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.     

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.     

Topographical organization of the efferent projections of the medial prefrontal cortex in the rat: an anterograde tract-tracing study with Phaseolus vulgaris leucoagglutinin

The purpose of the present investigation was to examine the topographical organization of efferent projections from the cytoarchitectonic divisions of the mPFC (the medial precentral, dorsal anterior cingulate and prelimbic cortices). We also sought to determine whether the efferents from different regions within the prelimbic division were organized topographically. Anterograde transport of Phaseolus vulgaris leucoagglutinin was used to examine the efferent projections from restricted injection sites within the mPFC. Major targets of the prelimbic area were found to include prefrontal, cingulate, and perirhinal cortical structures, the dorsomedial and ventral striatum, basal forebrain nuclei, basolateral amygdala, lateral hypothalamus, mediodorsal, midline and intralaminar thalamic nuclei, periaqueductal gray region, ventral midbrain tegmentum, laterodorsal tegmental nucleus, and raphe nuclei. Previously unreported projections of the prelimbic region were also observed, including efferents to the anterior olfactory nucleus, the piriform cortex, and the pedunculopontine tegmental-cuneiform region. A topographical organization governed the efferent projections from the prelimbic area, such that the position of terminal fields within target structures was determined by the rostrocaudal, dorsoventral, and mediolateral placement of the injection sites. Efferent projections from the medial precentral and dorsal anterior cingulate divisions (dorsomedial PFC) were organized in a similar topographical fashion and produced a pattern of anterograde labeling different from that seen with prelimbic injection sites. Target structures innervated primarily by the dorsomedial PFC included certain neocortical fields (the motor, somatosensory, and visual cortices), the dorsolateral striatum, superior colliculus, deep mesencephalic nucleus, and the pontine and medullary reticular formation. Previously unreported projections to the paraoculomotor central gray area and the mesencephalic trigeminal nucleus were observed following dorsomedial PFC injections. These results indicate that the efferent projections of the mPFC are topographically organized within and across the cytoarchitectonic divisions of the medial wall cortex. The significance of topographically organized and restricted projections of the rat mPFC is discussed in light of behavioral and physiological studies indicating functional heterogeneity of this region.     

Bursting-induced epileptiform EPSPs in slices of piriform cortex are generated by deep cells.

Previous study revealed that bursting activity generated by a variety of means in slices of piriform cortex induces persistent epileptiform EPSPs in superficial pyramidal cells by an NMDA-dependent process. The present study was undertaken to test the hypothesis that the observed epileptiform EPSPs in superficial pyramidal cells are driven by deep cells. This hypothesis was suggested by recent findings from in vitro studies of the properties of deep cells and in vivo studies indicating that the deep part of the piriform cortex or neighboring deep structures are involved in the generation of seizure activity in animal models of epilepsy. Results from simultaneous cell-pair recordings, examination of subdivided slices, and local application of excitatory and inhibitory agents provided strong evidence in support of this hypothesis. It was concluded that the endopiriform nucleus, a collection of cells immediately deep to the piriform cortex, plays a central role in generation, but that cells in the deep part of layer III and the claustrum may also contribute. Furthermore, it was found that generation of prolonged ictal-like activity only occurs in slices of piriform cortex in which the endopiriform nucleus is present. Implications of these findings for epileptogenesis are discussed.     

Connections of the auditory cortex with the claustrum and the endopiriform nucleus in the cat

We studied the connections of eleven auditory cortical areas with the claustrum and the endopiriform nucleus in the cat, by means of cortical injections of either wheat germ agglutinin conjugated to horseradish peroxidase, or biotinylated dextran amines. Unlike previously accepted reports, all auditory areas have reciprocal connections with the ipsi- and contralateral claustrum, though they differ in strength and/or topography. The areas that send the strongest projections are the intermediate region of the posterior ectosylvian gyrus and the insular cortex, followed by the primary auditory cortex and the dorsal portion of the posterior ectosylvian gyrus. The high degree of convergence of cortical axons in the intermediate region of the claustrum, arising from tonotopic and nontonotopic areas, suggests that claustral neurons are unlikely to be well tuned to the frequency of the acoustic stimulus. Corticoclaustral axons from any given area cover territories largely overlapping with those occupied by the claustrocortical neurons projecting back to the same area. The location of cortically projecting neurons in the claustrum matches the position of the target cortical area in the cerebral hemisphere, both rostrocaudally and dorsoventrally. These findings suggest that the intermediate region of the claustrum integrates inputs from all auditory cortical areas, and then sends the result of such processing back to every auditory cortical field. On the other hand, the endopiriform nucleus, a limbic-related structure thought to play a role in the acquisition of conditioned fear, would process mostly polymodal information, since it only receives projections from the insular and temporal cortices.     

