A topographical analysis of the origin of some efferent projections from the lateral hypothalamic area in the rat

Abstract

Some projections from the lateral hypothalamic area in the rat have been investigated, using combinations of fluorescent tracers, injected into several different parts of the central nervous system. Projections appear to arise from loosely organized assemblies of neurons, called sets and from more densely packed assemblies, called clusters. The sets and clusters vary considerably in position and in distinctness of their borderlines. Even within extensive and vaguely denned sets, however, high concentrations of labeled neurons may be present at specified sites in the lateral hypothalamus. Such concentrations are observed in the transitional area of the zona incerta and the dorsal part of the lateral hypothalamus and in the ventrolateral part of the hypothalamus, bordering the cerebral peduncle and the subthalamic nucleus, in both cases after injections into some “autonomic centers” in the brainstem, such as the parabrachial nuclei and the dorsal vagal complex. Sets and clusters may overlap considerably. Within the fields of overlap the number of double labeled neurons may vary from almost zero up to more than 50%, depending on the injection sites. The results show that different parts of the lateral hypothalamus in the rat have different efferent relationships. Combination of the results of the present study with known data concerning the afferent relationships, the cytoarchitecture and behavioral functions of the lateral hypothalamic area, suggests that different parts of this entity are involved in different regulatory and behavioral functions.