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.     

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.     

Fiber degeneration associated with hyperphagia-inducing knife cuts in the hypothalamus

The pattern of axonal degeneration associated with hyperphagia-producing hypothalamic knife transections was investigated using the Fink-Heimer method for staining of degenerating axons and their terminal endings. Histological analysis of silver-stained material after parasagittal knife cuts which result in hyperphagia and obesity revealed fiber degeneration coursing longitudinally in the medial forebrain bundle including the perifornical component to reach the nucleus accumbens, the diagonal band, the preoptic-anterior hypothalamic junction, the lateral hypothalamus, the zona incerta, the periventricular thalamus, the parafascicular thalamic nucleus, the substantia nigra pars compacta, the central gray matter, the ventral tegmental area of T'sai and the superior colliculus. The data obtained in the present study lend support to the suggestion that projections coursing in the medial forebrain bundle interconnect the anteriomedial hypothalamus and the midbrain tegmentum and may underlie the hyperphagia and obesity produced by hypothalamic knife cuts.     

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.     

Organization of inputs to the dorsomedial nucleus of the hypothalamus: a reexamination with Fluorogold and PHAL in the rat

Possible inputs to the DMH were studied first using the fluorescent retrograde tracer Fluorogold, and identified cell groups were then injected with the anterograde tracer PHAL to examine the distribution of labeled axons in and around the DMH. From this work, we conclude that the majority of inputs to the DMH arise in the hypothalamus, although there are a few significant projections from the telencephalon and brainstem. With few exceptions, each major nucleus and area of the hypothalamus provides inputs to the DMH. Telencephalic inputs arise mainly in the ventral subiculum, infralimbic area of the prefrontal cortex, lateral septal nucleus, and bed nuclei of the stria terminalis. The majority of brainstem inputs arise in the periaqueductal gray, parabrachial nucleus, and ventrolateral medulla. In addition, it now seems clear that inputs to the DMH use only a few discrete pathways. Descending inputs course through a periventricular pathway through the hypothalamic periventricular zone, a medial pathway that follows the medial corticohypothalamic tract, and a lateral pathway traveling through medial parts of the medial forebrain bundle. Ascending inputs arrive through a midbrain periventricular pathway that travels adjacent to the cerebral aqueduct in the periaqueductal gray, and through a brainstem lateral pathway that travels through central and ventral midbrain tegmental fields and enters the hypothalamus, and then the DMH from more lateral parts of the medial forebrain bundle. The results are discussed in relation to evidence for involvement of the DMH in ingestive behavior, and diurnal and stress-induced corticosterone secretion.     

Distribution of phenylethanolamine-N-methyltransferase-immunoreactive perikarya and fibers in the brain of the lizard Gekko gecko

The distribution of phenylethanolamine N-methyltransferase (PNMT)-immunoreactive (PNMTi) cell bodies and fibers in the brain of the lizard Gekko gecko was studied by antibodies raised in rabbits against purified bovine adrenal PNMT. The PNMTi cell bodies were observed in the ventrolateral rhombencephalic tegmentum at the level of the obex. No immunoreactive perikarya were found in the nucleus of the solitary tract, the medial longitudinal fascicle or the hypothalamus. An extensive network of PNMTi fibers is present throughout the brain, extending rostrally as far as the olfactory peduncle. In the telecenphalon, moderate to dense plexuses of PNMTi fibers were observed in the medial part of the nucleus accumbens, the medial septal nucleus, the nucleus of the diagonal band, the caudoventral septal region and the central amygdaloid nucleus. In the diencephalon, the periventricular and lateral zones of the preoptic and hypothalamic areas, the medial forebrain bundle and the dorsomedial thalamic nucleus contain many PNMTi fibers. Brainstem structures innervated by PNMTi fibers are the ventral tegmental area, the substantia nigra, the periaqueductal gray, the locus coeruleus, the parabrachial region, the nucleus of the solitary tract, the dorsal motor nucleus of the vagus and the ventrolateral region of the caudal brainstem. Although the brain of Gekko appears to lack PNMTi cells in areas comparable to the C2 and C3 cell groups in rats, the distribution of PNMTi fibers is nevertheless strikingly similar in both groups.     