Areal and laminar distribution of some pulvinar cortical efferents in rhesus monkey

The areal and laminar distribution of the cortical efferents of the medial, lateral and inferior pulvinar nuclei (PM, PL and PI respectively) were determined in rhesus monkey using autoradiography and Horseradish Peroxidase (HRP). The autoradiographic data indicated that: areas 8a, 45 and 46 on the convexity and 11 and 12 on the orbital surface of the frontal lobe received projections from PM; areas 20, 21 and 22 in temporal lobe received projections from PM primarily with caudal-medial parts of PM projecting to more rostral-dorsal parts of temporal lobe and rostral-lateral parts of PM projecting to more caudal-ventral parts of temporal lobe but PL also sends some efferents to caudal temporal lobe; areas 5 and 7 in parietal lobe and 18 and 19 in occipital lobe received projections primarily from the region in pulvinar comprising PL and PI with the more ventral parts of this region projecting to the ventral-lateral parts of occipital lobe and the more dorsal parts of this region projecting to the more dorsal-lateral and medial parts of parieto-occipital cortex and with PM contributing slightly to these projections rostrally. The autoradiographic information on the pulvinar projections to frontal lobe and temporal pole was supplemented by data derived from cortical HRP injections. These indicated that although only PM of the pulvinar subnuclei projected to these regions, three other caudal thalamic structures, i.e., medial dorsal nucleus, nucleus limitans and suprageniculate nucleus also projected to these regions raising some questions about the identity of the densocellular part of the medial dorsal nucleus which has also been considered to be part of pulvinar. The laminar distribution of pulvinar cortical efferents was uniformly similar regardless of the pulvinar recipient area examined. Elevated numbers of silver grains were observed over all cortical layers, but the silver grains were densest over the deep parts of layer III. The thalamic reticular nucleus was the only diencephalic structure observed to receive projections from pulvinar and it did so from PM, PL and PI. The pulvinar's efferents are to homotypical rather than heterotypical cortex and its connections are most extensive with cortex rather than with subcortical structures.     

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.     

Cortical efferents of the perirhinal,postrhinal, and entorhinal cortices of the rat

We investigated the cortical efferents of the parahippocampal region by placing injections of the anterograde tracers, Phaseolus vulgaris‐leuccoagglutinin, and biotinylated dextran amine, throughout the perirhinal (PER), postrhinal (POR), and entorhinal cortices of the rat brain. The resulting density of labeled fibers was evaluated in 25 subregions of the piriform, frontal, insular, temporal, cingulate, parietal, and occipital areas. The locations of labeled terminal fibers differed substantially depending on whether the location of the injection site was in PER area 35, PER area 36, POR, or the lateral or the medial entorhinal (LEA and MEA). The differences were greater for sensory regions. For example, the POR efferents preferentially target visual and spatial regions, whereas the PER efferents target all sensory modalities. The cortical efferents of each region largely reciprocate the cortical afferents, though the degree of reciprocity varied across originating and target regions. The laminar pattern of terminal fibers was consistent with the notion that the efferents are feedback projections. The density and amount of labeled fibers also differed substantially depending on the regional location of injection sites. PER area 36 and POR give rise to a greater number of heavy projections, followed by PER area 35. LEA also gives rise to widespread cortical efferents, arising mainly from a narrow band of cortex adjacent to the PER. In contrast, the remainder of the LEA and the MEA provides only weak efferents to cortical regions. Prior work has shown that nonspatial and spatial information is transmitted to the hippocampus via the PER‐LEA and POR‐MEA pathways, respectively. Our findings suggest that the return projections follow the same pathways, though perhaps with less segregration. © 2009 Wiley‐Liss, Inc.     

