Atypical antidepressants inhibit depolarization-induced calcium uptake in rat hippocampus synaptosomes

The effect of the atypical antidepressants mianserin, iprindole, and fluoxetine on synaptosomal calcium uptake was tested under conditions where a selective action on voltage-dependent calcium channels can be documented. Synaptosomes from rat hippocampus were incubated with 45calcium either in choline-rich medium or in depolarizing (60 mM K+) choline-rich medium, and drug effects on calcium uptake in these two conditions, as well as on the net depolarization-induced calcium uptake, were studied in the range of concentrations 0.6-200 microM. A concentration-dependent marked inhibition of uptake in depolarizing choline medium was observed for the three antidepressants, whereas only a minor degree of inhibition of uptake in resting choline medium was present at the highest drug concentration; as a result, the concentration-effect relationships exhibited a strong concentration-dependent inhibition of net depolarization-induced calcium uptake. The IC50 values, calculated by interpolation of the last three or four points of the concentration-effect relationships, were 27, 39, and 68 microM for fluoxetine, iprindole, and mianserin, respectively. Significant degrees of calcium channel inhibition are not expected at brain concentrations of mianserin and iprindole that are likely to be encountered during clinical use; however, the fluoxetine concentration-effect relationship established in the present study, coupled with the published ratio of 20:1 for brain:plasma concentrations of fluoxetine-norfluoxetine in humans, suggests that brain calcium channel function could be appreciably reduced in some patients treated with this atypical antidepressant.