DIFFERENTIAL-EFFECTS OF SPECIFIC OPIOID RECEPTOR AGONISTS ON RAT PUP ISOLATION CALLS

The possibility was investigated that specific opioid receptor types might selectively alter the production of isolation-induced ultrasonic vocalizations. Intracisternal injections of mu, delta and kappa-opioid receptor agonists were administered to isolated 10-day-old rat pups. The mu-receptor agonist [D-Ala2-NMe-Phe4-Gly-ol]-enkephalin (DAMGO) and delta-receptor agonist [D-Pen2, D-Pen5]-enkephalin (DPDPE) both reduced the rate of isolation-induced ultrasonic calling in the absence of sedation. The kappa-receptor agonist U50,488 had the opposite effect, significantly raising the rate of vocalization. Fourteen-day-old pups, with a larger delta-receptor population, showed a greater sensitivity to DPDPE than was seen in the younger animals.