Some anatomical observations on the projections from the hypothalamus to brainstem and spinal cord: an HRP and autoradiographic tracing study in the cat

The hypothalamus is closely involved in a wide variety of behavioral, autonomic, visceral, and endocrine functions. To find out which descending pathways are involved in these functions, we investigated them by horseradish peroxidase (HRP) and autoradiographic tracing techniques. HRP injections at various levels of the spinal cord resulted in a nearly uniform distribution of HRP-labeled neurons in most areas of the hypothalamus except for the anterior part. After HRP injections in the raphe magnus (NRM) and adjoining tegmentum the distribution of labeled neurons was again uniform, but many were found in the anterior hypothalamus as well. Injections of 3H-leucine in the hypothalamus demonstrated that: The anterior hypothalamic area sent many fibers through the medial forebrain bundle (MFB) to terminate in the ventral tegmental area of Tsai (VTA), the rostral raphe nuclei, the nucleus Edinger-Westphal, the dorsal part of the substantia nigra, the periaqueductal gray (PAG), and the interpeduncular nuclei. Further caudally a lateral fiber stream (mainly derived from the lateral parts of the anterior hypothalamic area) distributed fibers to the parabrachial nuclei, nucleus subcoeruleus, locus coeruleus, the micturition-coordinating region, the caudal brainstem lateral tegmentum, and the solitary and dorsal vagal nucleus. Furthermore, a medial fiber stream (mainly derived from the medial parts of the anterior hypothalamic area) distributed fibers to the superior central and dorsal raphe nucleus and to the NRM, nucleus raphe pallidus (NRP), and adjoining tegmentum. The medial and posterior hypothalamic area including the paraventricular hypothalamic nucleus (PVN) sent fibers to approximately the same mesencephalic structures as the anterior hypothalamic area. Further caudally two different fiber bundles were observed. A medial stream distributed labeled fibers to the NRM, rostral NRP, the upper thoracic intermediolateral cell group, and spinal lamina X. A second and well-defined fiber stream, probably derived from the PVN, distributed many fibers to specific parts of the lateral tegmental field, to the solitary and dorsal vagal nuclei, and, in the spinal cord, to lamina I and X, to the thoracolumbar and sacral intermediolateral cell column, and to the nucleus of Onuf. The lateral hypothalamic area sent many labeled fibers to the lateral part of the brainstem and many terminated in the caudal brainstem lateral tegmentum, including the parabrachial nuclei, locus coeruleus, nucleus subcoeruleus, and the solitary and dorsal vagal nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)     

Efferent projections of the intergeniculate leaflet and the ventral lateral geniculate nucleus in the rat

The intergeniculate leaflet (IGL) and the ventral lateral geniculate nucleus (VLG) are ventral thalamic derivatives within the lateral geniculate complex. In this study, IGL and VLG efferent projections were compared by using anterograde transport of Phaseolus vulgaris-leucoagglutinin and retrograde transport of FluoroGold. Projections from the IGL and VLG leave the geniculate in four pathways. A dorsal pathway innervates the thalamic lateral dorsal nucleus (VLG), the reuniens and rhomboid nuclei (VLG and IGL), and the paraventricular nucleus (IGL). A ventral pathway runs through the geniculohypothalamic tract to the suprachiasmatic nucleus and the anterior hypothalamus (IGL). A medial pathway innervates the zona incerta and dorsal hypothalamus (VLG and IGL); the lateral hypothalamus and perifornical area (VLG); and the retrochiasmatic area (RCA), dorsomedial hypothalamic nucleus, and subparaventricular zone (IGL). A caudal pathway projects medially to the posterior hypothalamic area and periaqueductal gray and caudally along the brachium of the superior colliculus to the medial pretectal area and the nucleus of the optic tract (IGL and VLG). Caudal IGL axons also terminate in the olivary pretectal nucleus, the superficial gray of the superior colliculus, and the lateral and dorsal terminal nuclei of the accessory optic system. Caudal VLG projections innervate the lateral posterior nucleus, the anterior pretectal nucleus, the intermediate and deep gray of the superior colliculus, the dorsal terminal nucleus, the midbrain lateral tegmental field, the interpeduncular nucleus, the ventral pontine reticular formation, the medial and lateral pontine gray, the parabrachial region, and the accessory inferior olive. This pattern of IGL and VLG projections is consistent with our understanding of the distinct functions of each of these ventral thalamic derivatives.     

