α‐Adrenergic control of gonadotropin secretion in the ewe

Abstract

The effect of the centrally acting α‐adrenoceptor agonist, clonidine, on plasma LH and FSH was studied in oestradiol‐primed and unprimed ewes and in oestrous ewes. In unprimed anoestrous ewes, clonidine stimulated LH and FSH release after a lag period of 18 h, and noradrenaline intracarotid injection or i.v. infusions immediately stimulated LH release. In oestradiol‐infused anoestrous ewes, clonidine produced either a delay or inhibition of the gonadotrophin surge and noradrenaline i.v. infusion advanced the LH surge. In oestrous ewes treated with clonidine, there was marked delay in the LH surge, but the magnitude of the LH and FSH surges were unaffected. Intravenous administration of α‐adrenoceptor blockers, phentolamine and phenoxybenzamine, blocked the oestradiol‐induced gondotrophin surge in anoestrous ewes. The effect of phenoxybenzamine on gonadotrophin surge was dose dependent in oestrous ewes. Small doses (4 mg/kg i.v.) of phenoxybenzamine delayed the synchronous LH and FSH surges. There was complete blockade of the LH surge and partial blockade of FSH surges in ewes given phenoxybenzamine (8 mg/kg i.v.) before the expected synchronous gonadotrophin surges. After this experiment, the initial rise of plasma progesterone concentrations did not occur until day 6 of oestrous cycle. Administration of phenoxybenzamine before the expected second FSH surge had no effect on the second FSH surge. Gonadotrophin release induced by gonadotrophin‐releasing hormone was attenuated by phenoxybenzamine, but not by clonidine. The results suggest that the LH surge is under α‐adrenergic control and the first FSH surge is under partial α‐adrenergic control, but the second FSH surge is not under α‐adrenergic control. The results also suggest oestradiol modulation of α‐adrenergic receptor action.