Pindolol suppresses serotonergic neuronal activity and does not block the inhibition of serotonergic neurons produced by 8-hydroxy-2-(di-n-propylamino)tetralin in awake cats.

C A Fornal, F J Martin, C W Metzler, B L Jacobs

Index: J. Pharmacol. Exp. Ther. 291 , 229, (1999)

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Abstract

Clinical studies have shown that pindolol can enhance the effects of antidepressant drugs, presumably by acting as an antagonist at somatodendritic 5-hydroxytryptamine (5-HT)(1A) autoreceptors, which regulate the firing rate of central serotonergic neurons. The current study characterized the action of pindolol on the single-unit activity of serotonergic neurons in the dorsal raphe nucleus of freely moving cats. (+/-)-Pindolol produced a dose-dependent inhibition of neuronal activity after i.v. (ED(50) = 0.25 mg/kg) and s.c. (ED(50) = 1.23 mg/kg) administration. The active enantiomer (-)-pindolol (1 mg/kg i.v.) also suppressed neuronal activity (maximal decrease, 88%). Upon p.o. administration, (+/-)-pindolol (10 mg/kg) produced a marked, long-acting suppression of neuronal activity similar to that observed after s.c. administration. In all cases, the reduction in firing rate produced by pindolol was completely reversed by low doses of N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)cycloh exanecarboxamide (WAY-100635) (0.1 mg/kg i.v. or 0.2 mg/kg s.c.), a selective 5-HT(1A) antagonist. Systemic administration of (-)-tertatolol (1-5 mg/kg i.v.), another beta-adrenoceptor blocker/putative 5-HT(1A) antagonist, had no significant effect on neuronal activity. The ability of i.v. (+/-)-pindolol (0.1-1 mg/kg) to reverse the suppression of serotonergic neuronal activity produced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10 microg/kg i.v.), a selective 5-HT(1A) agonist, also was examined. (+/-)-Pindolol had no appreciable effect on the action of 8-OH-DPAT. In contrast, the 5-HT(1A) antagonist drugs WAY-100635 (0.1 mg/kg i.v. ), 4-fluoro-N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl benzamide (0.1 mg/kg, i.v.), N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylprop anamid e [(S)-WAY-100135] (0.5 mg/kg i.v.), and (-)-tertatolol (1-5 mg/kg i. v.) reversed the effect of 8-OH-DPAT to varying degrees. Overall, these results indicate that pindolol acts as an agonist rather than an antagonist at 5-HT(1A) autoreceptors in awake animals.