrain at the level of caudal olfactory tubercle. Heavy labelling was seen in a distinct group of large, predominantly multipolar neurons (magnocellular preoptic area) that continued from the level of caudal olfactory tubercle to the level of the nLOT. This band of HRP-positive neurons could be followed more caudally to a position dorsal and medial to the nLOT near the lateral margin of the lateral anterior hypothalamic area. The midbrain projections to the MOB originated in the dorsal and median raphe nuclei. After injections of HRP into the AOB, centrifugal projections were identified from the nAOT and the posteromedial cortical amygdaloid nucleus. In addition, isolated neurons were labelled in the medial cortical amygdaloid nucleus but no labelled neurons were found in the nST. These results support the notion of two anatomically distinct olfactory systems and demonstrate two previously unreported pathways through which the limbic system may modulate sensory processing in the olfactory bulb.     

the connections of the mouse olfactory bulb: A study using orthograde and retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase

The efferent and centrifugal afferent connections of the main olfactory bulb (MOB) of the mouse were studied by orthograde and retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). MOB projects ipsilaterally to the anterior olfactory nucleus, taenia tecta, anterior hippocampal continuation, indusium grisium, olfactory tubercle, and the lateral and medial divisions of the entorhinal area. In the region of the anterior one-half to two-thirds of the posterior division of the insular cortex the projection from MOB extends into the insular cortex. The only efferent projection of MOB to the contralateral half of the brain was to the anterior olfactory nucleus. All efferent projections of MOB, thus, are to telencephalic structures. By contrast the centrifugal afferents to MOB originate from every major division of the neuraxis. Neurons projecting to the bulb were found ipsilaterally in all divisions of the anterior olfactory nucleus (AON). In some cases, labeling in the external division of AON was weak or absent. In the contralateral AON, pars externa was the most intensively labeled sub-division. Retrogradely labeled neurons were also present in all other subdivisions of the contralateral AON but were fewer in number and less heavily labeled than in the ipsilateral AON. Ipsilaterally, positive neurons were also present in taenia tecta, and the anterior hippocampal continuation. There was profuse retrograde labeling of neurons in the entire extent of the ipsilateral piriform cortex (PC). There was a rostral to caudal gradient of labeling in PC with more positive neurons in rostral than caudal parts. Labeled neurons were present in the lateral entorhinal cortex LEC and in the transitional cortex between LEC and PC. Very heavy retrograde labeling was present in the nuclei of the horizontal and vertical limbs of the diagonal band (HDB and VDB). More cells were labeled in HDB than in VDB. Neurons were labeled in the ipsilateral nucleus of the lateral olfactory tract (NLOT) and, when the injection spread into the accessory olfactory bulb, labeled neurons were present ventral to NLOT in accessory NLOT. A few lightly labeled neurons were always present in the posterolateral and medial cortical amygdaloid areas. Neurons were labeled in the zona inserta and scattered throughout several hypothalamic nuclei. There was massive retrograde labeling of neurons in the locus coeruleus and neurons were abundantly labeled in the dorsal and medial raphe nuclei and nucleus raphe pontis. In general, the labeling of MOB connections was more extensive than that which has been reported in closely related species.(ABSTRACT TRUNCATED AT 400 WORDS)     

Efferents from the medial anterior hypothalamic area in the guinea pig

Medial anterior hypothalamic connections were studied with H³-proline and autoradiography. Most of the axons projected to other hypothalamic nuclei. The major pathways were found ventral medial to the fornix and in the periventricular tract. Substantial projections were apparent in the ventromedial and dorsomedial nuclei with less label in the arcuate nucleus. The dorsal premammillary nuclei were labeled bilaterally, particularly with more caudal injections of anterior hypothalamus. Efferents were evident in the posterior hypothalamus and continued into the central gray of the midbrain. Labeled fibers reached the ventral tegmental area and in the reticular formation were traced only through pons. Rostral projections were to the medial and lateral preoptic areas and ventral lateral septum. The bed nucleus of stria terminalis was labeled and a very few fibers reached the medial amygdaloid nucleus. The periventricular nucleus of thalamus was labeled.     

