The κ opioid receptor and food intake

Many studies have suggested a role of opioid receptors in the modulation of food intake. Several distinct classes of opioid receptors have been postulated. In an attempt to establish which opioid receptor(s) modulate feeding we studied the effect of the κ agonist, bremazocine, on feeding and compared its effects to the preferential μ agonist, morphine, and the mixed κ-σ agonist, butorphanol and the κ agonist, ethylketocyclazocine. Bremazocine increased feeding to the same extent as morphine and was less potent than mixed agonist/antagonists. The bremazocine effect demonstrated a bell-shaped dose response curve. Daily admiistration of bremazocine or morphine enhances the effect on increasing food intake. However, this effect of daily injections on enhancing food intake is not present when animals receiving morphine are crossed over to bremazocine and vice versa. The bremazocine effect is enhanced by diprenorphine and not inhibited by naloxone. Low doses of the dopamine antagonist, haloperidol, enhance the bremazocine effect and higher doses inhibit it. Finally, using another κ agonist, tifluadom, we showed that the effect on food intake is stereospecific. Our studies provided further evidence for a role for the κ opiod receptor in feeding. However, they also suggest that more than one subpopulation of opioid receptors is involved in feeding modulation.