Central noradrenergic neurons: a locus for the functional interplay between alpha-2 adrenoceptors and opiate receptors.

The locus coeruleus (LC) contains the largest clusters of noradrenergic neurons in the brain. Single-cell recordings in rats show that LC neurons can be inhibited by clonidine acting via alpha-2 adrenoceptors; morphine and opiate peptides are also inhibitory but act via separate opiate receptors. During states of opiate withdrawal LC neurons become hyperactive; under these conditions clonidine can normalize activity via alpha-2 adrenoceptors even when opiate receptors are blocked. These single-cell studies provide a possible mechanism for the clinical efficacy of clonidine in relieving the signs and symptoms of opiate withdrawal in human addicts.