Systemic naloxone administration potentiates locus coeruleus noradrenergic neuronal activity under stressful but not non-stressful conditions.

When administered during non-stressful, quiet-waking conditions, i.v. naloxone (1.0 mg/kg) had no effect on the activity of locus coeruleus (LC) noradrenergic neurons in behaving cats. In contrast, the activation of LC noradrenergic unit activity produced by restraint stress was greatly potentiated by the same dose of naloxone. Indices of behavioral distress, vocalization and struggling, were also found to be significantly increased in animals given naloxone during stress. These results suggest that endogenous opioids have a moderating influence upon the level of activity of the LC which operates only under specific conditions such as stress.