Organization of subcortical pathways for sensory projections to the limbic cortex. I. Subcortical projections to the medial limbic cortex in the rat

Subcortical afferent projections to the medial limbic cortex were examined in the rat by the use of retrograde axonal transport of horseradish peroxidase. Small iontophoretic injections of horseradish peroxidase were placed at various locations within the dorsal and ventral cingulate areas, the dorsal agranular and ventral granular divisions of the retrosplenial cortex and the presubiculum. Somata of afferent neurons in the thalamus and basal forebrain were identified by retrograde labeling. Each of the anterior thalamic nuclei was found to project to several limbic cortical areas, although not with equal density. The anterior dorsal nucleus projects primarily to the presubiculum and ventral retrosplenial cortex; the anterior ventral nucleus projects to the retrosplenial cortex and the presubiculum with apparently similar densities; and the anterior medial nucleus projects primarily to the cingulate areas. The projections from the lateral dorsal nucleus to these limbic cortical areas are organized in a loose topographic fashion. The projection to the presubiculum originates in the most dorsal portion of the lateral dorsal nucleus. The projection to the ventral retrosplenial cortex originates in rostral and medial portions of the nucleus, whereas afferents to the dorsal retrosplenial cortex originate in caudal portions of the lateral dorsal nucleus. The projection to the cingulate originates in the ventral portion of the lateral dorsal nucleus. Other projections from the thalamus originate in the intralaminar and midline nuclei, including the central lateral, central dorsal, central medial, paracentral, reuniens, and paraventricular nuclei, and the ventral medial and ventral anterior nuclei. In addition, projections to the medial limbic cortex from the basal forebrain originate in cells of the nucleus of the diagonal band. Projections to the presubiculum also originate in the medial septum. These results are discussed in regard to convergence of sensory and nonsensory information projecting to the limbic cortex and the types of visual and other sensory information that may be relayed to the limbic cortex by these projections.     

Evidence for some collateralization between cortical and diencephalic efferent axons of the rat subicular cortex

The present study has used the fluorescent dye tracing technique in order to determine the exact location of neuronal somata within the subicular cortex which project to the diencephalon, telencephalon (entorhinal cortex), or to both via axonal collaterals. The greatest collateralization to the two sites is found in the neurons of the subiculum proper. In this region approximately one-third of all neurons project to both the entorhinal cortex and the hypothalamus (either the mammillary bodies or the ventral medial hypothalamic nucleus). The hypothalamic and cortical projection cell bodies in this region are intermingled extensively with each other. In the cytoarchitectonically more organized regions of the subicular cortex, i.e. the pre-, para- and postsubiculum, the situation is quite different. In these areas neurons project to the hypothalamus or entorhinal cortex but very seldom does a single neuron project to both areas, and the neuronal somata are spatially segregated according to their projections. The entorhinal cortex projecting somata are located in layer two whereas the hypothalamic neurons are in the deeper layer. The somata projecting to the thalamus are the most deeply located neurons in all regions of the subicular cortex, and extremely few collateralize to the entorhinal cortex.     

Organization of neural inputs to the suprachiasmatic nucleus in the rat

The circadian timing of the suprachiasmatic nucleus (SCN) is modulated by its neural inputs. In the present study, we examine the organization of the neural inputs to the rat SCN using both retrograde and anterograde tracing methods. After Fluoro-Gold injections into the SCN, retrogradely labeled neurons are present in a number of brain areas, including the infralimbic cortex, the lateral septum, the medial preoptic area, the subfornical organ, the paraventricular thalamus, the subparaventricular zone, the ventromedial hypothalamic nucleus, the posterior hypothalamic area, the intergeniculate leaflet, the olivary pretectal nucleus, the ventral subiculum, and the median raphe nuclei. In the anterograde tracing experiments, we observe three patterns of afferent termination within the SCN that correspond to the photic/raphe, limbic/hypothalamic, and thalamic inputs. The median raphe projection to the SCN terminates densely within the ventral subdivision and sparsely within the dorsal subdivision. Similarly, areas that receive photic input, such as the retina, the intergeniculate leaflet, and the pretectal area, densely innervate the ventral SCN but provide only minor innervation of the dorsal SCN. A complementary pattern of axonal labeling, with labeled fibers concentrated in the dorsal SCN, is observed after anterograde tracer injections into the hypothalamus and into limbic areas, such as the ventral subiculum and infralimbic cortex. A third, less common pattern of labeling, exemplified by the paraventricular thalamic afferents, consists of diffuse axonal labeling throughout the SCN. Our results show that the SCN afferent connections are topographically organized. These hodological differences may reflect a functional heterogeneity within the SCN. J. Comp. Neurol. 389:508–534, 1997. © 1997 Wiley-Liss, Inc.     

