A group of neurons highly reactive for enkephalins in the rat hypothalamus.

Enkephalin-like immunoreactivity was demonstrated in a group of highly reactive neurons (HRN) of the rat hypothalamus by the biotin-avidin immunohistochemical technique. The location of the HRN spans several nuclei but the consistent immunoreactivity, the constant topography, and the uniform dimensions of the neurons suggest that they belong to one group. At its caudal end the group appears within the rostral dorso-medial nucleus. At the level of the caudal paraventricular nucleus (PVH) the HRN are assembled in a spherical pattern around a subgroup of neurons in the anterior hypothalamus (AH). The HRN then are found in a position directly between the PVH and the fornix. Before the HRN disappear at the level of the caudal preoptic area, many of the HRN become associated with the fornix. The close association of the HRN with the AH subgroup and the fornix suggests that the HRN may influence the activity of these structures. The HRN are small to medium in size and their short processes suggest that the HRN communicate with other neurons in their vicinity. The areas of the hypothalamus in which the HRN are found are involved in neuroendocrine and thermoregulatory functions suggesting that the HRN may play a role in modifying the activity of neurons and fibres involved in these functions.