Suppression of progesterone-induced gonadotropin surge by adrenergic agonists in estrogen-primed ovariectomized rats

An involvement of catecholamines in the preovulatory gonadotropin surge has been proposed for many years. Specifically, an alpha-adrenergic mechanism has been implicated in the stimulation of ovulation and LH release. On the other hand, little is known regarding the possible role of adrenergic neurotransmission once a gonadotropin surge has been initiated. In this study, drugs which affect adrenergic receptors were administered centrally to estrogen-primed ovariectomized rats and the effects on serum concentrations of LH and FSH induced by treatment with progesterone at 08:00 h were examined. As expected, the progesterone treatment caused a significant (p less than 0.05) increase in blood LH and FSH levels by 14:00 and 15:00 h, respectively, when compared with those seen at 13:00 h. In control rats that received isotonic saline intraventricularly at 15:00-16:00 h, both LH and FSH levels continued to rise linearly and reached peak levels by 17:00 and 18:00 h. By contrast, intraventricular infusion of norepinephrine at 16:00 h caused a significant decrease (p less than 0.05) in LH levels when compared with the saline-infused control groups. Moreover, rats that received intraventricular infusion of phenylephrine (an alpha 1-adrenergic agonist) at 15:00 h failed to show a significant increase in LH and FSH levels until 19:00 h; this inhibitory effect was mimicked by a relatively more specific alpha 1-adrenergic agonist, methoxamine. Consistent with our previous findings, animals which received intraventricular isoproterenol (a specific beta-adrenergic agonist) at 15:00 h also responded with suppressed serum LH levels at 16:00-19:00 h when compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)