Estrogen decreases the sensitivity of anterior pituitary to the inhibitory effect of nitric oxide on prolactin release

OBJECTIVE: We investigated the effect of chronic estrogen treatment on the inhibitory action of nitric oxide (NO) on prolactin release. METHODS: The effect of NO on prolactin release was studied in anterior pituitaries of female Wistar rats, intact at random stages, ovariectomized (OVX), and OVX treated for 15 days with 17beta-estradiol (OVX-E(2)). RESULTS: Sodium nitroprusside (NP, 0.5 mM), a NO donor, inhibited prolactin release from anterior pituitaries and was able to stimulate cGMP synthesis in intact and OVX rats. Only a high, supraphysiological concentration of NP (2 mM) inhibited prolactin release from anterior pituitaries of OVX-E(2) rats and increased cGMP synthesis in OVX-E(2) rats. 8-Br-cGMP, a cGMP analogue, decreased prolactin release from anterior pituitaries of OVX rats but did not affect it in OVX-E(2) rats. CONCLUSION: Our results suggest that estrogen may modify the sensitivity of the anterior pituitary to the inhibitory effect of NO on prolactin release by affecting guanylyl cyclase activity and the cGMP pathway.