Effects of (-)-tertatolol, (-)-penbutolol and (±)-pindolol in combination with paroxetine on presynaptic 5-HT function:: an in vivo microdialysis and electrophysiological study

1 The antidepressant efficacy of selective serotonin reuptake inhibitors (SSRIs) might be enhanced by co-administration of 5-HT1A receptor antagonists. Thus, we have recently shown that the selective 5-HT1A receptor antagonist, WAY 100635, blocks the inhibitory effect of an SSRI on 5-HT cell firing, and enhances its ability to elevate extracellular 5-HT in the forebrain. Here we determined whether the beta-adrenoceptor/5-HT1A. receptor ligands (+/-)-pindolol, (-)-tertatolol and (-)penbutolol, interact with paroxetine in a similar manner. 2 Both (-)-tertatolol (2.4 mg kg(-1) i.v.) and (-)-penbutolol (2.4 mg kg(-1) i.v.) enhanced the effect of paroxetine (0.8 mg kg(-1) i.v.) on extracellular 5-HT in the frontal cortex, whilst (+/-)-pindolol (4 mg kg(-1) i.v.) did not. (-)-Tertatolol (2.4 mg kg-l i.v.) alone caused a slight increase in 5-HT however, (-)-penbutolol (2.4 mg kg(-1) i.v.) alone had no effect. 3 In electrophysiological studies (-)-tertatolol (2.4 mg kg(-1) i.v.) alone had no effect on 5-HT cell firing but blocked the inhibitory effect of paroxetine. In contrast, (-)-penbutolol (0.1-0.8 mg kg(-1) i.v.) itself inhibited 5-HT cell firing, and this effect was reversed by WAY 100635 (0.1-0.8 mg kg(-1) i.v.). We have recently shown that (+/-)-pindolol inhibits 5-HT cell firing via a WAY 100635-sensitive mechanism. 4 Our data suggest that (-)-tertatolol enhances the effect of paroxetine on forebrain 5-HT via blockade of 5-HT1A autoreceptors which mediate paroxetine-induced inhibition of 5-HT cell firing. In comparison, the mechanisms by which (-)-penbutolol enhances the effect of paroxetine on extracellular 5-HT is unclear, since (-)-penbutolol itself appears to have agonist properties at the 5-HT1A autoreceptor. Indeed, the agonist action of (+/-)-pindolol at 5-HT1A autoreceptors probably explains its inability to enhance the effect of paroxetine on 5-HT in the frontal cortex. 5 Overall, our data suggest that both (-)-tertatolol and (-)-penbutolol are superior to (+/-)pindolol in terms of enhancing the effect of an SSRI on extracellular 5-HT. Both (-)-tertatolol and (-)-penbutolol are worthy of investigation for use as adjuncts to SSRIs in the treatment of major depression.