Interaction of brain noradrenaline and the pituitary-adrenal axis in learning and extinction.

The effect of 6-hydroxydopamine-induced degeneration of the dorsal tegmental noradrenergic (NA) projection alone or in combination with the removal of the adrenal glands was examined on several behavioral tasks. No impairment of acquisition on a continuously reinforced lever pressing response for food reward was seen as a result of the combined treatment. However, resistance to extinction was observed after depletion of forebrain noradrenaline on its own and this effect was prevented by the adrenalectomy. Adrenalectomy on its own failed to affect extinction. Acquisition of a passive avoidance task was slightly impaired after forebrain noradrenaline depletion but only the group with combined noradrenaline loss and adrenalectomy showed a 24 hour retention deficit. No alteration in shock thresholds was found in any group although both adrenalectomized groups consumed less food and were slightly less active in locomotor cages. It is suggested that previous reports of acquisition and retention deficits in avoidance tasks after combined dorsal NA bundle lesions and adrenalectomy are due to alterations in fear motivation rather than to a general learning impairment.