The topographic organization of hypothalamic and brain stem projections to the hippocampus

Direct projections primarily ipsilateral to hippocampus from medial septal, diagonal band, supramammillary, submammillothalamic, locus coeruleus, and dorsal and medianus raphe nuclei were demonstrated. The locus coeruleus projects primarily through the cingulum and fornix superior to the dorsal posterior hippocampus, with its terminal fields in the stratum lacunosum moleculare of the subiculum and areas CA 1-CA 2 of the dorsal posterior hippocampus. LC projections to the granular layer of the dentate hilus were not found. Raphe nuclei project through the cingulum, fornix superior, and primarily the fimbria, to the dorsal and ventral posterior hippocampus, with their terminal fields in the stratum lacunosum moleculare of the dorsal posterior subicular region, stratum radiatum of CA 1-CA 3 in the dorsal hippocampus, and the stratum polymorph of the dentate gyrus, primarily in its superficial part. Raphe projections to the anterior hippocampal rudiment were found. However, no projection was found to the subiculum of the ventral posterior hippocampus, nor to stratum oriens. Hypothalamic nuclei project through the fornix superior and the fimbria, mainly to the dorsal posterior hippocampus with abundant terminal fibers in the depth of the dentate hilus. Smaller cells in these hypothalamic nuclei appear projecting to the ventral hippocampus. The number of neurons in the entorhinal area, the diagonal band, and the hypothalamic nuclei projecting to the hippocampus suggests these groups as the main sources of the extrinsic hippocampal afferents. In addition, they may also serve as relay stations for inputs from more caudal nuclei, and the topographic organization of their terminal fields as described herein may have important functional implications.     

An autoradiographic study of projections ascending from the midbrain central gray, and from the region lateral to it, in the rat

Ascending projections from the midbrain central gray (CG) and from the region lateral to it were traced in the rat using tritiated amino acid autoradiography. Leucine or a cocktail of amino acids (leucine, proline, lysine, histidine, and tyrosine) were used as tracers. In addition to projections within the midbrain, ascending fibers follow three trajectories. The ventral projection passes through the ventral tegmental region of Tsai and the medial forebrain bundle to reach the hypothalamus, preoptic area, caudoputamen, substantia innominata, stria terminalis, and amygdala. There are labeled fibers in the diagonal bands of Broca and medial septum, and terminal labeling in the lateral septum, nucleus accumbens, olfactory tubercle, and frontal cortex. The dorsal periventricular projection terminates in the midline and intralaminar thalamic nuclei. The ventral periventricular projection follows the ventral component of the third ventricle into the hypothalamus, passing primarily through the dorsal hypothalamic area and labeling the rostral hypothalamus and preoptic area. Projections from the region lateral to the CG are similar, but exhibit stronger proximal, and weaker distal, projections. Rostral levels of the CG send heavier projections to the fields of Forel and the zona incerta, but fewer fibers through the supraoptic decussation, than do caudal levels. Ascending projections from the CG are both strong and widespread. Strong projections to the limbic system and the intralaminar thalamic nuclei provide an anatomical substrate for CG involvement in nociception and affective responses.     

Biochemical mapping of the noradrenergic ventral bundle projection sites: Evidence for a noradrenergic-dopaminergic interaction

