Topographically specific hippocampal projections target functionally distinct prefrontal areas in the rhesus monkey

The sources of ipsilateral projections from the hippocampal formation, the presubiculum, area 29a-c, and parasubiculum to medial, orbital, and lateral prefrontal cortices were studied with retrograde tracers in 27 rhesus monkeys. Labeled neurons within the hippocampal formation (CA1, CA1', prosubiculum, and subiculum) were found rostrally, although some were noted throughout the entire rostrocaudal extent of the hippocampal formation. Most labeled neurons in the hippocampal formation projected to medial prefrontal cortices, followed by orbital areas. In addition, there were differences in the topography of afferent neurons projecting to medial when compared with orbital cortices. Labeled neurons innervating medial cortices were found mainly in the CA1' and CA1 fields rostrally, but originated in the subicular fields caudally. In contrast, labeled neurons which innervated orbital cortices were considerably more focal, emanating from the same relative position within a field throughout the rostrocaudal extent of the hippocampal formation. In marked contrast to the pattern of projection to medial and orbital prefrontal cortices, lateral prefrontal areas received projections from only a few labeled neurons found mostly in the subicular fields. Lateral prefrontal cortices received the most robust projections from the presubiculum and the supracallosal area 29a-c. Orbital, and to a lesser extent medial, prefrontal areas received projections from a smaller but significant number of neurons from the presubiculum and area 29a-c. Only a few labeled neurons were found in the parasubiculum, and most projected to medial prefrontal areas. The results suggest that functionally distinct prefrontal cortices receive projections from different components of the hippocampal region. Medial and orbital prefrontal cortices may have a role in long-term mnemonic processes similar to those associated with the hippocampal formation with which they are linked. Moreover, the preponderance of projection neurons from the hippocampal formation innervating medial when compared with orbital prefrontal areas followed the opposite trend from what we had observed previously for the amygdala (Barbas and De Olmos [1990] (J Comp Neurol 301:1-23). Thus, the hippocampal formation, associated with mnemonic processes, targets predominantly medial prefrontal cortices, whereas the amygdala, associated with emotional aspects of memory, issues robust projections to orbital limbic cortices. Lateral prefrontal cortices receive robust projections from the presubiculum and area 29a-c and sparse projections from the hippocampal formation. These findings are consistent with the idea that the role of lateral prefrontal cortices in memory is distinct from that of either medial or orbital cortices. The results suggest that signals from functionally distinct limbic structures to some extent follow parallel pathways to functionally distinct prefrontal cortices.     

Zinc-positive afferents to the rat septum originate from distinct subpopulations of zinc-containing neurons in the hippocampal areas and layers. A combined fluoro-gold tracing and histochemical study

The purpose of the present study was to examine whether zinc-positive and zinc-negative hippocampal neurons in rats differed with respect to their projections to the septum. By combining retrograde axonal transport of the fluorescent tracer Fluoro-Gold with histochemical demonstration of zinc selenide complexes in zinc-containing neurons after intraperitoneal injection of sodium selenite, we were able to visualize the distribution of retrogradely Fluoro-Gold labeled neurons and zinc-containing neurons in the same sections. After unilateral injection of Fluoro-Gold into the rat septum a few retrogradely labeled cells were observed in layer IV of the ipsilateral medial entorhinal area, and numerous labeled cells were observed mainly in the superficial layers of the ipsilateral subicular areas and throughout the CA1 and CA3 pyramidal cell layers, as well as in the contralateral CA3 pyramidal cell layer. Zinc-containing neurons were observed in layers IV-VI of the medial entorhinal area, layers II and III of the parasubiculum, layers II, III and V of presubiculum, and in the superficial CA1 and deep CA3 pyramidal cell layers. Cells double-labeled with Fluoro-Gold and zinc selenide complexes were primarily located in distal (relative to the area dentata) parts of the superficial CA1 pyramidal cell layer and distal parts of the deep CA3 pyramidal cell layer and in layers II and III of presubiculum. Only a very few double-labeled cells were seen in the contralateral CA3. The result demonstrates that the hippocampo-septal projection of rats is a mixture of zinc-positive and zinc-negative fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

