Regulation of Soluble Guanylyl Cyclase Activity by Oestradiol and Progesterone in the Hypothalamus But Not Hippocampus of Female Rats

Abstract

Oestradiol and progesterone act in the hypothalamus to coordinate the timing of lordosis and ovulation in female rats in part through regulation of nitric oxide (NO) and cyclic guanosine monophosphate (cyclic GMP) signalling pathways. Soluble guanylyl cyclase is an enzyme that produces cyclic GMP when stimulated by NO and plays a crucial role in the display of lordosis behaviour. We examined the effects of oestradiol and progesterone on the stimulation of cyclic GMP synthesis by NO-dependent and independent activators of soluble guanylyl cyclase in preoptic-hypothalamic and hippocampal slices. Ovariectomised Sprague-Dawley rats were injected with oestradiol (2 microg oestradiol benzoate, s.c.) or vehicle for 2 days. Progesterone (500 microg, s.c.) or vehicle was injected 44 h after the first dose of oestradiol. Rats were killed 48 h after the first oestradiol or vehicle injection, and hypothalamus and hippocampus were obtained. NO-dependent activation of soluble guanylyl cyclase was induced by NO donors, sodium nitroprusside or diethylamine NONOate; NO-independent activation of soluble guanylyl cyclase was induced with 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole and 5'-cyclopropyl-2-[1-2fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridine-3-yl]pyridine-4-ylamine. The NO-dependent activators of soluble guanylyl cyclase produced a concentration-dependent increase in cyclic GMP accumulation and induced significantly greater cyclic GMP accumulation in preoptic-hypothalamic slices from animals treated with oestradiol and progesterone than in slices from rats injected with vehicle, oestradiol or progesterone alone. Hormones did not modify soluble guanylyl cyclase activation by NO-independent stimulators or influence NO content in preoptic-hypothalamic slices. Oestradiol and progesterone did not affect activation of soluble guanylyl cyclase in hippocampal slices by any pharmacological agent, indicating a strong regional selectivity for the hormone effect. Thus, oestradiol and progesterone, administered in vivo, enhance the ability of NO to activate soluble guanylyl cyclase in brain areas modulating female reproductive function without an effect on production of NO itself.