5-HT1B receptor-mediated regulation of serotonin clearance in rat hippocampus in vivo

The 5-hydroxytryptamine (5-HT; serotonin) transporter (5-HTT) is important in terminating serotonergic neurotransmission and is a primary target for many psychotherapeutic drugs. Study of the regulation of 5-HTT activity is therefore important in understanding the control of serotonergic neurotransmission. Using highspeed chronoamperometry, we have demonstrated that local application of 5-HT1B antagonists into the CA3 region of the hippocampus prolongs the clearance of 5-HT from extracellular fluid (ECF). In the present study, we demonstrate that the 5-HT1B antagonist cyanopindolol does not produce this effect by increasing release of endogenous 5-HT or by directly binding to the 5-HTT. Dose-response studies showed that the potency of cyanopindolol to inhibit clearance of 5-HT was equivalent to that of the selective 5-HT reuptake inhibitor fluvoxamine. Local application of the 5-HT1A antagonist WAY 100635 did not alter 5-HT clearance, suggesting that the effect of cyanopindolol to prolong clearance is not via a mechanism involving 5-HT1A receptors. Finally, the effect of low doses of cyanopindolol and fluvoxamine to inhibit clearance of 5-HT from ECF was additive. These data are consistent with the hypothesis that activation of terminal 5-HT1B autoreceptors increases 5-HTT activity.