Down-regulation of serotonin2, but not of beta-adrenergic receptors during chronic treatment with amitriptyline is independent of stimulation of serotonin2 and beta-adrenergic receptors.
Antidepressant drugs down-regulate beta-adrenergic, alpha 2-adrenergic and serotonergic 5-HT2 receptors with a time course that parallels their clinical efficacy, i.e. chronic administration is required (Crews and Smith, 1978; Svensson and Usdin, 1978; Banerjee, Kung, Riggi and Chanda, 1979; Bergstrom and Keller, 1979; Peroutka and Snyder, 1980). In the present study, it was found that the 5-HT2 receptor antagonist, nefazadone (50 mg/kg per day) did not prevent the downregulation of 5-HT2 receptors in the cerebral cortex produced by amitriptyline (10 mg/kg per day), when administered for 3 weeks. Moreover, treatment with nefazadone (50 mg/kg per day) alone for 3 weeks decreased binding to 5-HT2 receptors in cerebral cortex. In contrast, administration of propranolol, the beta receptor antagonist, (10 mg/kg per day) with amitriptyline (10 mg/kg per day) for 3 weeks prevented the down-regulation of beta receptors, but did not alter the decrease in binding to 5-HT2 receptors. In addition, the depletion of central stores of norepinephrine and serotonin by a 4-day treatment with reserpine (5 mg/kg per day) increased binding to beta receptors in the cerebral cortex and hippocampus, but did not affect binding to 5-HT2 receptors in either region. These results suggest that the 5-HT2 receptor is not down-regulated by direct stimulation by serotonin agonists and that the down-regulation of 5-HT2 receptors by amitriptyline is independent of down-regulation of beta-adrenergic receptors.