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.     

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.     

The development of axonal connections in the central olfactory system of rats

The development of the cytoarchitecture and axonal connections of the central olfactory system were studied in fetal and neonatal rats from E16. In contrast to neocortical development, the olfactory cortex lacks a distinct cortical plate. In the piriform cortex and the olfactory tubercle the cellular laminae emerge simultaneously, while in the anterior olfactory nucleus, there are morphogenetic gradients from superficial to deep as well as from caudal to rostral which parallel the known cytogenetic gradients. Parallel morphogenetic and cytogenetic gradients are also present in the lateral to medial axis of the olfactory tubercle. The projection from the olfactory bulb and the associational projections from the piriform cortex begin to develop well before birth. At E17 fibers from the bulb are limited to the lateral olfactory tract (LOT) and the molecular layer just deep to it, and then spread out caudally, laterally, and medially away from the LOT. This sequence of innervation parallels and predicts the density of innervation in the adult: those areas which are innervated first (such as the piriform cortex deep to the LOT) ultimately receive the heaviest innervation; conversely, those areas which are innervated very late (such as the medial olfactory tubercle) receive the lightest projection. The intracortical projections from the anterior and posterior piriform cortex extend into layer I ipsilaterally by E20 and obtain their adult distribution by the middle of the first postnatal week. On the other hand, fibers from the anterior olfactory nucleus and the entorhinal area do not reach their full adult extent until the second postnatal week. Similarly, the crossed projection of the anterior piriform cortex to the contralateral posterior piriform cortex does not grow into layer I until this later time. The timing of fiber ingrowth showed no relation to the trajectory or eventual areal or laminar termination of fibers. As with the olfactory bulb projection, the timing may influence the density of termination. Centrifugal fibers to the bulb are demonstrable around the time of birth both by the retrograde transport of horseradish peroxidase (HRP) and by the anterograde transport of 3H-leucine. The arrival of additional fibers during the remainder of the first postnatal week parallels the known cytogenetic and morphogenetic gradients in the areas in which they arise. The projections of the olfactory cortex to the lateral hypothalamic area and the mediodorsal thalamic nucleus are evident before birth. This correlates with the early generation of the cells which give rise to these projections.     

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.     

Connections of the olfactory bulb and nucleus olfactorius anterior in the hedgehog (Erinaceus europaeus): fluorescent tracers and HRP study

The projections of the main olfactory bulbs (MOBs) and the dorsal part of the anterior olfactory nucleus (NOA) in the hedgehog (Erinaceus europaeus) have been studied by fluorescent tracers and the horseradish peroxidase method (HRP), respectively, to reveal the pattern of labeling from these structures. After different dye injections in both MOBs, labeled cells were present in the following structures: tenia tecta, vertical limb of the diagonal band of Broca, and medial septal nucleus in the ipsilateral injection site; and the NOA, piriform cortex, nucleus of the lateral olfactory tract, horizontal limb of the diagonal band of Broca, posterolateral cortical amygdaloid nucleus, anterior amygdaloid area, and dorsal raphe nucleus in both hemispheres. Structures showing double-labeled cells were the NOA, horizontal limb of the diagonal band of Broca, nucleus of the lateral olfactory tract, anterior amygdaloid area, and posterolateral cortical amygdaloid nucleus. After HRP injections in the dorsal part of the NOA, labeled cells were distributed in the NOA, nucleus of the lateral olfactory tract, posterolateral cortical amygdaloid nucleus, piriform cortex, horizontal and vertical limbs of the diagonal band of Broca, mitral cell layer of the MOB, tenia tecta, anterior amygdaloid area, and the contralateral NOA. We suggest that the contralateral projection nuclei to the MOB of the hedgehog, unusual in other mammals, and the large number of cells with axonal collaterals projecting to both hemispheres, may be a strategy in these animals to bilaterally integrate brain functions at the expense of its reduced corpus callosum.     

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.     

The development of lamination of afferent fibers to the olfactory cortex in rats, with additional observations in the adult

The complementary distribution of the fibers from the olfactory bulb and the intracortical associational fibers to layers Ia and Ib, respectively, of the olfactory cortex has been examined in both adult and neonatal rats, using horseradish peroxidase (HRP) and 3H-leucine as double tracers in the same animal. The observations presented here confirm and extend the previous demonstration (Price, '73) that in the adult the two projections are essentially nonoverlapping throughout the olfactory cortex. Indeed, when the distribution of axons from the olfactory bulb (labeled by HRP inserted into a cut in the LOT) is compared on the same section with that of associational fibers (labeled by 3H-leucine injected into the cortex), the overlap between the two projections is limited to a zone only 5-10 micron in width in both the piriform cortex and olfactory tubercle. In contrast, at P1 the two projections overlap throughout layer I, although the bulbar and associational fibers are slightly concentrated superficially and deeply in layer I, respectively. This overlap is especially prominent in the part of the anterior piriform cortex deep to the LOT. During the remainder of the first postnatal week, this overlap resolves and by P7 the segregation of the two sets of afferent fibers is nearly equivalent to that seen in the adult. However, there are several instances in adults where the segregation of these afferents does not develop. First, a relatively small population of aberrant axons derived from the LOT may be traced from layer Ia into layer Ib and then back to layer Ia. Most of these axons are large in diameter and lack the boutonlike varicosities found on smaller axons in layer Ia. They are most prominent in areas where the cortex is highly curved. Second, in layer I of the nucleus of the lateral olfactory tract, bulbar and associational fibers are extensively intermingled. In this case also, the bulbar fibers are large in diameter with only a few boutonlike varicosities. The developmental emergence of afferent segregation and its breakdown in cases where the fibers from the olfactory bulb do not form boutons suggest that an interaction between the two distinct sets of fibers and the dendritic field is responsible for the normal development of this segregation and that this interaction depends on the process of synaptogenesis.     