A putative striato-dorsal thalamic pathway in lizards

Striatal targets related to the dorsal thalamus were studied in reptiles. The lateral striatum projects to globus pallidus and to three cellular groups associated to the lateral forebrain bundle: the anterior entopeduncular nucleus, the suprapeduncular nucleus, and the ventromedial thalamic nucleus. The projection is heavier on the suprapeduncular nucleus, which in turn projects on nucleus rotundus in the dorsal thalamus. Nucleus rotundus is the origin of a prominent projection to the lateral striatum among other forebrain areas. The intermediomedial striatum projects also to globus pallidus and to the three cellular groups associated with the lateral forebrain bundle, but in this case, the projection is heavier on the ventromedial thalamic nucleus. The latter nucleus targets a number of nuclear aggregates in the ventral tier of the dorsal thalamus, which in turn project to the intermediomedial striatum. As in mammals, the striatum in reptiles may influence through these pathways its input from the dorsal thalamus.     

Collateral projections of single neurons in the posterior thalamic region to both the temporal cortex and the amygdala: a fluorescent retrograde double-labeling study in the rat

It has been reported that the acoustic thalamus of the rat sends projection fibers to both the temporal cortical areas and the lateral amygdaloid nucleus to mediate conditioned emotional responses to an acoustic stimulus. In the present study, fluorescent retrograde double labeling with Fast Blue and Diamidino Yellow has been used in the rat to examine whether single neurons in the posterior thalamic region send axon collaterals to both the temporal cortical areas and lateral amygdaloid nucleus. One of the tracers was injected into the lateral amygdaloid nucleus and the other into the temporal cortical areas close to the rhinal sulcus. Neurons double-labeled with both tracers were found mainly in the posterior intralaminar nucleus and suprageniculate nucleus, and to a lesser extent in the subparafascicular nucleus and medial division of the medial geniculate nucleus. No double-labeled neurons were seen in either the dorsal or ventral division of the medial geniculate nucleus. When one of the tracers was injected into the lateral amygdaloid nucleus and the other into either the dorsal portion of the temporal cortex, the dorsal portion of the entorhinal cortex, or the posterior agranular insular cortex, no double-labeled neurons were found in the posterior thalamic region. The present results indicate that a substantial number of single neurons in the acoustic thalamus project to both the limbic cortical areas and lateral amygdaloid nucleus by way of axon collaterals. These neurons may be implicated in affective and autonomic components of responses to multi-sensory stimuli, including acoustic ones. J. Comp. Neurol. 384:59-70, 1997. © 1997 Wiley-Liss, Inc.     

Parallel but separate inputs from limbic cortices to the mammillary bodies and anterior thalamic nuclei in the rat

The proposal that separate populations of subicular cells provide the direct hippocampal projections to the mammillary bodies and anterior thalamic nuclei was tested by placing two different fluorescent tracers in these two sites. In spite of varying the injection locations within the mammillary bodies and within the three principal anterior thalamic nuclei and the lateral dorsal thalamic nucleus, the overall pattern of results remained consistent. Neurons projecting to the thalamus were localized to the deepest cell populations within the subiculum while neurons projecting to the mammillary bodies consisted of more superficially placed pyramidal cells within the subiculum. Even when these two cell populations become more intermingled, e.g., in parts of the intermediate subiculum, almost no individual cells were found to project to both diencephalic targets. In adjacent limbic areas, i.e., the retrosplenial cortex, postsubiculum, and entorhinal cortex, populations of cells that project to the anterior thalamic nuclei and mammillary bodies were completely segregated. This segregated pattern included afferents to those nuclei comprising the head‐direction system. The sole exception was a handful of double‐labeled cells, mainly confined to the ventral subiculum, that were only found after pairs of injections in the anteromedial thalamic nucleus and mammillary bodies. The projections to the anterior thalamic nuclei also had a septal‐temporal gradient with relatively fewer cells projecting from the ventral (temporal) subiculum. These limbic projections to the mammillary bodies and anterior thalamus comprise a circuit that is vital for memory, within which the two major components could convey parallel, independent information. J. Comp. Neurol. 518:2334–2354, 2010. © 2010 Wiley‐Liss, Inc.     

