Hypothalamus

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It became clear from animal experiments that the small area of the hypothalamus contains a structural maze of

Figure 5 Photomicrographs of coronal sections from the human hypothalamus demonstrating the distribution of NK3 in different subcompartments of the paraventricular nucleus (A and B). Scale bar=0.35 mm. (C) The schematic diagram shows the parts of the Pa from which these sections were taken. (Reproduced from NeuroReport 11(14) with permission from Lippincott Williams & Wilkins, 2000).

Figure 5 Photomicrographs of coronal sections from the human hypothalamus demonstrating the distribution of NK3 in different subcompartments of the paraventricular nucleus (A and B). Scale bar=0.35 mm. (C) The schematic diagram shows the parts of the Pa from which these sections were taken. (Reproduced from NeuroReport 11(14) with permission from Lippincott Williams & Wilkins, 2000).

neural relays that integrate fundamental autonomic, endocrine, and behavioral responses into an elegant strategy regulating homeostasis and reproduction. These relays are cyto- and chemoarchitectonically distinct nuclei, which also differ with respect to their affiliations and functions. Extrapolations to humans of the conclusions obtained from studies on experimental animals have been based on the rationale that

Figure 6 These photomicrographs of coronal sections through the hippocampal region demonstrate an example of nonradioactive in situ hybridization signal obtained with two different probes directed against leptin receptor mRNA (A) and against CRF mRNA (B).

homologous structures have analogous function across mammals. The homologies are established on the basis of chemoarchitecture.

The past century has seen comprehensive cytoarch-itectonic and comparative studies, which have provided plans for the structural organization of the human hypothalamus. However, apart from the work of Saper and that of Braak and Braak, these studies relied on cytoarchitecture and myeloarchitecture for their delineations. Cytoarchitecture, when used as a single comparative criterion, has shortcomings because the cell groups in the adult human hypothalamus are dispersed and certainly less obvious with respect to their subnuclear boundaries than they are in the rat. As a consequence of ineffectual comparative investigations, there is confusion regarding the terminology of the human hypothalamic cell groups. The basic organizational plan of the hypothalamus is thought to be well preserved throughout the mammalian lineage. It became apparent that to produce a unified nomenclature it is necessary to study human and rat hypothalami in parallel, using the same criteria for the establishment of homologies. Thus, current neuroa-natomical studies of the human hypothalamus use cytoarchitecture, chemoarchitecture, topography, subnuclear organization, and pattern of development as criteria for cross-species homologies.

For example, the chemoarchitecture of the human paraventricular hypothalamic nucleus was recently studied with the aid of three-dimensional computer reconstruction (Fig. 7). Chemoarchitecture revealed five subnuclei in the adult human Pa. The most prominent of these is the magnocellular subnucleus (PaM), occupying the ventrolateral quadrant of the Pa and composed of a concentration of large arginine-vasopressin (AVP)- and acetylcholinesterase (AChE)-positive cells and small calbindin (Cb)-positive neurons. Rostrally, the PaM is succeeded by the small anterior parvicellular subnucleus (PaAP), which contains small AChE-, AVP-, and TH-positive cells. Dorsal to the PaM is the dorsal subnucleus (PaD), containing large spindle-shaped TH-, oxytocin (OXY)-, and AChE-positive cells as well as a population of Cb-positive neurons. Abutting the wall of the third ventricle and medial to PaD and PaM is the parvicellular subnucleus PaP. The PaP contains small cells immunoreactive for corticotropin-releasing factor (CRF), NK3, and nonphosphorylated neurofilament protein SM132. The posterior subnucleus (PaPo) is situated posterior to the descending column of fornix; it replaces all previously mentioned subdivisions caudally and is a chemoarchitectonic amalgam that includes dispersed large AChE-, TH-, OXY-, and AVP-positive cells as well as small NK3-, CRF-, SMI32-, and Cb-immunoreactive neurons. The distinctions between Pa subnuclei were further validated by the recent demonstration of different developmental patterns for the subnuclei during fetal gestation (Fig. 8). It is appropriate to mention that the latter study also relied on chemoarchitecture. These findings indicated the homology between the human PaM and PaD and the magnocellular subnuclei of the rat Pa and also between the human PaP and PaPo and the medial parvicellular and posterior subnuclei of the rat.

Apart from the already mentioned CRF, several other releasing hormones have been localized in the human hypothalamus. Most of these molecules are characterized by limited distribution, which benefits the accuracy of identification of the corresponding neuronal structure. Thus, neurons containing growth hormone-releasing hormone (GRH) in the human brain are found exclusively in the arcuate nucleus of

Figure 7 A model of the human Pa showing the subnuclei delineations based on computer-generated plots of six histochemical markers (AChE, SMI32, Cb, TH, NK3, and CRF) and verified with the distribution of AVP, OXY, and NPH. The third ventricle wall lies to the right. (Reproduced from J. Comp. Neurol. 423 with permission from John Wiley & Sons, Inc., 2000).

Figure 7 A model of the human Pa showing the subnuclei delineations based on computer-generated plots of six histochemical markers (AChE, SMI32, Cb, TH, NK3, and CRF) and verified with the distribution of AVP, OXY, and NPH. The third ventricle wall lies to the right. (Reproduced from J. Comp. Neurol. 423 with permission from John Wiley & Sons, Inc., 2000).

the hypothalamus. Some GRH-immunoreactive fibers are also found dorsally in the periventricular area, dorsomedial hypothalamic nucleus, and on the boundary of the ventromedial hypothalamic nucleus. Luteinizing hormone-releasing hormone (LHRH) has been found in the human hypothalamus within distinct neuronal groups, including the arcuate nucleus, the periventricular nucleus, the medial preoptic area, and the premammillary area.

Neurons of arcuate and periventricular hypothala-mic nuclei also contain somatostatin, whereas fibers containing somatostatin are also found extending into the median eminence. Somatostatin is a peptide that inhibits the release of growth hormone (somatotropin) from the anterior pituitary. As such, somatostatin is a release-inhibiting factor. The distribution of somatos-tatin-immunoreactive neurons extends beyond the hypothalamus, with somatostatin-containing neurons found in the diagonal band of Broca, medial septal nuclei, nucleus basalis of Meynert, striatum, bed nucleus of stria terminalis, amygdala, and as far caudal as the periaqueductal gray and brain stem reticular formation. Concurrent autoradiographic studies revealed somatostatin binding sites in the human brain stem tegmentum, basal forebrain, and striatum. A decade of research on the distribution of the hormone-releasing factors established the location of these distinct chemically coded networks and contributed to neuroanatomical delineation of the human brain.

Recent chemoarchitectonic studies demonstrated the presence of an important feeding regulatory molecule orexin (hypocretin) in the lateral hypotha-lamic neurons of humans, rats, and mice. The report also distinguished between two populations ofneurons

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