Medial forebrain bundle of the rat: III. Cytoarchitecture of the rostral (telencephalic) part of the medial forebrain bundle bed nucleus

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Abstract
The boundaries of the medial forebrain bundle (MFB) of the rat have been presented in previous work on the structure of this fiber system (Nieuwenhuys et al.: J. Comp. Neurol. 206:49-81, '82). Neuronal cell bodies within these outlines constitute the bed nucleus of the MFB. Many fiber components of the MFB appeared to be spatially arranged within the bundle and featured an orderly topography (Veening et al.: J. Comp. Neurol. 206:82-108, '82). As the fibers of the MFB are thought to be a major source of afferents to the bed nucleus (Millhouse: In P. J. Morgane and J. Panksepp (eds): Anatomy of the Hypothalamus, Vol. 1. New York: Marcel Dekker, pp. 221–265, '79), the latter has been subjected in this and the companion study (Geeraedts et al.:J. Comp. Neurol. 294:537-568, '90) to a detailed cytoarchitectonic analysis. This analysis is based on continuous series sectioned in the three conventional planes. On the basis of cytoarchitectonic characteristics, including size and shape, staining intensity, packing density, and spatial orientation of the cell bodies, it was found that the bed nucleus of the MFB as described in the literature is by no means a cytoarchitectonic unit per se. Rather, the neuronal cell population located within the telencephalic stream of the MFB can be parcellated into a number of cellular groups, which partly or entirely belong to more-or-less known basal telencephalic structres. These structures are designated here as the MFB-related areas. They correspond largely to the subcommissural substantia innominata (SIC), the sublenticular substantia innominata (SIL), the nucleus of the diagonal band of Broca, the olfactory tubercle, the magnocellular preoptic nucleus (POMA), the lateral preoptic area (LPOA), and the interstitial nucleus of the stria medullaris (ISM). The complex of the MFB-related areas is surrounded by the following cellular entities: the nucleus accumbens (ACB), the caudatus-putamen region (CPU), the globus pallidus (GP), the bed nucleus of the stria terminalis (BST), the anterior amygdaloid area (AAA), the amygdaloid nuclear complex (A), the medial preoptic area (MPOA)and the anterior hypothalamic area (AHA). Both MFB-related areas and their surroundings have been identified and delimited in this study. This resulted in a new cytoarchitectonic atlas of the rat's basal telencephalon. Our atlas does not only show the relative positions of the above mentioned cellular groups, but also those of their subdivisions. The SIC, the SIL, the POMA and the LPOA have been parcellated into four, three, three, and six subareas, respectively; the GP, the BST, the MPOA, and the AHA into four, seven, fourteen, and six subareas, respectively. Several of these subdivisions have not been identified previously. The amygdaloid nuclear complex was not further parcellated cytoarchitectonically, as this was felt to be beyond the scope of our study. In preparing the atlas, it was kept in mind that the basal telencephalon should be parcellated in cellular groups and subgroups easily recognizable even by investigators not well trained in cytoarchitectonics. When the outlines of the MFB on the levels 1 to 4 of the atlas of the bun- dle (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 rostral or telencephalic part of the bed nucleus of the MFR: the deep layer of the olfactory tubercle, the magnocellu- lar preoptic nucleus, the horizontal limb of the diagonal band of Broca, the lateral preoptic area and the sublenticular substantia innominata. After superimposing the areas of labeling of 21 autoradiographically identified fiber components of the MFB (Veening et al.: J. Comp. Neurol. 206:82-108, '82) upon our tracings of the rostral bed nucleus, a remarkable correspondence appeared with the borders of the cytoarchitectonically defined cellular entities and those of the traversing fiber components to and from specific origins and destinations. These findings can be summarized as follows: the ventrolateral or POMA-section of the MFB is occupied by olfactory related fibers; the ven- tromedial or LPOA-section of the MFB contains septal, preoptic, and medial hypothalamic fibers; and the dorsal or SIL-section of the MFB contains mesolimbic fibers, including fibers arising from the central amygdaloid nucleus, the bed nucleus of the stria terminalis, the ventral tegmental area, and the parabrachial nuclei of the brainstem region. Further morphological analysis of the connectivity of the neurons of the cellular entities of the rostral bed nucleus and their proposed dendritic rela- tionships with collaterals of the fibers of the MFB-components will provide more profound information on the intrinsic organization of the MFB and its so-called bed nucleus.