Interaction between opiates and hypothalamic dopamine on prolactin release.

Opiate stimulation of prolactin (PRL) release appears to involve a hypothalamic mechanism(s). The present study utilized both central acting drugs and direct measurement of hypothalamic dopamine (DA) to investigate this problem. Administration of L-dopa, the precursor of DA; piribedil, a DA agonist; or amineptine, a DA reuptake inhibitor, each decreased serum PRL concentrations. Morphine sulfate (MS) and haloperidol (HAL) significantly increased serum PRL levels. L-dopa and piribedil reversed the stimulatory effect of MS on serum PRL concentrations by increasing dopamine activity. MS blocked the inhibitory effects of amineptine on serum PRL release, possibly by decreasing the concentration of DA available for reuptake. Injection of subeffective doses of HAL concurrently with a subeffective dose of MS increased serum PRL concentrations, by an additive inhibitory action on dopaminergic activity. β-endorphin, an endogenous opioid peptide, decreased the rate of DA turnover in the median eminence, and increased serum PRL levels approximately 10 - fold. These observations indicate that opiates stimulate PRL release by decreasing DA activity in the median eminence.