Distributions of spinothalamic, spinohypothalamic, and spinotelencephalic fibers revealed by anterograde transport of PHA-L in rats.

Fibers projecting from several levels of the spinal cord to the diencephalon and telencephalon were labeled anterogradely with Phaseolus vulgaris leucoagglutinin injected iontophoretically. Labeled fibers in the thalamus confirmed projections previously observed. In addition, many labeled fibers were seen in the hypothalamus and in telencephalic areas not generally recognized previously as receiving such projections. In the hypothalamus, these areas included the lateral hypothalamus (including the medial forebrain bundle), the posterior hypothalamic area, the dorsal hypothalamic area, the dorsomedial nucleus, the paraventricular nucleus, the periventricular area, the suprachiasmatic nucleus, and the lateral and medial preoptic areas. In the telencephalon, areas with labeled fibers included the ventral pallidum, the globus pallidus, the substantia innominata, the basal nucleus of Meynert, the amygdala (central nucleus), the horizontal and vertical limbs of the diagonal band of Broca, the medial and lateral septal nuclei, the bed nucleus of the stria terminalis, the nucleus accumbens, infralimbic cortex, and medial orbital cortex. These results suggest that somatosensory, possibly including visceral sensory, information is carried directly from the spinal cord to areas in the brain involved in autonomic regulation, motivation, emotion, attention, arousal, learning, memory, and sensory-motor integration. Many of these areas are associated with the limbic system.     

A C-2-Deoxyglucose analysis of the neural pathways of the limbic forebrain in the rat: II. The hypothalamus

An attempt was made to characterize the nature of the functional organization of the hypothalamus by observing the patterns of uptake of ¹⁴C-2-deoxyglucose (2DG) following electrical stimulation of different regions within the preoptico-hypothalamus in the rat. The experimental paradigm consisted of electrical brain stimulation delivered continuously for periods of 30 sec on and 30 sec off for 45 minutes following injection of 2DG. Brains were removed and processed for autoradiography. Activation of the medial forebrain bundle was noted following stimulation of the nucleus accumbens and lateral preoptico-hypothalamus. Activated fibers could be followed only in a caudal direction through the medial forebrain bundle and into the ventral tegmental area as a result of nucleus accumbens stimulation. Stimulation of the lateral preoptic region or of the anterior half of lateral hypothalamus produced activation of the lateral septal nucleus, lateral habenular nucleus, perifornical region, midline thalamus and ventral tegmental area. Since stimulation of the perifornical hypothalamus significantly activated the rostro-caudal extent of the midbrain central gray, it is suggested that impulses from the lateral hypothalamus reach the lower brainstem via its connections with the perifornical hypothalamus. Ventromedial hypothalamic stimulation activated only the lateral septal nucleus, cortico-medial amygdala and medial preoptico-hypothalamus, while medial preoptico-hypothalamic stimulation resulted in increased 2DG uptake in the midbrain central gray, thus suggesting that medial hypothalamic impulses reach the brainstem by first ascending to the level of the preoptico-hypothalamus. Mammillary body stimulation orthodromically activated fibers in the mammillothalamic and mammillotegmental tracts and antidromically fibers in the fornix for a short distance.     

Ascending projections of the posterior nucleus of the hypothalamus: PHA-L analysis in the rat

With the exception of a report by R. B. Veazey, D. G. Amaral, and W. M. Cowan (1982, J. Comp. Neurol. 207:135–156) that examined the projections of the posterior hypothalamic area in the monkey by using the autoradiographic technique, the ascending projections of the posterior nucleus (PH) of the hypothalamus have not been systematically examined in any species. The present report describes the ascending projections of PH in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris -leucoagglutinin (PHA-L). The major ascending route for PH fibers is the medial forebrain bundle. PH fibers project densely to several subcortical and cortical sites. The subcortical sites are the subthalamus/hypothalamus (zona incerta, the supramammillary nucleus, lateral, perifornical, dorsal, and anterior nuclei/areas), the thalamus (lateroposterior, laterodorsal, parafascicular, reuniens, paraventricular, central medial, paracentral, central lateral and intermediodorsal nuclei), the amygdala (central, lateral, and medial nuclei), the septal area (bed nucleus of atria terminalis, medial and lateral septum), and the basal forebrain (horizontal/vertical limbs of diagonal band nuclei and lateral preoptic area). The cortical sites are the perirhinal, insular, frontal (lateral agranular), prelimbic, and infralimbic cortices. The diversity of PH projections to subcortical and cortical “limbic-related” sites and to several structures with direct input to the hippocampus (supramammillary nucleus, reuniens, paraventricular and laterodorsal nuclei of the thalamus, medial and lateral septum, and perirhinal cortex) suggest that the PH may serve a critical role in various components of emotional behavior, including mnemonic processes associated with significant emotional events. © 1995 Wiley-Liss, Inc.     

