The entorhinal cortex of the monkey: III. Subcortical afferents

The subcortical afferent connections of the entorhinal cortex of the Macaca fascicularis monkey were investigated by the placement of small injections of the retrograde tracer wheat germ agglutinin conjugated to horseradish peroxidase into each of its subdivisions. Retrogradely labeled cells were observed in several subcortical regions including the amygdaloid complex, claustrum, basal forebrain, thalamus, hypothalamus, and brainstem. In the amygdala, labeled cells were observed principally in the lateral nucleus, the accessory basal nucleus, the deep or paralaminar portion of the basal nucleus, and the periamygdaloid cortex. Additional retrogradely labeled cells were found in the endopiriform nucleus, the anterior amygdaloid area, and the cortical nuclei. Retrogradely labeled cells were observed throughout much of the rostrocaudal extent of the claustrum and tended to be located in its ventral half. In the basal forebrain, retrogradely labeled cells were observed in the medial septal nucleus, the nucleus of the diagonal band, and to lesser extent within the substantia innominata. Several of the cells in the latter region were large and located within the densely packed neuronal clusters of the basal nucleus of Meynert. Most of the labeled cells in the thalamus were located in the midline nuclei. Many were found in nucleus reuniens, but even greater numbers were located in the centralis complex. Additional labeled cells were located in the paraventricular and parataenial nuclei. In all cases, numerous retrogradely labeled cells were observed in the medial pulvinar. In the hypothalamus, most of the retrogradely labeled cells were located in the supramamillary area, though scattered cells were also observed in the perifornical region and in the lateral hypothalamic area. Caudal to the mamillary nuclei there were labeled cells in the ventral tegmental area. There were relatively few labeled cells in the brainstem and these were invariably located either in the raphe nuclei or locus coeruleus.     

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

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

Intrinsic and efferent connections of the endopiriform nucleus in rat.

The endopiriform nucleus is a large group of multipolar cells located deep to the piriform cortex. The function of this nucleus is unknown, but studies with animal models suggest that it plays an important role in temporal lobe epileptogenesis. To address questions concerning mechanisms of epileptogenesis and to gain insights into its normal function, efferent axons from the endopiriform nucleus were labeled by anterograde transport from small extracellular injections of Phaseolus vulgaris leucoagglutinin. Several principles of organization were derived: (1) heavy local and long intrinsic connections are present throughout the endopiriform nucleus; (2) endopiriform efferents target cortical rather than nuclear structures; (3) extensive projections from the endopiriform nucleus extend to most basal forebrain areas including the piriform cortex, entorhinal cortex, insular cortex, orbital cortex, and all cortical amygdaloid areas. The perirhinal cortex, olfactory tubercle, and most subdivisions of the hippocampal formation receive light projections; (4) projections are highly distributed spatially within all target areas; (5) efferent axons from the endopiriform nucleus are unmyelinated and give rise to boutons along their entire course rather than arborizing locally; and (6) the endopiriform nucleus and piriform cortex share target areas, but efferents from the endopiriform nucleus lack the precise laminar order of those from the piriform cortex, and provide a heavy caudal to rostral pathway that is lacking in the cortex. The significance of these findings for the triggering of generalized seizures from the deep piriform region are discussed. An hypothesis for a role of the endopiriform nucleus in memory storage is presented.     

A PHA-L analysis of ascending projections of the dorsal raphe nucleus in the rat.