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

We have divided the cortical regions surrounding the rat hippocampus into three cytoarchitectonically discrete cortical regions, the perirhinal, the postrhinal, and the entorhinal cortices. These regions appear to be homologous to the monkey perirhinal, parahippocampal, and entorhinal cortices, respectively. The origin of cortical afferents to these regions is well-documented in the monkey but less is known about them in the rat. The present study investigated the origins of cortical input to the rat perirhinal (areas 35 and 36) and postrhinal cortices and the lateral and medial subdivisions of the entorhinal cortex (LEA and MEA) by placing injections of retrograde tracers at several locations within each region. For each experiment, the total numbers of retrogradely labeled cells (and cell densities) were estimated for 34 cortical regions. We found that the complement of cortical inputs differs for each of the five regions. Area 35 receives its heaviest input from entorhinal, piriform, and insular areas. Area 36 receives its heaviest projections from other temporal cortical regions such as ventral temporal association cortex. Area 36 also receives substantial input from insular and entorhinal areas. Whereas area 36 receives similar magnitudes of input from cortices subserving all sensory modalities, the heaviest projections to the postrhinal cortex originate in visual associational cortex and visuospatial areas such as the posterior parietal cortex. The cortical projections to the LEA are heavier than to the MEA and differ in origin. The LEA is primarily innervated by the perirhinal, insular, piriform, and postrhinal cortices. The MEA is primarily innervated by the piriform and postrhinal cortices, but also receives minor projections from retrosplenial, posterior parietal, and visual association areas.     

Projections from the presubiculum and the parasubiculum to morphologically characterized entorhinal-hippocampal projection neurons in the rat

The relations between the inputs from the presubiculum and the parasubiculum and the cells in the entorhinal cortex that give rise to the perforant pathway have been studied in the rat at the light microscopical level. Projections from the presubiculum and the parasubiculum were labeled anterogradely, and, in the same animal, cells in the entorhinal cortex that project to the hippocampal formation were labeled by retrograde tracing and subsequent intracellular filling with Lucifer Yellow. The distribution and the number of appositions between the afferent fibers and hippocampal-projection neurons in the various layers of the entorhinal cortex were analyzed. The results show that layers I-IV of the entorhinal cortex contain neurons that give rise to projections to the hippocampal formation. The morphology of these projection neurons is highly variable and afferents from the presubiculum and the parasubiculum do not show a preference for any specific morphological cell type. Both inputs preferentially innervate the dendrites of their target cells. However, presubicular and parasubicular projections differ with respect to the layer of entorhinal cortex they project to. The number of appositions of presubicular afferents with cells that have their cell bodies in layer III of the entorhinal cortex is 2-3 times higher than with cells in layer II. In contrast, afferents from the parasubiculum form at least 2-3 times as many synapses on the dendrites of cells located in layer II than on neurons that have their cell bodies in layer III. Cells in layers I and IV of the entorhinal cortex receive weak inputs from the presubiculum and parasubiculum. Not only is the presubiculum different from the parasubiculum with respect to the distribution of projections to the entorhinal cortex, they also differ in their afferent and efferent connections. In turn, cells in layer II of the entorhinal cortex differ in their electrophysiological characteristics from those in layer III. Moreover, layer II neurons give rise to the projections to the dentate gyrus and field CA3/CA2 of the hippocampus proper, and cells in layer III project to field CA1 and the subiculum. Therefore, we propose that the interactions of the entorhinal-hippocampal network with the presubiculum are different from those with the parasubiculum.     

Hippocampal cells have behavioral correlates during the performance of an auditory working memory task in the rat.