A new subdivision of anterior piriform cortex and associated deep nucleus with novel features of interest for olfaction and epilepsy

The anterior part of the piriform cortex (the APC) has been the focus of cortical-level studies of olfactory coding and associative processes and has attracted considerable attention as a result of a unique capacity to initiate generalized tonic-clonic seizures. Based on analysis of cytoarchitecture, connections, and immunocytochemical markers, a new subdivision of the APC and an associated deep nucleus are distinguished in the rat. As a result of its ventrorostral location in the APC, the new subdivision is termed the APC(VR). The deep nucleus is termed the pre-endopiriform nucleus (pEn) based on location and certain parallels to the endopiriform nucleus. The APC(VR) has unique features of interest for normal function: immunostaining suggests that it receives input from tufted cells in the olfactory bulb in addition to mitral cells, and it provides a heavy, rather selective projection from the piriform cortex to the ventrolateral orbital cortex (VLO), a prefrontal area where chemosensory, visual, and spatial information converges. The APC(VR) also has di- and tri-synaptic projections to the VLO via the pEn and the submedial thalamic nucleus. The pEn is of particular interest from a pathological standpoint because it corresponds in location to the physiologically defined "deep piriform cortex" ("area tempestas") from which convulsants initiate temporal lobe seizures, and blockade reduces ischemic damage to the hippocampus. Immunostaining revealed novel features of the pEn and APC(VR) that could alter excitability, including a near-absence of gamma-aminobutyric acid (GABA)ergic "cartridge" endings on axon initial segments, few cholecystokinin (CCK)-positive basket cells, and very low gamma-aminobutyric acid transporter-1 (GAT1)-like immunoreactivity. Normal functions of the APC(VR)-pEn may require a shaping of neuronal activity by inhibitory processes in a fashion that renders this region susceptible to pathological behavior.     

Amygdaloid connections with posterior insular and temporal cortical areas in the rat

The connections of the amygdala with the insular and temporal cortices were examined by injecting wheat germ agglutinin conjugated to HRP (WGA-HRP) into the rat cortex. Following injections into the posterior agranular insular area (AIp) or perirhinal cortex (PR), bands of labeled neurons extending across nuclear boundaries were observed in the amygdala. These neuronal bands involved cells in the lateral, basolateral, and basomedial nuclei as well as the periamygdaloid cortex. Other nuclei of the corticomedial amygdala and the ventral endopiriform nucleus also exhibited retrogradely labeled cells. Anterograde label was observed in nuclei containing labeled neurons and in the central nucleus. Injections into gustatory, somatosensory, and auditory neocortical areas located dorsal to AIp and PR labeled small numbers of cells in the lateral and basolateral nuclei. Injections into AIp, PR, and, to a lesser extent, dorsally adjacent neocortical areas produced both retrograde and anterograde labeling in the contralateral amygdala. The main nuclei with contralateral insular and temporal projections are the basomedial nucleus, ventral endopiriform nucleus, and nucleus of the lateral olfactory tract. The contralateral central nucleus and to a lesser extent the lateral nucleus exhibited anterograde labeling. The pattern of retrograde labeling seen with injections at different rostrocaudal levels of the AIp-PR continuum indicates that amygdalocortical projections to these areas exhibit an overlapping topographical organization. Comparison of the results of this study with findings on amygdaloprefrontal cortical efferents suggests that amygdaloid projections to the entire fronto-insulo-temporal mesocortical field are topographically organized.     

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.     

Growth and targeting of subplate axons and establishment of major cortical pathways.