Ascending projections from the dorsal raphe nucleus (DR) were examined in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). The majority of labeled fibers from the DR ascended through the forebrain within the medial forebrain bundle. DR fibers were found to terminate heavily in several subcortical as well as cortical sites. The following subcortical nuclei receive dense projections from the DR: ventral regions of the midbrain central gray including the 'supraoculomotor central gray' region, the ventral tegmental area, the substantia nigra-pars compacta, midline and intralaminar nuclei of the thalamus including the posterior paraventricular, the parafascicular, reuniens, rhomboid, intermediodorsal/mediodorsal, and central medial thalamic nuclei, the central, lateral and basolateral nuclei of the amygdala, posteromedial regions of the striatum, the bed nucleus of the stria terminalis, the lateral septal nucleus, the lateral preoptic area, the substantia innominata, the magnocellular preoptic nucleus, the endopiriform nucleus, and the ventral pallidum. The following subcortical nuclei receive moderately dense projections from the DR: the median raphe nucleus, the midbrain reticular formation, the cuneiform/pedunculopontine tegmental area, the retrorubral nucleus, the supramammillary nucleus, the lateral hypothalamus, the paracentral and central lateral intralaminar nuclei of the thalamus, the globus pallidus, the medial preoptic area, the vertical and horizontal limbs of the diagonal band nuclei, the claustrum, the nucleus accumbens, and the olfactory tubercle. The piriform, insular and frontal cortices receive dense projections from the DR; the occipital, entorhinal, perirhinal, frontal orbital, anterior cingulate, and infralimbic cortices, as well as the hippocampal formation, receive moderately dense projections from the DR. Some notable differences were observed in projections from the caudal DR and the rostral DR. For example, the hippocampal formation receives moderately dense projections from the caudal DR and essentially none from the rostral DR. On the other hand, virtually all neocortical regions receive significantly denser projections from the rostral than from the caudal DR. The present results demonstrate that dorsal raphe fibers project significantly throughout widespread regions of the midbrain and forebrain.     

Specific retrograde transport of nerve growth factor (NGF) from neocortex to nucleus basalis in the rat

[¹²⁵I]labeled NGF injected in very small quantities into the frontal or dorsal anterior occipital cortex of adult rats, was specifically taken up and transported retrogradely to large, presumably cholinergic neurons in the nucleus basalis region (lateral preoptic nucleus, anterior lateral hypothalamic nucleus, substantia innominata, ventral globus pallidus and internal capsule), as revealed by light microscopic autoradiography. Cells projecting to the injection site in the frontal cortex were localized ipsilaterally in the more caudal parts of the nucleus basalis region, whereas cells projecting to the dorsal anterior occipital cortex could be found throughout the entire extent of the nucleus basalis and also in the vertical and horizontal limb of the nucleus of the diagonal band of Broca. Other nuclei known to project to the cortex (locus coeruleus, substantia nigra, nucleus raphe, thalamus) were consistently found to be unlabeled. In contrast to [¹²⁵I]NGF, injection of [¹²⁵I]cytochrome C failed to label any cell bodies in the basal forebrain nuclei by retrograde transport. This high selectivity for uptake and retrograde transport of NGF indicates the presence of membrane receptors for NGF or a closely related molecule on these cholinergic neurons of the basal forebrain innervating the cerebral cortex.     

Sources of presumptive glutamatergic/aspartatergic afferents to the magnocellular basal forebrain in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of [3H]-D-aspartate into the magnocellular basal forebrain of the rat was compared with the distribution of neurons labeled by comparable injections of the nonspecific retrograde axonal tracer wheat germ agglutinin conjugated to horseradish peroxidase. Cells retrogradely labeled by wheat germ agglutinin-horseradish peroxidase were found in a wide range of limbic and limbic-related structures in the forebrain and brainstem. In the telencephalon, labeled neurons were seen in the orbital, medial prefrontal, and agranular insular cortical areas, the amygdaloid complex, and the hippocampal formation. Labeled cells were also seen in the olfactory cortex, the lateral septum, the ventral striatopallidal region, and the magnocellular basal forebrain itself. In the diencephalon, neurons were labeled in the midline nuclear complex of the thalamus, the lateral habenular nucleus, and the hypothalamus. In the brainstem, labeled cells were found bilaterally in the ventral midbrain, the central gray, the reticular formation, the parabrachial nuclei, the raphe nuclei, the laterodorsal tegmental nucleus, and the locus coeruleus. A significant fraction of the afferents to the magnocellular basal forebrain appear to be glutamatergic and/or aspartatergic. Only a few of the regions labeled with wheat germ agglutinin-horseradish peroxidase were not also labeled with [3H]-D-aspartate in the comparable experiments. Most prominent among the non-glutamatergic/aspartatergic projections were those from fields CA1 and CA3 of the hippocampus, the hilus of the dentate gyrus, the dorsal subiculum, the tuberomammillary nucleus, and the ventral pallidum. In addition, most of the lateral hypothalamic and brainstem projections to the magnocellular basal forebrain were not significantly labeled with [3H]-D-aspartate. In addition to these inputs, a commissural projection from the region of the contralateral nucleus of the horizontal limb of the diagonal band was confirmed with both wheat germ agglutinin-horseradish peroxidase and the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin. This projection did not label with [3H]-D-aspartate or [3H]-GABA, suggesting that it is not glutamatergic/aspartatergic or GABAergic. Furthermore, double labeling experiments with the fluorescent retrograde tracer True Blue and antibodies against choline acetyltransferase indicate that the projection is not cholinergic.     