Ascending projections from the pedunculopontine tegmental nucleus and the adjacent mesopontine tegmentum in the rat

Ascending projections from the pedunculopontine tegmental nucleus (PPT) and the surrounding mesopontine tegmentum to the forebrain in the rat are here examined by using both retrograde and anterograde tracing techniques combined with choline acetyltransferase (ChAT) immunohistochemistry. The anterogradely transported lectin Phaseolus vulgaris-leukoagglutinin (PHA-L) was iontophoretically injected into the PPT in 12 rats. Anterogradely labelled fibers and varicosities were observed in the thalamic nuclei, confirming the findings of our previous retrograde studies (Hallanger et al: J. Comp. Neurol. 262:105-124, '87). In addition, PHA-L-labelled fibers and varicosities suggestive of terminal fields were observed in the anterior, tuberal, and posterior lateral hypothalamic regions, the ventral pallidum in the region of the nucleus basalis of Meynert, the dorsal and intermediate lateral septal nuclei, and in the central and medial nuclei of the amygdala. To determine whether these were cholinergic projections, the retrograde tracer WGA-HRP was injected into terminal fields in the hypothalamus, septum, ventral pallidum, and amygdala. Numerous ChAT-immunoreactive neurons in the PPT and laterodorsal tegmental nucleus (LDT) were retrogradely labelled from the lateral hypothalamus. These cholinergic neurons constituted over 20% of those retrogradely labelled in the dorsolateral mesopontine tegmentum; the balance consisted of noncholinergic neurons of the central tegmental field, retrorubral field, and cuneiform nucleus. Following placement of WGA-HRP into dorsal and intermediate lateral septal regions, the vast majority (greater than 90%) of retrogradely labelled neurons were cholinergic neurons of the PPT and LDT, with few noncholinergic retrogradely labelled neurons in the adjacent tegmentum. In contrast, fewer cholinergic neurons were retrogradely labelled following placement of tracer into the nucleus basalis of Meynert or into the central, medial, and basolateral nuclei of the amygdala, while numerous noncholinergic neurons of the central tegmental field rostral to the PPT and of the retrorubral field adjacent to the PPT were retrogradely labelled in these cases. These anterograde and retrograde studies demonstrate that cholinergic PPT and LDT neurons provide a substantial proportion of mesopontine tegmental afferents to the hypothalamus and lateral septum, while projections to the nucleus basalis and the amygdala are minimal.     

Efferent projections of infralimbic and prelimbic areas of the medial prefrontal cortex in the Japanese monkey, Macaca fuscata

The infralimbic area (IL) and prelimbic area (PL) have been postulated as an autonomic motor region in the medial prefrontal cortex. The present study was conducted to reveal the projection sites of IL and PL of the monkey, Macaca fuscata, using biotinylated dextran amine as an anterograde tracer. IL and PL projected densely to the ventromedial caudate nucleus, the core and shell of the nucleus accumbens (Acb), parvicellular lateral basal and magnocellular accessory basal nuclei of the amygdala, lateral preoptic area, ventromedial hypothalamic nucleus, tubero-mammillary nucleus (TM), medial part of the magnocellular and dorsal part of the parvicellular (MDpc) dorsomedial thalamic nuclei, reunience and medial part of the medial pulvinar nucleus, and dorso-lateral part of the periaqueductal gray (PAGdl) in the mesencephalon. Moderately to weakly projected areas were the intermediate and lateral parts of the agranular insular cortex, orbital part of area 12, agranular and dysgranular part of the temporal pole cortex (TPa-g), auditory temporal cortex, lateral and medial (MS) septal nuclei, bed nucleus of the stria terminalis, diagonal band of Broca, substantia innominata, and medial preoptic area, dorsomedial, lateral, and posterior hypothalamic nuclei, magnocellular lateral basal and lateral amygdaloid nuclei, paratenial, paraventricular (PV), inter-antero-medial (IAM), reticular, central medial (CeM), parafascicular (PF) and limitans nuclei of the thalamus, lateral habenular nucleus, pedunculo-pontine nucleus, dorsal part of the lateral lemniscal nucleus, ventral tegmental area (VTA), dorsal raphe, superior central nucleus, medial and lateral parabrachial nuclei (PBl) and nucleus locus coeruleus (LC). A few scattered terminals were observed in the perifornical nucleus of the hypothalamus and substantia nigra pars compacta. PL and area 24 were characterized by projections to the entorhinal (Ent) and piriform (Pir) cortex as well as to the magnocellular part of the ventral anterior thalamic nucleus (VAmc). The morphology of the terminal arborization in each nuclei was different in appearance, perhaps reflecting the synaptic interaction between the nerve terminals and postsynaptic dendrites. PL projected uniquely to Ent, Pir and VAmc and IL projected uniquely to TPa-g, MS, IAM, CeM, MDpc, PF, PBl and LC. IL projected more strongly than PL to the shell of Acb, amygdaloid nuclei, PV, TM, VTA and PAGdl. The present results support the hypothesis that IL is a major cortical autonomic motor area and PL integrates limbic and autonomic inputs in the primate.     