In the developing mammalian neocortex, the first postmitotic neurons form the "preplate" superficial to the neuroepithelium. The preplate is later split into a marginal zone (layer 1) and subplate by cortical plate neurons that form layers 2-6. Cortical efferent axons from layers 5 and 6 and cortical afferent axons from thalamus pass between cortex and subcortical structures through the internal capsule. Here, we identify in rats the axonal populations that establish the internal capsule, and characterize the potential role of subplate axons in the development of cortical efferent and afferent projections. The early growth of cortical efferent and afferent axons was studied using 1-1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) as an anterograde and retrograde tracer in aldehyde-fixed brains of embryonic rats. Cortical axons first enter the nascent internal capsule on embryonic day (E) 14 and originate from lateral and anterior cortex; axons from posterior cortex extend rostrally but do not yet exit cortex. The labeled axons, tipped by growth cones with complex morphologies, take a pathway deep to the preplate. Preplate neurons extend these early cortical efferents, based on the developmental stage of the cortex, and on their location and morphology. Most of these cells later occupy the subplate. Cortical plate neurons extend axons into the internal capsule by E16. En route to the internal capsule, cortical plate axons take the same path as the earlier-growing preplate axons, through the intermediate zone deep to subplate. Subplate axons reach thalamus by E16; the first cortical plate axons enter thalamus about a day later. Thalamic axons enter cortex by E16, prior to other cortical afferents. On E15, both preplate and thalamic axons reach the midpoint of the internal capsule. To determine the subcortical distribution of subplate axons, we used Dil as a retrograde tracer in aldehyde-fixed brains and fast blue and rhodamine-B-isothiocyanate as in vivo retrograde markers in neonatal rats. Tracers were injected into the superior colliculus, the principal midbrain target of layer 5 neurons, at times before, during, and after the arrival of cortical axons, or into the subcortical pathway of primary layer 5 axons at two points, the cerebral peduncle caudal to the internal capsule, and the pyramidal decussation at the junction of the hindbrain and spinal cord, at times shortly after the passing of cortical axons. In every case, the labeled neurons are confined to layer 5; subplate neurons are not labeled.(ABSTRACT TRUNCATED AT 400 WORDS)     

Projections from the amygdaloid complex to the claustrum and the endopiriform nucleus: a Phaseolus vulgaris leucoagglutinin study in the rat

The claustrum and the endopiriform nucleus contribute to the spread of epileptiform activity from the amygdala to other brain areas. Data of the distribution of pathways underlying the information flow between these regions are, however, incomplete and controversial. To investigate the projections from the amygdala to the claustrum and the endopiriform nucleus, we injected the anterograde tracer Phaseolus vulgaris leucoagglutinin into various divisions of the amygdaloid complex, including the lateral, basal, accessory basal, central, anterior cortical and posterior cortical nuclei, the periamygdaloid cortex, and the amygdalohippocampal area in the rat. Analysis of immunohistochemically processed sections reveal that the heaviest projections to the claustrum originate in the magnocellular division of the basal nucleus. The projection is moderate in density and mainly terminates in the dorsal aspect of the anterior part of the claustrum. Light projections from the parvicellular and intermediate divisions of the basal nucleus terminate in the same region, whereas light projections from the accessory basal nucleus and the lateral division of the amygdalohippocampal area innervate the caudal part of the claustrum. The most substantial projections from the amygdala to the endopiriform nucleus originate in the lateral division of the amygdalohippocampal area. These projections terminate in the central and caudal parts of the endopiriform nucleus. Lighter projections originate in the anterior and posterior cortical nuclei, the periamygdaloid cortex, the medial division of the amygdalohippocampal area, and the accessory basal nucleus. These data provide an anatomic basis for recent functional studies demonstrating that the claustrum and the endopiriform nucleus are strategically located to synchronize and spread epileptiform activity from the amygdala to the other brain regions. These topographically organized pathways also provide a route by means of which the claustrum and the endopiriform nucleus have access to inputs from the amygdaloid networks that process emotionally significant information.     

