Serotonin1B Receptor Activation Mimics Behavioral Effects of Presynaptic Serotonin Release

The locomotor hyperactivity induced by 3,4-methylene-dioxymethamphetamine (MDMA) and related drugs in rats appears to be due to the drug-induced release of presynaptic serotonin (5-HT). Thus, these drugs increase locomotor activity by acting as indirect 5-HT agonists. The subtype of 5-HT receptor upon which this released 5-HT acts postsynaptically to produce the activating effect of MDMA-like drugs is not known. When tested under conditions in which MDMA increases locomotion, direct agonists at both 5-HT1A and 5-HT1C/2 receptors consistently decrease locomotion. Hence, the present experiments tested the hypothesis that the hyperactivity produced by the release of endogenous 5-HT is due to the activation of 5-HT1B receptors. Using the Behavioral Pattern Monitor (BPM), the profile of behavioral effects of a 5-HT1B agonist, 5-methoxy-3(1,2,3,6)tetrahydropyridin-4yl)-1H-indole (RU 24969), was compared to that previously described for MDMA and related indirect 5-HT agonists. The BPM provided detailed information regarding the amount and qualitative patterning of locomotor activity and investigatory responses in rats. Various doses of RU 24969 (1.25 to 5 mg/kg) were administered to naive male rats 10 minutes prior to placement in the test chambers. As previously reported for MDMA, locomotor activity increased with dose, and investigatory rearings and holepokes decreased. The hyperactivity was characterized by repetitive spatial patterns of locomotion that were qualitatively similar to those produced by indirect 5-HT agonists such as MDMA and dissimilar to those produced by indirect dopamine (DA) agonists such as amphetamine. Pretreatment with racemic propranolol but not (+)propranolol antagonized the hyperactivity induced by RU 24959. Fluoxetine, a 5-HT reuptake inhibitor, failed to block the locomotor activating effects of RU 24969. These findings confirm the similarity between the behavioral effects of RU 24969 and indirect 5-HT agonists and suggest that the locomotor hyperactivity produced by both RU 24969 and MDMA is mediated by the activation of 5-HT1B receptors. Although the effects of MDMA on 5-HT1B receptors are secondary to its ability to release presynaptic 5-HT, the activation produced by RU 24969 appears to be a consequence of its direct agonist effects.