Efferent projections of infralimbic and prelimbic areas of the medial prefrontal cortex in the Japanese monkey, Macaca fuscata

The infralimbic area (IL) and prelimbic area (PL) have been postulated as an autonomic motor region in the medial prefrontal cortex. The present study was conducted to reveal the projection sites of IL and PL of the monkey, Macaca fuscata, using biotinylated dextran amine as an anterograde tracer. IL and PL projected densely to the ventromedial caudate nucleus, the core and shell of the nucleus accumbens (Acb), parvicellular lateral basal and magnocellular accessory basal nuclei of the amygdala, lateral preoptic area, ventromedial hypothalamic nucleus, tubero-mammillary nucleus (TM), medial part of the magnocellular and dorsal part of the parvicellular (MDpc) dorsomedial thalamic nuclei, reunience and medial part of the medial pulvinar nucleus, and dorso-lateral part of the periaqueductal gray (PAGdl) in the mesencephalon. Moderately to weakly projected areas were the intermediate and lateral parts of the agranular insular cortex, orbital part of area 12, agranular and dysgranular part of the temporal pole cortex (TPa-g), auditory temporal cortex, lateral and medial (MS) septal nuclei, bed nucleus of the stria terminalis, diagonal band of Broca, substantia innominata, and medial preoptic area, dorsomedial, lateral, and posterior hypothalamic nuclei, magnocellular lateral basal and lateral amygdaloid nuclei, paratenial, paraventricular (PV), inter-antero-medial (IAM), reticular, central medial (CeM), parafascicular (PF) and limitans nuclei of the thalamus, lateral habenular nucleus, pedunculo-pontine nucleus, dorsal part of the lateral lemniscal nucleus, ventral tegmental area (VTA), dorsal raphe, superior central nucleus, medial and lateral parabrachial nuclei (PBl) and nucleus locus coeruleus (LC). A few scattered terminals were observed in the perifornical nucleus of the hypothalamus and substantia nigra pars compacta. PL and area 24 were characterized by projections to the entorhinal (Ent) and piriform (Pir) cortex as well as to the magnocellular part of the ventral anterior thalamic nucleus (VAmc). The morphology of the terminal arborization in each nuclei was different in appearance, perhaps reflecting the synaptic interaction between the nerve terminals and postsynaptic dendrites. PL projected uniquely to Ent, Pir and VAmc and IL projected uniquely to TPa-g, MS, IAM, CeM, MDpc, PF, PBl and LC. IL projected more strongly than PL to the shell of Acb, amygdaloid nuclei, PV, TM, VTA and PAGdl. The present results support the hypothesis that IL is a major cortical autonomic motor area and PL integrates limbic and autonomic inputs in the primate.     

Efferent connections of the lateral hypothalamic area of the rat: An autoradiographic investigation