Rat subthalamic nucleus and zona incerta share extensively overlapped representations of cortical functional territories

The subthalamic nucleus (STN) and the zona incerta (ZI) are two major structures of the subthalamus. The STN has strong connections between the basal ganglia and related nuclei. The ZI has strong connections between brainstem reticular nuclei, sensory nuclei, and nonspecific thalamic nuclei. Both the STN and ZI receive heavy projections from a subgroup of layer V neurons in the cerebral cortex. The major goal of this study was to investigate the following two questions about the cortico‐subthalamic projections using the lentivirus anterograde tracing method in the rat: 1) whether cortical projections to the STN and ZI have independent functional organizations or a global organization encompassing the entire subthalamus as a whole; and 2) how the cortical functional zones are represented in the subthalamus. This study revealed that the subthalamus receives heavy projections from the motor and sensory cortices, that the cortico‐subthalamic projections have a large‐scale functional organization that encompasses both the STN and two subdivisions of the ZI, and that the group of cortical axons that originate from a particular area of the cortex sequentially innervate and form separate terminal fields in the STN and ZI. The terminal zones formed by different cortical functional areas have highly overlapped and fuzzy borders, as do the somatotopic representations of the sensorimotor cortex in the subthalamus. The present study suggests that the layer V neurons in the wide areas of the sensorimotor cortex simultaneously control STN and ZI neurons. Together with other known afferent and efferent connections, possible new functionality of the STN and ZI is discussed. J. Comp. Neurol. 522:4043–4056, 2014. © 2014 Wiley Periodicals, Inc.     

Connections of the retrosplenial dysgranular cortex in the rat.

Although the retrosplenial dysgranular cortex (Rdg) is situated both physically and connectionally between the hippocampal formation and the neocortex, few studies have focused on the connections of Rdg. The present study employs retrograde and anterograde anatomical tracing methods to delineate the connections of Rdg. Each projection to Rdg terminates in distinct layers of the cortex. The thalamic projections to Rdg originate in the anterior (primarily the anteromedial), lateral (primarily the laterodorsal), and reuniens nuclei. Those from the anteromedial nucleus terminate predominantely in layers I and IV-VI, whereas the axons arising from the laterodorsal nucleus have a dense terminal plexus in layers I and III-IV. The cortical projections to Rdg originate primarily in the infraradiata, retrosplenial, postsubicular, and areas 17 and 18b cortices. The projections arising from visual areas 18b and 17 predominantly terminate in layer I of Rdg, axons from contralateral Rdg form a dense terminal plexus in layers I-IV, with a smaller number of terminals in layers V and VI, afferents from postsubiculum terminate in layers I and III-V, and the projection from infraradiata cortex terminates in layers I and V-VI. The efferent projections from Rdg are widespread. The major cortical projections from Rdg are to infraradiata, retrosplenial granular, area 18b, and postsubicular cortices. Subcortical projections from Rdg terminate primarily in the ipsilateral caudate and lateral thalamic nuclei and bilaterally in the anterior thalamic nuclei. The efferent projections from Rdg are topographically organized. Rostral Rdg projects to the dorsal infraradiata cortex and the rostral postsubiculum, while caudal Rdg axons terminate predominantely in the ventral infraradiata and the caudal postsubicular cortices. Caudal but not rostral Rdg projects to areas 17 and 18b of the cortex. The Rdg projections to the lateral and anterior nuclei also are organized along the rostral-caudal axis. Together, these data suggest that Rdg integrates thalamic, hippocampal, and neocortical information.     

Afferent connections of the medial frontal cortex of the rat. II. Cortical and subcortical afferents

In order to compare the frontal cortex of rat and macaque monkey, cortical and subcortical afferents to subdivisions of the medial frontal cortex (MFC) in the rat were analyzed with fluorescent retrograde tracers. In addition to afferent inputs common to the whole MFC, each subdivision of the MFC has a specific pattern of afferent connections. The dorsally situated precentral medial area (PrCm) was the only area to receive inputs from the somatosensory cortex. The specific pattern of afferents common to the ventrally situated prelimbic (PL) and infralimbic (IL) areas included projections from the agranular insular cortex, the entorhinal and piriform cortices, the CA1–CA2 fields of the hippocampus, the subiculum, the endopiriform nucleus, the amygdalopiriform transition, the amygdalohippocampal area, the lateral tegmentum, and the parabrachial nucleus. In all these structures, the number of retrogradely labeled cells was larger when the injection site was located in area IL. The dorsal part of the anterior cingulate area (ACd) seemed to be connectionally intermediate between the adjacent areas PrCm and PL; it receives neither the somatosensory inputs characteristic of area PrCm nor the afferents characteristic of areas PL and IL, with the exception of the afferents from the caudal part of the retrosplenial cortex. A comparison of the pattern of afferent and efferent connections of the rat MFC with the pattern of macaque prefrontal cortex suggests that PrCm and ACd areas share some properties with the macaque premotor cortex, whereas PL and IL areas may have characteristics in common with the cingulate or with medial areas 24, 25, and 32 and with orbital areas 12, 13, and 14 of macaques. © 1995 Wiley-Liss, Inc.     