Connections of the parahippocampal cortex in the cat. II. Subcortical afferents

The organization of subcortical inputs to the parahippocampal cortex, which in the present study in the cat is considered to comprise the entorhinal and perirhinal cortices, was studied by using retrograde and anterograde tracing techniques. The results of the retrograde tracer horseradish peroxidase (HRP), HRP conjugated with wheat germ agglutinine (WGA-HRP), Fast Blue (FB) or Nuclear Yellow (NY] injections indicate that the entorhinal and perirhinal cortices receive inputs from the magnocellular basal forebrain and from distinct portions of the amygdaloid complex, the claustrum, and the thalamus. The two cortices are further projected upon by fibers from the supramamillary region of the hypothalamus, the ventral tegmental area of the mesencephalon, the dorsal raphe nucleus, the nucleus centralis superior, and the locus coeruleus. The entorhinal cortex, in addition, receives projections from the medial septum. As regards the projections from the amygdaloid complex, it was observed that the entorhinal cortex receives its heaviest input from the basolateral amygdaloid nucleus, whereas the perirhinal cortex receives a strong projection from the lateral nucleus and a weaker projection from the basomedial nucleus of the amygdala. Of the thalamic nuclei that project to the parahippocampal cortex, the nucleus reuniens is only connected with the entorhinal cortex, while fibers from the medial geniculate nucleus and the lateral posterior nucleus terminate in the perirhinal cortex. Injections of tritiated amino acid (3H-leucine) were placed in the medial septum, the dorsal and ventral claustrum, the basolateral and basomedial amygdaloid nuclei, and the nucleus reuniens of the thalamus. The results of these experiments demonstrate that, with the exception of the claustrum, these subcortical areas project mainly to the superficial layers I-III and the lamina dissecans of the parahippocampal cortex, and to a lesser degree to the deep layers V and VI.     

Estrogen receptor immunoreactivity within subregions of the rat forebrain: neuronal distribution and association with perikarya containing choline acetyltransferase

Administration of the neuroactive steroid hormone estrogen has been shown to effect cholinergic basal forebrain neuronal function. Antibodies directed against the estrogen receptor alpha (ERalpha) revealed dark (type 1) and light (type 2) nuclear positive neurons within the islands of Calleja, endopiriform nucleus, lateral septum, subfields of the cholinergic basal forebrain, bed nucleus of the stria terminalis, striohypothalamic region, medial preoptic region, periventricular, ventromedial, arcuate and tuberal mammillary nuclei of the hypothalamus, reuniens and anterior medial thalamic nuclei, amygdaloid complex, piriform cortex and subfornical organ. In contrast, only a few scattered ERalpha labeled neurons were found in cortex and hippocampus. ERalpha stained cell bodies were not seen in the striatum. Counts of ERalpha labeled neurons in intact female rats revealed significantly more type 2 neurons within the basal forebrain subfields. Quantitation of ERalpha immunoreactive neurons revealed a significant decrease in the relative number of type 1 neurons within the medial septum (MS), horizontal limb of the diagonal band (HDB) and substantia innominata/nucleus basalis (SI/NB) following ovariectomy. Quantitation following choline acetyltransferease (ChAT) immunohistochemistry revealed a significant decrease in the number of ChAT positive neurons within the MS, HDB and SI/NB, but not VDB following ovariectomy. Following ovx, the percentage of double labeled cholinergic basal forebrain neurons also declined significantly within the MS, VDB, HDB and SI/NB. These observations suggest that estrogen effects a subpopulation of cholinergic basal forebrain neurons and may provide insight into the biologic actions of this steroid in Alzheimer's disease.     