Efferent projections of the suprachiasmatic nucleus: II. Studies using retrograde transport of fluorescent dyes and simultaneous peptide immunohistochemistry in the rat

In a previous study (Watts et al., '87) we reexamined the projections of the suprachiasmatic nucleus (SCh) with the PHA-L method and found that they could be divided conveniently into six groups of fibers. By far the densest projection ends just dorsal to the SCh in a comma-shaped region designated the "subparaventricular zone," although some fibers continue on through the paraventricular nucleus of the hypothalamus to end in the overlying midline thalamus, and others continue on to end in the dorsomedial nucleus, the region around the ventromedial nucleus, and the posterior hypothalamic area. Other relatively sparse projections from the SCh were also described to the preoptic region, lateral septal nucleus, parataenial and paraventricular nuclei of the thalamus, and ventral lateral geniculate nucleus. In addition, the same method was used to show that the subparaventricular zone projects in turn massively to these same regions, as well as back to the SCh itself and to the periaqueductal gray. The present series of experiments was designed to confirm these observations with retrograde tracer injections and to investigate the cellular and possible neurotransmitter organization of the major projections from the SCh and subparaventricular zone with a combined retrograde tracer-immunohistochemical method. For this, the distribution of neuronal cell bodies within the SCh that stain with antisera to vasopressin, vasoactive intestinal polypeptide (VIP), corticotropin-releasing factor, bombesin, substance P, neurotensin, somatostatin, thyrotropin-releasing hormone, and angiotensin II was described in detail first. Then the distribution of retrogradely labeled neurons that were also stained for one or another of these peptides was described after injections of true blue, or in some cases SITS, into the regions of the subparaventricular zone, the paraventricular and parataenial nuclei of the thalamus, the ventromedial nucleus, the dorsomedial nucleus, and the periaqueductal gray. The results confirm previous immunohistochemical and anterograde tracing studies and in addition indicate that cells in dorsal as well as ventral parts of the SCh project to each of the terminal fields examined, as do many cells in surrounding areas, including the subparaventricular zone. Our results also suggest that, at the very least, vasopressin-, VIP-, and neurotensin-stained cells in the SCh project to the subparaventricular zone, midline thalamus, and dorsomedial nucleus, and that the vasopressin and VIP-stained fiber systems are partially segregated at the level of the subparaventricular zone.(ABSTRACT TRUNCATED AT 400 WORDS)     