An autoradiographic study of complementary laminar patterns of termination of afferent fibers to the olfactory cortex

The afferent projections to the olfactory cortical areas from the olfactory bulb and the prepiriform cortex have been studied in the rat, using the autoradiographic method for demonstrating axonal connections. In order to relate closely the results of these experiments to the structure of the olfactory cortical areas, the cytoarchitectonic characteristics of these areas have also been described. The olfactory cortical areas, which receive a direct input from the olfactory bulb, include the anterior olfactory nucleus, the ventral portion of the tenia tects, the olfactory tubercle, the prepiriform cortex, the nucleus of the lateral olfactory tract, the cortical amygdaloid nucleus, and the lateral entorhinal area. All of these areas are composed primarily of pyramidal cells and have three basic layers: a superficial plexiform layer containing the apical dendrites of the pyramidal cells (layer I), a pyramidal cell layer (layer II), and a deeper polymorphic cellular layer (layer III). In each area layer I may be divided into a superficial portion (IA) and a deeper portion (IB). The autoradiographic experiments have shown that all of the olfactory cortical areas receive projections from the prepiriform cortex as well as from the olfactory bulb, and that these two projections have complementary laminar patterns of termination which are the same in every area. Throughout the olfactory cortex the fibers from the olfactory bulb terminate exclusively in layer IA, in relation to the distal segments of the apical dendrite of the pyramidal cells, whereas the fibers from the prepiriform cortex terminate in layer IB, in relation to more proximal segments of the apical dendrites, and also in layer III. The boundary between the two projections within layer I is very sharp, with minimal overlap. In contrast to this precise laminar organization, there is little evidence for a topographical organization within these Projections.     

Projections from the amygdaloid complex to the piriform cortex: A PHA-L study in the rat

Projections from the amygdala to the piriform cortex are proposed to provide a pathway via which the emotional system can modulate the processing of olfactory information as well as mediate the spread of seizure activity in epilepsy. To understand the details of the distribution and topography of these projections, we injected the anterograde tracer Phaseolus vulgaris-leucoagglutinin into different nuclear divisions of the amygdaloid complex in 101 rats and analyzed the distribution and density of projections in immunohistochemically processed preparations. The heaviest projections from the amygdala to the piriform cortex originated in the medial division of the lateral nucleus, the periamygdaloid and sulcal subfields of the periamygdaloid cortex, and the posterior cortical nucleus. The heaviest terminal labeling was observed in layers Ib and III of the medial aspect of the posterior piriform cortex. Lighter projections to the posterior piriform cortex originated in the dorsolateral division of the lateral nucleus, the magnocellular and parvicellular divisions of the basal and accessory basal nuclei, and the anterior cortical nucleus. The projections to the anterior piriform cortex were light and originated in the dorsolateral and medial divisions of the lateral nucleus, the magnocellular division of the basal and accessory basal nuclei, the anterior and posterior cortical nuclei, and the periamygdaloid subfield of the periamygdaloid cortex. The results indicate that only selective amygdaloid nuclei or their subdivisions project to the piriform cortex. In addition, substantial projections from several amygdaloid nuclei converge in the medial aspect of the posterior piriform cortex. Via these projections, the amygdaloid complex can modulate the processing of olfactory information in the piriform cortex. In pathologic conditions such as epilepsy, these connections might provide pathways for the spread of seizure activity from the amygdala to extra-amygdaloid regions.     

Efferent and afferent connections of the olfactory bulb and prepiriform cortex in the pigeon (Columba livia)

Although olfaction in birds is known to be involved in a variety of behaviors, there is comparatively little detailed information on the olfactory brain. In the pigeon brain, the olfactory bulb (OB) is known to project to the prepiriform cortex (CPP), piriform cortex (CPi), and dorsolateral corticoid area (CDL), which together are called the olfactory pallium, but centrifugal pathways to the OB have not been fully explored. Fiber connections of CPi and CDL have been reported, but those of other olfactory pallial nuclei remain unknown. The present study examines the fiber connections of OB and CPP in pigeons to provide a more detailed picture of their connections using tract‐tracing methods. When anterograde and retrograde tracers were injected in OB, projections to a more extensive olfactory pallium were revealed, including the anterior olfactory nucleus, CPP, densocellular part of the hyperpallium, tenia tecta, hippocampal continuation, CPi, and CDL. OB projected commissural fibers to the contralateral OB but did not receive afferents from the contralateral olfactory pallium. When tracers were injected in CPP, reciprocal ipsilateral connections with OB and nuclei of the olfactory pallium were observed, and CPP projected to the caudolateral nidopallium and the limbic system, including the hippocampal formation, septum, lateral hypothalamic nucleus, and lateral mammillary nucleus. These results show that the connections of OB have a wider distribution throughout the olfactory pallium than previously thought and that CPP provides a centrifugal projection to the OB and acts as a relay station to the limbic system. J. Comp. Neurol. 522:1728–1752, 2014. © 2013 Wiley Periodicals, Inc.     

