Combined Treatment with Citalopram and Buspirone: Effects on Serotonin 5-HT_2A and 5-HT_2C Receptors in the Rat Brain

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 [³H]ketanserin and [¹²⁵I]DOI binding to 5-HT_2A receptors. Repeated administration of buspirone and buspirone-citalopram combination increased the affinity of [³H]mesulergine toward 5-HT_2C receptors, and buspirone-citalopram combination also decreased [¹²⁵I]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

References

• 1 Appel N M, Mitchell W M, Garlick R K, Glennon R A, Teitler M, De Souza E B. Autoradiographic characterization of (+-)-1-(2,5-dimethoxy-4-[125I] iodophenyl)-2-aminopropane ([125I]DOI) binding to 5-HT2 and 5-HT1c receptors in rat brain.  J Pharmacol Exp Ther. 1990;  255 843-857
• 2 Appelberg B G, Syvälahti E K, Koskinen T E, Mehtonen O P, Muhonen T T, Naukkarinen H H. Patients with severe depression may benefit from buspirone augmentation of selective serotonin reuptake inhibitors: results from a placebo-controlled, randomized, double-blind, placebo wash-in study.  J Clin Psychiatry. 2001;  62 448-452
• 3 Ashby C RJ, Edwards E, Wang R Y. Electrophysiological evidence for a functional interaction between 5-HT1A and 5-HT2A receptors in the rat medial prefrontal cortex: an iontophoretic study.  Synapse. 1994;  17 173-181
• 4 Aznar S, Qian Z, Shah R, Rahbek B, Knudsen G M. The 5-HT1A serotonin receptor is located on calbindin- and parvalbumin-containing neurons in the rat brain.  Brain Res. 2003;  959 58-67
• 5 Baldwin D, Rudge S. The role of serotonin in depression and anxiety.  Int Clin Psychopharmacol. 1995;  9 41-45
• 6 Barker E L, Westphal R S, Schmidt D, Sanders-Bush E. Constitutively active 5-hydroxytryptamine2C receptors reveal novel inverse agonist activity of receptor ligands.  J Biol Chem. 1994;  269 11 687-11 690
• 7 Blier P, Ward N M. Is there a role for 5-HT1A agonists in the treatment of depression?.  Biol Psychiatry. 2003;  53 193-203
• 8 Bouwer C, Stein D J. Buspirone is an effective augmenting agent of serotonin selective reuptake inhibitors in severe treatment-refractory depression.  S Afr Med J. 1997;  87 534-537, 540
• 9 Dhaenen H. Imaging the serotonergic system in depression.  Eur Arch Psychiatry Clin Neurosci. 2001;  251 1176-1180
• 10 Dimitriou E C, Dimitriou C E. Buspirone augmentation of antidepressant therapy.  J Clin Psychopharmacol. 1998;  18 465-469
• 11 Dolberg O T, Klag E, Gross Y, Schreiber S. Relief of serotonin selective reuptake inhibitor induced sexual dysfunction with low-dose mianserin in patients with traumatic brain injury.  Psychopharmacology (Berl). 2002;  161 404-407
• 12 Egan C, Grinde E, Dupre A, Roth B L, Hake M, Teitler M, Herrick-Davis K. Agonist high and low affinity state ratios predict drug intrinsic activity and a revised ternary complex mechanism at serotonin 5-HT(2A) and 5-HT(2C) receptors.  Synapse. 2000;  35 144-150
• 13 Fava M. Augmentation and combination strategies in treatment-resistant depression.  J Clin Psychiatry. 2001;  62 4-11
• 14 Frank E, Karp J F, Rush A J. Efficacy of treatments for major depression.  Psychopharmacol Bull. 1993;  29 457-475
• 15 Gibson E L, Barnfield A M, Curzon G. Evidence that mCPP-induced anxiety in the plus-maze is mediated by postsynaptic 5-HT2C receptors but not by sympathomimetic effects.  Neuropharmacology. 1994;  33 457-465
• 16 Gobert A, Rivet J M, Cistarelli L, Melon C, Millan M J. Alpha2-adrenergic receptor blockade markedly potentiates duloxetine- and fluoxetine-induced increases in noradrenaline, dopamine, and serotonin levels in the frontal cortex of freely moving rats.  J Neurochem. 1997;  69 2616-2619
• 17 Gobert A, Rivet J M, Cistarelli L, Melon C, Millan M J. Buspirone modulates basal and fluoxetine-stimulated dialysate levels of dopamine, noradrenaline and serotonin in the frontal cortex of freely moving rats: activation of serotonin1A receptors and blockade of alpha2-adrenergic receptors underlie its actions.  Neuroscience. 1999;  93 1251-1262