Projections of the median raphe nucleus in the rat

No previous report in any species has examined comprehensively the projections of the median raphe (MR) nucleus with modern tracing techniques. The present report represents an in depth analysis of the projections of MR by use of the anterograde anatomical tracer Phaseolus vulgaris-leucoagglutinin. MR fibers descend along the midline within the brainstem and mainly ascend within the medial forebrain bundle in the forebrain. MR fibers distribute densely to the following brainstem/forebrain sites: caudal raphe nuclei, laterodorsal tegmental nucleus, dorsal raphe nucleus, interpeduncular nucleus, medial mammillary body, supramammillary nucleus, posterior nucleus and perifornical region of the hypothalamus, midline and intralaminar nuclei of thalamus, dopamine-containing cell region of medial zona incerta, lateral habenula, horizontal and vertical limbs of the diagonal band nuclei, medial septum, and hippocampal formation. Virtually all of these structures lie on or close to the midline, indicating that the MR represents a midline/para-midline system of projections. Overall, MR projections to the cortex are light. MR projects moderately to the perirhinal, entorhinal and frontal cortices, but sparingly to remaining regions of cortex. A comparison of MR with dorsal raphe (DR) projections (Vertes RP. 1991. J Comp Neurol 313:643-668) shows that these two major serotonin-containing cell groups of the midbrain distribute to essentially nonoverlapping regions of the forebrain; that is, the MR and DR project to complementary sites in the forebrain. A direct role for the MR in the desynchronization of the electroencephalographic activity of the hippocampus and its possible consequences for memory-associated functions of the hippocampus is discussed.     

Growth of medial forebrain bundle axons into peripheral nerve grafts in the rat

In rats, we intercepted medial forebrain bundle axons just lateral to the hypothalamus with peripheral nerve grafts which terminated extracranially. The neurons which grew into the nerve grafts were labeled with retrogradely transported fluorescent dyes. Catecholamines were labeled with glyoxylic acid histofluorescence. Most nuclei, particularly the raphe complex and locus coeruleus, which project rostrally into the medial forebrain bundle were labeled. Many catecholamine fibers were observed in the graft even after removal of the superior cervical ganglions. Thus, monoaminergic neurons which were located relatively remotely from the implant site exhibited rather selective regrowth into the nerve grafts.     

Transneuronal pathways to the vestibulocerebellum

The α-herpes virus (pseudorabies, PRV) was used to observe central nervous system (CNS) pathways associated with the vestibulocerebellar system. Retrograde transneuronal migration of α-herpes virions from specific lobules of the gerbil and rat vestibulo-cerebellar cortex was detected immunohistochemically. Using a time series analysis, progression of infection along polyneuronal cerebellar afferent pathways was examined. Pressure injections of >20 nanoliters of a 10⁸ plaque forming units (pfu) per ml solution of virus were sufficient to initiate an infectious locus which resulted in labeled neurons in the inferior olivary subnuclei, vestibular nuclei, and their afferent cell groups in a progressive temporal fashion and in growing complexity with increasing incubation time. We show that climbing fibers and some other cerebellar afferent fibers transported the virus retrogradely from the cerebellum within 24 hours. One to three days after cerebellar infection discrete cell groups were labeled and appropriate laterality within crossed projections was preserved. Subsequent nuclei labeled with PRV after infection of the flocculus/paraflocculus, or nodulus/uvula, included the following: vestibular (e.g., z) and inferior olivary nuclei (e.g., dorsal cap), accessory oculomotor (e.g., Darkschewitsch n.) and accessory optic related nuclei, (e.g., the nucleus of the optic tract, and the medial terminal nucleus); noradrenergic, raphe, and reticular cell groups (e.g., locus coeruleus, dorsal raphe, raphe pontis, and the lateral reticular tract); other vestibulocerebellum sites, the periaqueductal gray, substantia nigra, hippocampus, thalamus and hypothalamus, amygdala, septal nuclei, and the frontal, cingulate, entorhinal, perirhinal, and insular cortices. However, there were differences in the resulting labeling between infection in either region. Double-labeling experiments revealed that vestibular efferent neurons are located adjacent to, but are not included among, flocculus-projecting supragenual neurons. PRV transport from the vestibular labyrinth and cervical muscles also resulted in CNS infections. Virus propagation in situ provides specific connectivity information based on the functional transport across synapses. The findings support and extend anatomical data regarding vestibulo-olivo-cerebellar pathways. © 1996 Wiley-Liss, Inc.     

