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.     

Dual olfactory representation in the rat thalamus: an anatomical and electrophysiological study

A combination of electrophysiological and anatomical techniques was used to determine the sites of termination of olfactory projections to the thalamus and the distribution of the cells of origin of these projections within the olfactory cortex. Following electrical stimulation of the olfactory bulb, short-latency unit responses were recorded not only in the central segment of the mediodorsal thalamic nucleus but also in the ventral and anterior parts of the submedial thalamic nucleus. Responses were not obtained in the ventral or lateral parts of the mediodorsal nucleus, in the dorsal part of the submedial nucleus, or in the intralaminar nuclei between the mediodorsal and submedial nuclei. The cells of origin of the projection were identified by making injections of horseradish peroxidase conjugated to wheat germ agglutinin (HRP WGA) into the thalamus and examining the olfactory cortex for retrogradely labeled cells. Following injections into the mediodorsal nucleus, labeled cells were found in the polymorphic cell zone deep to the olfactory tubercle, in the ventral endopiriform nucleus deep to the piriform cortex, and in an equivalent position deep to the periamygdaloid and lateral entorhinal cortices. After injections into the submedial nucleus, a smaller number of labeled cells were found in similar locations, except that they were restricted to the rostral olfactory cortical areas and were not found deep to the lateral part of the piriform cortex. Retrogradely labeled cells and anterogradely labeled axons were also found in the lateral orbital and ventral agranular insular areas of the prefrontal cortex with injections into the mediodorsal nucleus, and in the ventrolateral orbital area with injections into the submedial nucleus. Anterograde tracing experiments, using the autoradiographic method, have confirmed these results. Injections of 3H-leucine deep to the junction between the anterior piriform cortex and the olfactory tubercle label axons in both the central segment of the mediodorsal nucleus and the ventral part of the submedial nucleus, while injections deep to the posterior piriform cortex label axons in the mediodorsal nucleus only. Within the mediodorsal nucleus, the projection also appears to be organized so that fibers which arise more rostrally terminate ventrolaterally in the central segment, while fibers which arise more caudally terminate more dorsomedially. These results indicate that there is a substantial and possibly dual thalamocortical mechanism available for processing of olfactory stimuli.     

Catecholamine innervation of the piriform cortex: a tracing and immunohistochemical study in the rat

In order to determine the origin of the Catecholamine innervation of the rat piriform cortex (PC), we combined retrograde transport of the B subunit of the cholera toxin (CTb) with tyrosine hydroxylase (TH) immunohistochemistry. A substantial number of CTb retrogradely labeled cells was found in the parabrachial pigmented, paranigral and interfascicular nuclei of the ventral tegmental area and the dorsal part of the locus coeruleus, whereas nearly no labeling was noted in the substantia nigra. Following TH immunohistochemistry on the same sections, most if not all of the CTb labeled cells were also TH immunoreactive. Occasional double-labeled cells were also observed in the anterior part of the raphe dorsal nucleus. As visualized with dopamine β-hydroxylase, dopamine or TH immunohistochemistry, che noradrenaline fibers were homogeneously distributed whereas the dopamine fibers showed rostro-caudal and latero-medial differences. The distribution of TH fibers overlapped both patterns. Our report suggests that the heterogeneous distribution of the DA fibers could support a differential centrifugal modulation of the olfactory information processing throughout the PC.     

Anatomical and experimental study of certain association fascicles in the cortex of the rabbit (Oryctalagus cuniculus).

1. All lesions resulted in degeneration of the short intracortical association fibers in cortical layer I and of the short subcortical fibers which extended to the corona radiata before ending in the deeper layers of the overlying neopallium. 2. From all the lesions fibers were traced through the corona radiata to the subcallosal or the so-called superior fronto-occipital association bundle. This bundle had projection fibers to the orbitofrontal cortex. 3. From the lesion in the orbitofrontal neopallium, the orbitofrontal-pyriform connections were established. Such fibers coursed on the dorsal edge of the lateral olfactory tract and distributed to the pyriform cortex and to the nucleus of the lateral olfactory tract. 4. The uncinate fasciculus of man derived its name from its arching course from the base of the frontal lobe to the temporal lobe. Because of the more caudal position of the amygdala in the rabbit, the comparable fasciculus passed directly caudally and exhibited only slight arching. This fasciculus in the rabbit had the typical dorsal and ventral parts. The dorsal part arose from the orbitofrontal cortex to distribute to the pyriform and the temporal lobe cortices. The ventral portion extended into the olfactory tuberculum and the anterior amygdaloid area. 5. The paraventricular component of the transverse frontal fasciculus interconnected the neopallium with the medial part of the olfactory tuberculum. It had origins in the frontal and possibly in other neocortical areas. 6. The cingulum interconnected the medial portion of the olfactory tubercle, the septum, the various cingulate areas and areas of the neopallium with each other. 7. Therefore, the New Zealand white rabbit had short association fibers which were mainly neopallial in origin and termination and long association fibers which had both a neopallial and a limbic component.     

