Oxytocin induces mating behavior in rats of both sexes. Previous experiments revealed that progesteroneinduced sex behavior in ovariectomized, estrogen-primed rats was caused by release of NO from NOergic neurons that stimulated the release of luteinizing hormone-releasing hormone (LHRH). The LHRH activated brain-stem neurons that initiated the lordosis ref lex. We hypothesized that oxytocin might similarly release NO in the medial basal hypothalamic region that would stimulate release of LHRH into the hypophyseal portal vessels to release luteinizing hormone. To investigate this hypothesis, medial basal hypothalamic explants were preincubated in Krebs-Ringer bicarbonate buffer for 30 min, followed by a 30-min incubation in fresh Krebs-Ringer bicarbonate buffer containing the compounds to be tested. Oxytocin stimulated LHRH release 3-to 4-fold at the lowest concentration tested (10 ؊10 M). Values remained at a plateau as the concentration was increased to 10 ؊7 M and then declined in a concentration-dependent manner, so that there was no stimulation with a concentration of 10 ؊5 M. Oxytocin (10 ؊7 M) stimulated release of prostaglandin E 2 into the medium, a finding consistent with a role of NO in the response. That NO indeed mediated the action of oxytocin was supported by blockade of the action of oxytocin by the competitive inhibitor of NO synthase (NOS), N G -monomethyl-L-arginine (300 M). Furthermore, oxytocin (10 ؊9 to 10 ؊7 M) activated NOS as measured at the end of the experiments. Oxytocin appeared to act to stimulate norepinephrine terminals in the medial basal hypothalamus, which activated NOS by ␣ 1 -adrenergic receptors, because prazocine, an ␣ 1 receptor blocker, inhibited the LHRH-releasing action of oxytocin. Finally, incubation of neural lobe explants with sodium nitroprusside, a NO releasor, revealed that nitroprusside (300-600 M, but not 900 M) inhibited oxytocin release. Therefore, the NO released by oxytocin also diffuses into the oxytocin neuronal endings and inhibits oxytocin release, forming a negative feedback loop. The results indicate that oxytocin is important not only in induction of mating, but also in stimulating LHRH release with subsequent luteinizing hormone discharge that plays a crucial role in reproduction.Although oxytocin is one of the most abundant peptides in the body, little is known about its function. In the female, it is critical for milk ejection during lactation and plays an important role in parturition by inducing uterine contractions (1, 2). Oxytocin also appears to influence maternal and mating behavior, because intraventricular injection of the peptide induces these behaviors in female rats (3, 4).Although the amounts of oxytocin present in the neurohypophysis of male and female rats are similar, its function in the male has not been established (1). It induces natriuresis in both sexes (1), and recent evidence indicates that it may do so by releasing atrial natriuretic peptide (5).Intraventricular injection of oxytocin causes penile erection in male ra...
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