Effect of antidepressants on intracellular Ca++ mobilization in human frontal cortex

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.

Effect of fluoxetine on intracellular Ca2+ levels in bladder female transitional carcinoma (BFTC) cells

The effect of the antidepressant fluoxetine on Ca2+ signaling in cultured cells was largely unknown. The effect of various concentrations of fluoxetine on [Ca 2+] i in populations of bladder female transitional cancer (BFTC) cells was evaluated by using fura-2 as a Ca2+ probe. Fluoxetine increased [Ca 2+] i concentration dependently (20-100 microM) with an EC50 value of 30 microM. The response was inhibited by 50-60% on extracellular Ca2+ removal. In Ca2+ -free medium, pretreatment with 1 microM thapsigargin (an inhibitor of the endoplasmic reticulum Ca2+ pump) abolished 50 microM fluoxetine-induced Ca2+ release; whereas pretreatment with fluoxetine did not alter the thapsigargin-induced Ca2+ response. Addition of 3 mM Ca2+ increased [Ca 2+] i after pretreatment with 50 microM fluoxetine in Ca2+ -free medium, suggestive of fluoxetine-induced capacitative Ca2+ entry. Suppression of inositol 1,4,5-trisphosphate formation by 2 microM U73122 (a phospholipase C inhibitor) did not affect 50 microM fluoxetine-induced Ca2+ release. Collectively, this study shows that fluoxetine increased [Ca 2+] i in bladder cancer cells in a concentration-dependent fashion, by releasing Ca2+ from thapsigargin-sensitive Ca2+ stores in an IP3-independent manner, and by inducing Ca2+ influx from extracellular medium.

Fluoxetine dilates isolated small cerebral arteries of rats and attenuates constrictions to serotonin, norepinephrine, and a voltage-dependent Ca(2+) channel opener

BACKGROUND AND PURPOSE

Recent clinical observations question that the antidepressant effect of fluoxetine (Prozac) can be explained solely with serotonin reuptake inhibition in the central nervous system. We hypothesized that fluoxetine affects the tone of vessels and thereby modulates cerebral blood flow.

METHODS

A small branch of rat anterior cerebral artery (195+/-15 microm in diameter at 80 mm Hg perfusion pressure) was isolated, cannulated, and pressurized (at 80 mm Hg), and changes in diameter were measured by videomicroscopy.

RESULTS

Fluoxetine dilated small cerebral arteries with an EC(50) of 7.7+/-1.0x10(-6) mol/L, a response that was not affected by removal of the endothelium or application of 4-aminopyridine (an inhibitor of aminopyridine-sensitive K(+) channels), glibenclamide (an inhibitor of ATP-sensitive K(+) channels), or tetraethylammonium (a nonspecific inhibitor of K(+) channels). The presence of fluoxetine (10(-6) to 3x10(-5) mol/L) significantly attenuated constrictions to serotonin (10(-9) to 10(-5) mol/L) and norepinephrine (10(-9) to 10(-5) mol/L). Increasing concentrations of Bay K 8644 (a voltage-dependent Ca(2+) channel opener, 10(-10) to 10(-6) mol/L) elicited constrictions, which were markedly reduced by 2x10(-6) and 10(-5) mol/L fluoxetine, whereas 3x10(-5) mol/L fluoxetine practically abolished the responses.

CONCLUSIONS

Fluoxetine elicits substantial dilation of isolated small cerebral arteries, a response that is not mediated by endothelium-derived dilator factors or activation of K(+) channels. The finding that fluoxetine inhibits constrictor responses to Ca(2+) channel opener, as well as serotonin and norepinephrine, suggests that fluoxetine interferes with the Ca(2+) signaling mechanisms in the vascular smooth muscle. We speculate that fluoxetine increases cerebral blood flow in vivo, which contributes to its previously described beneficial actions in the treatment of mental disorders.