The connections of the septal region in the rat

The efferent, afferent and intrinsic connections of the septal region have been analyzed in the rat with the autoradiographic method. The lateral septal nucleus, which can be divided into dorsal, intermediate and ventral parts, receives its major input from the hippocampal formation and projects to the medial septal-diagonal band complex. The ventral part of the nucleus also sends fibers through the medial forebrain bundle to the medial preoptic and anterior hypothalamic areas, to the lateral hypothalamic area and the dorsomedial nucleus, to the mammillary body (including the supramammillary region), and to the ventral tegmental area. The medial septal nucleus/diagonal band complex projects back to the hippocampal formation by way of the dorsal fornix, fimbria, and possibly the cingulum. Both nuclei also project through the medial forebrain bundle to the medial and lateral preoptic areas, to the lateral hypothalamic area, and to the mammillary complex. The medial septal nucleus also sends fibers to the midbrain (the ventral tegmental area and raphe nuclei) and to the parataenial nucleus of the thalamus, while the nucleus of the diagonal band has an additional projection to the anterior limbic area. Ascending inputs to the medial septal nucleus/diagonal band complex arise in several hypothalamic nuclei and in the brainstem aminergic cell groups. The posterior septal nuclei (the septofimbrial and triangular nuclei) receive their major input from the hippocampal formation, and project in a topographically ordered manner upon the habenular nuclei and the interpeduncular nuclear complex. The bed nucleus of the stria terminalis receives its major input from the amygdala (Krettek and Price, '78); but other afferents arise from the ventral subiculum, the ventromedial nucleus, and the brainstem aminergic cell groups. The principal output of the bed nucleus is through the medial forebrain bundle to the substantia innominata, the nucleus accumbens, most parts of the hypothalamus and the preoptic area, the central tegmental fields of the midbrain, the ventral tegmental area, the dorsal and median nuclei of the raphe, and the locus coeruleus. The bed nucleus also projects to the anterior nuclei of the thalamus, the parataenial and paraventricular nuclei, and the medial habenular nucleus, and through the stria terminalis to the medial and central nuclei of the amygdala, and to the amygdalo-hippocampal transition area.     

Septal complex of the telencephalon of lizards: III. Efferent connections and general discussion

The projections of the septum of the lizard Podarcis hispanica (Lacertidae) were studied by combining retrograde and anterograde neuroanatomical tracing. The results confirm the classification of septal nuclei into three main divisions. The nuclei composing the central septal division (anterior, lateral, medial, dorsolateral, and ventrolateral nuclei) displayed differential projections to the basal telencephalon, preoptic and anterior hypothalamus, lateral hypothalamic area, dorsal hypothalamus, mammillary complex, dorsomedial anterior thalamus, ventral tegmental area, interpeduncular nucleus, raphe nucleus, torus semicircularis pars laminaris, reptilian A8 nucleus/ substantia nigra and central gray. For instance, only the medial septal nucleus projected substantially to the thalamus whereas the anterior septum was the only nucleus projecting to the caudal midbrain including the central gray. The anterior and lateral septal nuclei also differ in the way in which their projection to the preoptic hypothalamus terminated. The midline septal division is composed of the dorsal septal nucleus, nucleus septalis impar and nucleus of the posterior pallial commissure. The latter two nuclei projected to the lateral habenula and, at least the nucleus of the posterior pallial commissure, to the mammillary complex. The dorsal septal nucleus projected to the preoptic and periventricular hypothalamus and the anterior thalamus, but its central part seemed to project to the caudal midbrain (up to the midbrain central gray). Finally, the ventromedial septal division (ventromedial septal nucleus) showed a massive projection to the anterior and the lateral tuberomammillary hypothalamus. Data on the connections of the septum of P. hispanica and Gecko gekko are discussed from a comparative point of view and used for better understanding of the functional anatomy of the tetrapodian septum. J. Comp. Neurol. 401:525–548, 1998. © 1998 Wiley-Liss, Inc.     

Efferent connections of the hippocampal formation in the rat.

In this investigation the projections of the hippocampal formation to the septal area and hypothalamus were studied in the rat with the combined use of 3H-amino acid radioautography and horseradish peroxidase histochemistry. The results indicate that all of the fibers which project to the hypothalamus and the majority of fibers which project to the septum arise from the subicular cortex and not from hippocampal pyramidal cells. The projection to both of these areas are topographically organized along the longitudinal axis of the hippocampal formation. Specifically, fibers from subicular cortical cells situated at the septal end of the hippocampal formation which project through the medial part of the dorsal fornix terminate in the dorsomedial quadrant of the lateral septal nucleus and in the dorsal portion of the pars posterior of the medial mammillary nucleus. Fibers from progressively more posteroventral levels of the hippocampal formation which project through more lateral portions of the dorsal fornix and fimbria terminate in progressively lateral and ventral quadrants of the lateral septal nucleus and in progressively more ventral portions of the pars posterior. Concerning the specific origin of the fornix system, fibers from only the prosubiculum and subiculum project through both the pre- and postcommissural fornix. Hippocampal pyramidal cells from all CA fields have a restricted projection through the precommissural fornix and terminate in the caudal half of the septum while the presubiculum projects solely through the postcommissural fornix. The medial corticohypothalamic tract (MCHT) was found to arise from cells located in anterior ventral levels of the subicular cortex. Fibers from this tract appeared to be distributed throughout the pericellular region of the entire ventromedial extent of the hypothalamus from the level of the suprachiasmatic nucleus through the level of the medial mammillary nucleus. In this way, the mammillary bodies receive input from the subicular cortex via two routes: the descending column of the fornix and the MCHT.     

