Opioid induction of Fos in the female rat brain : modulation by ovarian hormones

Previous research has shown that opioid drugs have profound effects on female reproduction. It has also been established that ovarian hormones exert control over opioid peptide synthesis and opioid receptor gene expression. The present thesis investigated functional pathways by which opioids affect the sexual behaviour of female rats, by examining the induction of Fos protein. Ovariectomized, sexually experienced Long Evans rats (n = 48) were implanted stereotaxically with guide cannulae aimed at the right lateral ventricle. One group of rats was treated with estradiol benzoate (10 og) 48 hr and progesterone (250 og) 4 hr before testing, another group was treated with EB-alone and a third group was treated with oil. Females were infused with the selective o-receptor agonist, DAMGO, the selective e-receptor agonist, DPDPE, the selective m-receptor agonist, U50-488, or saline. All rats were sacrificed 75 min after the drug infusion. The induction of Fos immunoreactivity was analyzed in the lateral septum, medial preoptic area, ventromedial hypothalamus, medial amygdala, and mesencephalic central gray. The present study found that Fos immunoreactivity was induced differentially by the opioid agonists within the different brain regions, and the hormones either augmented or had no effect on the induction. These data demonstrate that estrogen and progesterone alter the ability of opioid agonists to induce Fos protein within different regions of the female rat brain. These results contribute to the understanding of opioid actions within the female rat brain