LY393558, a 5-hydroxytryptamine reuptake inhibitor and 5-HT 1B/1D receptor antagonist: effects on extracellular levels of 5-hydroxytryptamine in the guinea pig and rat

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The stimulation of terminal 5-HT 1B/1D autoreceptors limits the effects of selective serotonin reuptake inhibitors on extracellular levels of 5-hydroxytryptamine (5-HT, serotonin) in vivo. Microdialysis studies show that acute oral administration of LY393558—a 5-HT reuptake inhibitor and antagonist at both the human 5-HT 1B and 5-HT 1D receptor—in the dose range 1–20 mg/kg, increases extracellular levels of 5-HT in both the guinea pig hypothalamus and rat frontal cortex. In both species, the levels of 5-HT that were attained were higher than following an acute, maximally effective dose of fluoxetine (20 mg/kg orally), reaching approximately 1500% in the guinea pig hypothalamus and 700% in the rat frontal cortex. In both species, the response to LY393558 (10 mg/kg p.o.) was impulse dependent, being absent in the presence of tetrodotoxin delivered at 1 μM via the microdialysis probe. The sensitivity to tetrodotoxin contrasted with the effects seen with dl-fenfluramine. Studies in rats showed that the microdialysate 5-HT concentration achieved in the frontal cortex after an acute challenge with LY393558 (5 mg/kg p.o.) was significantly greater than following a chronic regime of fluoxetine treatment (10 mg/kg/day orally for 21 days). Moreover, in rats chronically treated with LY393558 (5 mg/kg/day orally for 21 days), the mean basal concentration, 24 h after the final pretreatment dose, was of the same magnitude as that following chronic fluoxetine. However, in contrast to the response seen in fluoxetine-pretreated animals, a challenge dose of LY393558 still elicited a further increase in extracellular 5-HT in LY393558-pretreated animals. LY393558 is a potent 5-HT reuptake inhibitor and 5-HT 1B/1D receptor antagonist. Microdialysis studies show that acute oral administration increases extracellular levels of 5-HT, by an impulse-dependent mechanism, above those produced by a maximally effective dose of fluoxetine, and in rats to levels only achieved following chronic fluoxetine treatment. Its neurochemical profile in vivo suggests that it may be a more effective antidepressant with the potential for producing an earlier onset of clinical activity than selective serotonin reuptake inhibitors.