Ultrasonic vocalization in rat pups as a marker of behavioral development: An investigation of the effects of drugs influencing brain opioid system

Early postnatal exposure to morphine significantly influenced the ultrasonic vocalization of rat pups removed from their nest. In particular, a significant decrease in the rate of calling, sound pressure level and range of frequency was found in morphine-treated animals; moreover, the duration of calls was significantly increased by morphine administration. Conversely, neither β-casomorphins (βCMS), which are opioid peptides derived from the enzymatic digestion of milk protein (β-casein), nor an opioid antagonist, like naloxone, significantly affected ultrasonic emission. The results are discussed with particular reference to the role of the opioid system in separation distress-induced vocalization in young animals.