Stimulation of food intake following opiate agonists in rats but not hamsters.

The proposed mu and kappa opiate receptor agonists morphine and ketocyclazocine, as well as meperidine, were compared for their ability to stimulate feeding and drinking by male rats and hamsters that were not deprived of food or water. Morphine (8.0 mg/kg) and ketocyclazocine (0.5-4.0 mg/kg), but not meperidine (0.5-64.0 mg/kg), increased 3-h food intake by rats. By 6 h the hyperphagic responses were less pronounced. However, 6-h water intake was increased by all three agonists. In contrast to rats, hamsters failed to increased food or water intake over an 8-h period following morphine (6.25-800 mg/kg), ketocyclazocine (0.25-16.0 mg/kg), or meperidine (1.0-128 mg/kg) administration. Thus, kappa or mu opiate receptors may mediate the observed hyperphagic effect of opiate agonists on rat food intake. In addition, these results are consistent with our earlier suggestion that hamsters lack an opiate-sensitive feeding system.