DISCRIMINATIVE STIMULUS EFFECTS OF A CENTRALLY ADMINISTERED, DELTA-OPIOID PEPTIDE (D-PEN(2)-D-PEN(5)-ENKEPHALIN) IN PIGEONS

The present study assessed the discriminative stimulus effects of the delta-opioid agonist [D-Pen(2)-D-Pen(5)]enkephalin (DPDPE) in pigeons. Food-restricted pigeons were trained to discriminate between ICV inje ctions of 100 mu g [D-Pen(2)-D-Pen(5)]enkephalin (DPDPE) and saline in a two-key operant procedure; acquisition of discriminative control wa s rapid (14-28 daily sessions). [D-Ser(2), Leu(5), Thr(6)]enkephalin ( DSLET) and [D-Ala(2)]deltorphin II, peptides selective for delta-opioi d receptors, produced discriminative stimulus effects similar to DPDPE , and were approximately equipotent to DPDPE. The non-peptidic, delta- opioid agonist BW373U86 (0.032-100 mg/kg, IM) partially generalized to DPDPE. The kappa-opioid agonist U69,593 (0.01-1 mg/kg, IM), and the m u-opioid agonists, DAMGO (0.1-3.2 mu g, ICV) and morphine (1-10 mg/kg, IM), did not produce discriminative stimulus effects similar to DPDPE , up to doses that markedly decreased response rates. Naltrindole (0.1 mg/kg, IM), an antagonist selective for deltaopioid receptors, produc ed approximately a 30-fold reduction in the potency of DPDPE. DPDPE's discriminative stimulus effect in pigeons appears to be mediated throu gh a delta-opioid receptor; this effect may provide a procedure for as sessing delta-opioid receptor function in vivo.