Determination of the course of brainstem catecholamine fibers mediating amphetamine anorexia

Previous studies suggest that brainstem catecholamine (CA) fibers which mediate amphetamine (AMPH)-induced anorexia ascend through the midlateral medial forebrain bundle and perifornical region and terminate in the perifornical hypothalamic region (PFH) at the level of the hypothalamic ventromedial nucleus⁴³. Through studies of wire-knife cuts (KCs) placed in the lower brainstem, the present paper further delineates the course of fibers mediating AMPH feeding suppression, as they ascend through the medullary, pontine and midbrain tegmentum. The results indicate that the crucial CA fibers ascend through the ventrolateral medulla just dorsal to the nucleus of the seventh cranial nerve, 1.1–1.9 mm lateral to midline. In their rostral course, these fibers apparently maintain a relatively straight position in the ventral pons and then enter the ventrolateral midbrain just dorsal to the medial lemniscus, between 0.7 and 1.1 mm lateral to midline. These medullary fibers, possibly originating from the norepinephrine/epinephrine-containing ventrolateral cell group (A1/C1), then appear to join additional fibers from the scattered dopamine-containing neurons positioned in the caudal midbrain (A8 CA cell group). Together, these dopamine, epinephrine and norepinephrine systems are believed to ascend into the medial aspect of the medial forebrain bundle on their way to the PFH at the level of the ventromedial nucleus.     

The morphology and connections of the posterior hypothalamus in the cynomolgus monkey (Macaca fascicularis). II. Efferent connections

The efferent connections of the posterior hypothalamus have been analyzed autoradiographically in a series of eight cynomolgus monkey (Macaca fascicularis) brains with injections of ³H-amino acids in different regions of the mamillary complex and the surrounding areas. The medial mamillary nucleus was found to project through the mamillothalamic tract to the ipsilateral anteroventral, anteromedial, and interanteromedial nuclei, and by way of the mamillotegmental tract principally to the deep tegmental nucleus (of Gudden). It also appears to contribute fibers to the medial forebrain bundle, some of which reach as far rostrally as the medial septal nucleus. The lateral mamillary nucleus projects through the mamillothalamic tract bilaterally upon the anterodorsal nuclei of the thalamus, and through the mamillotegmental system to the dorsal tegmental nucleus; it also appears to contribute fibers to the medial forebrain bundle. The supramamillary area has extensive ascending and descending connections that are distributed with the medial forebrain bundle to the hypothalamus and rostral midbrain; in addition, it gives rise to an unusually well-defined projection to field CA2 of the hippocampus and to a narrow zone overlying the outer part of the granule cell layer and the adjoining part of the molecular layer of the dentate gyrus. We have not been able to distinguish the connections of the posterior hypothalamic nucleus from those of the caudal part of the lateral hypothalamic area: they both appear to contribute substantially to the ascending components of the medial forebrain bundle, and through its descending projection to the tegmental fields of the midbrain, the nucleus centralis superior of the raphe complex, the locus coeruleus, and the central gray as far caudally as the facial nerve. Their further projections to the spinal cord were not examined. Viewed broadly, and in the light of previous work, our observations confirm, once again, the constancy of the connections of the hypothalamus in the mammalian brain, and the pivotal position that the posterior hypothalamus occupies in the elaborate system of connections that links the limbic areas of the forebrain with the complex of structures that Nauta has aptly designated the “midbrain limbic region”.     

Brain reward circuitry: Four circuit elements “wired” in apparent series

Activation of a variety of anatomically distinct sites in the central nervous system can produce rewarding states. Four central reward phenomena are amphetamine injections into nucleus accumbens, morphine injections into the ventral tegmental area, electrical stimulation of the ventral tegmental area, and electrical stimulation of the lateral hypothalamic medial forebrain bundle. Current evidence suggests that these four rewarding events trigger activity in elements of a common reward circuit and that the elements are connected in series. The four partially identified elements in this circuit are

1. (1) descending, fast-recovering, short refractory period fibers of the medial forebrain bundle,

2. (2) separate, opioid peptide-containing afferents to the ventral tegmental area,

3. (3) the dopaminergic cells projecting from the ventral tegmental area to nucleus accumbens, and

4. (4) the dopaminoceptive cells of nucleus accumbens.

