Effects of opiate antagonists and putative mu- and kappa-agonists on milk intake in rat and squirrel monkey

The effects of a number of relatively pure opiate antagonists (naloxone, naltrexone, diprenorphine), and putative mu- (morphine, etorphine) and kappa- (ketocyclazocine, ethylketocyclazocine) receptor agonists on sweetened condensed milk intake were examined over a broad range of doses in non-deprived rats and squirrel monkeys. The antagonists consistently decreased milk intake in both the rat and squirrel monkey. There were, however, species differences: diprenorphine was 30 times more potent than either naloxone or naltrexone in the squirrel monkey, but was of similar potency in the rat. The effects of the opiate agonists were more variable than those of the antagonists. In both species, all agonists decreased milk intake at high doses that also produced behavioral depression. Significant increases in drinking were produced only by low doses of ketocyclazocine and ethylketocyclazocine in the rat. The suppression of milk intake by the antagonists supports a modulatory role of opiate receptors in the control of drinking behavior, however, the effects of the agonists on drinking are less easily interpreted within this conceptual framework.