Single unit activity was recorded from the rat hippocampal CA1 and CA3 subfields, dentate gyrus (DG), entorhinal cortex, subicular complex, motor cortex (MC), prefrontal cortex, and dorsomedial thalamus during performance of a continuous nonmatching-to-sample task. The rats made go and no-go responses to indicate whether the current tone was the same as (match) or different from (nonmatch) the preceding tone. About half of the units from the MC, CA1, CA3, and DG had motor correlates, increments of activity immediately prior to a go response. This suggests that those regions are involved in making a response. The CA1 and CA3 had more units with comparison–motor correlates, increases of activity prior to a correct go response on nonmatch trials. This pattern of activity suggests that the CA1 and CA3 are involved in a mnemonic process of comparison between retained and current stimuli in a nonspatial auditory task. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Direct projections from the entorhinal cortical layers to the dentate gyrus, hippocampus, and subicular complex in the cat

Projections from each layer of the entorhinal cortex (EC) of the cat were traced to the dentate gyrus (DG), Ammon's horn (CA), prosubiculum (ProSb), subiculum (Sb), presubiculum (PreSb) and parasubiculum (ParaSb); the anterograde or retrograde labeling method was used after stereotaxic injection of wheat germ agglutinin-horseradish peroxidase, cholera toxin B subunit, or Phaseolus vulgaris leucoagglutinin. On the side ipsilateral to the tracer-injection, layer II of the EC projected most abundantly to the outer half of the molecular layer (ML) of the DG, less abundantly to the almost entire thickness of the stratum lacunosum-moleculare (SLM) of CA2-3, moderately to the almost entire thickness of the SLM of CA1, and less to the outer part of the ML of the ProSb and Sb. Layer III projected abundantly to the almost entire thickness of the SLM of CA1 and outer part of the ML of the ProSb and Sb, and sparsely to the SLM of CA2-3. Layer IV projected sparsely to the pyramidal cell layer of the ProSb and Sb; Layer IV of the medial part (toward the ParaSb) of the EC projected further to the ML of the DG. Layer VI projected sparsely to the outer part of the ML of the DG, almost entire thickness of the SLM of CA1-3, and outer part of the ML of the ProSb and Sb. More temporal parts of the hippocampal region received the projections from progressively more medial and more rostral parts of layers II and III, and from progressively more rostral parts of layers IV and VI. The ML of the PreSb and ParaSb received projections from all layers of the medial part of the ipsilateral EC. The SLM of CA1 and ML of the ProSb, Sb and ParaSb received projections from layer II and/or III of the contralateral medial entorhinal area.     

Descending projections from the spinal and mesencephalic nuclei of the trigeminal nerve to the spinal cord in the cat. A study with the horseradish peroxidase technique

Descending projections from the spinal (Vsp) and the mesencephalic nuclei (Vme) of the trigeminal nerve to the spinal cord were studied by means of the retrograde horseradish peroxidase technique in the cat. The number of labeled neurons was largest in the case of high cervical injections and decreased as the injections were placed caudally. Small laminae III and IV neurons of the nucleus caudalis (Vc) were labeled ipsilaterally following injections placed as caudally as the middle cervical segments (C4-C5). Lamina I (marginal) neurons of the Vc were labeled ipsilaterally after injections at the middle thoracic level (T6) but those of C1 were labeled after lumbar injections (L3). Lamina V neurons of C1 and the medullary counterparts were labeled bilaterally after injections placed caudally to thoracic segments. A few small neurons were labeled in the ipsilateral nucleus interpolaris (Vi) after injections placed as caudally as the middle cervical segments (C6). Among the subdivisions of the Vsp, the labeled neurons were most numerous in the nucleus oralis (Vo). They were medium-sized and large, and appeared bilaterally, with an ipsilateral predominance at the level of the superior olive. The great majority projected to the cervical segments but a few also projected to the lower cervical to the thoracic segments (C8-T9). Neurons of the Vme projected ipsilaterally to the upper cervical segments (C1-C3). No projections were found from the principal sensory nucleus. The present study suggests that the trigeminospinal projections of the Vsp and the Vme are composed of various cells of origin and thereby subserve not only the trigeminospinal reflex but other unknown functions.     

