Effects of aggressive encounters on pineal melatonin formation in male gerbils (Meriones unguiculatus, Cricetidae).

Pineal N-acetyl-transferase activity and radioimmunoassayable melatonin levels were determined in adult male gerbils subjected to aggressive encounters using the intruder-model. In the first experiment, a single encounter of 3 min was applied in the afternoon to intact and to animals with sympathetically denervated pineal organs. Compared with controls, both stressed groups demonstrated a drastic decrease in N-acetyl-transferase activity followed by a slow recovery. In both groups there also occurred a marked change in pineal melatonin content: in intact animals pineal melatonin levels were elevated immediately after the encounter; thereafter, melatonin values decreased. In animals bearing denervated pineal organs melatonin levels fell as a consequence of the encounter. In a second experiment, intact gerbils experienced four daily encounters of 1 min for one week. Thereafter the nocturnal formation of melatonin was studied. In comparison with untreated controls, the repeatedly stressed animals demonstrated a temporal delay in the rise of both N-acetyl-transferase activity and melatonin. Since the pineal organ is able to transduce events of the social environment into an endocrine message--as set forth by both our experiments--the pineal organ might play an important role within central processing of social stress.