Evidence That Opioid Peptides and Dopamine Participate in the Suckling-Induced Release of Prolactin in the Ewe

A pharmacological approach was used to study the involvement of opioid peptides and dopamine in mediating the suckling-induced release of prolactin in the lactating ewe (10-20 days post partum). To promote reliable and predictable suckling activity lambs were fitted with elasticated masks to prevent sucking for 4.5 h. After a 1-hour control period of frequent blood sampling, ewes were treated (i.v. injections every 5 min) for a further 75 min with either saline vehicle, an opioid antagonist (naloxone; 4.17 mg/5 min), a dopamine antagonist (metoclopramide; 1.25 mg/5 min), a mixture of naloxone + metoclopramide or a dopamine agonist (apomorphine; 6.6 mg/5 min). Blood was withdrawn at 5-min intervals for determination of plasma prolactin and luteinizing hormone (LH) by radioimmunoassay. Plasma LH concentrations (less than or equal to 1 microgram/l) were not significantly affected by any of the drug treatments and there was no evidence for an acute fall in LH associated with suckling- or TRH-induced increases in prolactin secretion. Naloxone significantly (p less than 0.05) reduced the mean incremental change in prolactin concentration (delta PRL) in response to suckling (+7 +/- 18 micrograms/ml) compared with saline-infused controls (+79 +/- 26 micrograms/ml), an effect which was completely reversed by combined treatment with naloxone and metoclopramide (+146 +/- 56 micrograms/ml). Metoclopramide alone raised basal prolactin levels by 46% (p less than 0.01) but did not affect delta PRL in response to suckling (+115 +/- 52 micrograms/ml). Neither naloxone, metoclopramide nor a combination of the two drugs affected the subsequent prolactin to TRH (10 micrograms). Apomorphine, however, completely abolished both the suckling- and TRH-induced release of prolactin.(ABSTRACT TRUNCATED AT 250 WORDS)