The isolation and companion comfort responses of 7- 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 either the administration of 0.125 mg/kg morphine (MOR) or the presence of a littermate; these effects were both reversed by naltrexone (NLX), an opioid receptor blocker. The present study reports that the same dose of MOR produced NLX-antagonized quieting without sedation in 7- and 3-day-old pups; higher doses of MOR decreased USV but produced motor deficits as well. The 0.125 mg/kg dose of MOR is less effective in reducing USV in 3- and 7-day-olds; calling rates declined by no more than 42%, compared with 65% at 10 days of age. The presence of a companion also lowered the USV of 3- and 7-day-olds by a lesser amount (55-57%) than the 67% seen in 10-day-olds or the 90% decline when pups are 2 weeks old. This suggests that age-related changes in the opioid system may be relevant to the increased salience of a social companion that comes with maturity.
Carden
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Division of Developmental Psychobiology, New York State Psychiatric Institute, New York 10032, USA. sec1@columbia.edu
Hernandez
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Hofer
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D000367:Age Factors; D000818:Animals; D000831:Animals, Newborn / Q000502:physiology*; D001143:Arousal / Q000187:drug effects / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D009020:Morphine / Q000494:pharmacology*; D009271:Naltrexone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D011957:Receptors, Opioid / Q000187:drug effects / Q000502:physiology*; D012919:Social Behavior; D012934:Social Isolation; D014828:Vocalization, Animal / Q000187:drug effects / Q000502:physiology
D009292:Narcotic Antagonists; D011957:Receptors, Opioid; D009271:Naltrexone; D009020:Morphine