Effect of naloxone on the suckling induced prolactin release in rats.

2.0 mg/kg b.w. of naloxone--a specific opiate antagonist--injected i.p. 10 min before a brief (lasting for 10 min) suckling and 10 min before a second suckling 30 min after the first suckling significantly inhibited the rise in plasma prolactin levels induced by the suckling stimuli. 0.2 mg/kg of naloxone caused only a slight inhibition on the pituitary prolactin response, the inhibitory effect being more evident on the response to the second suckling stimulus. The data are consistent with the view that endogenous opioids might be involved in the control of prolactin secretion of lactating rats.