Alterations in brain norepinephrine metabolism and behavior induced by environmental stimuli previously paired with inescapable shock.

After exposure to a single session of inescapable footshock, rats show deficits in escape performance 24 h later when required to lever press on a fixed ratio (FR-3) schedule. Footshock stress produces an immediate increase in brain levels of 3-methoxy-4-hydroxyphenylglycol sulfate (MHPG-SO4), a major metabolite of norepinephrine in rat brain. Twenty-four hours after a single or repeated session(s) of footshock stress, when levels of MHPG-SO4 returned to baseline, increases in brain levels of MHPG-SO4, crouching and defecation behavior were elicited in rats by neutral environmental stimuli that had been previously paired with inescapable footshock stress. These results suggest that sensitization or conditioning of noradrenergic neuronal systems may be induced by environmental stimuli previously paired with stress, and may help to explain, at least in part, the deficits in escape performance observed 24 h after exposure to inescapable shock.