ISOLATION ALTERS STRIATAL MET-ENKEPHALIN IMMUNOREACTIVITY IN RAT PUPS

Isolated preweanling rats emit ultrasonic vocalizations. Mu- and delta -opioid agonists quiet isolated pups; naltrexone, an opioid receptor b locker, prevents this quieting. A littermate companion is as effective as morphine in quieting vocalizations, and naltrexone also blocks com panion quieting. We have now quantified methionine enkephalin (Met-ENK ) immunoreactivity in the brains of 10-day-old Wistar rat pups taken d irectly from the home cage or kept either alone or with a companion fo r a brief or prolonged period. Met-ENK is an endogenous ligand that bi nds to the mu- and delta-opioid receptors. Striatal peptide levels wer e higher when pups were with a companion than when they were kept alon e; the peptide level of pups in the home cage did not differ from eith er. Comparisons of pups in the brief (5 min) and prolonged (60 min) se paration conditions showed significantly higher peptide levels followi ng a brief period out of the nest than at the end of an hour. In hypot halamus, hippocampus, and frontal cortex neither social condition nor duration of separation significantly altered peptide quantity. Larger amounts of Met-ENK in pups provided with a companion could reflect an increase in posttranslational cleavage of the precursor molecule leadi ng to stimulation of receptors that act to diminish USV. Reduced level s following 60 min out of the home cage might reflect depletion of the peptide following an initial release during the period when the pup's vocal response is most vociferous.