Cyanopindolol is a highly potent and selective antagonist at the presynaptic serotonin autoreceptor in the rat brain cortex

Rat brain cortex slices preincubated with 3H-serotonin were superfused with physiological salt solution containing the serotonin uptake blocker DU 24565 (6-nitroquipazine). The effects of (+/-)-cyanopindolol and its enantiomers, of ICI 118,551 (erythro-dl-1-(7-methylindan-4-yloxy)-3-isopropylaminobut an-2-ol) and of isoprenaline on the electrically (3 Hz) evoked 3H overflow were studied. (+/-)-Cyanopindolol increased the evoked 3H overflow; this effect was prevented by preexposure to the previously characterized serotonin receptor antagonist metitepin. The concentration-response curve of unlabelled serotonin for its inhibitory effect on the electrically evoked 3H overflow was shifted to the right by (+/-)-cyanopindolol (apparent pA2 value: 8.29), whereas that of noradrenaline (determined in the absence of DU 24565) was not affected (apparent pA2 value: less than 6.0). The concentration-response curve of serotonin was also shifted to the right by (-)-cyanopindolol (apparent pA2 value: 8.30) and (+)-cyanopindolol (6.83) but not by ICI 118,551 (less than 5.5). Isoprenaline (up to 10 mumol/l; examined in the absence of DU 24565) did not influence the electrically evoked 3H overflow. The present results show that the presynaptic serotonin autoreceptor is blocked by cyanopindolol in a stereoselective way. This drug is 20 times more potent than metitepin as an antagonist at the presynaptic serotonin autoreceptor, and, in contrast to the latter, it does not act as an antagonist at the presynaptic alpha 2-adrenoceptor on the serotoninergic neurone.