Many patients with major depression do not respond to selective serotonin reuptake inhibitors (SSRIs). Lack of response could be due to inhibition of dopamine (DA) release by serotonin (5-HT) through 5-HT2C receptors. Combining an SSRI with a 5-HT2C antagonist may result in improved efficacy by causing simultaneous increases of 5-HT and DA. In order to test this augmentation strategy, male Wistar rats were treated (s.c.) with an acute dose of the SSRI citalopram (Cit, 5 mg/kg), the 5-HT2C antagonist SB 242084 (SB, 2 mg/kg), or Cit + SB, and the effect on 5-HT and DA release in the nucleus accumbens (NAcc) was assessed by microdialysis. In a separate experiment, animals were treated with vehicle, Cit (20 mg/kg/d), SB (2 mg/kg/d) or Cit + SB for a period of 2 days (s.c.), and the impact on the release of 5-HT and DA in the ventral tegmental area (VTA) and NAcc was studied. On the day of microdialysis, 5-HT2C receptor sensitivity was assessed with an SB challenge. Acutely administered Cit + SB increased 5-HT release in the NAcc more than Cit alone. SB alone increased DA release in the NAcc (not in the VTA), but when administered together with Cit, this effect was abolished. A 2-day treatment with Cit or Cit + SB increased 5-HT release in both VTA and NAcc. Combining Cit with SB augmented the effect of Cit in the VTA. DA release in VTA and NAcc was only significantly increased after 2-days of treatment with Cit + SB. In conclusion, Cit + SB had synergistic effects on 5-HT and DA release after 2-days of treatment, probably related to a decreased tonic inhibition of DA release via 5-HT2C receptors. Regional differences occur and future studies should elucidate if this augmentation strategy is beneficial at the behavioral level.
Keywords: 5-HT2C receptor; Dopamine; Microdialysis; SSRI augmentation; Serotonin.
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