Association and commissural fiber systems of the olfactory cortex of the rat. I. Systems originating in the piriform cortex and adjacent areas

The association and commissural fiber systems arising in the olfactory cortical areas caudal to the olfactory peduncle (the piriform cortex, nucleus of the lateral olfactory tract, anterior cortical nucleus of the amygdala, periamygdaloid cortex and entorhinal cortex) have been studied utilizing horseradish peroxidase as both an anterograde and a retrograde axonal tracer. In the piriform cortex two sublaminae within layer II (IIa and IIb) and layer III have been found to give rise to distinctly different projections. Retrograde cell labeling experiments indicate that the association fiber projection from layer IIb is predominantly caudally directed, while the projection from layer III is predominantly rostrally directed. Cells in layer IIa project heavily to areas both caudal and rostral to the piriform cortex. The commissural fibers from the piriform cortex are largely restricted in their origin to layer IIb of the anterior part of the piriform cortex and in their termination on the contralateral side to the posterior part of the piriform cortex and adjacent olfactory cortical areas. A projection to the olfactory bulb has also been found to arise from cells in layers IIb and III of the ipsilateral piriform cortex, but not in layer IIa. In addition to those from the piriform cortex, association projections have also been found from other olfactory cortical areas. The nucleus of the lateral olfactory tract has a heavy bilateral projection to the medial part of the anterior piriform cortex and the lateral part of the olfactory tubercle (as well as a lighter projection to the olfactory bulb); both the anterior cortical nucleus of the amygdala and the periamygdaloid cortex project ipsilaterally to several olfactory cortical areas. The entorhinal cortex has been found to project to the medial parts of the olfactory tubercle and the olfactory peduncle. The olfactory tubercle is the only olfactory cortical area from which no association fiber systems (instrinsic or extrinsic) have been found to originate. A broad topographic organization exists in the distribution of the fibers from several of the olfactory areas. This is most obvious in the anterior part of the olfactory cortex, in which fibers from the more rostral areas (the anterior olfactory nucleus and the anterior piriform cortex) terminate in regions near the lateral olfactory tract, while those from more caudal areas (the posterior piriform cortex and the entorhinal cortex) terminate in areas further removed, both laterally and medially, from the tract. Projections to olfactory areas from the hypothalamus, thalamus, diagonal band, and biogenic amine cell groups have been briefly described.     

On the scent of human olfactory orbitofrontal cortex: Meta-analysis and comparison to non-human primates

It is widely accepted that the orbitofrontal cortex (OFC) represents the main neocortical target of primary olfactory cortex. In non-human primates, the olfactory neocortex is situated along the basal surface of the caudal frontal lobes, encompassing agranular and dysgranular OFC medially and agranular insula laterally, where this latter structure wraps onto the posterior orbital surface. Direct afferent inputs arrive from most primary olfactory areas, including piriform cortex, amygdala, and entorhinal cortex, in the absence of an obligatory thalamic relay. While such findings are almost exclusively derived from animal data, recent cytoarchitectonic studies indicate a close anatomical correspondence between non-human primate and human OFC. Given this cross-species conservation of structure, it has generally been presumed that the olfactory projection area in human OFC occupies the same posterior portions of OFC as seen in non-human primates. This review questions this assumption by providing a critical survey of the localization of primate and human olfactory neocortex. Based on a meta-analysis of human functional neuroimaging studies, the region of human OFC showing the greatest olfactory responsivity appears substantially rostral and in a different cytoarchitectural area than the orbital olfactory regions as defined in the monkey. While this anatomical discrepancy may principally arise from methodological differences across species, these results have implications for the interpretation of prior human lesion and neuroimaging studies and suggest constraints upon functional extrapolations from animal data.     

