A selective role for brain histamine in prolactin release induced by opiates

We studied the effects of histamine (HA) antagonists on the facilitatory action of morphine (M) and-endorphin (E) on prolactin (PRL) release and the effect of -fluoromethylhistidine (-FMH, inhibitor of HA synthesis) onE-induced PRL secretion. Male rats were injected intracerebroventricularly (i.c.v.) with mepyramine (MEP, H₁-antagonist, 0.8 mol/rat) or ranitidine (RAN, H₂-antagonist, 0.4 mol/rat) 10 min before M (6 mg/kg, intracarotid, i.a.) orE (0.25 g/rat, i.c.v.). -FMH (200 g/rat, i.c.v.) was administered 3 h beforeE. Plasma PRL levels were measured at various times before and after drug treatment. RAN but not MEP significantly reduced PRL release induced by M whereas neither HA-antagonists nor -FMH modifiedE-induced PRL release. The results obtained show that brain HA contributes through activation of H₂-receptors to the PRL facilitatory action of M but not ofE.