Mice (BALB/C or NItt, C57BR/cdJ and A/HeJ) were isolated for 2 weeks then exposed to another mouse for 10 rain daily for 1 week. Isolated, unexposed and grouped controls were also studied. Aggression, defined as stereotyped attack behavior, was produced only in the NIH mice which were subsequently used as aggressors. When attacked, the victims, C57BR/cdJ mice defended themselves whereas A/HeJ mice remained submissive.
Isolation alone did not alter adrenal medullary levels of tyrosine hydroxylase (TOH), phenylethanolamine-N-methyltransferase (PNMT) or catecholamines. NIH mice showed an increase in TOIt but not in PNMT when they fought with NIH or C57BR/cdJ mice, whereas both victims showed increases in TOH and PNMT. It was concluded therefore that fright or active fighting induces increases in adrenal medullary enzymes.
Drugs were injected into C57BR/cdJ mice at doses which did not impair their defense behavior, using untreated NIIt mice as aggressors. Phenobarbital fully inhibited both TOtI and PNMT rise while chlorpromazine only partly suppressed PNMT increase. Methamphetamine was less effective than phenobarbital. Tranylcypromine increased both enzymes in control animals and partly suppressed TOH rise induced by defense.
Thus, a reliable model of mild stress was produced which appears suitable for screening psychoactive drugs.