Distribution of catecholamine neurons in the hypothalamus and preoptic region of mouse

The distribution and morphology of cells containing tyrosine hydroxylase (TH) were mapped by using the immunoperoxidase technique in the hypothalamus and preoptic area in two strains of mouse, CBA/J and BALB/cJ. On the basis of rostral‐caudal contiguities between cell aggregates, hypothalamic‐preoptic neurons were subdivided into three arbitrary groups: (1) dorsal, (2) intermediate, and (3) ventral. New or more prominent collections of TH cells were observed, and in some regions, cells were more complexly organized than originally described. In the dorsal group , a rostral collection of small ovoid cells, previously not described, were located in the anterior preoptic nucleus (APN) of Loo ('31) and extended rostrally and ventrally into the preoptic periventricular gray. The next constituent occupied the paraventricular nucleus (PVN), and was composed of two classes of cells: (1) a small ovoid cell within anterior and medial parvocellular PVN in contiguity rostrally with a similar cell in APN and (2) a larger, angular cell within and adjacent to the lateral PVN in contiguity caudally with cells in the zona incerta (ZI). Further caudally, a larger and more pleomorphic collection of TH neurons was localized to the medial ZI, particularly at midtuberal levels. These cells were not scattered, as previously reported, but were differentiated into two clear‐cut densities, a larger medial island and a more elongated lateral island. Cells of ZI, both large and small, extended caudally into the dorsal hypothalamic and subparafascicular nuclei and periventricular gray. In contrast to previous descriptions, no cells were seen in the nucleus reuniens. In the intermediate group , the most rostral constituent occupied the preoptic periventricular gray, extended as far as the lamina terminalis, and merged dorsocaudally with cells in APN. While the variably shaped cells of the hypothalamic periventricular gray (PVG) were still present in the retrochiasmatic region, a striking absence of these cells was noted at midtuberal levels between the dorsomedial and arcuate hypothalamic nuclei. At this level, a new group of small‐round TH cells, resembling those of the arcuate nucleus, was identified in the dorsomedial hypothalamic nucleus (DMN). At caudal tuberal levels, similar neurons were found in the posterior hypothalamic nucleus (PH). These neurons overflowed medially into the PVG and caudoventrally into the arcuate nucleus. In the ventral group , the most rostral constituent, composed of both small and ovoid cells in the retrochiasmatic area, appeared to represent the rostral commissural portion of the arcuate nucleus (Arc). In Arc, TH cells were more complexly organized than previously described and formed three morphologically distinct substructures. The density of TH cells, in general, was greatest in the midtuberal region. The observation that few or no cells were located in ventromedial or caudal aspects of Arc indicates that cells synthesizing other neurotransmitters occupy these regions. The remarkable overlap of TH‐immunoreactive cells with recently described peptidergic cells in Arc and other hypothalamic substructures suggests the coexistence of two or more putative neurotransmitters within single neurons.