Norepinephrine (NE) and dopamine (DA) concentration and dopamine turnover were measured 12 days after a unilateral or bilateral noradrenergic ventral bundle (VB) transection to determine the noradrenergic projection sites and possible interactions with dopaminergic systems.Both bilateral and unilateral VB transection resulted in a significant reduction of NE of the nucleus accumbens, lateral septal nucleus, medial forebrain bundle, ventromedial nucleus, dorsomedial nucleus and medial amygdaloid nucleus. Bilateral transection also decreased NE content of the median eminence and the periventricular and arcuate nuclei. In the medial preoptic nucleus, the nucleus interstitialis striae terminalis and the central gray catecholamine area, bilateral transection significantly decreased NE concentrations while unilateral lesions had no significant effect. The anterior hypothalamic, lateral preoptic, and paraventricular nuclei responded to bilateral VB transection with a decrease in NE concentration and to unilateral lesion with a bilateral increase in NE. In the dorsal hippocampus and the caudate nucleus, bilateral lesions had no effect on NE concentrations while unilateral transection significantly decreased NE concentrations. Regions in which neither bilateral nor unilateral VB transection produced a significant change in NE content are the olfactory tubercle, the nucleus tractus diagonalis, substantia nigra pars compacta and reticulata, ventral tegmental area, habenula, superior colliculus, and the cingulate and piriform cortices.Transection of the noradrenergic ventral bundle also produced changes in dopaminergic systems suggesting a noradrenergic-dopaminergic interaction. Bilateral VB transection decreased the dopamine concentration and turnover in the nucleus accumbens, increased steady-state levels and turnover in the nucleus tractus diagonalis and increased dopamine concentration in the lateral septum. Unilateral VB transection decreased DA concentration bilaterally in the caudate nucleus, olfactory tubercle, nucleus accumbens and the nucleus interstitialis striae terminalis but increased concentrations in the substantia nigra pars reticulata (ipsilateral) and in the ventral tegmental area (bilateral). These results indicate a broad projection field for the noradrenergic ventral bundle and suggest a noradrenergic-dopaminergic interaction.     

Efferent connections of septal nuclei of the domestic chick (Gallus domesticus): an anterograde pathway tracing study with a bearing on functional circuits

Small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were placed in different subregions of the septum of domestic chicks. The main targets of septal projections comprised the ipsi- and contralateral septal nuclei, including the nucleus of the diagonal band, basal ganglia, including the ventral paleostriatum, lobus parolfactorius, nucleus accumbens, and olfactory tubercle, archistriatum, piriform cortex, and anterior neostriatum. Further diencephalic and mesencephalic septal projections were observed in the ipsilateral preoptic region, hypothalamus (the main regions of afferentation comprising the lateral hypothalamic nuclei, ventromedial, paraventricular and periventricular nuclei, and the mammillary region), dorsal thalamus, medial habenular and subhabenular nuclei, midbrain central gray, and ventral tegmental area. Contralateral projections were also encountered in the septal nuclei, ventral paleostriatum, periventricular and anteromedial hypothalamic nuclei, suprachiasmatic nucleus, and the lateral hypothalamic area. Avian septal efferents are largely similar to those of mammals, the main differences being a relatively modest hippocampal projection arising mainly from the nucleus of the diagonal band (as confirmed by a specific experiment with the retrograde pathway tracer True blue), the lack of interpeduncular projection, and a greater contingent of amygdalar efferents arising from the lateral septum rather than the nucleus of the diagonal band. This pattern of connectivity is likely to reflect an important role of the avian septal nuclei in the coordination of limbic circuits and the integration of a wide variety of information sources modulating the appropriate behavioral responses: attention and arousal level, memory formation, hormonally mediated behaviors, and their affective components (such as ingestive, reproductive, and parental behaviors), social interaction, locomotor modulation, and circadian rhythm.     

Hypothalamic afferent connections mediating adrenocortical responses that follow hippocampal stimulation

This study identified some neural pathways which mediate the adrenocortical responses that follow hippocampal stimulation. The increase in plasma corticosterone following dorsal hippocampus stimulation, in rats with electrodes chronically implanted under pentobarbital anesthesia, was blocked by dorsal fornix and lateral septal lesions and by small posterior hypothalamic deafferentation. Fimbria transection, lateral septal lesions, and posterior hypothalamic deafferentation, but not midbrain reticular formation lesions, also blocked the adrenocortical responses to ventral hippocampus stimulation. Our present and previous studies indicate that the dorsal and ventral hippocampal effects on the hypothalamus, which increase plasma corticosterone concentrations, are mediated by the dorsal fornix and fimbria, respectively, as well as by the lateral septum. A posterior hypothalamic input, which does not involve the medial forebrain bundle or the midbrain reticular formation is also essential for the activation of this response.     

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.     