Contralateral projections of the rat anterior olfactory nucleus

The anterior olfactory nucleus (AON) is a central olfactory cortical structure that has heavy reciprocal connections with both the olfactory bulb (OB) and piriform cortex. While it has been firmly established that the AON is a primary source of bilateral projections in the olfactory system through extensive connections with both the ipsilateral and contralateral OB, AON, and piriform cortex, few studies have examined this circuitry in detail. In the present study we used small injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) and the retrograde tracer FluoroGold in specific subregions of the AON to explore the topography of the interconnections between the left and right AONs. Labeled fibers were found in the contralateral AON following injections in all areas. However, detailed quantitative analyses revealed that different regions of the AON have distinct patterns of interhemispheric innervation; contralateral fibers were most heavily targeted to dorsal and lateral AON subregions, while the medial and ventral areas received relatively light projections. These results demonstrate important features of the interhemispheric circuitry of the AON and suggest separate functional roles for subregions of the AON in olfactory information processing.     

Topographical organization of the inferior collicular projection and other connections of the ventral nucleus of the lateral lemniscus in the cat

The topographic distribution of projections from the ventral nucleus of the lateral lemniscus (VNLL) in the cat was investigated with the autoradiographic tracing method. The origin of minor projections was verified by retrograde tracing methods. Small injections of tritiated leucine were placed in restricted zones of VNLL. A major afferent fiber system to the inferior colliculus was labeled in all cases. From the injection site labeled fibers coursed through and around the nuclei of the lateral lemniscus to enter the ipsilateral inferior colliculus. Regardless of the position or small size of the injection, labeled fibers distributed widely in the inferior colliculus. Fibers ended in the central nucleus and deeper layers of the dorsal cortex in most cases. There was also labeling in the ventrolateral nucleus, but very few fibers ended as lateral as the lateral nucleus. A small number of labeled fibers passed from the inferior colliculus into the nucleus of the brachium of the inferior colliculus and adjacent tegmental areas. Some labeled fibers entered the commissure of the inferior colliculus where they were traced into the dorsal cortex and rostral pole of the inferior colliculus on the side contralateral to the injection site. Though the projections labeled in individual cases were similar in their divergent pattern within the central nucleus of the inferior colliculus, specific variations in the pattern were found. The dorsal zone of VNLL projected more heavily to the deeper layers of the dorsal cortex and an adjacent field in the central nucleus than the other zones. Dorsal injections in the middle zone of VNLL, on the other hand, labeled the medial part of the central nucleus more heavily, whereas ventral injections in the middle zone resulted in heavier lateral labeling. The ventral zone of VNLL projected heavily to a central field in the central nucleus. In addition to this major afferent system of VNLL to the inferior colliculus, a smaller descending projection was found. The descending projection ended mainly in the dorsomedial periolivary region and ventral nucleus of the trapezoid body. However, in some cases a few fibers were traced to the cochlear nuclei. Finally, we observed projections to the medial geniculate body from the dorsal and ventral zones of VNLL that ended diffusely in the medial division of the medial geniculate body. Possibly some fibers from the dorsal zone contribute to a broader projection of the lateral tegmentum to the dorsal division of the medial geniculate body.     

The laminar distribution of intracortical fibers originating in the olfactory cortex of the rat

In this study, the autoradiographic method for tracing axonal connections was used to identify the laminar distribution of intracortical fibers originating in the olfactory cortical areas of the rat. Most of the projections can be divided into two major fiber systems with different laminar patterns of termination. The first of these, termed the layer Ib fiber system, arises in the anterior olfactory nucleus, the anterior and posterior piriform cortex, and the lateral entorhinal cortex, and terminates predominantly in layer Ib and, in many cases, layer III of the entire olfactory cortex. The second system, termed the layer II-deep Ib fiber system, originates in three relatively small olfactory cortical areas-the dorsal peduncular cortex, the ventral tenia tecta, and the periamygdaloid cortex and terminates in and around the cells of layer II in most parts of the olfactory cortex. There is significant overlap in the laminar distribution of the two systems, although the distinction between them is readily apparent. Within the layer Ib fiber system there are relatively slight but consistent differences in the lamination of fibers from different areas. The fibers from the anterior olfactory nucleus are concentrated in the deep part of layer Ib while those from the anterior piriform cortex are concentrated in the superficial part of this layer. The fibers from the posterior piriform cortex tend to be densest in the middle of layer Ib. These differences are maintained in all areas of termination of each set of fibers, both ipsilaterally and contra-laterally. In addition, intracortical fibers from the anterior cortical nucleus of the amygdala are distributed throughout layer I, including layer la and Ib. Fibers from the nucleus of the lateral olfactory tract terminate bilaterally around the cells of the islands of Callej a and the medial edge of the anterior piriform cortex.     

