ESTRADIOL INCREASES THE BEHAVIORAL-RESPONSE TO ARGININE-VASOPRESSIN (AVP) IN THE MEDIAL PREOPTIC-ANTERIOR HYPOTHALAMUS

Flank marking, a form of hamster scent marking controlled by arginine vasopressin (AVP) in the medial preoptic-anterior hypothalamus (MPOA-A H), is altered by circulating levels of gonadal hormones. We hypothesi zed that gonadal hormones influence flank marking either by altering t he availability of AVP for release in the MPOA-AH or by altering the s ensitivity or responsiveness of MPOA-AH neurons to AVP. We examined th e levels of AVP immunoreactivity (AVP-IR) over the estrous cycle in th e MPOA-AH and other areas. Arginine vasopressin immunoreactivity did n ot vary in the areas examined as a function of the stage of the estrou s cycle. In Experiment 2 we measured flank marking after MPOA-AH micro injection of AVP in ovariectomized hamsters receiving estradiol or emp ty Silastic capsules. Hamsters implanted with estradiol capsules marke d significantly more in response to AVP than did hamsters receiving no hormone replacement. These results support the hypothesis that estrad iol influences flank marking by altering the sensitivity or responsive ness of the MPOA-AH or its efferents to AVP. Additionally, we observed an unexpected effect of AVP in estradiol-treated hamsters. After micr oinjection with 90 muM AVP. lordosis occurred spontaneously in 60% of the hamsters even though no male was present. We suggest that female h amsters may be a useful model to further investigate the role of AVP a nd AVP-like peptides in female sexual behavior.