Behavioral depression produced by an uncontrollable stressor: Relationship to norepinephrine, dopamine, and serotonin levels in various regions of rat brain

This paper presents two experiments that continue efforts to determine the neurochemical changes responsible for stress-induced behavioral depression. These expriments measured active motor behavior in a swim tank as well as levels of norepinephrine (NE), dopamine (DA), and serotonin (5-HT) in various brain regions of rats after the animals had (a) been exposed to electric shocks they could control (Avoidance-escape condition), or (b) received the same shocks with no control over them (Yoked condition), or (c) received no shock (No-shock condition). In the first experiment, measures were taken 90 min after the shock session ended. In the swim test, Yoked animals showed a depression of active behavior relative to the other groups. From measures of monoamine levels, the change found to be most closely related to this post-stress behavioral depression was in NE in the locus coeruleus (LC), where Yoked animals showed a considerable depletion of NE. In the second study, the same measures were taken 48 h and 72–96 h after the stress session. Yoked animals tested at 48 h post-stress showed motor depression, but those tested after 72–96 h did not. NE in the LC was significantly depleted in Yoked animals tested at 48 h post-stress but showed only slight (and non-significant) depletion in those tested 72–96 h post-stress. These results, together with others, suggest that large stress-induced depletion of NE in the LC is involved in mediating behavioral depression brought about by severe stress. It is further suggested that the time course for behavioral recovery and for the disappearance of NE depletion in the LC that was seen in Yoked animals after stress parallels the time course previously reported by other investigators for induction of catecholamine-synthesizing enzymes — tyrosine hydroxylase (TH) and dopamine-β-hydroxylase (DBH) — in the LC, so that induction of TH and DBH activity may be a neurochemical mechanism to bring about recovery from poststress behavioral depression.