The role of amygdala norepinephrine in memory formation: involvement in the memory enhancing effect of peripheral epinephrine.

The present study examined the roles of amygdala alpha 1 and beta noradrenergic receptors in memory formation as well as their involvement in the memory enhancing effect of peripheral epinephrine (E). Male Sprague-Dawley rats with cannulae implanted into the amygdala were trained on the one-trial inhibitory avoidance task and tested for retention 24 hrs later. Immediately after training, they received various treatments to alter amygdala noradrenergic functions and/or peripheral adrenergic functions. Separate groups of animals were decapitated 10 min after training for assays of monoamine levels in various brain regions by the HPLC-EC method. Results indicated that, when infused into the amygdala immediately after training, isoproterenol and 8-bromo-cAMP enhanced retention, while propranolol impaired retention. On the other hand, phenylephrine or prazosin failed to produce statistically significant effects. Posttraining intra-amygdala infusion of propranolol, but not prazosin, abolished the memory enhancing effects of norepinephrine (NE) infused into the amygdala or E given subcutaneously to the adrenal demedullated rats. Depletion of amygdala NE by the selective neurotoxin DSP-4 also abolished the memory modulatory effects of E. These findings support that amygdala noradrenergic beta, but not alpha 1, receptors are involved in both central and peripheral memory modulatory processes. However, since the postmortem tissue NE levels in the amygdala and other brain regions did not differ among various groups, the inhibitory avoidance training and peripheral E may only activate a transient functional increase in the amygdala NE activity.