Development of long-distance efferent projections from fetal hippocampal grafts depends upon pathway specificity and graft location in kainate-lesioned adult hippocampus

Fetal hippocampal cells grafted into the excitotoxically lesioned hippocampus of adult rats are capable of extending axonal projections into the host brain. We hypothesize that the axonal growth of grafted fetal cells into specific host targets, and the establishment of robust long-distance efferent graft projections, require placement of fetal cells in close proximity to appropriate axon guidance pathways. Intracerebroventricular administration of kainic acid in adult rats leads to a specific loss of hippocampal CA3 pyramidal neurons. We grafted 5'-bromodeoxyuridine-labeled embryonic day 19 hippocampal cells into adult hippocampus at four days post-kainic acid lesion, and quantitatively measured the projection of grafted cells into the contralateral hippocampus and the septum after three to four months survival using Fluoro-Gold and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (Dil) tracing. Grafts located in or near the degenerated CA3 cell layer exhibited numerous neurons which established commissural projections with the contralateral hippocampus. However, such projection did not occur in intrahippocampal grafts located away from the CA3 cell layer. In contrast, neurons in all grafts established robust projections into the septum regardless of location within hippocampus although grafts located near the degenerated CA3 cell layer displayed more neurons with such projections. Location of grafted cells clearly influences the development of efferent graft projections into distant targets in the adult host brain, particularly access to axon guidance pathways to facilitate the formation of projections. The establishment of robust long-distance commissural projections of fetal hippocampal grafts is clearly dependent on their placement in or near the degenerated CA3 cell layer, suggesting that appropriate axon guidance pathways for commissural pathways are tightly focussed near this cell layer. However, the establishment of septal projections of these grafts was not dependent on specific location within the CA3 cell layer, suggesting that axonal guidance mechanisms to the septum are more diffuse and not limited to the CA3 dendritic layers. The results underscore that fetal hippocampal grafts are capable of partly restoring lesioned hippocampal circuitry in adult animals when appropriately placed in the host hippocampus.     

Topographic organization of connections between the hypothalamus and prefrontal cortex in the rhesus monkey

Prefrontal cortices have been implicated in autonomic function, but their role in this activity is not well understood. Orbital and medial prefrontal cortices receive input from cortical and subcortical structures associated with emotions. Thus, the prefrontal cortex may be an essential link for autonomic responses driven by emotions. Classic studies have demonstrated the existence of projections between prefrontal cortex and the hypothalamus, a central autonomic structure, but the topographic organization of these connections in the monkey has not been clearly established. We investigated the organization of bidirectional connections between these areas in the rhesus monkey by using tracer injections in orbital, medial, and lateral prefrontal areas. All prefrontal areas investigated received projections from the hypothalamus, originating mainly in the posterior hypothalamus. Differences in the topography of hypothalamic projection neurons were related to both the location and type of the target cortical area. Injections in lateral eulaminate prefrontal areas primarily labeled neurons in the posterior hypothalamus that were equally distributed in the lateral and medial hypothalamus. In contrast, injections in orbitofrontal and medial limbic cortices labeled neurons in the anterior and tuberal regions of the hypothalamus and in the posterior region. Projection neurons targeting orbital limbic cortices were more prevalent in the lateral part of the hypothalamus, whereas those targeting medial limbic cortices were more prevalent in the medial hypothalamus. In comparison to the ascending projections, descending projections from prefrontal cortex to the hypothalamus were highly specific, originating mostly from orbital and medial prefrontal cortices. The ascending and descending connections overlapped in the hypothalamus in areas that have autonomic functions. These results suggest that specific orbitofrontal and medial prefrontal areas exert a direct influence on the hypothalamus and may be important for the autonomic responses evoked by complex emotional situations.     

Projections from the lateral, basal, and accessory basal nuclei of the amygdala to the hippocampal formation in rat