The accessory olfactory bulbs and the lateral telencephalic wall of the rat-fish, Chimaera

The lateral telencephalon of Chimaera possesses several unique features but also has nuclei and fiber systems homologous with those of other sub-mammalian vertebrates. Ventricular ridges, similar to those of reptiles, are quite evident. Accessory olfactory bulbs are associated with the dorsal and ventral parts of each olfactory bulb. These contribute to the lateral olfactory tract. The internal granular layer caudal to the olfactory and the accessory bulbs blends with the anterior olfactory nucleus. Caudal to this nuclear area, the nuclei of the rostral telencephalon are well differentiated. Nuclear areas distinguishable in the lateral hemisphere include: the primordial dorsal pallium, the primordial piriform cortex, the primordial striatal and amygdaloid nuclei, and the lateral zone of the olfactory tubercle. These areas replace dorsal, dorsolateral, ventrolateral and ventral parts of the anterior olfactory nucleus, respectively. The primordial striatum is subdivided into hyperstriatum, neostriatum, paleostriatum augmentatum and paleostriatum primitivum. The amygdaloid area has anterior, corticomedial and basolateral nuclear groups. The basolateral area is best differentiated. The hyperstriatum forms a rostral ventricular eminence; the basolateral amygdaloid nucleus is present in a larger caudal ventricular ridge. Fiber tracts of the lateral wall include the lateral olfactory tract, the lateral corticohabenular tract, the lateral forebrain bundle and the stria terminalis. Nuclei of medial and lateral walls are interrelated through the hippocampal and the anterior commissures.     

Convergence of autonomic and limbic connections in the insular cortex of the rat

The connnections of the insular cortex in the rate were studied by using the anterograde and retrograde transport of wheat germ agglutinin-conjugated-horseradish peroxidase. Both anterograde and retrogrde transport were seen in the ipsilateral lateral frontal, infralimbic, piriform, and perirhinal cortical areas and in the contralateral insular cortex. In the thalaamus, both types of labeling were seen in the mediodorsal and ventroposteromedial parvocellular nuclei; primarily retrograde labeling was seen in the centromedial and paracentral nuclei. In the basal forebrain, anterograde labeling was seen in the lateralpart of the bed nucleus of the stria terminalis and in the central nucleus of the amygdala, while retrogradely labeled neurons were found in the magnocellular basal nucleus and in the lateral and basolateral amygdaloid nuclei. Both types of labeling were seen in the posterior lateral hypothalamic area; the tuberomammillary nucleus contained retrogradely labeled neurons bilaterally. In the midbrain, retrogradely labeled neurons were found in the ventral tegmental area and in the dorsal and superior central raphe nuclei. In the pons, both retrogradely and anterogradely transported label was seen bilaterally in the parabrachial nucleus, primarily in the ventromedial caudal part of the medial subnucleus. Retrogradely labeled neurons were found bilaterally in the locus coeruleus. Anterograde transport was followed into the medulla, bilaterally but more heavily in the contralateral side. Labeled axons appeared to terminate in a topographic pattern in the nucleus of the solitary tract. These results indicate that the insular cortex of the rat is an important part of the highly interconnected central autonomic system. Furthermore, the autonomic representation in the insular cortex may be organized in a viscerotopic manner. The insular cortex also has connections with the limbic system and with the lateral frontal cortical system. Although it is not yet clear whether these connections converge upon the same neurons within the insular cortex, earlier physiological data suggest that each of the diverse systems of connections of this area receives relayed vagal inputs. The insular cortex of the rat may contain a primary cortical visceral representation, and its connections may underlie autonomic integration with behavioral and emotional events.     