The histaminergic system in the guinea pig central nervous system: an immunocytochemical mapping study using an antiserum against histamine

Using an antiserum against conjugated histamine we mapped the histaminergic somata and their fiber projection areas in carbodiimide-fixed guinea pig central nervous system. The neurons were large and they were found exclusively in the posterior hypothalamus, as in the rat, but in the guinea pig they were more numerous and distributed more widely in thin layer around the posterior mammillary nucleus, scattered between and within the medial mammillary nuclei, and in a dense cell cluster emerging from the caudal magnocellular nucleus and extending to the medial preoptic area. The density of histamine-immunopositive fibers was very high in the olfactory tubercle, diagonal band of Broca, nucleus accumbens, medial and cortical amygdaloid nuclei, periventricular and lateral basal hypothalamus, paraventricular thalamus, and in a region from the medial central gray to the locus coeruleus and the parabrachial nucleus. Dense fiber networks were found in the piriform and entorhinal cortex, septum, dentate gyrus, and subiculum, in most parts of amygdala, and in many areas of the hypothalamus, thalamus, substantia nigra, raphe nuclei, inferior olivary, solitary tract and medial vestibular nuclei, and neurohypophysis. Medium fiber density was observed in the internal layers of the olfactory bulb, anterior olfactory nuclei, neocortex, zone CA1 of hippocampus, and many midbrain and hindbrain regions. Low density was present in the outer layers of the olfactory bulb, other parts of hippocampus, the globus pallidus, most of the caudatus-putamen, the cerebellar cortex, and the dorsal horn of the spinal cord. The retina and most of the myelinated white matter had single or no histaminergic fibers. It may be concluded from the results that most fibers seem to follow a ventromedial route to the forebrain, reaching the amygdala ventral to the medial forebrain bundle, the hippocampus via subiculum, and the hindbrain structures via the medial central gray. As compared to the rat, the fiber projections in the guinea pig brain were denser, particularly in the hippocampus, thalamus, pons-medulla, and neurohypophysis. The fiber densities in various regions of the guinea pig brain are compared to histamine receptor densities and the possible functions of histamine are discussed.     

Organization of cerebral cortical afferent systems in the rat. II. Magnocellular basal nucleus

The organization of the magnocellular basal nucleus (MBN) projection to cerebral cortex in the rat has been studied by using cytoarchitectonic, immunohistochemical, and retrograde and anterograde transport methods. The distribution of retrogradely labeled basal forebrain neurons after cortical injections of wheat germ agglutinin-horseradish peroxidase conjugate was essentially identical to that of neurons staining immunohistochemically for choline acetyltransferase. These large (20-30 micrometers perikaryon diameter) multipolar neurons were found scattered through a number of basal forebrain cell groups: medial septal nucleus, nucleus of the diagonal band of Broca, magnocellular preoptic nucleus, substantia innominata, and globus pallidus. This peculiar distribution mimics the locations of pathways by which descending cortical fibers enter the diencephalon. Each cortical area was innervated by a characteristic subset of MBN neurons, always located in close association with descending cortical fibers. In many instances anterogradely labeled descending cortical fibers appeared to ramify into diffuse terminal fields among MBN neurons which were retrogradely labeled by the same cortical injection. Double label experiments using retrograde transport of fluorescent dyes confirmed that MBN neurons innervate restricted cortical fields. Anterograde autoradiographic transport studies after injections of 3H-amino acids into MBN revealed that MBN axons reach cerebral cortex primarily via two pathways: (1) The medial pathway, arising from the medial septal nucleus, nucleus of the diagonal band, and medial substantia innominata and globus pallidus MBN neurons, curves dorsally rostral to the diagonal band nucleus, up to the genu of the corpus callosum. Most of the fibers either directly enter medial frontal cortex or turn back over the genu of the corpus callosum into the superficial medial cingulate bundle. Many of these fibers enter anterior cigulate or retrosplenial cortex, but some can be traced back to the splenium of the corpus callosum, where a few enter visual cortex but most turn ventrally and sweep into the hippocampal formation. Here they are joined by other fibers which, at the genu of the corpus callosum, remain ventrally located and run caudally through the dorsal fornix into the hippocampus. (2) The lateral pathway arises in part from medial septal, diagonal band, and magnocellular preoptic neurons whose axons sweep laterally through the substantia innominata to innervate primarily piriform, perirhinal, and endorhinal cortex. Some of these fibers may also enter the hippocampal formation from the entorhinal cortex via the ventral subiculum.(ABSTRACT TRUNCATED AT 400 WORDS)     

