DEFICITS IN CONDITIONED AVOIDANCE RESPONDING FOLLOWING ADRENALECTOMY AND CENTRAL NOREPINEPHRINE DEPLETION ARE DEPENDENT ON POSTSURGICAL RECOVERY PERIOD AND PHASE OF THE DIURNAL CYCLE

Rats that had undergone combined dorsal noradrenegic bundle lesion (DNBL) and bilateral adrenalectomy were impaired in acquiring a conditioned avoidance response when tested 1 wk following surgery. Normal acquisition was observed, however, when testing occurred 3 wk or more after surgery despite low levels of both plasma corticosterone and brain norepinephrine at that time. Furthermore, neither neonatal systemic administration of 6-hydroxydopamine to deplete forebrain norepinephrine, combined with the corticosterone inhibitor metyrapone, nor the pharmacological blockade of noradrenergic receptors, combined with adrenalectomy, disrupted acquisition of the avoidance response. Thus, the combination of forebrain norepinephrine loss and low plasma corticosterone does not inevitably impair avoidance acquisition. Rather, the determining factor for such impairment seems to be the interval between surgery and testing. The impairment at 1 wk following DNBL and adrenalectomy occurred only for rats tested during the dark phase of their light cycle. In addition, the DNBL abolished the effect of the light/dark cycle on posttraining plasma corticosterone. These results demonstrate the importance of the phase of the rat''s diurnal rhythm on both the hormonal and the behavioral effects of altering the pituitary-adrenal axis and/or forebrain norepinephrine. Because adrenocorticotropin, corticosterone and vasopressin all show diurnal patterns of release, it cannot be determined at this time which, if any, of these hormones is most important for the behavioral results reported here.