Organization of the pallium in the fire-bellied toad Bombina orientalis. I: Morphology and axonal projection pattern of neurons revealed by intracellular biocytin labeling

The cytoarchitecture and axonal projection pattern of pallial areas was studied in the fire-bellied toad Bombina orientalis by intracellular injection of biocytin into a total of 326 neurons forming 204 clusters. Five pallial regions were identified, differing in morphology and projection pattern of neurons. The rostral pallium receiving the bulk of dorsal thalamic afferents has reciprocal connections with all other pallial areas and projects to the septum, nucleus accumbens, and anterior dorsal striatum. The medial pallium projects bilaterally to the medial pallium, septum, nucleus accumbens, mediocentral amygdala, and hypothalamus and ipsilaterally to the rostral, dorsal, and lateral pallium. The ventral part of the medial pallium is distinguished by efferents to the eminentia thalami and the absence of contralateral projections. The dorsal pallium has only ipsilateral projections running to the rostral, medial, and lateral pallium; septum; nucleus accumbens; and eminentia thalami. The lateral pallium has ipsilateral projections to the olfactory bulbs and to the rostral, medial, dorsal, and ventral pallium. The ventral pallium including the striatopallial transition area (SPTA) has ipsilateral projections to the olfactory bulbs, rostral and lateral pallium, dorsal striatopallidum, vomeronasal amygdala, and hypothalamus. The medial pallium can be tentatively homologized with the mammalian hippocampal formation, the dorsal pallium with allocortical areas, the lateral pallium rostrally with the piriform and caudally with the entorhinal cortex, the ventral pallium with the accessory olfactory amygdala. The rostral pallium, with its projections to the dorsal and ventral striatopallidum, resembles the mammalian frontal cortex.     

Descending projections to the mammillary nuclei in the rat, as studied by retrograde and anterograde transport of wheat germ agglutinin-horseradish peroxidase

The cells of origin and projection fields of the descending afferents to the mammillary nuclei were studied in the rat with retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. The subiculum projects bilaterally to the entire medial mammillary nucleus (MM) in a topographic fashion along the two axes: 1) the proximal part of the subiculum along the presubiculo-CA1 axis projects to the caudal and lateral regions of the MM whereas the more distal part of the subiculum projects to the medial region; 2) the septal part of the subiculum projects to the caudodorsal region of the MM whereas the more temporal part projects progressively to the more rostroventral regions. The ventral subiculum also projects ipsilaterally to the ventral and lateral margin of the lateral mammillary nucleus (LM). The presubiculum projects bilaterally to the dorsolateral region of the pars posterior of the MM and ipsilaterally to the LM. The infra-limbic cortex projects bilaterally to the rostrodorsal region of the MM, whereas the retrosplenial cortex (areas 29a and 29b) projects bilaterally to the medial region at the midrostrocaudal and middorsoventral levels of the MM. The nucleus of the diagonal band projects bilaterally to the caudomedial region of the MM, whereas the lateral septal nucleus projects bilaterally to the pars mediana and the mammillary fiber capsule. A part of the anterior hypothalamic area ventromedial to the fornix projects predominantly ipsilaterally to the rostroventral part of the MM, whereas other basal forebrain regions such as the bed nucleus of the stria terminalis, the medial preoptic and anterior hypothalamic areas, and the area of the tuber cinereum send fibers predominantly ipsilaterally to the mammillary fiber capsule. The results reveal a complex organization of the descending projections to the mammillary nuclei, which may reflect the complex functions of these nuclei within the limbic circuitry.     