The amygdaloid complex and hippocampal formation mediate functions involving emotion and memory. To investigate the connections that regulate the interactions between these regions, we injected the anterograde tracer Phaseolus vulgaris-leucoagglutinin into various divisions of the lateral, basal, and accessory basal nuclei of the rat amygdala. The heaviest projection to the entorhinal cortex originates in the medial division of the lateral nucleus which innervates layer III of the ventral intermediate and dorsal intermediate subfields. In the basal nucleus, the heaviest projection arises in the parvicellular division and terminates in layer III of the amygdalo-entorhinal transitional subfield. In the accessory basal nucleus, the parvicellular division heavily innervates layer V of the ventral intermediate subfield. The most substantial projection to the hippocampus originates in the basal nucleus. The caudomedial portion of the parvicellular division projects heavily to the stratum oriens and stratum radiatum of CA3 and CA1. The accessory basal nucleus projects to the stratum lacunosum-moleculare of CA1. The subiculum receives a substantial input from the caudomedial parvicellular division. The parasubiculum receives dense projections from the caudal portion of the medial division of the lateral nucleus, the caudomedial parvicellular division of the basal nucleus, and the parvicellular division of the accessory basal nucleus. Our data show that select nuclear divisions of the amygdala project to the entorhinal cortex, hippocampus, subiculum, and parasubiculum in segregated rather than overlapping terminal fields. These data suggest that the amygdaloid complex is in a position to modulate different stages of information processing within the hippocampal formation.     

GABAergic presubicular projections to the medial entorhinal cortex of the rat

We characterized presubicular neurons giving rise to bilateral projections to the medial entorhinal cortex (MEA) of the rat. Retrograde labeling of presubiculo-entorhinal projections with horseradish peroxidase and subsequent GABA immunocytochemistry revealed that 20-30% of the ipsilaterally projecting neurons are GABAergic. No GABAergic projections to the contralateral MEA were observed. GABAergic projection neurons were observed only in the dorsal part of the presubiculum, which, when taking into account the topography of presubicular projections to MEA, indicates that only the dorsal part of MEA receives GABAergic input. The GABAergic projection neurons constitute approximately 30-40% of all GABAergic neurons present in the superficial layers of the dorsal presubiculum. Using double-label fluorescent retrograde tracing, we found that the ipsilateral and contralateral presubiculo-entorhinal projections originate from different populations of neurons. Anterograde labeling of presubiculo-entorhinal projections and electron microscopical analysis of labeled terminals substantiated the presence of a restricted GABAergic presubiculo-entorhinal projection. A small fraction of afferents to only ipsilateral dorsal MEA formed symmetrical synapses with dendritic shafts. No symmetrical synapses on spines were noted. Most afferents to the dorsal part of ipsilateral MEA, as well as all afferents to the remaining ipsilateral and contralateral MEA, formed asymmetrical synapses with both spines and dendritic shafts in an almost equal ratio. Thus, we conclude that the majority of the presubiculo-entorhinal projections exert an excitatory effect on both principal neurons and interneurons. The projections from the dorsal part of the presubiculum comprise a small inhibitory component that originates from GABAergic neurons and targets entorhinal interneurons.     

Precocious development of parvalbumin-like immunoreactive interneurons in the hippocampal formation and entorhinal cortex of the fetal cynomolgus monkey

The calcium-binding protein parvalbumin (PV), a reliable marker of the hippocampal basket and chandelier cells, is first expressed on embryonic day 83 (E83), corresponding to midgestation of the macaque monkey, in restricted hippocampal groups of immature neurons (Berger and Alvarez [1996] J. Comp. Neurol. 366:674-699). In the present study, PV-like immunoreactivity (LIR) was used to follow the further development of this subclass of interneurons. Asynchronous area-specific developmental sequences were observed, predominating initially in the caudal half of the hippocampal formation and the laterocaudal division of the entorhinal cortex and occurring relatively simultaneously in the interconnected hippocampal and entorhinal subfields. Dendritic elongation of PV-like immunoreactive interneurons and perisomatic distribution of PV-like immunoreactive terminal boutons on their cellular targets were first observed in the subiculum around E127; then from E127 to E142 in CA3/CA2 and layers III-V of the entorhinal cortex and, to a lesser extent in CA1, the dentate hilus and deep granule cell layer; and finally from E156 to postnatal day 12 in the rest of the dentate gyrus, the presubiculum and parasubiculum, and layers III-II-I of the entorhinal cortex. These data provide the first indication that a population of basket cells, a major gamma-aminobutyric acid (GABA)ergic component of the hippocampal intrinsic inhibitory circuitry, reaches its cellular targets several weeks before birth in primates in contrast to rodents. The role of the prenatal PV expression in the hippocampal formation of nonhuman primates and whether it coincides with the onset of postsynaptic inhibitory potentials or is accompanied or preceded by a period of gamma-aminobutyric acid-mediated excitatory effects as in rat pups, are crucial questions. They underline the need to pursue direct investigations on primates to be able to legitimately extrapolate the data obtained in rodents.     

