Publications

Abstract

The role of dopaminergic mechanisms in the control of gonadotrophin secretion in normal and hyperprolactinaemic subjects is controversial. Whilst bromocriptine, a potent dopamine agonist, has been used to restore normal gonadotrophin secretion in subjects with pathological hyperprolactinaemia (PHP), dopamine and dopamine agonists have been reported to suppress basal and stimulated gonadotrophin release. We therefore investigated the importance of dopaminergic control of gonadotrophin secretion by studying LH, FSH and PRL responses in normal and PHP subjects to central dopamine synthesis inhibition using monoiodotyrosine (MIT) and to a 4 h dopamine infusion designed to elevate peripheral plasma dopamine concentration to levels reported for pituitary portal plasma (1-6 ng/ml). MIT administration resulted in a significant release of PRL (peak increment 520 +/- 84% above basal) but not of LH or FSH in normal subjects. In PHP subjects there was a blunted PRL response (peak 13.3 +/- 3.5%) to MIT administration and significant LH (P less than 0.05) but not FSH release. Dopamine infusion (0.5 microgram/kg/min) resulted in suppression of PRL (min 19 +/- 3% of basal) but not of LH or FSH. A rebound of PRL (peak 188 +/- 68% of basal) but not LH or FSH occurred on cessation of dopamine. There was an apparent rise in LH (P less than 0.02 vs. normals) but not FSH in PHP patients during dopamine infusion. Plateau dopamine levels achieved during the infusion were 2.9 +/- 0.3 ng/ml and 5.9 +/- 0.8 ng/ml in normal and PHP subjects respectively. The responses to MIT show that dopamine functions as an inhibitor of PRL but not of LH or FSH in normal subjects. In PHP patients the responses suggest increased dopaminergic inhibition of LH release but loss of inhibitory control of PRL release. Physiological concentrations of plasma dopamine do not significantly inhibit LH or FSH release in normal subjects but paradoxically results in an apparent release of LH in PHP patients. We conclude that dopamine mechanisms do not play a significant role in modulating gonadotrophin release in normal subjects. In PHP patients, PRL feedback results in increased hypothalamic dopamine activity which in turn inhibits LH release. We conclude that the inhibitory action of dopamine on PRL release restores LH secretion by removing central dopaminergic inhibition through hypothalamic feedback of PRL.