The efferent projections of the lateral hypothalamic area (LHA) at mid-tuberal levels were examined with the autoradiographic tracing method. Connections were observed to widespread regions of the brain, from the telencephalon to the medulla. Ascending fibers course through LHA and the lateral preoptic area and lie lateral to the diagonal band of Broca. Fibers sweep dorsally into the lateral septal nucleus, cingulum bundle and medial cortex. Although sparse projections are found to the ventromedial hypothalamic nucleus, a prominent pathway courses to the dorsal and medial parvocellular subnuclei of the paraventricular nucleus. Labeled fibers in the stria medullaris project to the lateral habenular nucleus. The central nucleus of the amygdala is encapsulated by fibers from the stria terminalis and the ventral amygdalofugal pathway. The substantia innominate, nucleus paraventricularis of the thalamus, and bed nucleus of the stria terminalis also receive LHA fibers. Three descending pathways course to the brainstem: (1) periventricular system, (2) central tegmental tract (CTT), and (3) medial forebrain bundle (MFB). Periventricular fibers travel to the ventral and lateral parts of the midbrain central gray, dorsal raphe nucleus, and laterodorsal tegmental nucleus of the pens. Dorsally coursing fibers of CTT enter the central tegmental field and the lateral and medial parabrachial nuclei. The intermediate and deep layers of the superior colliculus receive some fibers. Fibers from CTT leave the parabranchial region by descending in the ventrolateral pontine and medullary reticular formation; some of these fibers sweep dorsomedially into the nucleus tractus solitarius, dorsal motor nucleus of the vagus, and nucleus commissuralis. From MFB, fibers descend into the ventral tegmental area and to the border of the median raphe and raphe magnus nuclei.     

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.     

The efferent connections of the ventromedial nucleus of the hypothalamus of the rat.

The efferent connections of the ventromedial nucleus of the hypothalamus (VMH) of the rat have been examined using the autoradiographic method. Following injections of small amounts (0.4-2.0 muCi) of tritium labeled amino acids, fibers from the VMH can be traced forward through the periventricular region, the medial hypothalamus and the medial forebrain bundle to the preoptic and thalamic periventricular nuclei, to the medial and lateral preoptic areas, to the bed nucleus of the stria terminalis and to the ventral part of the lateral septum. Some labeled axons continue through the bed nucleus of the stria terminalis into the stria itself, and hence to the amygdala, where they join other fibers which follow a ventral amygdalopetal route from the lateral hypothalamic area and ventral supraoptic commissure. These fibers terminate in the dorsal part of the medial amygdaloid nucleus and in the capsule of the central nucleus. A lesser number of rostrally directed fibers from the VMH crosses the midline in the ventral supraoptic commissure and contributes a sparse projection to the contralateral amygdala. Descending fibers from the VMH take three routes: (i) through the medial hypothalamus and medial forebrain bundle; (ii) through the periventricular region; and (iii) bilaterally through the ventral supraoptic commissure. These three pathways are interconnected by labeled fibers so that it is not possible to precisely identify their respective terminations. However, the periventricular fibers seem to project primarily to the posterior hypothalamic area and central gray, as far caudally as the anterior pole of the locus coeruleus, while the medial hypothalamic and medial forebrain bundle fibers apparently terminate mainly in the capsule of the mammillary complex, in the supramammillary nucleus and in the ventral tegmental area. The ventral supraoptic commissure fibers leave the hypothalamus closely applied to the medial edges of the two optic tracts. After giving off their contributions to the amygdala, they continue caudally until they cross the dorsal edge of the cerebral peduncle to enter the zona incerta. Some fibers probably terminate here, but others continue caudally to end in the dentral tegmental fields, and particularly in the peripeduncular nucleus. Within the hypothalamus, the VMH appears to project extensively to the surrounding nuclei. However, we have not been able to find evidence for a projection from the VMH to the median eminence. Isotope injections which differentially label the dorsomedial or the ventrolateral parts of the VMH have shown that most of the long connections (to the septum, amygdala, central tegmental fields and locus coeruleus) originate in the ventrolateral VMH, and there is also some evidence for a topographic organization within the projections of this subdivision of the nucleus.     

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.     

Projections of the ventral premammillary nucleus.