The origins of descending projections to the lumbar spinal cord at different stages of development in the North American opossum

We have employed the retrograde transport of fast blue (FB) to identify the origins of descending projections to the lumbar cord of the opossum from postnatal day (PD)1, 12-13 days after conception, to maturity. When FB injections were made into the lumbar cord at PD1, supraspinal labeling was sparse and limited to the hypothalamus, the reticular formation, the coeruleus complex, the caudal raphe, and, in one case, the interstitial nucleus of the medial longitudinal fasciculus and the lateral vestibular nucleus. Only a few propriospinal neurons were labeled at cervical and thoracic levels. By PD3, however, supraspinal and propriospinal labeling was abundant and present in most of the areas labeled in the adult animal. A notable exception was the red nucleus which was not labeled until approximately PD10. Our results have been compared with those described in other species and discussed in light of their relevance to the development of descending control over hindlimb movement and developmental plasticity of descending spinal pathways.     

Empirical comparisons of proportional hazards, poisson, and logistic regression modeling of occupational cohort data

Protein deprivation experienced in adult life leads to deficits in the number of hippocampal granule and CA3-CA1 pyramidal cells and to changes in the dendritic domain of granule cells and CA3 pyramids. To obtain a more complete insight into the effects of malnutrition on the limbic system of the adult rat we have analyzed the subiculum and the entorhinal cortex (neuronal layers II, III, and V-VI) in groups of 8-month-old rats fed with a low-protein diet (8% casein) since the age of 2 months and in age-matched control rats. Stereological methods were employed to estimate the total number of neurons in the subiculum and layers II, III, and V-VI of the entorhinal cortex and the volume of the respective cell layers. Moreover, to evaluate whether protein deprivation affects the dendritic domains of the neurons from these regions we have analyzed, in Golgi-impregnated material, the dendritic trees of the pyramidal cells of the subiculum and of the stellate neurons of the entorhinal cortex layer II applying quantitative and metric methods. The volume of the subiculum and the total number of its neurons were reduced in malnourished animals. In these animals we also found marked regressive changes in the apical and basal dendritic trees of the pyramidal subicular neurons. However, the spine density was increased in malnourished rats. No differences in the volume of the neuronal layers of the entorhinal cortex or in the total number of their neurons were found between protein-deprived and control rats, and no alterations were depicted in the dendritic trees of the stellate neurons of layer II. We can thus conclude that the effects of long-term protein deprivation are region specific and that the resulting structural alterations are confined to the three-layered components of the hippocampal region.     

Comparison of ventral subicular and hippocampal neuron spatial firing patterns in complex and simplified environments.

Activity from ventral subicular and hippocampal CA1 neurons was recorded in rats exploring a 4-arm radial maze in which the local and distal cues could be manipulated. Cells from both regions exhibited place fields, although ventral subicular neurons had larger fields than hippocampal cells. Rotation of the local and distal cues in opposite directions produced movement of the place fields in either direction or a complete change in firing pattern. Simplifying the environment also produced changes in place field location. Despite similarities between regions, subiculum fields decreased in size whereas hippocampal fields increased in the simple environment. These findings suggest that subicular cells may receive converging input from several hippocampal neurons and code more complex configurations of the cues. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dopamine D2 receptor expression in hippocampus and parahippocampal cortex of rat, cat, and human in relation to tyrosine hydroxylase-immunoreactive fibers

