The objective of this project is to gain insight into the mechanisms underlying the changes in expression of the beta-adrenergic receptors induced by various treatments. In our investigation of agonist- and antagonist-induced changes in beta-adrenergic receptor mRNA levels, we have found that perturbation of the microtubule cytoskeleton can alter both basal and agonist-stimulated cAMP levels in C-6 glioma cells. Microtubule disruption in the unstimulated state, however, does not alter basal cAMP levels, yet beta-adrenergic receptor subtype mRNAs and binding sites change dramatically. Thus, a mode of regulation of beta-adrenergic receptor expression exists which is sensitive to the state of microtubule assembly. This may explain why the two subtypes are differentially regulated, even though they both cause increases in cAMP production. It may also serve to explain how antagonists can alter beta-adrenergic receptor expression. In a related finding, c- fos expression was probed as an indicator of threshold cAMP levels necessary for changes in beta-adrenergic receptor expression. Antagonists which are able to alter beta-adrenergic receptor expression do not also induce c-fos mRNA increases, indicating that cAMP cannot mediate antagonist effects. We continue to characterize the effects of agonists of cholesterol transport on beta-adrenergic receptor expression. The effects of chronic treatment with the antidepressant desimipramine to down-regulate beta-adrenergic receptor binding and mRNA while increasing cellular sensitivity to cAMP suggest that depression might be characterized by either low basal cAMP levels or a relative insensitivity to beta-adrenergic receptor stimulation.
