Receptor alterations associated with serotonergic agents: an autoradiographic analysis

Controversy exists concerning whether receptor down-regulation is involved in the efficacy of antidepressants. Many investigators believe that norepinephrine (NE) receptor down-regulation is more important than serotonin (5-HT) receptor down-regulation. The ability to accurately determine which receptor types or subtypes have been down-regulated has been impaired by the lack of sufficiently specific ligands for labeling these receptor subtypes. Studies that have attempted to examine 5-HT2 receptor down-regulation have used [3H]-ketanserin as the ligand of choice to label 5-HT2 receptors, but this ligand also labels a nondescript site. The binding of [3H]-ketanserin to sites other than 5-HT2 receptors can be examined and controlled for by autoradiographic techniques. The authors briefly review potential problems involved in analyzing receptor binding after antidepressant treatment and present new findings of receptor alterations in rat brain as examined by autoradiographic techniques following chronic exposure to fluoxetine (a selective 5-HT uptake inhibitor that has been shown to be an effective antidepressant). Laboratory animals injected with fluoxetine showed receptor down-regulation (reduced density) in the serotonergic system. A provocative and potentially important finding of this study is that this selective 5-HT uptake blocker also down-regulates beta-adrenergic receptors in the CNS.