Projection from nucleus reuniens thalami to piriform cortex: A tracing study in the rat

To study the cells of origin and area of termination of the projection from the nucleus reuniens thalami (NRe) to the piriform cortex (PC) we used anterograde and retrograde tracing with the B subunit of the cholera toxin. Tracer injections in the NRe resulted in anterogradely labeled fibers in the dorsolateral part of the PC layers I and III. Following injections in the PC, retrogradely labeled cells were observed primarily in the dorsal subdivision of the NRe. Moreover, a topographical organization was observed in this subdivision: its anterior part projects to the posterior part of the PC, whereas its middle part projects to the anterior part of the PC. The present findings suggest that the NRe may exert different modulatory influences on the dorsolateral part of both anterior and posterior PC areas. The possible role of the NRe in the olfactory information processing is discussed.     

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.     

Cells of origin,course, and termination patterns of the ventral,uncrossed component of the mature rat corticospinal tract

The cells of origin, the course, and termination patterns of the ventral, uncrossed component of the rat corticospinal tract (CST) was investigated by using retrograde and anterograde tracing methods. Anterograde tracing with biotin dextran-amine (BDA) revealed the position and detailed morphology of CST fibers in the spinal cord. Cross sections on spinal levels C4, T8, and L4 showed labeled fibers in the ipsilateral ventral funiculus on all levels. Although ipsilateral ventral CST fibers run close to the midline in the cervical cord, they were found to disperse more in the ventromedial funiculus at lower spinal levels. To study the termination patterns of the ipsilateral ventral projection, a dorsal spinal cord hemisection was performed at level T8, severing the crossed dorsomedial and dorsolateral components but leaving ipsilateral ventral running fibers intact. These fibers were observed to have sometimes several collaterals with terminal arbors extending into different spinal segments, innervating mostly laminae III–VI. Structures closely resembling synaptic boutons were identified in these arbors. By retrograde tracing in animals with dorsal spinal cord hemisection, we found labeled cells equally distributed throughout the spinally projecting cortical areas corresponding to the level of tracer injection. Labeled cells were found in layer V. The diameter of the labeled cells was not significantly different from other spinally projecting cortical neurons. In summary, a neuroanatomically complete ipsilateral, ventral corticospinal projection down to low spinal levels was found. The large extension of the terminal arborizations in intermediate laminae of the spinal cord suggests a modulatory role of this CST component. J. Comp. Neurol. 386:293–303, 1997. © 1997 Wiley-Liss, Inc.     

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)     

Cytoarchitecture and neural afferents of orbitofrontal cortex in the brain of the monkey.

The orbitofrontal cortex of the monkey can be subdivided into a caudal agranular sector, a transitional dysgranular sector, and an anterior granular sector. The neural input into these sectors was investigated with the help of large horseradish peroxidase injections that covered the different sectors of orbitofrontal cortex. The distribution of retrograde labeling showed that the majority of the cortical projections to orbitofrontal cortex arises from a restricted set of telencephalic sources, which include prefrontal cortex, lateral, and inferomedial temporal cortex, the temporal pole, cingulate gyrus, insula, entorhinal cortex, hippocampus, amygdala, and claustrum. The posterior portion of the orbitofrontal cortex receives additional input from the piriform cortex and the anterolateral portion from gustatory, somatosensory, and premotor areas. Thalamic projections to the orbitofrontal cortex arise from midline and intralaminar nuclei, from the anteromedial nucleus, the medial dorsal nucleus, and the pulvinar nucleus. Orbitofrontal cortex also receives projections from the hypothalamus, nucleus basalis, ventral tegmental area, the raphe nuclei, the nucleus locus coeruleus, and scattered neurons of the pontomesencephalic tegmentum. The non-isocortical (agranular-dysgranular) sectors of orbitofrontal cortex receive more intense projections from the non-isocortical sectors of paralimbic areas, the hippocampus, amygdala, and midline thalamic nuclei, whereas the isocortical (granular) sector receives more intense projections from the dorsolateral prefrontal area, the granular insula, granular temporopolar cortex, posterolateral temporal cortex, and from the medial dorsal and pulvinar thalamic nuclei. Retrograde labeling within cingulate, entorhinal, and hippocampal cortices was most pronounced when the injection site extended medially into the dysgranular paraolfactory cortex of the gyrus rectus, an area that can be conceptualized as an orbitofrontal extension of the cingulate complex. These observations demonstrate that the orbitofrontal cortex has cytoarchitectonically organized projections and that it provides a convergence zone for afferents from heteromodal association and limbic areas. The diverse connections of orbitofrontal cortex are in keeping with the participation of this region in visceral, gustatory, and olfactory functions and with its importance in memory, motivation, and epileptogenesis.     

