Chronic amitriptyline exposure reduces 5-HT3 receptor-mediated cyclic GMP formation in NG 108-15 cells

In the present study, we investigated the effects of chronic in vitro administration of amitriptyline, a tricyclic antidepressant, on cyclic GMP formation stimulated by 5-hydroxytryptamine (5-HT) in the neuroblastoma x glioma hybrid cell line, NG 108-15, 5-HT (0.01-100 microM)-stimulated cyclic GMP formation was concentration-dependent and was sensitive to ICS 205-930, a 5-HT3 receptor antagonist. Exposure of NG 108-15 cells to 5 microM amitriptyline for 3 days significantly reduced 5-HT-stimulated cyclic GMP formation. Acute treatment with amitriptyline had no effect on 5-HT-stimulated cyclic GMP formation. The reduction by chronic amitriptyline exposure of 10 microM 5-HT-stimulated cyclic GMP formation was concentration-dependent over the concentration range examined (0.5 to 10 microM). The IC50 of amitriptyline was 1.9 microM. In contrast, amitriptyline exposure, even at a concentration of 8 microM, failed to modify cyclic GMP formation stimulated by bradykinin, sodium nitroprusside, or atrial natriuretic peptide. Increases in intracellular Ca2+ concentration ([Ca2+]i) evoked by 10 microM 5-HT were attenuated in amitriptyline-exposed cells, while 100 nM bradykinin-induced [Ca2+]i increases were not affected. In addition, chronic exposure to 5 microM amitriptyline caused a decrease in affinity (Kd) of [3H]zacopride specific binding to 5-HT3 recognition sites. The Bmax for the labelled ligand remained unchanged. These results suggest that chronic amitriptyline exposure reduces 5-HT-stimulated cyclic GMP formation and [Ca2+]i increases, and this may reflect the functional changes of 5-HT3 receptors.