Steroid priming promotes oxytocin-induced norepinephrine release in the ventromedial hypothalamus of female rats

Abstract

In vivo microdialysis was used to detect norepinephrine (NE) release in the ventromedial hypothalamus of estradiol (E 2)- or E 2 plus progesterone (P)-treated female rats injected with 1.0 IU of oxytocin (OXY). Dialysates were collected before and after OXY administration on 3 consecutive days and analyzed for NE content by high performance liquid chromatography with electrochemical detection. After the last sample was collected on day 1, animals were injected with 3 μg E 2 benzoate or oil. On day 3, E 2-primed animals received 200 μg of P and control females received oil prior to OXY administration. OXY administration did not induce NE release on day 1. When OXY was administered to animals that received E 2 approximately 20 h earlier, increased release of NE was not consistently seen. In contrast, E 2-primed animals that received P on day 3 displayed significant increases in the release of NE after OXY administration compared to their own basal levels and to NE levels in control animals. To distinguish whether E 2 priming is sufficient to promote OXY-induced release of NE without the addition of P, NE content of VMH dialysates in a second group of animals was examined following exposure to vehicle or E 2 alone. When OXY was administered 24 or 48 h after estrogen priming, only 1 of 4 E 2-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 injections of OXY, the α 1-adrenoceptor antagonist prazosin, or both OXY and prazosin. OXY enhanced lordosis behavior in females primed with subthreshold doses of E 2 and P. Prazosin abolished lordosis behavior in rats primed with behaviorally effective doses of E 2 and P and significantly inhibited lordosis in steroid-primed females given OXY. These data suggest that after priming with both E 2 and P together, but not with E 2 alone, OXY may facilitate lordosis behavior through activation of NE transmission.