Ontogeny of oxytocin receptors in rat forebrain: a quantitative study

The ontogeny of oxytocin receptors in rat forebrain was studied using the selective oxytocin receptor antagonist 125I-d(CH2)5[Tyr(Me)2, Thr4, Tyr-NH29]OVT [( 125I]-OTA). With in vitro receptor autoradiography, binding wa noted on the first postnatal day in dorsal subiculum and thalamus. On postnatal days 5-18, intense labeling was evident in posterior cingulate cortex, dorsal subiculum, lateral septum, and the CA1 subfield of hippocampus. Of these regions only the lateral septum expressed oxytocin receptors in adult brain. Competition studies on coronal sections through posterior cingulate, septum, and dorsal subiculum at P10 demonstrated that transient binding sites in these areas were indeed oxytocin selective (OXY greater than AVP greater tha V1 greater than V2). Result of saturation studies on cingulate membranes from 10-day-old pups agreed favorably with previous reports of the kinetics of [125I]-OTA binding to adult oxytocin receptors (Kd = 0.1 nM in P10 cingulate cortex vs. 0.07 nM for adult ventral subiculum). In contrast to these evanescent developmental sites, oxytocin receptors in the bed nucleus of the stria terminalis and the ventromedial nucleus of the hypothalamus only appeared in adulthood, presumably in response to the surge of gonadal steroids at puberty.     

The connections of the nucleus reuniens thalami: evidence for a direct thalamo-hippocampal pathway in the rat

The afferent and efferent connections of the rat's midline nucleus reunions thalami (reuniens) were studied by experiments using the methods of retrograde cell marking by horseradish peroxidase (HRP) and anterograde fiber tracing by autoradiography. A microelectrophoretic deposit of tritiated amino acids in reuniens provided the first evidence of a direct thalamo-hippocampal connection. Labeled reuniens efferents ascend to the genu of the corpus callosum and turn caudally in the cingulate fasciculus, from which fibers distribute to layer I of the anterior medial, cingulate, and retrosplenial cortices. A longer component of this system curves around the callosal splenium and forms a massive rostrally directed fiber sheet that innervates entorhinal and parahippocampal areas and Ammon's horn. Entorhinal afferents are localized to layers I and III, whereas the hippocampal afferent plexus is remarkably restricted to the stratum lacunosum-moleculare of the CA1 field and the corresponding stratum of the ventral subiculum. Reuniens projects more sparsely and diffusely to many subcortical structures, a number of which lie in the limbic domain: the anterior olfactory nucleus, nucleus accumbens, olfactory tubercle, amygdala, claustrum, septum, preoptic area, medial and lateral hypothalamic regions, deep portions of the pretectum and superior colliculus, rostral levels of the ventral tegmental area and central gray substance and, perhaps, the median eminence. The efferent connections of reuniens were examined with HRP. HRP deposited in the nucleus labeled small to moderate numbers of neurons in many structures extending from the frontal cortex to caudal midbrain levels. The appearance of cell labeling in regions projected upon by reuniens suggests a reciprocity of connections between it and the medial cortex, septum, preoptic area, amygdala, medial and lateral hypothalamic regions, ventral tegmental area, central gray substance, pretectum, superior colliculus and the subiculum. Cell labeling in regions not receiving its efferents – the ventral thalamus, midbrain tegmentum, mesencephalic raphe, and parabrachial nuclei – may hold another clue to the future understanding of the role of the nucleus reuniens in limbic functions.     

Efferents from the medial anterior hypothalamic area in the guinea pig

Medial anterior hypothalamic connections were studied with H³-proline and autoradiography. Most of the axons projected to other hypothalamic nuclei. The major pathways were found ventral medial to the fornix and in the periventricular tract. Substantial projections were apparent in the ventromedial and dorsomedial nuclei with less label in the arcuate nucleus. The dorsal premammillary nuclei were labeled bilaterally, particularly with more caudal injections of anterior hypothalamus. Efferents were evident in the posterior hypothalamus and continued into the central gray of the midbrain. Labeled fibers reached the ventral tegmental area and in the reticular formation were traced only through pons. Rostral projections were to the medial and lateral preoptic areas and ventral lateral septum. The bed nucleus of stria terminalis was labeled and a very few fibers reached the medial amygdaloid nucleus. The periventricular nucleus of thalamus was labeled.     

