Spatial organization and morphometric characteristics of histaminergic neurons in the rat brain.

We report here a study addressing the spatial organization, densities, numbers, sizes, and shapes of histaminergic neurons in the rat hypothalamus. Studies were performed on 50 rats using histochemical and morphometric methods, computer image analysis, and three-dimensional reconstruction of the histaminergic nuclei of the hypothalamus. The results demonstrated that the total bilateral volume of the histaminergic nuclei of the rat brain amounts to 0.5 mm(3): the E2 nucleus occupies 40% of the total volume, E4 occupies 35%, E3 occupies 13%, E5 occupies 9%, and E1 occupies 3%. The distribution density of monoamine oxidase B neurons in the histaminergic nuclei of the rat hypothalamus decreased in the order E1 > E3 > E2 (the "compact" nuclei) > E4 (an "intermediate" density nucleus) > E5 (the "diffuse part"). The mean number of histaminergic neurons in the rat hypothalamus was 37200 +/- 2800; the E2 nucleus contained 54% of these cells, E3 contained 23%, E4 contained 6%, E1 contained 7%, and E5 contained 0.32%. The E1-E3 nuclei were dominated by small and intermediate-sized neurons, round in shape, and the E5 nucleus was dominated by intermediate and large neurons, fusiform in shape. A subpopulation of giant histaminergic neurons was detected in the E4-E5 nuclei.