The projections of the ventral premammillary nucleus (PMv) have been examined with the Phaseolus vulgaris leucoagglutinin (PHAL) method in adult male rats. The results indicate that the nucleus gives rise to two major ascending pathways and a smaller descending pathway. One large ascending pathway terminates densely in most regions of the periventricular zone of the hypothalamus, with the notable exception of the suprachiasmatic, suprachiasmatic preoptic, and median preoptic nuclei. This pathway is in a position to influence directly many cell groups known to regulate anterior pituitary function. The second large pathway ascends through the medial zone of the hypothalamus and densely innervates the ventrolateral part of the ventromedial nucleus and adjacent basal parts of the lateral hypothalamic area, medial preoptic nucleus, principal nucleus of the bed nuclei of the stria terminalis, ventral lateral septal nucleus, posterodorsal part of the medial nucleus of the amygdala, posterior nucleus, and immediately adjacent regions of the posterior cortical nucleus of the amygdala. It is already known that these regions are major components of the sexually dimorphic circuit, and, interestingly, that they provide the major neural inputs to the PMv. The smaller descending projection from the PMv seems to innervate preferentially the posterior hypothalamic nucleus, although a small number of fibers appear to end in the tuberomammillary nucleus, supramammillary nucleus, specific regions of the medial mammillary nucleus, interfascicular nucleus, interpeduncular nucleus, periaqueductal gray, dorsal nucleus of the raphe, laterodorsal tegmental nucleus, Barrington's nucleus, and locus coeruleus. Relatively sparse terminal fields associated with ascending fibers were also observed in the dorsomedial nucleus of the hypothalamus; in the nucleus reuniens, parataenial nucleus, paraventricular nucleus of the thalamus, and mediodorsal nucleus; in the central nucleus of the amygdala, anterodorsal part of the medial nucleus of the amygdala, posterior part of the basomedial nucleus of the amygdala; and in the ventral subiculum and adjacent parts of hippocampal field CA1, and the infralimbic and prelimbic areas of the medial prefrontal cortex. Taken as a whole, the evidence suggests that the PMv receives two major inputs--one from the sexually dimorphic circuit, and the other from the blood in the form of gonadal steroid hormones--and gives rise to two major outputs: one (perhaps feed-forward) to the neuroendocrine (periventricular) zone of the hypothalamus, and the other (perhaps feed-back) to the sexually dimorphic circuit.     

An autoradiographic study of the efferent connections of the lateral hypothalamic area in the rat.

The efferent connections of the lateral hypothalamic area (LHA) have been analyzed in a series of 30 rat brains with injections of 3H-amino acids into different parts of the area and the surrounding regions. Our findings indicate that all parts of the LHA contribute ascending and descending fibers to the medial forebrain bundle, and also project medially to certain of the adjoining hypothalamic nuclei. All levels of the LHA appear to send some fibers to a continuous group of structures that extends from the medial septal-diagonal band complex rostrally, through the lateral preoptic and lateral hypothalamic areas to the mammillary complex and the ventral tegmental area caudally. In addition, it is evident that cells at different levels within the LHA may have differential projections. Thus, the anterior and lateral parts of the LHA also appear to project substantially to the anterior hypothalamic area, the ventromedial and dorsomedial hypothalamic nuclei, the parataenial and paraventricular nuclei of the thalamus, and the medial part of the lateral habenular nucleus. Similarly, cells in the tuberal and posterior parts of the LHA project to the central gray, the longest projections from the posterior region reaching as far caudally as the central tegmental field, the parabrachial nucleus, the locus coeruleus, and the superior central and dorsal nuclei of the raphe. Viewed as a whole, the LHA is therefore well-suited to integrate inputs from the limbic system and brainstem and to relay them on the one hand to the medial zone of the hypothalamus and on the other to virtually every structure closely associated with the medial forebrain bundle and to the nuclei of origin of the major ascending monoaminergic systems.     

Cytoarchitecture and efferent projections of the nucleus incertus of the rat

The nucleus incertus is located caudal to the dorsal raphe and medial to the dorsal tegmentum. It is composed of a pars compacta and a pars dissipata and contains acetylcholinesterase, glutamic acid decarboxylase, and cholecystokinin-positive somata. In the present study, anterograde tracer injections in the nucleus incertus resulted in terminal-like labeling in the perirhinal cortex and the dorsal endopyriform nucleus, the hippocampus, the medial septum diagonal band complex, lateral and triangular septum medial amygdala, the intralaminar thalamic nuclei, and the lateral habenula. The hypothalamus contained dense plexuses of fibers in the medial forebrain bundle that spread in nearly all nuclei. Labeling in the suprachiasmatic nucleus filled specifically the ventral half. In the midbrain, labeled fibers were observed in the interpeduncular nuclei, ventral tegmental area, periaqueductal gray, superior colliculus, pericentral inferior colliculus, pretectal area, the raphe nuclei, and the nucleus reticularis pontis oralis. Retrograde tracer injections were made in areas reached by anterogradely labeled fibers including the medial prefrontal cortex, hippocampus, amygdala, habenula, nucleus reuniens, superior colliculus, periaqueductal gray, and interpeduncular nuclei. All these injections gave rise to retrograde labeling in the nucleus incertus but not in the dorsal tegmental nucleus. These data led us to conclude that there is a system of ascending projections arising from the nucleus incertus to the median raphe, mammillary complex, hypothalamus, lateral habenula, nucleus reuniens, amygdala, entorhinal cortex, medial septum, and hippocampus. Many of the targets of the nucleus incertus were involved in arousal mechanisms including the synchronization and desynchronization of the theta rhythm.     