Fos expression following self-stimulation of the medial prefrontal cortex

Self-stimulation of the medial prefrontal cortex and medial forebrain bundle appears to be mediated by different directly activated fibers. However, reward signals from the medial prefrontal cortex do summate with signals from the medial forebrain bundle, suggesting some overlap in the underlying neural circuitry. We have previously used Fos immunohistochemistry to visualize neurons activated by rewarding stimulation of the medial forebrain bundle. In this study, we assessed Fos immunolabeling after self-stimulation of the medial prefrontal cortex. Among the structures showing a greater density of labeled neurons in the stimulated hemisphere were the prelimbic and cingulate cortex, nucleus accumbens, lateral preoptic area, substantia innominata, lateral hypothalamus, anterior ventral tegmental area, and pontine nuclei. Surprisingly, little or no labeling was seen in the mediodorsal thalamic nucleus or the locus coeruleus. Double immunohistochemistry for tyrosine hydroxylase and Fos showed that within the ventral tegmental area, a substantial proportion of dopaminergic neurons did not express Fos. Despite previous suggestions to the contrary, comparison of the present findings with those of our previous Fos studies reveals a number of structures activated by rewarding stimulation of both the medial prefrontal cortex and the medial forebrain bundle. Some subset of activated cells in the common regions showing Fos-like immunoreactivity may contribute to the rewarding effect produced by stimulating either site.     

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 organization of the hypothalamic pathways mediating affective defense behavior in the cat

The purpose of this study was to describe the hypothalamic pathways which mediate affective defense in the cat utilizing the methods of [14C]2-deoxyglucose (2-DG) and [3H]leucine radioautography in concert with the technique of electrical brain stimulation. The feline affective defense response, characterized by pupillary dilatation, piloerection, ear retraction, hissing, growling and striking with the forepaws, was elicited consistently by stimulation of sites within the ventromedial hypothalamus and anterior aspect of the medial hypothalamus. In one series of experiments, 2-DG autoradiography was employed to describe the brain regions activated following stimulation of sites in the region of the ventromedial hypothalamus from which affective defense had been elicited. Ventromedial hypothalamic stimulation produced activation primarily in forebrain regions situated rostral to the level of the stimulating electrode. These structures included principally the anteromedial hypothalamus and medial preoptic area, as well as the bed nuclei of the stria terminalis and anterior commissure, diagonal band and lateral septal area. The caudal extent of activation included only the dorsal and perifornical hypothalamus at the level of the stimulation site. In a second series of experiments, affective defense sites in the anteromedial hypothalamus were stimulated and the regional distribution of 2-DG label was identified. In contrast to the results obtained from ventromedial hypothalamic stimulation, these experiments revealed a marked descending distribution of label within the posterior hypothalamus, midbrain central gray and ventral tegmental area. Results obtained from studies in which tritiated amino acids were injected into affective defense sites in both the ventromedial nucleus and anteromedial hypothalamus confirmed the general findings observed with 2-DG autoradiography. From these observations, we have concluded that the organization of the pathway mediating affective defense behavior from the ventromedial hypothalamus to the midbrain involves an initial synapse within the region of the anteromedial hypothalamus and a second synapse in the midbrain central gray substance. The significance of the anteromedial hypothalamus for the expression of affective defense behavior was considered in the Discussion.     

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.     

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

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

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.     

Overlapping distributions of orexin/hypocretin- and dopamine-beta-hydroxylase immunoreactive fibers in rat brain regions mediating arousal,motivation, and stress

A double-label immunohistochemical study was carried out to investigate overlap between dopamine-beta-hydroxylase (DbetaH) -immunopositive projections and the projections of hypothalamic neurons containing the arousal- and feeding-related peptide, orexin/hypocretin (HCRT), in rat brain. Numerous intermingled HCRT-immunopositive and DbetaH-immunopositive fibers were seen in a ventrally situated corridor extending from the hypothalamus to deep layers of the infralimbic cortex. Both fiber types avoided the nucleus accumbens core, caudate putamen, and the globus pallidus. In the diencephalon, overlap was observed in several hypothalamic areas, including the perifornical, dorsomedial, and paraventricular nuclei, as well as in the paraventricular thalamic nucleus. Intermingled HCRT-containing and DbetaH-containing fibers extended from the hypothalamus into areas within the medial and central amygdala, terminating at the medial border of the lateral subdivision of the central nucleus of the amygdala. Dense overlap between the two fiber types was also observed in the periaqueductal gray, particularly in the vicinity of the dorsal raphe, as well as (to a lesser extent) in the ventral tegmental area, the retrorubral field, and the pedunculopontine tegmental nucleus. Hypocretin-containing cell bodies, located in the perifornical and lateral hypothalamus, were embedded within a dense plexus of DbetaH-immunopositive fibers and boutons, with numerous cases of apparent contacts of DbetaH-containing boutons onto HCRT-immunopositive soma and dendrites. HCRT-containing fibers were observed amid the noradrenergic cells of the locus coeruleus, and in the vicinity of the A1, A2, and A5 cell groups. Hence, the projections of these two arousal-related systems, originating in distinctly different parts of the brain, jointly target several forebrain regions and brainstem monoaminergic nuclei involved in regulating core motivational processes.     