Branched connections to the septum and to the entorhinal cortex from the hippocampus, amygdala, and diencephalon in the rat

Neuronal cell populations giving origin to bifurcating projections to the septum and the entorhinal cortex were studied in the rat by means of double retrograde labeling using the fluorescent tracers Fast Blue and Diamidino Yellow. Double labeled pyramidal neurons were consistently detected in the temporal level of the CA1 area and subiculum of the hippocampal formation, where they represented at least 50% of the cells retrogradely labeled from the entorhinal injections. Double labeled neurons were also detected in the amygdala, where they prevailed in the basal complex. Scattered double labeled neurons were observed in a number of hypothalamic nuclei, with a slight predominance in the preoptic region. Finally, a few double labeled cells were detected in the midline thalamus, and especially in the thalamic paraventricular nucleus. In all these structures, double labeled neurons were located ispilaterally to the injection sites. The present data indicate that the septum and entorhinal cortex are tightly interconnected by axonal bifurcations deriving from a variety of telencephalic and diencephalic sources.     

Afferents to the hippocampus of the rat studied with the method of retrograde transport of horseradish peroxidase

Horseradish peroxidase (HRP) injected into rat hippocampus was transported to the perikarya of neurons which project to the hippocampus. HRP-labeled cells were present in both medial and lateral entorhinal cortex; cells of the medial entorhinal cortex appeared to be topographically organized. The mediaal septal nucleus contained stained cells; its mediaal aspect was labeled after dorsal hippocampal injections, while ventral hippocampal injections resulted in the labeling of more laterally located cells. Stained cells were also observed in the ipsilateral nucleus locus coeruleus, dorsal and median raphe nuclei and areas CA3–4 of the contralateral hippocampus. In additions, cells in the supramammillary region, an area not previously recognized to project to the hippocampus, were labeled. Finally, the mossy fiber terminal zone and the CA3–4 terminal zone in the dentate molecular layer of the ipsilateral hippocampus demonstrated HRP activity, presumably the result of orthograde axonal transport from the injection site.     

Afferent connections of the entorhinal area in the rat as demonstrated by retrograde cell-labeling with horseradish peroxidase

The entorhinal cortex (EC) of the rat has been divided into medial (MEA) and lateral (LEA) subdivisions. In order to analyze its afferent connections, small deposits of horseradish peroxidase (HRP) were placed at various loci within EC. The patterns of retrograde cell-labeling charted in 18 such cases suggested that EC is projected upon by several allocortical and subcortical structures and that there are differences in the afferent connections of the two subdivisions. Thus, although HRP injection of either division of EC led to cell-labeling in the hippocampal formation, most in ammonic field CA1 and the subiculum, several cells of the presubiculum were preferentially labeled by injection of MEA. Injections of LEA, but not those in MEA, resulted in substantial cell-labeling in the anterior piriform cortex of both hemispheres. Regardless of the location of its injection site within EC, the enzyme labeled cells in the diagonal band nucleus of Broca, amygdala and claustrum. The pattern of cell-labeling in the diagonal band nucleus extended into the ventrolaterally contiguous nucleus basalis after injection of LEA and into the dorsally contiguous medial septal nucleus after injection of MEA Whereas HRP deposits in either division of EC resulted in cell-labeling in the cortical and medial nuclei of the amygdala, only those deposits which involved LEA led to cell-labeling in the posterior part of the lateral nucleus. In the thalamus, labeled cells were found in the rostral part of the paratenial, periventricular and reuniens nuclei. Finally, at midbrain levels, numerous labeled cells appeared in the dorsal raphe nucleus, a few in the median raphe and locus coeruleus, and, only after rostral EC injection, in the ventral tegmental area.     