Projections from the amygdaloid complex and adjacent olfactory structures to the entorhinal cortex and to the subiculum in the rat and cat

Axonal projections are described from the lateral and hasolateral nuclei of the amygdaloid complex, and from the overlying periamygdaloid and pre-piriform cortices and the endopiriform nucleus, to the lateral entorhinal area, the ventral part of the subiculum, and the parasubiculum in the cat and rat. All of these projections have well-defined laminar patterns of termination, which are complementary to those of other projections to the same structure. Based on these results, and on cytoarchitectonic distinctions, the lateral entorhinal area has been divided into dorsal, ventral, and ventromedial subdivisions. The olfactory bulb and prepiriform cortex project to layers IA and IB, respectively, of all three subdivisions, but the lateral amygdaloid nucleus has a restricted projection to layer 111 of the ventral subdivision only. The periamygdaloid cortex projects to layer II of the ventromedial and adjoining parts of the ventral subdivisions. The ventral part of the subiculum receives fibers from the posterior division of the hasolateral nucleus, which terminate in the cellular layer and the deep half to one-third of the plexiform layer. The periamygdaloid cortex and the endopiriform nucleus also project to the same part of the subiculum, but these fibers terminate in the outer part of the plexiform layer. None of these projections extend into the dorsal part of the subiculum. The posterior division of the basolateral nucleus also projects to the posterodorsal part of the parasubiculum (“parasubiculum a” of Blackstad, 1956). These fibers end in the deeper part of the plexiform layer and the superficial part of the cellular layer.     

Organization of ascending hypothalamic projections to the rostral forebrain with special reference to the innervation of cholinergic projection neurons

Axonal projections from hypothalamic nuclei to the basal forebrain, and their relation to cholinergic projection neurons in particular, were studied in the rat by using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) in combination with choline acetyltransferase (ChAT) immunocytochemistry. Discrete iontophoretic PHA-L injections were delivered to different portions of the caudal lateral hypothalamus, as well as to various medial hypothalamic areas, including the ventromedial, dorsomedial, and paraventricular nuclei, and anterior hypothalamic and medial preoptic areas. The simultaneous detection of PHA-L-labeled fibers/terminals and ChAT-positive neurons was performed by using nickel-enhanced diaminobenzidine (DAB) and nonenhanced DAB as chromogens. Selected cases were investigated at the electron microscopic level. Ascending hypothalamic projections maintained an orderly lateromedial arrangement within the different components of the medial forebrain bundle, as well as with respect to their terminal projection fields (e.g., within the bed nucleus of the stria terminalis and lateral septal nucleus). The distribution pattern of hypothalamic inputs to cholinergic projection neurons corresponded to the topography of ascending hypothalamic axons. Axons originating from neurons in the far-lateral hypothalamus reached cholinergic neurons in a zone that extended from the dorsal part of the sublenticular substantia innominata (SI) caudolaterally, to the lateral portion of the bed nucleus of the stria terminalis rostromedially, encompassing a narrow band along the ventral part of the globus pallidus and medial portion of the internal capsule. Axons originating from cells in the medial portion of the lateral hypothalamus reached cholinergic cells primarily in more medial and ventral parts of the SI, and in the magnocellular preoptic nucleus and horizontal limb of the diagonal band nucleus (HDB). Axons from medial hypothalamic cells appeared to contact cholinergic neurons primarily in the medial part of the HDB, and in the medial septum/vertical limb of the diagonal band complex. Electron microscopic double-labeling experiments confirmed contacts between labeled terminals and cholinergic cells in the HDB and SI. Individual hypothalamic axons established synapses with both cholinergic and noncholinergic neuronal elements in the same regions. These findings have important implications for our understanding of the organization of afferents to the basal forebrain cholinergic projection system.     

Ascending projections of the locus coeruleus in the rat. II. Autoradiographic study.