The organization of centrifugal projections from the anterior olfactory nucleus, ventral hippocampal rudiment, and piriform cortex to the main olfactory bulb in the hamster: an autoradiographic study

The centrifugal projections from the various subdivisions of the anterior olfactory nucleus (AON) can be categorized into four groups based on the organization of terminal fields in the main olfactory bulb (MOB). Pars lateralis and dorsalis have bilaterally asymmetric laminar projections to the MOB. The ipsilateral projections terminate primarily in the superficial half of the granule cell layer and in the deep third of the glomerular layer, whereas the contralateral projections terminate primarily in the superficial half of the granule cell layer and do not extend into the glomerular layer. Pars ventralis and posterior have bilaterally symmetric laminar projections with heavy terminations both in the superficial half of the granule cell layer and in the deep third of the glomerular layer. Pars medialis sends predominantly ipsilateral projections to the deep half of the granule cell layer. Pars externa has predominantly contralateral projections with a very narrow terminal field immediately deep to the internal plexiform layer. The projections to the MOB from the ventral hippocampal rudiment (HR) and the piriform cortex (PC) are exclusively ipsilateral. The projections from the ventral HR terminate primarily in the deep half of the granule cell layer. The projections from the PC also terminate predominantly in the granule cell layer, but there is a progressive shifting of terminal fields from the superficial half of this layer toward deeper regions for centrifugal axons arising from progressively more caudal levels of the PC. The laminar termination patterns of cortical afferents to the ipsilateral MOB thus are correlated with the mediolateral axis of the olfactory peduncle and the rostrocaudal axis of the piriform cortex. The centrifugal axons from these various sources enter directly into the granule cell layer of the caudal MOB or pass through the internal plexiform layer of the accessory olfactory bulb to reach the middle and anterior part of the MOB. We have termed these two routes the final common bulb pathway. The centrifugal axons from the laterally situated sources join the anterior and bulbar limbs of the anterior commissure before entering the final common bulbar pathway. In contrast, the centrifugal axons from pars medialis and the ventral HR travel diffusely in the cellular layer of the ipsilateral olfactory peduncle. A small component of the centrifugal projections from the PC travels in association with the lateral olfactory tract.     

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.     

New features of connectivity in piriform cortex visualized by intracellular injection of pyramidal cells suggest that "primary" olfactory cortex functions like "association" cortex in other sensory systems.

Associational connections of pyramidal cells in rat posterior piriform cortex were studied by direct visualization of axons stained by intracellular injection in vivo. The results revealed that individual cells have widespread axonal arbors that extend over nearly the full length of the cerebral hemisphere. Within piriform cortex these arbors are highly distributed with no regularly arranged patchy concentrations like those associated with the columnar organization in other primary sensory areas (i.e., where periodically arranged sets of cells have common response properties, inputs, and outputs). A lack of columnar organization was also indicated by a marked disparity in the intrinsic projection patterns of neighboring injected cells. Analysis of axonal branching patterns, bouton distributions, and dendritic arbors suggested that each pyramidal cell makes a small number of synaptic contacts on a large number (>1000) of other cells in piriform cortex at disparate locations. Axons from individual pyramidal cells also arborize extensively within many neighboring cortical areas, most of which send strong projections back to piriform cortex. These include areas involved in high-order functions in prefrontal, amygdaloid, entorhinal, and perirhinal cortex, to which there are few projections from other primary sensory areas. Our results suggest that piriform cortex performs correlative functions analogous to those in association areas of neocortex rather than those typical of primary sensory areas with which it has been traditionally classed. Findings from other studies suggest that the olfactory bulb subserves functions performed by primary areas in other sensory systems.