Efferent connections of the lateral cortex of the lizard gekko gecko: Evidence for separate origins of medial and lateral pathways from the lateral cortex to the hypothalamus

The lateral cortex of the lizard Gekko gecko is composed of three parts: a dorsal and ventral part located rostrally and a posterior part located caudally. In order to obtain detailed information about the efferent connections of these lateral cortex subdivisions, iontophoretic injections of the anterograde tracers Phaseolus vulgaris leucoagglutinin and biotinylated dextran were made in the various parts. The main projection from the dorsal part terminates in the caudal part of the medial cortex. Other cortical projections were noted to the ipsi- and contralateral lateral cortex, the large-celled part of the medial cortex, and the dorsal cortex. Additional fibers were found bilaterally in the anterior olfactory nucleus and the external amygdaloid nucleus. The ventral part of the lateral cortex projects mainly to the ipsilateral, posterior part of the dorsal ventricular ridge and the external amygdaloid nucleus. Minor contralateral projections to these nuclei were also found. Other projections were observed to travel to the caudal part of the medial cortex, to the nucleus sphericus, and bilaterally to the lateral cortex and the anterior olfactory nucleus. The posterior part of the lateral cortex has similar efferent connections as the dorsal part and should be regarded as the caudal continuation of the dorsal part. Because previous studies have shown that the medial cortex and the amygdaloid complex project to different hypothalamic areas, we conclude that the dorsal and ventral parts of the lateral cortex transmit olfactory information to separate hypothalamic areas that are probably involved with different types of behavior. © 1995 Wiley-Liss, Inc.     

Basal forebrain efferents to the medial dorsal thalamic nucleus in the rhesus monkey

Thalamic efferent connections of the basal forebrain (BF); medial septal nucleus (MS), vertical limb of the diagonal band (VDB), horizontal limb of the diagonal band (HDB), nucleus basalis (NB), and ventral pallidum (VP) were investigated in twelve rhesus monkeys. In five animals, injections of radioactively labeled amino acids were placed in the BF. In four animals, the injections involved different divisions of the NB, HDB, and the most ventral part of the VDB. In those four cases, labeled fibers in the medial forebrain bundle were observed traveling caudally towards the hypothalamus where some turned dorsally to enter the inferior thalamic peduncle. These fibers terminated in the ventral half of the magnocellular part of the medial dorsal thalamic nucleus (MDmc). In a fifth case, the amino acid injection involved most of the MS and the VDB. Labeled fibers traveled caudally from the injection site and entered the stria medullaris. These fibers then traveled caudally before turning ventrally to terminate in the dorsal half of MDmc. To determine which of the diverse neuronal types in the BF gives rise to these thalamic projections, in two monkeys injections of horseradish peroxidase (HRP) were placed into MDmc. Labeled neurons were observed throughout the full extent of the NB, the VDB, the MS, and part of the VP. In order to determine the extent of the cholinergic input to MDmc from the BF, one of the HRP cases was processed for the simultaneous visualization of HRP, and acetylcholinesterase (AChE), the hydrolytic enzyme for acetylcholine, and a second case was processed for simultaneous visualization of HRP, and choline acetyltransferase (ChAT), the synthetic enzyme for acetylcholine. We observed that 30-50% of the HRP-labeled neurons were putatively cholinergic. In order to determine if the NB projection to MD is a collateral of the NB projection to orbital frontal cortex, one fluorescent retrograde tracer was injected into the orbital frontal cortex and one into MD. This case showed that approximately 5% of the BF neurons that project to MDmc also project to the orbital frontal cortex. These results confirm a significant subcortical projection by which the cholinergic system of the basal forebrain may influence higher cortical functions through the thalamus.     

The striato-entopeduncular pathway in the rat. A retrograde transport study with wheatgerm-agglutinin-horseradish peroxidase

The projection from the caudate-putamen to the entopeduncular nucleus was studied in rats using the wheatgerm-agglutinin--horseradish peroxidase (WGA-HRP) tracing technique. After iontophoretic injections of WGA-HRP into subregions of the entopeduncular nucleus, numerous HRP-labeled cells were found in the ipsilateral caudate-putamen. In all cases, the labeled neurons were localized in a restricted part of the caudate-putamen rostral to the cross-over of the anterior commissure. Labeled cells in the nucleus accumbens or the olfactory tubercle were never observed. The striato-entopeduncular projection was topographically organized: dorsal caudate-putamen projects to the dorsal part of the entopeduncular nucleus and ventral caudate-putamen to the ventral part. These results demonstrate that the striato-entopeduncular pathway is topographically organized as are a number of other neuronal circuits of the basal ganglia system.     

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.     

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.