Apparent efficacy of kappa-opioid receptor ligands on serum prolactin levels in rhesus monkeys.

These studies investigated whether serum prolactin levels could be a quantitative marker of the apparent efficacy of kappa-opioid receptor ligands in primates. The effects of s.c. bremazocine and U50,488 (trans-(+/-)-3, 4-Dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamid e; agonists), nalorphine (partial agonist) and nalmefene (antagonist) on prolactin levels were studied in intact female rhesus monkeys. The above compounds, except nalmefene, increased prolactin levels, and their actions conformed to sigmoidal dose-effect curves. The rank order of the compounds' maximum effects in this neuroendocrine endpoint is similar to that in cloned kappa-receptors in vitro, and in a presently studied thermal antinociception assay in vivo. Prolactin may therefore be a quantitative marker of the apparent efficacy of kappa-opioid receptor ligands in primates.