Brainstem Projections of the Dorsolateral Hypothalamic Area

This chapter highlights about Brainstem projections of the dorsolateral hypothalamic area. The dorsolateral hypothalamic area (DLH) covers a relatively large, but not strictly outlined area in the posterior part of the hypothalamus. This area extends rostrocaudally from the level of the caudal aspect of the paraventricular nucleus (about 2.4 mm caudal to the bregma level, in adult rats) until the level of the caudal end of the third ventricle (3.4 mm caudal to the bregma). The dorsolateral hypothalamic area can be divided into 3 areas (dorsomedial hypothalamus, perifornical area, lateral hypothalamic area), and further subdivided into 8 subdivisions. After unilateral stereotaxic injections of biotinylated dextran amine (BDA) into individual DLH subdivisions, the distribution patterns of BDA-positive fibers were mapped on serial sections between the hypothalamus and spinal cord in 22 rats. BDA-labeled fibers were observable over 100 different brainstem areas, nuclei or subdivisions. Each DLH subdivisions established distinct topographical patterns, however generally the density of labeled fibers was low in the lower brainstem. The present findings confirm a significant portion of the data reported about the innervation pattern of the dorsolateral hypothalamus (DLH) to the lower brainstem in general, the evaluation of the projections of the 8 individual subgroups provides us new information. No definitive descending BDA-positive pathway, but long-run solitaire BDA-labeled fibers were seen. Some of the major tracts or fasciculi in the brainstem were connected by these descending fibers. Only 4 of the 116 locations had high fiber densities: the Barrington's and pedunculopontine tegmental nuclei, and the ventrolateral and lateral portions of the periaqueductal grey. Knowing the precise locations of their neuronal inputs is important for understanding how the lower brainstem functions because it contains significant visceral and autonomic regulatory areas and nuclei, the reticular formation, central noradrenaline and adrenaline neurons (including premotor sympathetic ones), and serotonin-expressing raphe nuclei. Some of the tegmental nuclei and nuclei of the reticular formation were widely innervated, although the density of the BDA-labeled fibers was generally low.