The ascending projections of the locus coeruleus were studied using an autoradiographic method. The major projection of locus coeruleus neurons ascends in a dorsal pathway traversing the midbrain tegmentum in a position ventrolateral to the periaqueductal gray. At the caudal diencephalon the locus coeruleus axons descend to enter the medial forebrain bundle at a caudal tuberal hypothalamic level. They are jointed in the medial forebrain bundle by a much smaller locus coeruleus projection which takes a ventral course through the midbrain tegmentum and enters the medial forebrain bundle via the mammillary peduncle and ventral tegmental area. Terminal projections are evident in the midbrain to the periaqueductal gray, tegmentum and raphe nuclei. There are widespread projections to the dorsal thalamus. The heaviest of these are to the intralaminar nuclei, the anteroventral and anteromedial nuclei, the dorsal lateral geniculate and the paraventricular nucleus. In the hypothalamus the largest projections are to the lateral hypothalamic area, periventricular nucleus, supraoptic nucleus and paraventricular nucleus. As the locus coeruleus projection ascends in the medial forebrain bundle, fibers leave it to traverse the lateral hypothalamus and zona incerta and enter the internal capsule, the ventral amygdaloid bundle and ansa peduncularis. These appear to terminate in the amygdaloid complex and, via the external capsule, in the lateral and dorsal neocortex. At the level of the septum 4 projections are evident. One group of fibers enters the stria medullaris to terminate in the paraventricular nucleus and habenular nuclei. A second group joins the stria terminalis to terminate in the anygdaloid complex. The third group turns into the diagonal band and medial septum; some fibers terminate in the septal nuclei and others continue into the fornix to termimate in hippocampus. A large component continues around the corpus callosum into the cingulum to terminate in the cingulate and adjacent neocortex, the subiculum and hippocampus. The remaining fibers continue rostrally in the medial forebrain bundle to terminate in olfactory forebrain and frontal neocortex. Commissural projections arise at 4 locations. The first decussation occurs in the dorsal tegmentum just below the central gray rostral to the locus coeruleus. The crossing fibers enter the contralateral dorsal bundle. A second group of fibers leaves the ipsilateral dorsal pathway, crosses in the posterior commissure and enters the contralateral dorsal pathway at the level. The third commissural projection arises more rostrally and crosses in the dorsal supraoptic commissure to enter the contralateral medial forebrain bundle. The fourth commissural projection is through the anterior commissure. The termination of the contralateral projection appears similar to that of the ipsilateral projection.     

Septal complex of the telencephalon of the lizard Podarcis hispanica. II. afferent connections

The afferent connections to the septal complex were studied in the lizard Podarcis hispanica (Lacertidae) by means of a combination of retrograde and anterograde tracing. The results of these experiments allow us to classify the septal nuclei into three main divisions. The central septal division (anterior, lateral, dorsolateral, ventrolateral, and medial septal nuclei plus the nucleus of the posterior pallial commissure) receives a massive, topographically organized, cortical projection (medial, dorsal, and ventral areas) and widespread afferents from the tuberomammillary hypothalamus and the basal telencephalon. Moreover, it receives discrete projections from the dorsomedial anterior thalamus, the ventral tegmentum, the midbrain raphe, and the locus coeruleus. The ventromedial septal division (ventromedial septal nucleus) receives a massive projection from the anterior hypothalamus, dense serotonergic innervation, and a faint amygdalohypothalamic projection, but it is devoid of direct cortical input. The midline septal division (nucleus septalis impar and dorsal septal nucleus) receives a nontopographic cortical projection (dorsomedial and dorsal cortices) and afferents from the preoptic hypothalamus, the dorsomedial anterior thalamus, the midbrain central gray, and the reptilian A8 nucleus/substantia nigra. Our results indicate that the cortex provides a physiologically complex, massive input to the septum that terminates over the whole dendritic tree of septal cells. In contrast, most of the ascending afferents make axosomatic contacts by means of pericellular nests. The chemical nature of the main septal afferents and the comparative implications of the available hodological data on the organization of the septal complex of tetrapod vertebrates are discussed. J. Comp. Neurol. 383:489-511, 1997. © 1997 Wiley-Liss, Inc.     

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.