Differential projections of habenular nuclei

Lesions were made in the lateral and medial habenular nuclei of the cat. Subsequent degeneration of nerve fibers and terminalis was studied using Nauta-Gygax silver technique. The medial and lateral habenular nuclei project differentially to the septum, olfactory, tubercle, thalamus, midbrain tegmentum and tectum. The diffuse part of the habenulopeduncular tract rises from the lateral habenular nucleus and the compact part rises from both nuclei. Degenerating terminals were seen caudally in the following nuclei: interpeduncular, central superior, dorsal raphae, ventral tegmental (from the medial habenular nucleus), dosral tegmental (from the lateral habenular nucleus), pretectal area, superior colliculus and inferior colliculus (from the lateral habenular nucleus). Rostral projections course in the medial part of the stria medullaris from the medial habenular nucleus and in the lateral part of the stria medullaris from the lateral habenular nucleus: Degenerating terminals were seen rostrally in the following nuclei: dorsomedial, anteroventral, anterodorsal, paraventricular, posterior medial septal (from the medial habenular nucleus) and preoptic area (from the lateral habenular nucleus). Projections occur from the medial habenular nucleus to the amygdala via the stria terminalis. The habenular nuclei are considered to be structures of the limbic system which are differentially related to midbrain, thalamic, amygdaloid, septal and preoptic structures via feedback circuits.     

An analysis of the efferent connections of the septal area in the cat

The neuroanatomical organization of the efferent connections of the septal area in the cat was analyzed by the use of anterograde ([3H]leucine radioautography) and retrograde (horseradish peroxidase histochemistry) tracing techniques. The results indicate that the lateral septal nucleus projects to the nuclei of the diagonal band, preoptic area, lateral hypothalamus, and supramammillary region. The projections of the septofimbrial nucleus supply the nuclei of the diagonal band and the medial habenular nucleus. Projection targets of the vertical limb of the diagonal band are widespread and include the preoptic area, lateral hypothalamus, anterior limbic cortex, amygdala, medial habenular nucleus, interpeduncular nucleus and hippocampal formation. The projection from the vertical limb to the hippocampal formation is organized in a topographical manner in such a fashion that cells positioned near the midline project to the dorsal hippocampus and adjoining subicular cortex while fibers originating from cells situated more laterally project to more ventral parts of the hippocampal formation. In general, the projections from the horizontal limb were similar to those from the vertical limb, but several differences were noted. Fibers arising from the horizontal limb are distributed to the ventral tegmental area and interpeduncular nucleus but this region seems to lack a projection to either the habenular complex or to the ventral aspect of the hippocampal formation. Fibers arising from the bed nucleus of the anterior commissure are distributed to the preoptic region, lateral hypothalamus, supramammillary region, posterior aspect of the medial mammillary nucleus and lateral habenular nucleus.     

Afferent connections of septal nuclei of the domestic chick (Gallus domesticus): a retrograde pathway tracing study

The afferents to the septum of the domestic chicken were studied using retrograde tracers, rhodamine conjugated latex bead or Fast Blue, placed in different septal subregions. The results were verified by anterograde tracer injections deposited to selected areas. The main telencephalic afferents to the septum arise ipsilaterally from the hippocampal formation, dorsolateral corticoid area, piriform cortex, amygdaloid pallium, and the ventral pallidum. Contralateral afferents originate from the lateral septum and the amygdaloid pallium. A massive bilateral projection arises from the lateral hypothalamus. Other hypothalamic afferents arise from the periventricular, paraventricular and anterior medial nuclei, and the premammillary and mammillary areas. The dorsal thalamic nuclei (dorsal medial anterior and posterior) and the reticular dorsal nuclei also contribute septal afferents. Brainstem afferents arise bilaterally from the ventral tegmental area, substantia nigra, central gray, A8, locus coeruleus, ventral subcoeruleus nucleus, and raphe nuclei. The main terminal fields for septal afferents lie in the lateral septal nucleus and the belt of medial septal nucleus. The core of the latter is invaded mainly by fibers from the brainstem, presumably belonging to the ascending activating system. The septal afferents of the chicken are largely similar to those of other avian and nonavian species. The most prominent differences with previous pigeon data were found in the subregional selectivity of the hippocampal formation, dorsolateral corticoid area, mammillary nuclei, some dorsal thalamic nuclei, substantia nigra, and subcoeruleus nuclei in their projections to defined septal nuclei.     

