Influence of oestrogen and noradrenergic afferent neurones on the response of LH and oxytocin to immobilization stress.

The influence of oestrogen on LH and oxytocin responses to immobilization stress, and the involvement of noradrenergic afferent neurones in these responses, was examined in ovariectomized rats with or without pretreatment with oestrogen or after noradrenergic transmitter blockade. Immobilization of the rats on a board in a supine position for 1 h brought about a rapid decrease in LH levels and an increase in oxytocin levels in the blood of ovariectomized rats. Oestradiol benzoate (20 micrograms) injected s.c. the day before immobilization, decreased basal LH levels but had no effect on basal oxytocin levels. Immobilization stress applied to oestrogen-treated rats induced a small but significant increase in LH concentrations and a rise in oxytocin concentrations similar to that in rats without oestrogen pretreatment. A dopamine-beta-hydroxylase inhibitor or phenoxybenzamine (alpha-adrenoceptor blocker) injected into ovariectomized rats reduced basal LH levels and increased basal oxytocin levels in the blood. Immobilization stress induced an increase in LH levels in rats treated with dopamine-beta-hydroxylase inhibitor, but had no effect in rats treated with phenoxybenzamine. Stress induced a larger increase in blood oxytocin levels in rats treated with either drug compared with that in control rats injected with vehicle. On the other hand, propranolol (beta-adrenoceptor blocker) had no effect on basal or stress-induced changes in LH or oxytocin levels in the blood. These results indicate that the LH response to stress, which might be mediated through alpha-adrenergic neurones, depends on the circulating oestrogen or LH levels before the stress. In contrast, the oxytocin response to stress may not be mediated by noradrenergic neurones and may not be influenced by oestrogen.