We have previously shown that desensitization of beta-adrenergic receptor (BAR) in a model system of from erythrocytes is associated with internalization of BAR, resulting in down-regulation of the receptor binding sites. Internalized BAR sites are sequestered in endocytotic vesicles which can be revealed by immunohistochemical staining using BAR-specific antibodies. The internalized BARs can be recycled to plasma membrane during resensitization that occurs after removal to the stimulating BAR agonist isoproterenol. We are addressing the question of whether desensitization and internalization involve a change in the transcription rate of BAR mRNA. For this study, we used the BAR present in C6-glioma cells which express both beta- and beta2-AR with a fast turnover rate. preliminary results using beta2-AR selective cDNA show that, indeed, beta2 AR mRNA levels decrease rapidly in response to stimulation with isoproterenol. The modulation of BAR in the CNS is also under investigation. We have found BARs in clathrin-coated vesicles isolated from bovine brain, suggesting that these receptors are internalized in vivo. Down-regulation of BAR by chronic antidepressant has been implicated in the therapeutic effect of this drug. We have therefore initiated a study to investigate the effect of chronic administration of the tricyclic antidepressant imipramine on the level of BAR mRNA in order to shed light on molecular mechanisms underlying the regulation of gene expression elicited by antidepressant-mediated changes of synaptic activity.