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.     

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.     

Bulbar projecting subcortical GABAergic neurons send collateral branches extensively and selectively to primary olfactory cortical regions

Circuit operations of the olfactory bulb are modulated by higher order projections from multiple regions, many of which are themselves targets of bulbar output. Multiple glutamatergic regions project to the olfactory bulb, including the anterior olfactory nucleus (AON), prefrontal cortex (PFC), piriform cortex (PC), entorhinal cortex (EC), and tenia tecta (TT). In contrast, only one region provides GABAergic projections to the bulb. These GABA neurons are located in the horizontal limb of the diagonal band of Broca extending posteriorly through the magnocellular preoptic nucleus to the nucleus of the lateral olfactory bulb. However, it was unclear whether bulbar projecting GABAergic neurons collaterallize projecting to other brain regions. To address this, we mapped collateral projections from bulbar projecting GABAergic neurons using intersectional strategies of viral and traditional tract tracers. This approach revealed bulbar projecting GABAergic neurons show remarkable specificity targeting other primary olfactory cortical regions exhibiting abundant collateral projections into the accessory olfactory bulb, AON, PFC, PC, and TT. The only "nonolfactory" region receiving collateral projections was sparse connectivity to the medial prefrontal orbital cortex. This suggests that basal forebrain inhibitory feedback also modulates glutamatergic feedback areas that are themselves prominent bulbar projection regions. Thus, inhibitory feedback may be simultaneously modulating both synaptic processing of olfactory information in the bulb and associational processing of olfactory information from primary olfactory cortex. We hypothesize that these olfactory GABAergic feedback neurons are a regulator of the entire olfactory system.     

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 mouse olfactory peduncle

The olfactory peduncle, the region connecting the olfactory bulb with the basal forebrain, contains several neural areas that have received relatively little attention. The present work includes studies that provide an overview of the region in the mouse. An analysis of cell soma size in pars principalis (pP) of the anterior olfactory nucleus (AON) revealed considerable differences in tissue organization between mice and rats. An unbiased stereological study of neuron number in the cell-dense regions of pars externa (pE) and pP of the AON of 3-, 12-, and 24-month-old mice indicated that pE has about 16,500 cells in 0.043 mm(3) and pP about 58,300 cells in 0.307 mm(3) . Quantitative Golgi studies of pyramidal neurons in pP suggested that mouse neurons are similar to although smaller than those of the rat. An immunohistochemical analysis demonstrated that all peduncular regions (pE, pP, the dorsal peduncular cortex, ventral tenia tecta, and anterior olfactory tubercle and piriform cortex) have cells that express either calbindin, calretinin, parvalbumin, somatostatin, vasoactive intestinal polypeptide, neuropeptide Y, or cholecystokinin (antigens commonly co-expressed by subspecies of γ-aminobutyric acid [GABA]ergic neurons), although the relative numbers of each cell type differ between zones. Finally, an electron microscopic comparison of the organization of myelinated fibers in lateral olfactory tract in the anterior and posterior peduncle indicated that the region is less orderly in mice than in rats. The results provide a caveat for investigators who generalize data between species, as both similarities and differences between the laboratory mouse and rat were observed.     

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.     

Intrinsic association fiber system of the piriform cortex: A quantitative study based on a cholera toxin B subunit tracing in the rat

By using retrograde and anterograde transport of the B subunit of cholera toxin (CTb), we examined quantitatively the association fiber systems, i.e., the collaterals of pyramidal cell axons, that reciprocally connect both the rostral and the caudal parts of the piriform cortex (PC). Well-defined CTb injections were obtained in layers Ib or II-III of the rostral and the caudal parts of the PC. Using precision counting, we determined the proportion of cellular profiles in layers II and III that gave rise to association fibers and thus demonstrated a predominance of rostrocaudal fibers over the caudorostral ones. Our data also support a precise laminar organization of the PC in which the rostrocaudal fibers originated mainly from layer II and the caudorostral fibers primarily from layer III. Cholera toxin injections into layer Ib produced a peak of labeled profiles 2 mm from the site, indicating that a large proportion of the association fibers from layer II travel for at least 2 mm and then synapse in layer Ib. At either end of the PC, the association projections are concentrated laterally. The functional significance of these anatomical features is discussed with respect to olfactory processing, propagation of the activity within the PC, and the possible role of intrinsic fibers in olfactory memory. © 1996 Wiley-Liss, Inc.     

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.     

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.