The stimulation of food intake by selective agonists of mu, kappa and delta opioid receptors

It is known that under some conditions the administration of opioid agonists will stimulate food intake. However, the lack of receptor selectivity of some of the agonists which produce this effect leaves open the question of which receptor types are actually involved. In the experiments presented here, rats were given intracerebroventricular injections of Dynorphin 1-17 (DYN), [D-ala²₆MePhe⁴, GlGly-ol⁵] enkephalin (DAGO), and [D-ser², leu⁵]enkephalin-thr (DSLET); these peptides are thought to be selective agonists at kappa, mu and delta opioid receptors, respectively. All three peptides stimulated food intake in non-deprived rats at doses in the 3 - 10 nmol range; water intake was also increased in some cases. Generally, DYN stimulated feeding at a lowet dose than DAGO or DSLET and the magnitude of the effect tended to be greater. On the other hand, DAGO more consistently increased water intake. In some cases, DYN also caused episodes of "barrel-rolling" and postural abnormalities, whereas DAGO had sedative and/or cataleptic effects. These results are interpreted as an involvement of more than one opioid receptor types in the regulation of appetite, possibly with separate opioid systems contributing to food and water intake.