Efferent projections of the infralimbic (area 25) region of the medial prefrontal cortex in the rat: an anterograde tracer PHA-L study

The efferent projections of the infralimbic region (IL) of the medial prefrontal cortex of the rat were examined by using the anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L). Major targets of the IL were found to include the agranular insular cortex, olfactory tubercle, perirhinal cortex, the whole amygdaloid complex, caudate putamen, accumbens nucleus, bed nucleus of the stria terminalis, midline thalamic nuclei, the lateral preoptic nucleus, paraventricular nucleus, supramammillary nucleus, medial mammillary nucleus, dorsal and posterior areas of the hypothalamus, ventral tegmental area, central gray, interpeduncular nucleus, dorsal raphe, lateral parabrachial nucleus and locus coeruleus. Previously unreported projections of the IL to the anterior olfactory nucleus, piriform cortex, anterior hypothalamic area and lateroanterior hypothalamic nucleus were observed. The density of labeled terminals was especially high in the agranular insular cortex, olfactory tubercle, medial division of the mediodorsal nucleus of the thalamus, dorsal hypothalamic area and the lateral division of the central amygdaloid nucleus. Several physiological and pharmacological studies have suggested that the IL functions as the 'visceral motor' cortex, involved in autonomic integration with behavioral and emotional events. The present investigation is the first comprehensive study of the IL efferent projections to support this concept.     

Afferent connections of the anterior thalamus in rabbits

This study was designed to determine whether axons of cholinergic dorsal tegmental neurons terminate on cells in the anterior thalamus in rabbits as in other species, and to localize projecting tegmental cells for future studies of their contributions to anterior thalamic learning-relevant neuronal activity. The distribution of retrogradely labeled neurons was examined following injections of wheat germ agglutinin horseradish peroxidase (WGA-HRP) centered in the anterior ventral (AV) thalamic nucleus. The results confirm past findings in rabbits indicating projections to anterior thalamus from the mammillary nuclei, the posterior cingulate cortex, presubiculum and postsubiculum. Demonstrated for the first time in rabbits were projections from the lateral dorsal and the pedunculopontine tegmental nuclei, locus coeruleus, dorsal raphe nucleus, Gudden's dorsal tegmental nucleus, pretectum and reticular thalamic nucleus.     

An experimental study of the ventral striatum of the golden hamster. I. Neuronal connections of the nucleus accumbens

As part of an experimental study of the ventral striatum, the horseradish peroxidase (HRP) method was used to examine the afferent and efferent neuronal connections of the nucleus accumbens. Following iontophoretic applications or hydraulic injections of HRP in nucleus accumbens, cells labeled by retrograde transport of HRP were observed in the ipsilateral telencephalon in the posterior agranular insular, perirhinal, entorhinal, and primary olfactory cortices, in the subiculum and hippocampal field CA1, and in the anterior and posterior divisions of the basolateral amygdaloid nucleus. In the diencephalon, labeled neurons were present ipsilaterally in the central medial, paracentral and parafascicular intralaminar nuclei, and in the midline nuclei parataenialis, paraventricularis, and reuniens. Retrograde labeling was observed in the ipsilateral brainstem in cells of the ventral tegmental area and dorsal raphe. Many of these projections to nucleus accumbens were found to be topographically organized. Anterograde transport of HRP from nucleus accumbens demonstrated ipsilateral terminal fields in the ventral pallidum and substantia nigra, pars reticulata. The afferent projections to nucleus accumbens from the posterior insular and perirhinal neocortices, intralaminar thalamus, and the dopamine-containing ventral tegmental area are analogous to the connections of the caudatoputamen, as are the efferents from nucleus accumbens to the substantia nigra and ventral globus pallidus. These connections substantiate the classification of nucleus accumbens as a striatal structure and provide support for the recently proposed concept of the ventral striatum. Furthermore, the demonstration that a number of limbic system structures, including the amygdala, hippocampal formation, entorhinal cortex, and olfactory cortex are important sources of afferents to the nucleus accumbens, suggests that the ventral striatum may serve to integrate limbic information into the striatal system.