Medial forebrain bundle of the rat: IV. Cytoarchitecture of the caudal (lateral hypothalamic) part of the medial forebrain bundle bed nucleus

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Abstract
In the preceding study (Geeraedts et al.: J. Comp. Neurol. 294:507-536, '90), the rostral or telencephalic portion of the rat's bed nucleus of the medial forebrain bundle (MFB) has been parcellated into several cytoarchitectonically distinct cellular groups and subgroups. The purpose of the present investigation is to subject the caudal or lateral hypothalamic (LH) portion of the MFB bed nucleus to a detailed cytoarchitectonic analysis. This analysis is based on the same materials, methods, and cytoarchitectonic criteria that were also employed in the preceding study. In contrast to descriptions in the literature, it was found that the LH-region constitutes a very heterogeneous population of neurons with an evident arrangement into groups, several of which have not been identified previously. Many of these cellular groups are partly or entirely located within the boundary of the LH-trajectory of the MFB as previously established by Nieuwenhuys et al. (J. Comp. Neurol. 206:49-81, '82). These groups are designated here as the MFB-related cellular groups. They appear to be arranged into two longitudinal zones. Both zones are caudally replaced by the ventral tegmental area (VTA) and a part of the mesencephalic tegmentum (TEGM1). The lateral zone lies in close proximity to the internal capsule/cerebral peduncle and comprises the following cellular groups: the ventrolateral subarea of the lateral hypothalamic area (LHVL), the anterolateral subarea of the lateral hypothalamic area (LHAL), the lateral tuberal nucleus (TUL), the pre-subthalamic nucleus (PSUT), the retro-subthalamic nucleus (RSUT), the anterodorsal subarea of the lateral hypothalamic area (LHAD), and the lateral hypothalamic nucleus (LHN). The medial zone consists of the following cellular groups: the intermediate hypothalamic area (IHA), the medial tuberal nucleus (TUM), the perifornical nucleus (PFX), the lateral supramammillary nucleus (SUL), the submammillothalamic nucleus (SMT), and the nucleus geminus posterior (GEP). The cellular groups of the medial zone together with the tuberomammillary nucleus (TUMM) are positioned at the interface between the lateral and the medial hypothalamus, and form an array of cellular groups indicated in our study as the intermediate division of the hypothalamus. The MFB-related cellular groups are dorsally, medially, ventrally, and laterally surrounded by rather well-known brain structures. Both the MFB-related cellular groups and the surrounding structures have been identified and delimited. This resulted in a new, elaborate cytoarchitectonic atlas of the rat's lateral hypothalamic region. When the outlines of the MFB on the levels 5-9 of the atlas of the bundle (Nieuwenhuys et al.: J. Comp. Neurol. 206:49-81, '82) are superimposed upon the corresponding tracings of our cytoarchitectonic atlas, the following cellular groups appear to constitute the caudal or lateral hypothalamic portion of the bed nucleus of the MFR: the LHVL, the LHAL, the LHAD, the TUL, the PSUT, the RSUT, the LHN. The GEP, the TEGM1, and, moreover, the lateral portions of the IHA, the TUM, the PFX, the SUL, and the SMT. After super- imposing the areas of labeling of 21 autoradiographically identified fiber com- ponents of the MFB (Veening et al.: J. Camp. Neurol. 206:82-108, '82) upon the corresponding tracings of our atlas, we noted a certain correspondence with the borders of the cytoarchitectonically defined cellular groups and those of the fiber components. These data can be summarized as follows: the inter- mediate division of the hypothalamus contains septal, medial preoptic, and medial hypothalamic fibers; the ventrolateral or TUL/LHVL-part of the MFB is occupied by olfactory related fibers; the dorsolateral part of the MFB, occupying the dorsolateral portion of the LHN-region, is traversed by meso- limbic fibers; the central and dorsomedial portions of the LHN-region are occupied by fibers arising from the lateral hypothalamus, the bed nucleus of the stria terminalis, the ventral tegmental area, and presumably also by fibers from (until now) unknown origin or destination.

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