Differential effects of specific opioid receptor agonists on rat pup isolation calls

Abstract

The possibility was investigated that specific opioid receptor types might selectively alter the production of isolation-induced ultrasonic vocalizations. Intracisternal injections of μ, δ and κ opioid receptor agonists were administered to isolated 10-day-old rat pups. The μ receptor agonist [ d-Ala 2-NMe-Phe 4-Gly-ol]-enkephalin (DAMGO) and δ receptor agonist [ d-Pen 2, d-Pen 5]-enkephalin (DPDPE) both reduced the rate of isolation-induced ultrasonic calling in the absence of sedation. The κ receptor agonist U50,488 had the opposite effect, significantly raising the rate of vocalization. Fourteen-day-old pups, with a larger δ receptor population, showed a greater sensitivity to DPDPE than was seen in the younger animals.