A detailed study comparing the distribution of D2 receptors and tyrosine hydroxylase-immunoreactive fibers in the hippocampus and parahippocampal cortices of the rat, cat, and human was conducted. The distribution of [125I]epidepride binding to D2 receptors along the transverse and longitudinal axes of the hippocampus and parahippocampus differed among the species. In rat hippocampus, the number of sites was highest in septal portions of lacunosum-moleculare of CA1 and stratum moleculare of the subiculum. Virtually no binding to D2 receptors existed in the temporal hippocampus. For the cat hippocampus, the highest binding existed in the inner one-third of the molecular layer of the dentate gyrus (DG). There were also significant numbers of D2 receptors in strata radiatum and oriens of the CA subfields, with almost undetectable levels in lacunosum moleculare and subiculum. The number of sites was higher in the septal than temporal hippocampus. In the human hippocampus, highest binding was observed in the molecular layer of DG and the subiculum, with lower levels in strata oriens and lacunosum-moleculare of CA3, and very low binding in CA1. The histochemical demonstration of the pattern of mossy fibers revealed an organization complementary to that of D2 receptors in cat and human. In none of the species was there significant expression of D2 receptors in the entorhinal cortex, except in the caudal extreme of this region in the rat. In that region a trilaminar pattern was exhibited that continued into the perirhinal cortex. A trilaminar pattern of D2 receptor expression was observed in the perirhinal cortex of all species, with the highest values in the external and deep laminae and low expression in the middle laminae. The organization of dopamine fibers was assessed by comparing the distribution of tyrosine hydroxylase-positive and dopamine beta-hydroxylase-immunoreactive fibers in these same regions. It revealed consistent mismatches between the pattern of D2 receptor expression and dopaminergic innervation in all three species. The implications for this mismatch are discussed. It is hypothesized that the distribution of D2 receptors, and not of dopamine fibers, determines what neural systems dopamine influences in the hippocampal complex.     

Neocortex in the hippocampus: an anatomical and functional study of CA1 heterotopias after prenatal treatment with methylazoxymethanol in rats

Migration disorders cause neurons to differentiate in an abnormal heterotopic position. Although significant insights have been gained into the etiology of these disorders, very little is known about the anatomy of heterotopias. We have studied heterotopic masses arising in the hippocampal CA1 region after prenatal treatment with methylazoxymethanol (MAM) in rats. Heterotopic cells were phenotypically similar to neocortical supragranular neurons and exhibited the same temporal profile of migration and neurogenesis. However, they did not express molecules characteristic of CA1 neurons such as the limbic-associated membrane protein. Horseradish peroxidase injections in heterotopia demonstrated labeled fibers not only in the neocortex and white matter but also in the CA1 stratum radiatum and stratum lacunosum. To study the pathophysiological consequences of this connectivity, we compared the effects of neocortical and limbic seizures on the expression of Fos protein and on cell death in MAM animals. After metrazol-induced seizures, Fos-positive cells were present in CA1 heterotopias, the only hippocampal region to be activated with the neocortex. By contrast, kainic acid-induced seizures caused a prominent delayed cell death in limbic regions and in CA1 heterotopias. Together, these results suggest that neocortical heterotopias in the CA1 region are integrated in both the hippocampal and neocortical circuitry.     

Vestibular nucleus projections to nucleus tractus solitarius and the dorsal motor nucleus of the vagus nerve: potential substrates for vestibulo-autonomic interactions

Autonomic effects of vestibular stimulation are important components of phenomena as diverse as acute vestibular dysfunction and motion sickness. However, the organization of neural circuits mediating these responses is poorly understood. This study presents evidence for direct vestibular nucleus projections to brain stem regions that mediate autonomic function. One group of albino rabbits received injections of Phaseolus vulgaris leucoagglutinin into the vestibular nuclei. The tracer was visualized immunocytochemically with standard techniques. Anterogradely labeled axons from the caudal medial vestibular nucleus (cMVN) and inferior vestibular nucleus (IVN) could be traced bilaterally to nucleus tractus solitarius (NTS). Fewer axons ended near the somata of neurons in the dorsal motor nucleus of the vagus nerve (DMX). A second group of rabbits received pressure or iontophoretic injections of cholera toxin B-HRP or Fluoro-Gold into a region including NTS and DMX. Retrogradely labeled neurons were observed bilaterally in the caudal half of cMVN and ipsilaterally in IVN. The labeled somata were small and they tended to occupy the center of cMVN in transverse sections. These previously unreported vestibular nucleus projections to NTS and DMX are a potential substrate for vestibular influences on autonomic function. In particular, they may contribute to both cardiovascular control during head movements (e.g., orthostatic reflexes) and autonomic manifestions of vestibular dysfunction, motion sickness and exposure to altered gravitational environments.     