Efferent connections of the substantia innominata in the rat

The efferent connections of the substantia innominata (SI) were investigated employing the anterograde axonal transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The projections of the SI largely reciprocate the afferent connections described by Grove (J. Comp. Neurol. 277:315-346, '88) and thus further distinguish a dorsal and a ventral division in the SI. Efferents from both the dorsal and ventral divisions of the SI descend as far caudal as the ventral tegmental area, substantia nigra, and peripeduncular area, but projections to pontine and medullary structures appear to originate mainly from the dorsal SI. Within the amygdala and hypothalamus, which receive widespread innervation from the SI, the dorsal SI projects preferentially to the lateral part of the bed nucleus of the stria terminalis; the lateral, basolateral, and central nuclei of the amygdala; the lateral preoptic area; paraventricular nucleus of the hypothalamus; and certain parts of the lateral hypothalamus, prominently including the perifornical and caudolateral zones described previously. The ventral SI projects more heavily to the medial part of the bed nucleus of the stria terminalis; the anterior amygdaloid area; a ventromedial amygdaloid region that includes but is not limited to the medial nucleus; the lateral and medial preoptic areas; and the anterior hypothalamus. Modest projections reach the lateral hypothalamus, with at least a slight preference for the medial part of the region, and the ventromedial and arcuate hypothalamic nuclei. Both SI divisions appear to innervate the dorsomedial and posterior hypothalamus and the supramammillary region. In the thalamus, the subparafascicular, gustatory, and midline nuclei receive a light innervation from the SI, which projects more densely to the medial part of the mediodorsal nucleus and the reticular nucleus. Cortical efferents from at least the midrostrocaudal part of the SI are distributed primarily in piriform, infralimbic, prelimbic, anterior cingulate, granular and agranular insular, perirhinal, and entorhinal cortices as well as in the main and accessory olfactory bulbs. The cells of origin for many projections arising from the SI were identified as cholinergic or noncholinergic by combining the retrograde transport of WGA-HRP with histochemical and immunohistochemical procedures to demonstrate acetylcholinesterase activity or choline acetyltransferase immunoreactivity. Most of the descending efferents of the SI appear to arise primarily or exclusively from noncholinergic cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Differential projections of the main and accessory olfactory bulb in the frog.

The central projections of the main olfactory bulb and the accessory olfactory bulb of the adult leopard frog (Rana pipiens) were reexamined, by using a horseradish peroxidase anterograde tracing method that fills axons with a continuous deposit of reaction product. The fine morphology preserved by this method allowed the terminal fields of the projection tracts to be delineated reliably, and for the first time. Herrick's amygdala has been newly subdivided into cortical and medial nuclei on the basis of cytoarchitecture, dendritic morphology, and the differential projections of the main and accessory olfactory tracts. The main olfactory bulb projects through the medial and lateral olfactory tracts to the postolfactory eminence, the rostral end of the medial cortex, the rostral end of the medial septal nucleus, the cortical amygdaloid nucleus, the nucleus of the hemispheric sulcus, and both the dorsal and ventral divisions of the lateral cortex, including its retrobulbar fringe. The lateral olfactory tract overlaps the dorsal edge of the striatal plate along the ventral border of the lateral cortex, but it is not certain whether any striatal cells are postsynaptic to the tract fibers. The lateral cortex is the largest of these territories, and receives the terminals of the main olfactory projection throughout its extent. It extends from the olfactory bulb to the posterior pole, and from the striatum to the summit of the hemisphere, where it borders the dorsal cortex. The medial and lateral olfactory tracts combine in the region of the amygdala to form a part of the stria medullaris thalami. These fibers cross in the habenular commissure and terminate in the contralateral cortical amygdaloid nucleus and periamygdaloid part of the lateral cortex. Cells projecting to the main olfactory bulb are found in the diagonal band and adjacent cell groups, but there is no evidence of an interbulbar projection arising from either the olfactory bulb proper or a putative anterior olfactory nucleus. The accessory olfactory bulb projects through the accessory olfactory tract to the medial and cortical amygdaloid nuclei. A fascicle of the tract crosses in the anterior commissure to terminate in the contralateral amygdala. While the main and accessory olfactory projections may converge in the cortical amygdaloid nucleus, the medial amygdaloid nucleus is connected exclusively with the accessory olfactory bulb.     

