An observational analysis of the effect of the selective kappa opioid agonist, U-50,488H, on feeding and related behaviours in the rat.

The behaviour of partially pre-satiated rats consuming a sweet palatable food and treated with either vehicle or the specific kappa receptor agonist U-50,488H (0.1-3 mg/kg) was recorded on videotape. Analysis revealed that the hyperphagia induced by the kappa agonist (0.3-3 mg/kg) resulted from an increase in the duration of feeding and not from an increase in the local rate of eating. The increase in duration was due, in turn, to a greater frequency of bouts of feeding. The kappa agonist also increased the latency to the final feeding bout. The effect of U-50,488H was consistent with de-satiation, so that the increase in feeding duration was in evidence from the start of the test period, while the temporal pattern of later satiation was preserved but lagged behind that of control animals. At the largest dose, other recorded activities (rearing, locomotor activity, grooming) were suppressed, with a marked increased in inactivity. At the lowest dose (0.1 mg/kg) there was a significant increase in grooming behaviour. The results are discussed with reference to an hypothesis of opioid function in the control of food intake.