STEROID PRIMING PROMOTES OXYTOCIN-INDUCED NOREPINEPHRINE RELEASE IN THE VENTROMEDIAL HYPOTHALAMUS OF FEMALE RATS

In vivo microdialysis was used to detect norepinephrine (NE) release i n the ventromedial hypothalamus of estradiol (E2)- or E2 plus progeste rone (P)-treated female rats injected with 1.0 IU of oxytocin (OXY). D ialysates were collected before and after OXY administration on 3 cons ecutive days and analyzed for NE content by high performance liquid ch romatography with electrochemical detection. After the last sample was collected on day 1, animals were injected with 3 mug E2 benzoate or o il. On day 3, E2-primed animals received 200 mug of P and control fema les received oil prior to OXY administration. OXY administration did n ot induce NE release on day 1. When OXY was administered to animals th at received E2 approximately 20 h earlier, increased release of NE was not consistently seen. In contrast, E2-primed animals that received P on day 3 displayed significant increases in the release of NE after O XY administration compared to their own basal levels and to NE levels in control animals. To distinguish whether E2 priming is sufficient to promote OXY-induced release of NE without the addition of P, NE conte nt of VMH dialysates in a second group of animals was examined followi ng exposure to vehicle or E2 alone. When OXY was administered 24 or 48 h after estrogen priming, only 1 of 4 E2-primed females had modestly elevated dialysate NE levels. To evaluate the interactions between OXY and NE in the regulation of reproductive behavior, lordosis responses were observed in hormone-primed female rats receiving systemic inject ions of OXY, the alpha1-adrenoceptor antagonist prazosin, or both OXY and prazosin. OXY enhanced lordosis behavior in females primed with su bthreshold doses of E2 and P. Prazosin abolished lordosis behavior in rats primed with behaviorally effective doses of E2 and P and signific antly inhibited lordosis in steroid-primed females given OXY. These da ta suggest that after priming with both E2 and P together, but not wit h E2 alone, OXY may facilitate lordosis behavior through activation of NE transmission.