Ventral temporal cortex in the rat: connections of secondary auditory areas Te2 and Te3.

The present study in the rat deals with the hodological organization of two cytoarchitectonically distinct areas lying caudoventrally (Te2) or ventrally (Te3) to the primary auditory area (Te1). The afferent and efferent systems of connections were identified by using the properties of retrograde and anterograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP). Large tracer deposits in the ventral temporal cortex involving Te2, Te3, and the dorsal bank of the perirhinal cortex induced a dense retrograde and anterograde pattern of labeling in the following nuclei of the medial geniculate (MG) complex: caudodorsal (MGCD), dorsal (MGD), medial (MGM), suprageniculate (SG), and peripeduncular area (PPA). The ventral nucleus (MGV) was only slightly labeled in its caudal division. Several extrageniculate structures were also labeled. Retrograde cell labeling occurred in centers giving rise to ascending systems of diffuse projections: locus coeruleus (LC), dorsal raphe nucleus (DR), and basal magnocellular nucleus (B). Slight anterograde labeling was present in the dorsal and external cortices of the inferior colliculus (IC), central gray, deep layers of the superior colliculus (SC), reticular thalamic nucleus (RT), and caudate putamen (CPU). Callosal connections were also noted with the contralateral homotopic cortex. In the cases in which there was a notable extension of the zone of diffusion of the tracer into the dorsal bank of the perirhinal cortex, a characteristic pattern of labeling in the subparafascicular, reuniens and paraventricular thalamic nuclei, mammillary complex, lateral and dorsal hypothalamic nuclei, amygdaloid complex, laterodorsal tegmental nucleus, subiculum, and retrosplenial cortex was displayed. Tracer deposits restricted to Te2 induced a dense labeling of the caudal, ventrolateral MGD, lateral PPA and, to a lesser extent, MGCD. The MGM and SG were only slightly labeled. Extrageniculate afferents essentially consist of sparse projections from LC, DR, and B, whereas efferent fibers are directed to the dorsal cortex of the IC, central gray, deep SC layers, and CPU. Callosal connections were also identified. Following tracer deposits restricted to Te3, dense labeling occurred in the MGD, mostly in its medial division, in the caudal MGM, and in the PPA. The MGCD, SG, and MGV were only sparsely labeled. Extrageniculate afferents arise from LC, DR, and B, and efferents are directed to the RT and dorsal cortex of the IC. Contralateral connections with the homotopic cortical area were also noted. Te2 and Te3 share some degree of similitude in their pattern of connections with the MG complex.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

Efferent connections of the nucleus of the lateral olfactory tract in the rat

The efferent connections of the nucleus of the lateral olfactory tract (LOT) were examined in the rat with the Phaseolus vulgaris leucoagglutinin (PHA-L) technique. Our observations reveal that layers II and III of LOT have largely segregated outputs. Layer II projects chiefly ipsilaterally to the olfactory bulb and anterior olfactory nucleus, bilaterally to the anterior piriform cortex, dwarf cell cap regions of the olfactory tubercle and lateral shell of the accumbens, and contralaterally to the lateral part of the interstitial nucleus of the posterior limb of the anterior commissure. Layer III sends strong bilateral projections to the rostral basolateral amygdaloid complex, which are topographically organized, and provides bilateral inputs to the core of the accumbens, caudate-putamen, and agranular insular cortex (dorsal and posterior divisions). Layer II projects also to itself and to layers I and II of the contralateral LOT, whereas layer III projects to itself, to ipsilateral layer II, and to contralateral layer III of LOT. In double retrograde labeling experiments using Fluorogold and cholera toxin subunit b tracers, LOT neurons from layers II and III were found to provide collateral projections to homonymous structures on both sides of the brain. Unlike other parts of the olfactory amygdala, LOT neither projects directly to the extended amygdala nor to the hypothalamus. Thus, LOT seemingly influences nonpheromonal olfactory-guided behaviors, especially feeding, by acting on the olfactory bulb and on ventral striatal and basolateral amygdaloid districts that are tightly linked to lateral prefrontal cortical operations.     

