ENHANCEMENT OF IMIPRAMINE-INDUCED RAT-BRAIN BETA-ADRENOCEPTOR DESENSITIZATION BY SUBACUTE COADMINISTRATION OF TRAZODONE, ZIMELIDINE, QUIPAZINE OR 5-HYDROXYTRYPTOPHAN

The present work was undertaken to characterize the role of serotonin in the regulations of beta-adrenoceptors utilizing isoprenaline-induced water drinking in the rat. For this purpose, a serotonin precursor, 5-hydroxytryptophan (24.3 mg/kg/day, PO), the serotonin neuronal uptake blockers, trazodone (18.5 mg/kg/day, PO), or zimelidine (14.6 mg/kg/day, PO) or a serotonin agonist, quipazine (12.6 mg/kg/day, PO) were administered either alone or in combination with imipramine for a period of 4 days. While none of these drugs alone showed any significant effect in attenuating the effects of isoprenaline-induced water drinking, their co- administration with imipramine did produce a significant reduction in isoprenaline-induced drinking. Simultaneous injection of the serotonin synthesis inhibitor, p-chlorophenylalanine (200 mg/kg/day, IP), has resulted in blockade of this acceleration of desensitization of beta-adrenoceptors produced by the subacute co-administration of trazodone or quipazine with imipramine. The selective 5HT2 receptor antagonist ketanserin (4 mg/kg/day/ IP) significantly inhibited the attenuation of the isoprenaline-induced drinking attained by the co-administration of quipazine with imipramine, while methysergide (2 mg/kg/day, IP) which blocks both 5HT1 and 5HT2 receptors failed to produce a significant effect on this response. These results indicate that the inhibition of the synaptosomal uptake of serotonin by quipazine seems to be more pertinent than its serotoninergic agonistic effect in the desensitization of central beta-adrenoceptors in the rat. Thus, it can be concluded that noradrenaline and serotonin are both required for the process of the desensitization of central beta-adrenoceptor systems by antidepressants.