The effects of desmethylimipramine on cyclic AMP-stimulated gene transcription in a model cell system.

The present study utilised an in vitro cell model of the cAMP signalling pathway to investigate the actions of desipramine (DMI) and other psychoactive agents on cAMP-driven gene transcription. The model comprised CHObeta2 SPAP cells; Chinese hamster ovary cells expressing human beta2 adrenoceptors and a secreted placental alkaline phosphatase (SPAP) reporter gene with multiple cAMP response elements (CREs) in its promoter region. SPAP assays showed DMI to inhibit isoprenaline or forskolin-enhanced gene transcription in a time and concentration-dependent manner (IC50=16.6+/-2.0 microM after 18 h). This effect of DMI was not dependent upon activity at the levels of the beta2 receptor, cAMP accumulation or phosphorylation of the transcription factor, cAMP response element binding protein (CREB). The inhibitory effects were maintained in the presence of DMI for at least 3 weeks and were mimicked by exposure to norfluoxetine (the major metabolite of fluoxetine; IC50=7.2+/-1.8 microM) and the neuroleptics, chlorpromazine and clozapine, all at a concentration of 10 microM. Amphetamine (10 microM, 18 h) enhanced SPAP gene transcription. Ca2+ imaging experiments ruled out an inhibitory effect of DMI on Ca2+ influx as concluded by previous studies. The results suggest a molecular target for DMI that lies downstream of CREB phosphorylation. Whether the inhibitory action of DMI is common to naturally expressed CRE-driven genes involved in adaptive responses to antidepressants in vivo remains to be determined.