Neurotransmitter specificity of cells and fibers in the medial preoptic nucleus: An immunohistochentical study in the rat

Abstract

The medial preoptic nucleus (MPN) is a sexually dimorphic complex with three major subdivisions. The cell‐dense central (MPN c) and medial (MPN m) subdivisions are larger in male rats, while the cell‐sparse lateral subdivision (MPN l) occupies a majority of the nucleus in females. In the present study we evaluated the distribution of possible monoaminergic and peptidergic cells and fibers within the MPN, as well as in adjacent regions of the medial preoptic area of the adult male rat. For this, we used an indirect immunohistochemical method with antisera to serotonin (5HT), dopamine β‐hydroxylase (DBH), tyrosine hydroxylase (TH), neuropeptide Y (NPY), cholecystokinin (CCK), vasoactive intestinal polypeptide (VIP), substance P (SP), neurotensin (NT), corticotropin‐releasing factor (CRF), luteotropin‐releasing hormone (LRH), somatostatin (SS), thyrotropin‐releasing hormone (TRH), oxytocin (OXY), vasopressin (VAS), adrenocorticotropic hormone (1‐24; ACTH), alpha‐melanocyte‐stimulating hormone (c:‐MSH), leucine‐enkephalin (L‐ENK), and calcitonin gene‐related peptide (CGRP). The results suggest that cell bodies and/or fibers crossreacting with all of these putative neurotransmitters are differentially distributed within the MPN. Within the MPN m, the densest plexuses of fibers were stained with antisera to SP and NPY, while moderate densities of fibers were stained with anti‐DBH, SS, CCK, CGRP, ACTH, and α‐MSH, and only a few fibers were stained with anti‐5HT, TH, NT, VAS, and L‐ENK. Moderate numbers of SP‐ and L‐ENK‐immunoreactive cell bodies, and a few SS‐, NT‐, CRF‐, and TRH‐stained cell bodies were also found within the MPN m. The MPN c contained a dense plexus of CCK‐immunoreactlve fibers, as well as a few CRF‐immunoreactive fibers. Both fiber types were localized almost exclusively to this subdivision, while most of the others studied here appeared to avoid it selectively. This suggests that there are relatively few inputs to the MPN c, and that they tend to avoid other parts of the nucleus, although moderate densities of DBH‐ and NPY‐immunoreactive fibers were found in both the MPN m and MPN c, The MPN c contained several CCK‐immunoreactive cell bodies as well as a moderate number of TRH‐stained cell bodies. Both cell types were nearly completely localized to the MPN c. The major inputs to the MPN 1 studied here appear to be stained with antisera to 5HT and L‐ENK, although moderate numbers of NT‐ sad CRF‐immunoreactive fibers were also found in this part of the nucleus. The MPN 1 also contained many NT and CRF‐stained cell bodies, and these two peptides appear to be co‐expressed within a sizable proportion of these cells. Finally, several CGRP‐ and VIP‐immunoreactive cell bodies were found in the MPN 1. Thus, it appears that each subdivision of the MPN receives a unique pattern of inputs and contains specific subpopulations of cell bodies. Since each part of the MPN is sexually dimorphic, it seems likely that the distributions of at least some of these neurotransmitter‐specific cell bodies and terminal fields within the MPN may also sexually differentiate under the influence of the perinatal steroid hormone environment.