THE ISOLATION AND COMPANION COMFORT RESPONSES OF 7-DAY-OLD AND 3-DAY-OLD RAT PUPS ARE MODULATED BY DRUGS ACTIVE AT THE OPIOID RECEPTOR

Rat pups emit ultrasonic vocalizations (USVs) when isolated in a novel environment. In 10-day-olds, USV has been shown to be reduced by eith er the administration of 0.125 mg/kg morphine (MOR) or the presence of a littermate; these effects were both reversed by naltrexone (NLX), a n opioid receptor blocker. the present study reports that the same dos e of MOR produced NLX-antagonized quieting without sedation on 7-and 3 -day-old pups; higher doses of MOR decreased USV but produced motor de ficits as well. The 0.125 mg/kg dose of MOR is less effective in reduc ing USV in 3- and 7-day-old; calling rates declined by no more than 42 %, compared with 65% at 10 days of age. The presence of a companion al so lowered the USV of 3- and 7-day-olds by a lesser amount (55-57%) th an the 67% seen in 10-day-olds or the 90% decline when pups are 2 week s old. This suggests that age-related changes in the opioid system may be relevant to the increased salience of a social companion that come s with maturity.