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.     

Afferent connections to the lateral hypothalamus: A horseradish peroxidase study in the rat

Horseradish peroxidase, 13% Sigma Type VI, was administered iontophoretically to the mid lateral hypothalamus (LH) of male hooded rats. Animals were perfused intracardially on the following day and brains were removed and sliced in the coronal or sagittal planes into 30–50 μm sections. Sections were processed with DAB and BDH for the brown and blue reaction products and later examined by bright and dark field microscopy for the presence and location of retrogradely labeled neurons. Results indicate that a significant number of afferent connections to the LH originate in the olfactory and accumbens nuclei, pyriform cortex, olfactory tracts, magnocellular and medial preoptic and anterior hypothalamic regions, stria terminalis, stria hypothalamic tract, diagonal tract of Broca, caudate-putamen and globus pallidus, internal capsule, lateral septal nuclei, lateral preoptic area and anterior medial forebrain bundle, the various amygdaloid nuclei, zona incerta, perifornical region, dorsal and ventral medial hypothalamic areas, supraoptic, paraventricular and periventricular nuclei, posterior hypothalamus and medial forebrain bundle, ventral thalamic nuclei, the fields of Forel, arcuate and mammillary nuclei, adjacent to the fasciculus retroflexus, in the ventral tegmental area of Tsai, interpeduncular nucleus, substantia nigra, mesencephalic reticular formation, periaqueductal gray, locus coeruleus and parabrachial region. Results are discussed in terms of previous anatomical and neurophysiological data, probable pathways, and the function of LH neurons.     

Efferent connections of the striatum and the nucleus accumbens in the lizard Gekko gecko

The efferent connections of the striatum and the nucleus accumbens of the lizard Gekko gecko were studied with the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). These structures were found to have segregated output systems. The striatum shows a major projection to the globus pallidus. Striatal fibers which are more caudally directed run through the lateral forebrain bundle and can be traced as far caudally as the pars reticularis of the substantia nigra where they exhibit many varicosities. Along its course, the lateral forebrain bundle issues fibers with varicosities to the anterior and posterior entopeduncular nuclei. The major recipient structure of the nucleus accumbens is the ventral pallidum. The nucleus accumbens, in addition, projects to the portion of the lateral hypothalamus in the path of the medial forebrain bundle and to the ventral tegmental area, which is its most caudal target. Subsequently, the same technique was used in an attempt to study the efferents of the globus pallidus and the ventral pallidum, the major recipient structures of the striatum and the nucleus accumbens. The globus pallidus was found to project to the rostral part of the suprapeduncular nucleus in the ventral thalamus and, in addition, may distribute fibers to the same structures as does the striatum. The ventral pallidum distributes fibers to the ventromedial thalamic nucleus. It probably also projects diffusely to the hypothalamus, the habenula, and the mesencephalic tegmentum.     

Limbic connections to the lateral preoptic area: a horseradish peroxidase study in the rat

Horseradish peroxidase, 13% Sigma Type VI, was administered iontophoretically to the lateral preoptic area (LPA) of male hooded rats. Animals were perfused intracardially on the following day and brains were removed and sliced in the coronal plane into 50 microns sections. Alternate sections were processed with DAB and BDH for the brown and blue reaction products and later examined by bright and dark field microscopy for the presence and location of retrogradely labeled neurons. Results indicate that there are a significant number of limbic efferent connections to the LPA. Afferents to the LPA originate in the prefrontal corex, nucleus accumbens, diagonal band and olfactory structures, lateral and medial septum, stria hypothalamic tract and stria terminalis, the magnocellular and medial preoptic nuclei, along the extent of the medial forebrain bundle in the LPA and LH, anterior and basolateral amygdala, ventromedial caudate-putamen, stria medullaris and lateral habenula, the stellatocellular-periventricular, ventromedial, arcuate and anterior hypothalamic nuclei, the perifornical area, zona incerta, ventral medial thalamic area, ventral tegmental area of Tsai, interpeduncular nucleus, reticular zone of the substantia nigra, mesencephalic periaqueductal gray and reticular formation, all aspects of the raphe nuclei and the locus coeruleus. Results are discussed in terms of known anatomical and neurophysiological data and the similar limbic inputs observed for lateral hypothalamic neurons which are found along the extent of the medial forebrain bundle.