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.     

Amygdaloid projections to subcortical structures within the basal forebrain and brainstem in the rat and cat

The efferent fiber connections of the nuclei of the amygdaloid complex with subcortical structures in the basal telencephalon, hypothalamus, midbrain, and pons have been studied in the rat and cat, using the autoradiographic method for tracing axonal connections. The cortical and thalamic projections of these nuclei have been described in previous papers (Krettek and Price, ′77b,c). Although the subcortical connections of the amygdaloid nuclei are widespread within the basal forebrain and brain stem, the projections of each nucleus have been found to be well defined, and distinct from those of the other amygdaloid nuclei. The basolateral amygdaloid nucleus projects heavily to the lateral division of the bed nucleus of the stria terminalis (BNST), to the caudal part of the substantia innominata, and to the ventral part of the corpus striatum (nucleus accumbens and ventral putamen) and the olfactory tubercle; it projects more lightly to the lateral hypothalamus. The central nucleus also projects to the lateral division of the BNST and the lateral hypothalamus, but in addition it sends fibers to the lateral part of the substantia nigra and the marginal nucleus of the brachium conjunctivum. The basomedial nucleus has projections to the ventral striatum and olfactory tubercle which are similar to those of the basolateral nucleus, but it also projects to the core of the ventromedial hypothalamic nucleus and the premammillary nucleus, and to a central zone of the BNST which overlaps the medial and lateral divisions. The medial nucleus also projects to the core of the ventromedial nucleus and the premammillary nucleus, but sends fibers to the medial division of the BNST and does not project to the ventral striatum. The posterior cortical nucleus projects to the premammillary nucleus and to the medial division of the BNST, but a projection from this nucleus to the ventromedial nucleus has not been demonstrated. Projections to the “shell” of the ventromedial nucleus have been found only from the ventral part of the subiculum and from a structure at the junction of the amygdala and the hippocampal formation, which has been termed the amygdalo-hippocampal area (AHA). The AHA also sends fibers to the medial part of the BNST and the premammillary nucleus. Virtually no subcortical projections outside the amygdala itself have been demonstrated from the lateral nucleus, or from the olfactory cortical areas around the amygdala (the anterior cortical nucleus, the periamygdaloid cortex, and the posterior prepiriform cortex). However, portions of the endopiriform nucleus deep to the prepiriform cortex project to the ventral putamen, and to the lateral hypothalamus.     

Localization of preproenkephalin mRNA in the rat brain and spinal cord by in situ hybridization

To determine the localization in rat brain and spinal cord of individual neurons that contain the messenger RNA coding for the opioid peptide precursor preproenkephalin, we performed in situ hybridization with a tritiated cDNA probe complementary to a protion of preproenkephalin mRNA. We observed autoradiographic signal over the cytoplasm of neurons of many regions of the central nervous system. Several types of controls indicated specificity of the labeling. Neurons containing preproenkephalin mRNA were found in the piriform cortex, ventral tenia tecta, several regions of the neocortex, nucleus accumbens, olfactory tubercle, caudate-putamen, lateral septum, bed nucleus of the stria terminalis, diagonal band of Broca, preoptic area, amygdala (especially central nucleus, with fewer labeled neurons in all other nuclei), hippocampal formation, anterior hypothalamic nucleus, perifornical region, lateral hypothalamus, paraventricular nucleus, dorsomedial and ventromedial hypothalamic nuclei, arcuate nucleus, dorsal and ventral premamillary nuclei, medial mamillary nucleus, lateral geniculate nucleus, zona incerta, periaqueductal gray, midbrain reticular formation, ventral tegmental area of Tsai, inferior colliculus, dorsal and ventral tegmental nuclei of Gudden, dorsal and ventral parabrachial nuclei, pontine and medullary reticular formation, several portions of the raphe nuclei, nucleus of the solitary tract, nucleus of the spinal trigeminal tract (especially substantia gelatinosa), ventral and dorsal cochlear nuclei, medial and spinal vestibular nuclei, cuneate and external cuneate nuclei, gracile nucleus, superior olive, nucleus of the trapezoid body, some deep cerebellar nuclei, Golgi neurons in the cerebellum, and most laminae of the spinal cord. In most of these brain regions, the present results indicate that many more neurons contain preproenkephalin mRNA than have been appreciated previously on the basis of immunocytochemistry.     