Organization of the pallium in the fire-bellied toad Bombina orientalis. I: Morphology and axonal projection pattern of neurons revealed by intracellular biocytin labeling

The cytoarchitecture and axonal projection pattern of pallial areas was studied in the fire-bellied toad Bombina orientalis by intracellular injection of biocytin into a total of 326 neurons forming 204 clusters. Five pallial regions were identified, differing in morphology and projection pattern of neurons. The rostral pallium receiving the bulk of dorsal thalamic afferents has reciprocal connections with all other pallial areas and projects to the septum, nucleus accumbens, and anterior dorsal striatum. The medial pallium projects bilaterally to the medial pallium, septum, nucleus accumbens, mediocentral amygdala, and hypothalamus and ipsilaterally to the rostral, dorsal, and lateral pallium. The ventral part of the medial pallium is distinguished by efferents to the eminentia thalami and the absence of contralateral projections. The dorsal pallium has only ipsilateral projections running to the rostral, medial, and lateral pallium; septum; nucleus accumbens; and eminentia thalami. The lateral pallium has ipsilateral projections to the olfactory bulbs and to the rostral, medial, dorsal, and ventral pallium. The ventral pallium including the striatopallial transition area (SPTA) has ipsilateral projections to the olfactory bulbs, rostral and lateral pallium, dorsal striatopallidum, vomeronasal amygdala, and hypothalamus. The medial pallium can be tentatively homologized with the mammalian hippocampal formation, the dorsal pallium with allocortical areas, the lateral pallium rostrally with the piriform and caudally with the entorhinal cortex, the ventral pallium with the accessory olfactory amygdala. The rostral pallium, with its projections to the dorsal and ventral striatopallidum, resembles the mammalian frontal cortex.     

Direct projections from the non-laminated divisions of the medial geniculate nucleus to the temporal polar cortex and amygdala in the cat

The medial geniculate nucleus (MG) is well known to send projection fibers not only to the auditory cortex, but also to the limbic structures of the forebrain including the perirhinal cortex and amygdala. In the cat, the non-laminated portions of the MG are also known to project to the amygdala, as well as to the auditory cortical areas surrounding the primary auditory area. On the other hand, projections from the non-laminated MG to the limbic cortical areas have not so far been studied systematically. Thus, in the present study, direct projections from the non-laminated portions of the medial geniculate nucleus to the temporal polar cortex and amygdala were examined in the cat by retrograde and anterograde tract-tracing techniques. The temporal polar cortex is the ventral polar region of the posterior sylvian and posterior ectosylvian gyri, which is located dorsal to the posterior rhinal sulcus and includes the ectorhinal area. After injection of cholera toxin B subunit into the temporal polar cortex, retrogradely labeled neurons were seen in the caudal two-thirds of the medial geniculate nucleus ipsilateral to the injection; they were distributed in the non-laminated portions of the MG (the dorsal and medial divisions and the ventromedial part of the ventral division), but not in the laminated portion (the principal part of the ventral division). These findings were confirmed by injecting Phaseolus vulgaris leucoagglutinin into each division of the MG. After the injection into each non-laminated division, terminal labeling was observed in the temporal polar cortex. Terminal labeling was further found in the lateral amygdaloid nucleus ipsilateral to the injection. Then, cholera toxin B subunit was injected into the lateral amygdaloid nucleus; retrogradely labeled neurons were observed ipsilaterally in the non-laminated portions of the MG, as well as in the temporal polar cortex. The results indicate that the non-laminated portions of the MG send projection fibers to the temporal polar cortex and lateral amygdaloid nucleus, and that the non-laminated portions of the MG and temporal polar cortex give rise to overlapping projections to the lateral amygdaloid nucleus. These connections appear to constitute neuronal links in “emotional” and/or “motivational” circuitry in the forebrain. © Wiley-Liss, Inc.