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

Footshock stress produced an immediate increase in brain concentrations of 3-methoxy-4-hydroxyphenylglycol sulfate (MHPG-SO4), a major metabolite of the neurotransmitter norepinephrine, in the rat brain. Twenty-four hours after footshock stress, when concentrations had returned to baseline, increases in MHPG-SO4 and emotional behavior could be elicited by previously neutral environmental stimuli that had been paired with the stress.