Afferent connections to the amygdaloid complex of the rat and cat: II. Afferents from the hypothalamus and the basal telencephalon

The projections from the basal telencephalon and hypothalamus to each nucleus of the amygdaloid complex of the rat, and to the central amygdala of the cat, were investigated by the use of retrograde transport of horseradish peroxidase (HRP). The enzyme was injected stereotaxically by microiontophoresis, using three different approaches. The ventral pallidum (Heimer, '78) and ventral part of the globus pallidus were found to project to the lateral and basolateral nuclei of the amygdala. The substantia innominata projects diffusely to the entire amygdaloid complex, except to the lateral nucleus and the caudal part of the medial nucleus. The anterior amygdaloid area shows a similar projection field, the only difference being that this structure does not project to any parts of the medial nucleus. The dorsal subdivision of the nucleus of the lateral olfactory tract sends fibers to the ipsilateral as well as the contralateral basolateral nucleus, and possibly to the ipsilateral basomedial and cortical amygdala. The ventral subdivision of the nucleus of the lateral olfactory tract was massively labeled after an injection in the ipsilateral central nucleus, but this injection affected the commissural component of the stria terminalis. The nucleus of the horizontal limb of the diagonal band of Broca connects with the medial, central, and anterior cortical nuclei, whereas the bed nucleus of stria terminalis and medial preoptic area are related to the medial nucleus predominantly. The lateral preoptic area is only weakly labeled after intra-amygdaloid HRP injections. The hypothalamo-amygdaloid projections terminate preponderantly in the medial part of the amygdaloid complex. Thus, axons from neurons in the area dorsal and medial to the paraventricular nucleus of the hypothalamus distribute to the medial nucleus and intra-amygdaloid part of the bed nucleus of stria terminalis. Most of the amygdalopetal fibers from the ventromedial, ventral premammillary, and arcuate nuclei of the hypothalamus end in the medial nucleus, but some extend into the central nucleus. A few fibers from the ventromedial nucleus of the hypothalamus reach the basolateral nucleus. The lateral hypothalamic area projects heavily to the central nucleus, and more sparsely to the medial and basolateral nuclei. The dorsal hypothalamic area and supramammillary nucleus show restricted projections to the central and basolateral nuclei, respectively. There are only a modest number of crossed hypothalamo-amygdaloid fibers. Most of these originate in the ventromedial nucleus of the hypothalamus and terminate in the contralateral medial nucleus. The projections from the basal telencephalon and hypothalamus to the central nucleus of the amygdala of the cat are similar to the corresponding projections in the rat.     

Organization of atriopeptin-like immunoreactive neurons in the central nervous system of the rat

The atrial natriuretic peptide, atriopeptin, is a circulating hormone that plays an important role in the regulation of fluid and electrolyte homeostasis. Several recent studies have shown that atriopeptin-like immunoreactivity is present within the central nervous system as well as peripheral tissues. In the present report, we describe in detail the organization of atriopeptin-like immunoreactive (APir) perikarya and fibers in the central nervous system of the rat. The most prominent collection of APir perikarya was found in the hypothalamus, adjacent to the anteroventral tip of the third ventricle. Additional groups of APir perikarya were observed along the wall of the third ventricle and in the paraventricular and arcuate nuclei. Separate, smaller groups with distinctive morphology were seen in the lateral hypothalamic area, in the supra-mammillary, medial, and lateral mammillary nuclei, medial habenular nucleus, bed nucleus of the stria terminalis, and the central nucleus of the amygdala. In the pons and brain-stem, APir neurons were observed in the pedunculopontine and laterodorsal tegmental nuclei, as well as in the ventral tegmental area, Barrington's nucleus, the parabrachial nucleus, and the nucleus of the solitary tract. The densest terminal fields of APir fibers were found in the paraventricular nucleus of the hypothalamus, the bed nucleus of the stria terminalis, the median eminence, and the interpeduncular nucleus. The presence of atriopeptin immunoreactivity within the central nervous system suggests that atriopeptin may function as a central neuromediator. Potential functions of this candidate neuromediator deduced from its anatomical distribution are discussed, including the possibility that atriopeptin may function as both a central neuromediator and a systemic hormone in the regulation of the cardiovascular system.