The paraventricular nucleus of the hypothalamus: Cytoarchitectonic subdivisions and organization of projections to the pituitary, dorsal vagal complex, and spinal cord as demonstrated by retrograde fluorescence double‐labeling methods

TLDR

The study investigates whether distinct or overlapping populations of neurons in the paraventricular nucleus of the hypothalamus project to the pituitary gland, dorsal vagal complex, and spinal cord. Retrograde tracing was performed in rats using two fluorescent dyes, bisbenzimide‑true blue and Evans blue‑granular blue, to map these projections. Cells projecting to the pituitary are concentrated in the magnocellular core, while descending connections arise mainly from the parvocellular division, with 10–15 % of parvocellular neurons sending divergent collaterals to both the dorsal vagal complex and spinal cord, at least 1,500 PVH cells innervating the medulla/spinal cord, and similar divergent projections from the lateral hypothalamic area and zona incerta, revealing an eight‑subdivision organization of the PVH.

Abstract

Abstract Experiments using two retrogradely transported fluorescent dyes (bisbenzimide‐true blue, and Evans blue‐granular blue) were performed in order to determine whether the same or different populations of neurons in the paraventricular nucleus of the hypothalamus (PVH) project to the pituitary gland, dorsal vagal complex, and spinal cord in the rat. The results suggest that cells projecting to the pituitary gland are concentrated in the magnocellular core of the nucleus, while the descending connections arise primarily from the surrounding parvocellular division. The occurrence of neurons double‐labeled with both dyes further indicate that at least 10–15% of the labeled cells in the parvocellular division send divergent axon collaterals to the dorsal vagal complex and to the spinal cord. Cell counts suggest that at least 1,500 cells in the PVH project to the medulla and/or spinal cord. These results, combined with a cytoarchitectonic analysis, show that the PVH consists of eight distinct subdivisions, three magnocellular and five parvocellular. The lateral hypothalamic area and zona incerta also contain a large number of cells projecting to the dorsomedial medulla and spinal cord; approximately 15% of such cells are double‐labeled following injections of separate tracers into these two regions of the same animal.