Efferent connections of the olfactory bulb in the cat. An experimental study with silver impregnation methods

After lesions of the olfactory bulb with some involvement of the anterior olfactory nucleus in the cat the ensuing degeneration was studied with both the Nauta and the Glees silver impregnation techniques. Preterminal and terminal degeneration was found in the anterior olfactory nucleus, pars lateralis and pars externa; the rostrolateral part of the olfactory tubercle; prepyriform and periamygdaloid cortices; anterior amygdaloid area; nucleus of the lateral olfactory tract and nucleus corticalis of amygdala, homolaterally. On the contralateral side the degeneration was restricted to the pars externa of the anterior olfactory nucleus and to the granular layer and the inner part of the internal plexiform layer of the olfactory bulb. In the olfactory tubercle and pyriform cortex the degeneration was located in the plexiform layer in which there were seen degenerating axo-dendritic synaptic contacts on the ascending dendrites of the pyramidal cells. The olfactory projection fibers which reach the contralateral anterior olfactory nucleus and bulb course through the anterior limb of the anterior commissure. In the olfactory bulb the fibers were found to end not only among the neuropil of the internal pleximorm and granular layers but also as axo-somatic contacts on the granular cells. The mitral cells and the periglomerular layer were completely free from degeneration. No degeneration was observed homolaterally in the rest of the amygdaloid complex, septal areas, bed nucleus of the anterior commissure, interstitial nucleus of the stria terminalis or hippocampus.     

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.     

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.     

Electrophysiological studies of the septal nuclei: I. The lateral septal region

Electrophysiological studies of the lateral septal region were performed on acutely prepared cats. The data indicate that the lateral septal region consists of two functionally distinct zones: a dorsal zone (i.e., dorsal septal nucleus) and a ventral zone (i.e., lateral septal nucleus). The dorsal septal nucleus receives a heavy ipsilateral fimbria projection, but receives no projection via the ventral septal afferent system. There is no return projection through the fimbria. Test responses recorded from the dorsal septal nucleus show prolonged periods of suppression. The lateral septal nucleus receives a lesser share of the ipsilateral fimbria input, but does receive input via the ventral septal afferent system. Convergence upon single cells between ipsilateral fimbria and ventral septal afferent input was an outstanding feature of lateral septal nucleus organization. Cells in the lateral septal nucleus project out of the lateral septal region in both dorsal and ventral directions.     

Single unit analysis of the hippocampal projections to the septum in the cat

The purpose of this study was to map the hippocampal efferent projections to the septum and to determine the synaptic organization of the hippocampal-septal system in the cat. Single unit responses were recorded in the septum with tungsten microelectrodes following electrical stimulation of the dorsal or ventral hippocampus in the anesthetized cat. Dorsal hippocampal stimulation produced excitatory unit driving at short latencies in the medial septum while ventral hippocampal stimulation produced short latency excitation in the lateral septum. The excitatory phase was invariably followed by a period of inhibition. Inhibition without a prior excitatory phase was seen in widespread regions of the septum upon either dorsal or ventral hippocampal stimulation. It was concluded that efferents from the dorsal and ventral hippocampus terminate in a topographic manner in the medial and lateral septum, respectively, and have excitatory effects upon these neurons as well. An interneuronal inhibitory network capable of influencing large regions of the septum was suggested from the data.     

