Role of extracellular serotonin levels in the effect of 5-HT1B receptor blockade

The release of serotonin (5-HT) at serotonergic nerve terminals is regulated by 5-HT1B autoreceptors. Several studies have reported that the effects of selective 5-HT reuptake inhibitors (SSRIs) on extracellular 5-HT are augmented by 5-HT1B receptor antagonists, whereas administration of these antagonists alone do not enhance 5-HT levels. It has been suggested that 5-HT1B receptors have low basal endogenous activity and therefore elevated endogenous 5-HT levels are needed to elicit an effect of 5-HT1B receptor antagonists. To test this hypothesis, different strategies were used to enhance 5-HT levels in the rat frontal cortex to assess the effects of locally applied NAS-181, a new selective 5-HT1B receptor antagonist. Blockade of 5-HT1B receptors with NAS-181 dose dependently augmented 5-HT levels when 5-HT levels were enhanced by a SSRI. No additional effect of NAS-181 on 5-HT output was found when 5-HT levels were enhanced by KCl depolarization-induced release or by preventing degradation of 5-HT with the monoamine oxidase inhibitor pargyline. In the presence of fluvoxamine, the increased 5-HT release evoked by KCl depolarization was augmented by NAS-181, supporting the idea that blockade of 5-HT transporters is necessary to measure an effect of 5-HT1B receptor blockade. In conclusion, the results provide circumstantial evidence that the effect of a 5-HT1B receptor antagonist depends on extracellular 5-HT levels, but strongly suggest that additional 5-HT reuptake inhibition is required to detect any effect of 5-HT1B receptor antagonist on 5-HT levels by in vivo microdialysis.