Combined Treatment with Citalopram and Buspirone: Effects on Serotonin 5-HT2A and 5-HT2C Receptors in the Rat Brain
Syvalahti E, Penttila J, Majasuo H, Palvimaki EP, Laakso A, Hietala J.
Department of Pharmacology and Clinical Pharmacology,
University of Turku, 20520 Turku, Finland.
Pharmacopsychiatry. 2006 Jan;39(1):1-8.


INTRODUCTION: We wanted to elucidate whether the proposed advantages of citalopram-buspirone combination treatment are related to changes in 5-HT (2A/C) receptor-mediated neurotransmission. METHODS: The affinity of buspirone to 5-HT (2A) and 5-HT (2C) receptors was measured in vitro, and the influence of buspirone on 5-HT (2C) receptor-mediated phosphoinositide hydrolysis was estimated. Four groups of rats received citalopram (10 mg/kg), buspirone (6 mg/kg), citalopram-buspirone combination, or saline once a day s. c. for 14 days. Treatment effects on 5-HT (2A) and 5-HT (2C) receptors were investigated by receptor autoradiography with antagonist and agonist radioligands. RESULTS: Buspirone was found to be a weak 5-HT (2C) receptor antagonist, with a low affinity for 5-HT (2A) and 5-HT (2C) receptors. Repeated buspirone-citalopram combination treatment markedly decreased [ (3)H]ketanserin and [ (125)I]DOI binding to 5-HT (2A) receptors. Repeated administration of buspirone and buspirone-citalopram combination increased the affinity of [ (3)H]mesulergine toward 5-HT (2C) receptors, and buspirone-citalopram combination also decreased [ (125)I]DOI binding to 5-HT (2C) receptors. DISCUSSION: We suggest that downregulation of brain 5-HT (2A) receptors and possibly of 5-HT (2C) receptor agonist sites is involved in the beneficial clinical effects of buspirone-SSRI augmentation treatment. Furthermore, a conversion of brain 5-HT (2C) receptors from high- to low-affinity state may provide an additional mechanism for the anti-anxiety effects of buspirone. Abbreviations. B (max):maximum receptor density DOI:(+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane 5-HT:serotonin (5-hydroxytryptamine) IP:inositol monophosphate K(d):equilibrium dissociation constant (affinity) K(i):inhibition constant KRB:Krebs-bicarbonate PI:phosphoinositide 1-PP:1-(2-pyrimidinyl)piperazine SSRI:selective serotonin reuptake inhibitor.
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