Autoradiographic visualization of sex differences in the pattern and density of opiate receptors in hamster hypothalamus

Abstract

Slide-mounted brain sections were used to visualize the distribution of opiate receptors in the hypothalamus of male and female hamsters using in vitro film autoradiography. Sex differences were found in the binding density and patterns of [ 3H]naloxone-labeled receptors. The distribution and density of [ 3H][ d-Ala 2, d-Leu 5enkephalin-labeled δ-receptors in adjacent brain sections were similar in males and females. Male hamsters showed a U-shaped pattern of [ 3H]naloxone binding in the sexually dimorphic nuclear complex with 28% and 34% greater labeling of the sesually dimorphic nucleus (SDN) and bed nucleus/stria terminalis (BNST), respectively, than periovulatory estrous females. Estrous and diestrous females showed a V-shaped pattern of [ 3H]naloxone binding in the same region, but binding density was higher at diestrus. Greater specific [ 3H]naloxone binding in diestrous females was also evident following extensive prewashing of slide-mounted tissue sections indicating that residual endogenous opioids were not occupying receptors, and thus, reducing the labeling of receptors in tissue from estrous females. An estrous-linked change in the affinity of hypothalamic opiate receptors was suggested by findings that [ 3H]naloxone binding density was greater in tissue from diestrous females when incubations were conducted in the presence of a 1-nM, but not a 10-nM, concentration of the labeled antagonist. Finally, a dense area of [ 3H]naloxone binding was discovered in the dorso-suprachiasmatic region of the hypothalamus. These data provide evidence for a sexual dimorphism in the distribution and density of opiate receptors in hamsters. The data suggest that μ- or κ-receptors are more likely than δ-receptors to be involved in mediating hypothalamic effects of endogenous and exogenous opiods on reproductive functions in this species.