A neurobiological basis for mood disorders has long been postulated, but is yet to be conclusively established. Earlier studies on the monoaminergic neurotransmitter systems in mood disorder have been very suggestive, although not conclusive, of alterations in these systems (noradrenergic, dopaminergic, serotonergic, and cholinergic) possibly owing to changes in receptor sensitivity (Post and Ballenger, 1984). These data have resulted in a recent and relatively extensive field of research investigating mechanisms that regulate receptor responsivity, which has focused to a large extent on the G protein-coupled signal transduction pathways.