An experimental study of the ventral striatum of the golden hamster. II. Neuronal connections of the olfactory tubercle

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 olfactory tubercle. Following iontophoretic applications or hydraulic injections of HRP in the tubercle, neurons labeled by retrograde transport of HRP were observed ipsilaterally in the telencephalon in the main olfactory bulb, the medial, lateral, ventral, and posterior divisions of the anterior olfactory nucleus, and in the orbital, ventral, and posterior agranular insular, primary olfactory, perirhinal, and entorhinal cortices. Labeled cells were also present in the basolateral, basomedial, anterior cortical, and posterolateral cortical amygdaloid nuclei, and bilaterally in the nucleus of the lateral olfactory tract. In the diencephalon, ipsilateral HRP-containing neurons were observed in the midline nuclei paraventricularis, parataenialis, and reuniens, and in the parafascicular intralaminar nucleus. Retrograde labeling was present in the ipsilateral brainstem in cells of the ventral tegmental area, substantia nigra, and dorsal raphe. Many of the above projections to the tubercle were found to be topographically organized. Anterograde axonal transport of HRP from the olfactory tubercle labeled terminal fields ipsilaterally in all parts of the anterior olfactory nucleus, in the ventral pallidum, and in the substantia nigra, pars reticulata. Contralaterally, terminal fields were present in the dorsal and lateral divisions of the anterior olfactory nucleus. The projections to the tubercle from the orbital, ventral, and posterior agranular insular, and perirhinal neocortices, intralaminar thalamus, and dopamine-containing areas of the ventral mesencephalon are analogous to the connections of the caudatoputamen, as are the efferents from the tubercle to the ventral globus pallidus and substantia nigra. These connections substantiate the recent suggestion that the olfactory tubercle is a striatal structure, and provide support for the ventral striatal concept. In the present study of the olfactory tubercle, and in the first study in this series on the nucleus accumbens, the ventral striatum was found to receive projections from a number of limbic system structures, including the main olfactory bulb, anterior olfactory nucleus, amygdala, hippocampus, and subiculum, and the entorhinal and primary olfactory cortices. These findings suggest that the ventral striatum is concerned with integrating limbic information into the striatal system.     

The luteinizing hormone-releasing hormone (LH-RH) neuronal networks of the guinea pig brain. I. Intra- and extra-hypothalamic projections.

In the guinea pig brain, LH-RH-containing cell bodies are located not only within the classical hypophysiotrophic area but also in the medial preoptic area, septum and olfactory tubercle. LH-RH fiber tracts project not only to the primary portal plexus in the median eminence but also throughout the limbic forebrain and limbic midbrain regions. Using radiofrequency lesions in different brain regions, the projections of LH-RH cell bodies were determined. Cells in the medial preoptic area project ot the organum vasculosum of the lamina terminalis (OVLT), the suprachiasmatic nucleus, the mammillary body complex and the ventral tegmental area. LH-RH neurons in both the medial septal nucleus and medial preoptic area project via the stria medullaris to the medial habenular nucleus and from there via the fasciculus retroflexus to the interpeduncular nucleus of the midbrain. Other LH-RH neurons in the medial septal nucleus, nucleus of the diagonal band of Broca and olfactory tubercle are congregated in small clusters around large blood vessels which penetrate into this area, and they do not appear to send axons outside their immediate vicinity. The types of LH-RH axonal terminations and the roles of these peptide-containing neurons are discussed.     

Differential projections to the hippocampus by neurons of the medial septum and vertical limb of the diagonal band

Neurons of the medial septum and vertical limb of the diagonal band of Broca project topographically to the hippocampus through the fornix and fimbria, the supracallosal stria and via a ventral route through the amygdala. Injection of the fluorescent dyes Fluoro-Gold or Diamidino yellow into the fimbria-fornix retrogradely labeled neurons in the medial septum and in the ventromedial portion of the vertical limb of the diagonal band. Injection of True blue into the vicinity of the supracallosal stria labeled only neurons in the dorsalateral portion of the diagonal band. No double labeled neurons were observed. These results indicate that (1) neurons of the medial septum and the ventromedial diagonal band project to (or towards) the hippocampus via the fornix and fimbria, (2) neurons in the dorsolateral portion of diagonal project via the supracallosal stria, and (3) neurons of the medial septum and diagonal band do not send collaterals via both routes. The differential projection of the two groups of neurons may explain the differences in degenerative changes in the two nuclei after damage to their axons.     

