Effect of dopamine receptor stimulation on the inhibition of LH pulsatility by a met-enkephaline (FK 33-824).

Plasma LH fluctuations reflect pulsatile hypothalamic GnRH activity. In order to investigate a possible interaction between opiate and dopaminergic pathways in the control of LH and PRL release, a met-enkephalin analogue (FK 33-824) was administered intravenously to female volunteers at a rate of 0.01 mg/kg/h for 4 h. LH pulses as recorded by plasma measurements every 20' were significantly (p less than 0.01) inhibited whereas PRL concentrations were increased. Pre-treatment with bromocriptine, 1.25 mg b.i.d. for 3 days, counteracted the stimulatory effect of FK 33-824 on PRL secretion but the inhibitory effect on LH episodic release was not modified by this dopamine agonist. Results indicate that opiate receptor stimulation selectively depresses hypothalamic GnRH activity and enhances PRL release from pituitary lactotrops. The GnRH lowering effect does not seem to be mediated by dopaminergic mechanisms which govern PRL secretion. Also transient, opioid-induced hyperprolactinemia is not causally related to the suppression of hypothalamic GnRH activity.