ESTROGEN-RECEPTOR IMMUNOREACTIVITY IN SPECIFIC BRAIN-AREAS OF THE PRAIRIE VOLE (MICROTUS-OCHROGASTER) IS ALTERED BY SEXUAL RECEPTIVITY AND GENETIC SEX

The prairie vole is a small rodent with an unusual reproductive strate gy. A sexually naive female vole requires male contact to initiate the maturation of her reproductive functions. Contact with an unfamiliar adult male vole increases blood estrogen levels, reproductive tissue w eights, and brain nuclear estrogen receptor binding levels of female v oles. What is not known is: 1) What is the precise distribution of est rogen receptor containing neurons in the prairie vole brain? 2) Does m ale induced sexual receptivity alter the distribution or number of est rogen receptors in specific brain areas of the female vole? 3) Do male and female voles differ in the distribution or number of estrogen rec eptor containing neurons? We compared sexually receptive-male-exposed females, sexually naive females, and sexually naive males, for the pre sence of estrogen receptor immunoreactive (ER-IR) neurons in specific cell groups of the brain. The number of ER-IR neurons per cell group w as counted and the relative amount of immunoreactivity per neuron was measured by densitometry. The neuroanatomical distribution of estrogen receptor containing neurons in the vole was similar to the distributi on of estrogen receptors in most rodents. The mean number of ER-IR neu rons did not differ between naive and male-exposed females. The induct ion of sexual receptivity however significantly decreased the concentr ation of estrogen receptor immunoreactivity per neuron in the medial p reoptic nucleus, the medial preoptic area, the encapsulated bed nucleu s of the stria terminalis, and the ventromedial nucleus of the hypotha lamus. Compared with naive males, the mean number of ER-IR neurons was up to four fold greater in naive females in the medial preoptic nucle us, anteroventral periventricular preoptic nucleus, the encapsulated b ed nucleus of the stria terminalis, the medial amygdala, and the ventr omedial nucleus of the hypothalamus. Additionally the amount of estrog en receptor immunoreactivity per neuron was considerably greater in th e medial preoptic nucleus, the medial preoptic area, the encapsulated bed nucleus of the stria terminalis, and the ventromedial nucleus of t he hypothalamus of naive females. If the amount of estrogen receptor p er cell is a determinant of a tissue's responsiveness to estrogen, red uced estrogen receptor immunoreactivity in males, and in females expos ed to males suggests that they may be less responsive to estrogen than naive females. We propose that this reduced estrogen receptor immunor eactivity in males is a result of reduced estrogen receptor protein le vels. Currently, we cannot definitively prove our working hypothesis t hat decreased estrogen receptor immunoreactivity in females exposed to males is due to reduced receptor levels, and not due to ligand altere d epitope availability. Our working hypothesis is supported by the bra in region-specific nature of our findings in the females. Experiments using additional antibodies directed against different epitopes of the estrogen receptor and examining ER mRNA will pursue this hypothesis. Brain regions in which estrogen receptor content differs depending upo n genetic sex and experiential factors may be particularly important i n the regulation of reproduction.