The brainstem projection of the vestibular nerve in the cat

The projection of the vestibular nerve to the brainstem of the cat was re-examined with silver degeneration methods after complete lesions of the vestibular ganglion. Whereas previous investigations emphasize that only parts of the major vestibular nuclei are innervated by the vestibular nerve, the present investigation shows that only the dorsal division of the lateral vestibular nucleus is uninnervated. Thus, the terminal field of the vestibular nerve extends to the cytoarchitectonic boundaries of the superior, medial, descending, and the ventral division of the lateral vestibular nuclei. Vestibular nerve fibers were also traced to discrete terminal fields in the reticular formation lateral to the abducens nucleus and in the rostral, lateral parts of the accessory cuneate nucleus. These observations indicate a more widespread distribution of vestibular nerve fibers in the brainstem of the cat than previously believed.     

Differential projections of the main and accessory olfactory bulb in the frog.

The central projections of the main olfactory bulb and the accessory olfactory bulb of the adult leopard frog (Rana pipiens) were reexamined, by using a horseradish peroxidase anterograde tracing method that fills axons with a continuous deposit of reaction product. The fine morphology preserved by this method allowed the terminal fields of the projection tracts to be delineated reliably, and for the first time. Herrick's amygdala has been newly subdivided into cortical and medial nuclei on the basis of cytoarchitecture, dendritic morphology, and the differential projections of the main and accessory olfactory tracts. The main olfactory bulb projects through the medial and lateral olfactory tracts to the postolfactory eminence, the rostral end of the medial cortex, the rostral end of the medial septal nucleus, the cortical amygdaloid nucleus, the nucleus of the hemispheric sulcus, and both the dorsal and ventral divisions of the lateral cortex, including its retrobulbar fringe. The lateral olfactory tract overlaps the dorsal edge of the striatal plate along the ventral border of the lateral cortex, but it is not certain whether any striatal cells are postsynaptic to the tract fibers. The lateral cortex is the largest of these territories, and receives the terminals of the main olfactory projection throughout its extent. It extends from the olfactory bulb to the posterior pole, and from the striatum to the summit of the hemisphere, where it borders the dorsal cortex. The medial and lateral olfactory tracts combine in the region of the amygdala to form a part of the stria medullaris thalami. These fibers cross in the habenular commissure and terminate in the contralateral cortical amygdaloid nucleus and periamygdaloid part of the lateral cortex. Cells projecting to the main olfactory bulb are found in the diagonal band and adjacent cell groups, but there is no evidence of an interbulbar projection arising from either the olfactory bulb proper or a putative anterior olfactory nucleus. The accessory olfactory bulb projects through the accessory olfactory tract to the medial and cortical amygdaloid nuclei. A fascicle of the tract crosses in the anterior commissure to terminate in the contralateral amygdala. While the main and accessory olfactory projections may converge in the cortical amygdaloid nucleus, the medial amygdaloid nucleus is connected exclusively with the accessory olfactory bulb.     

The olfactory bulb and medial hemisphere wall of the rat-fish, Chimaera

The Chimaerae are phylogenetically old cartilagenous fish which are included in the class Chondrichthyes with the sharks and rays. The telencephalon of these fish is connected to the diencephalon by a unique fibrous stalk. The telencephalic hemispheres are united only ventrally at midhemisphere levels in the region of the hippocampal and anterior commissures. The lateral ventricles join the midline ventricle through the interventricular foramen at this level. Each olfactory bulb is divided into a dorsal and ventral part and each has a dorsal and a ventral accessory olfactory bulb. A dorsal and a ventral olfactory nerve unite the respective parts of the bulb with a nasal sac. The cup-shaped mitral cell laminae share a common granular cell layer where they are in apposition. The granular lamina is replaced caudally by the anterior olfactory nucleus. The medially situated primordial hippocampal formation consists of the anterior continuation, the primordial subiculum, the cornu ammonis and the dentate gyrus. Septal nucleu recognizable in this form include: a medial and a lateral septal nucleus, a septohippocampal nucleus, a nucleus triangularis, an accumbens nucleus, and a nucleus of the diagonal band. The medial zone of the olfactory tubercle occupies the ventromedial hemisphere wall. Major fiber connections of the medial limbic lobe include: the medial olfactory tract, the fornix, the medial forebrain bundle and the medial corticohabenular tract.