ANTAGONIST PROPERTIES OF (-)-PINDOLOL AND WAY-100635 AT SOMATODENDRITIC AND POSTSYNAPTIC 5-HT1A RECEPTORS IN THE RAT-BRAIN

1 The aim of the present work was to characterize the 5-hydroxytryptam ine(1A) (5-HT1A) antagonistic actions of (-)-pindolol and WAY 100635 e thoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxam ide). Studies were performed on 5-HT1A receptors located on 5-hydroxyt ryptaminergic neurones in the dorsal raphe nucleus (DRN) and on pyrami dal cells in the CA1 and CA3 regions of the hippocampus in rat brain s lices. 2 Intracellular electrophysiological recording of CA1 pyramidal cells and 5-hydroxytryptaminergic DRN neurones showed that the 5-HT1A receptor agonist 5-carboxamidotryptamine (5-CT) evoked in both cell t ypes a concentration-dependent cell membrane hyperpolarization and a d ecrease in cell input resistance. On its own, (-)-pindolol did not mod ify the cell membrane potential and resistance at concentrations up to 10 mu M, but it antagonized the 5-CT effects in a concentration-depen dent manner. Similar antagonism of 5-CT effects was observed in the CA 3 hippocampal region. (-)-Pindolol also prevented the 5-HT1A receptor- mediated hyperpolarization of CAI pyramidal cells due to 5-HT (15 mu M ). In contrast, the 5-HT-induced depolarization mediated by presumed 5 -HT4 receptors persisted in the presence of 3 mu M (-)-pindoloi. 3 In the hippocampus, (-)-pindolol completely prevented the hyperpolarizati on of CA1 pyramidal cells by 100 nM 5-CT (IC50=92 nM; apparent K-B=20. 1 nM), and of CA3 neurones by 300 nM 5-CT (IC50=522 nM; apparent K-B=1 15.1 nM). The block by (-)-pindolol was surmounted by increasing the c oncentration of 5-CT, indicating a reversible and competitive antagoni stic action. 4 Extracellular recording of the firing rate of 5-hydroxy tryptaminergic neurones in the DRN showed that (-)-pindolol blocked, i n a concentration-dependent manner, the decrease in firing elicited by 100 nM 5-CT (IC50=598 nM; apparent K-B=131.7 nM) or 100 nM ipsapirone (IC50=132.5 nM; apparent K-B=124.9 nM). The effect of (-)-pindolol wa s surmountable by increasing the concentration of the agonist. Intrace llular recording experiments showed that 10 mu M (-)-pindolol were req uired to antagonize completely the hyperpolarizing effect of 100 nM 5- CT. 5 In vivo labelling of brain 5-HT1A receptors by i.v. administrati on of [H-3]-WAY 100635 ([O-methyl-H-3]-N-(2-(4-(2-methoxyphenyl)-1 zin yl)ethyl-N-(2-pyridyl)cyclo-hexane-carboxamide) was used to assess the ir occupancy following in vivo treatment with (-)-pindolol. (-)-Pindol ol (15 mg kg(-1)) injected i.p. either subchronically (2 day-treatment before i.v. injection of [H-3]-WAY 100635) or acutely (20 min before i.v. injection of [H-3]-WAY 100635) markedly reduced [H-3]-WAY 100635 accumulation in all 5-HT1A receptor-containing brain areas. In particu lar, no differences were observed in the capacity of (-)-pindolol to p revent [H-3]-WAY 100635 accumulation in the DRN and the CA1 and CA3 hi ppocampal areas. 6 Intracellular electrophysiological recording of 5-h ydroxytryptaminergic DRN neurones showed that WAY 100635 prevented the hyperpolarizing effect of 100 nM 5-CT in a concentration-dependent ma nner (IC50=4.9 nM, apparent K-B=0.25 nM). In CA1 pyramidal cells, hype rpolarization induced by 50 nM 5-CT was also antagonized by WAY 100635 (IC50=0.80 nM, apparent K-B=0.28 nM).