Contralateral cortical projection to the mediodorsal thalamic nucleus: origin and synaptic organization in the rat

The origin of the corticothalamic projections to the contralateral mediodorsal nucleus, the collateralization of cortical fibers and their synaptic organization in the ipsi- and contralateral mediodorsal nuclei were investigated in adult rats with double retrograde fluorescent and anterograde tracing. After tracer injections in the mediodorsal nuclei on either side, neurons were retrogradely labeled in all the areas of the contralateral prefrontal cortex in which ipsilateral labeling was also observed. Contralateral corticothalamic cells accounted for 15% of the labeled neurons in the orbital and agranular insular areas, while their proportion was lower (3%) in the anterior cingulate cortex. Up to 70% of the contralateral cortical neurons were double labeled by bilateral injections in the mediodorsal nuclei. At the electron microscopic level, unilateral injections of biotinylated dextran-amine in the orbitofrontal cortex resulted in anterograde labeling of small terminals and a few large boutons in the ipsilateral mediodorsal nucleus, while only small boutons were identified contralaterally. The diameter of postsynaptic dendritic profiles contacted by labeled small cortical endings was significantly larger in the ipsilateral mediodorsal nucleus than contralaterally. These findings demonstrate that dense contralateral cortical projections to the mediodorsal nucleus derive from the orbital and agranular insular areas, and that crossed corticothalamic afferents are mostly formed by collaterals of the ipsilateral connections. Our observations also point out the heterogeneity of corticothalamic boutons in the rat mediodorsal nucleus and morphological differences in the synaptic organization of prefrontal fibers innervating the two sides, indicating that ipsilateral cortical afferents may be more proximally distributed than crossed cortical fibers on dendrites of mediodorsal neurons.     

Origin, course, and laterality of spinocerebellar axons in the North American opossum, Didelphis virginiana

Inhibition of neurons containing gamma-aminobutyric acid (GABA) may underlie some of the excitatory effects of opioids in the central nervous system (CNS). In the present study, we examined the relationship of the cloned mu- and delta-opioid receptors (MOR1 and DOR1, respectively) to GABAergic neurons in brain and spinal cord. This was done by combining immunofluorescent staining for MOR1 or DOR1 with that for GABA or glutamic acid decarboxylase (GAD); fluorescent retrograde tract-tracing was used in some cases to identify neurons with particular projections. In rats, cells double labeled for GABA and MOR1 were observed in layers II-VI of the parietal cortex and in layers II-IV of the piriform cortex. In the hippocampus, double labeling was observed in the dentate gyrus and in regions CA1 and CA3. Double labeling was very prominent in the striatum and in the reticular nucleus of the thalamus; it was also observed in other portions of the diencephalon. However, double labeling for GABA and MOR1 was never observed in the cerebellar cortex. Cells double labeled for GABA and MOR1 were common in the periaqueductal gray (PAG) and the medial rostral ventral medulla (RVM) of both rats and monkeys, suggesting that involvement of GABAergic neurons with supraspinal opioid antinociception may extend to primates. In the RVM of rats, many of those double-labeled neurons were retrogradely labeled from the dorsal spinal cord. In contrast, double-labeled neurons in the PAG were almost never retrogradely labeled from the RVM. No unequivocal examples of double labeling for DOR1 and GAD were found in any region of the CNS that we examined in either rats or monkeys. However, GABAergic neurons were often apposed by DOR1 immunoreactive varicosities. Our findings suggest that activation of mu-opioid receptors directly modulates the activity of GABAergic neurons throughout the CNS, including neurons involved in the supraspinal component of opioid analgesia. In contrast, delta-opioid receptors appear to be positioned to modulate the activity of GABAergic neurons indirectly.