• 18 Golembiowska K, Dziubina A. Effect of acute and chronic administration of citalopram on glutamate and aspartate release in the rat prefrontal cortex.  Pol J Pharmacol. 2000;  52 441-448
• 19 Graeff F G, Guimaraes F S, De Andrade T G, Deakin J F. Role of 5-HT in stress, anxiety, and depression.  Pharmacol Biochem Behav. 1996;  54 129-141
• 20 Gray J A, Roth B L. Paradoxical trafficking and regulation of 5-HT(2A) receptors by agonists and antagonists.  Brain Res Bull. 2001;  56 441-451
• 21 Haddjeri N, Blier P, de Montigny C. Long-term antidepressant treatments result in a tonic activation of forebrain 5-HT1A receptors.  J Neurosci. 1998;  18 10 150-10 156
• 22 Harvey K V, Balon R. Augmentation with buspirone: a review.  Ann Clin Psychiatry. 1997;  7 143-147
• 23 Hietala J, Salonen I, Lappalainen J, Syvälahti E. Ethanol administration does not alter dopamine D1 and D2 receptor characteristics in rat brain.  Neurosci Lett. 1990;  108 289-294
• 24 Hjorth S, Auerbach S B. Lack of 5-HT1A autoreceptor desensitization following chronic citalopram treatment, as determined by in vivo microdialysis.  Neuropharmacology. 1994;  33 331-334
• 25 Hoyer D, Clarke D E, Fozard J R, Hartig P R, Martin G R, Mylecharane E J, Saxena P R, Humphrey P P. VII. International Union of Pharmacology classification of receptors for 5-hydroxytryptamine (Serotonin).  Pharmacol Rev. 1994;  46 157-203
• 26 Jenck F, Moreau J L, Mutel V, Martin J R, Haefely W E. Evidence for a role of 5-HT1C receptors in the antiserotonergic properties of some antidepressant drugs.  Eur J Pharmacol. 1993;  231 223-229
• 27 Joffe R T, Schuller D R. An open study of buspirone augmentation of serotonin reuptake inhibitors in refractory depression.  J Clin Psychiatry. 1993;  54 269-271
• 28 Julius D, MacDermott A B, Axel R, Jessell T M. Molecular characterization of a functional cDNA encoding the serotonin 1c receptor.  Science. 1988;  241 558-564
• 29 Kennett G A. 5-HT1C receptor antagonists have anxiolytic-like actions in the rat social interaction model.  Psychopharmacology (Berl). 1992;  107 379-384
• 30 Klimek V, Zak-Knapik J, Mackowiak M. Effects of repeated treatment with fluoxetine and citalopram, 5-HT uptake inhibitors, on 5-HT1A and 5-HT2 receptors in the rat brain.  J Psychiatry Neurosci. 1994;  19 63-67
• 31 Kuoppamäki M, Pälvimäki E P, Syvälahti E, Hietala J. 5-HT1C receptor-mediated phosphoinositide hydrolysis in the rat choroid plexus after chronic treatment with clozapine.  Eur J Pharmacol. 1994;  255 91-97
• 32 Kuoppamäki M, Seppälä T, Syvälahti E, Hietala J. Regulation of serotonin 5-HT2C receptors in the rat choroid plexus after acute clozapine treatment.  Eur J Pharmacol. 1994;  269 201-208
• 33 Kuoppamäki M, Syvälahti E, Hietala J. Clozapine and N-desmethylclozapine are potent 5-HT1C receptor antagonists.  Eur J Pharmacol. 1993;  245 179-182
• 34 Laakso A, Pälvimäki E P, Kuoppamäki M, Syvälahti E, Hietala J. Chronic citalopram and fluoxetine treatments upregulate 5-HT2c receptors in the rat choroid plexus.  Neuropsychopharmacology. 1996;  15 143-151
• 35 Lafaille F, Welner S A, Suranyi-Cadotte B E. Regulation of serotonin type 2 (5-HT2) and beta-adrenergic receptors in rat cerebral cortex following novel and classical antidepressant treatment.  J Psychiatry Neurosci. 1991;  16 209-214
• 36 Landen M, Bjorling G, Agren H, Fahlen T. A randomized, double-blind, placebo-controlled trial of buspirone in combination with an SSRI in patients with treatment-refractory depression.  J Clin Psychiatry. 1998;  59 664-668
• 37 Landen M, Eriksson E, Agren H, Fahlen T. Effect of buspirone on sexual dysfunction in depressed patients treated with selective serotonin reuptake inhibitors.  J Clin Psychopharmacol. 1999;  19 268-271
• 38 Lucaites V L, Nelson D L, Wainscott D B, Baez M. Receptor subtype and density determine the coupling repertoire of the 5-HT2 receptor subfamily.  Life Sci. 1996;  59 1081-1095
• 39 Maj J, Moryl E. Effects of sertraline and citalopram given repeatedly on the responsiveness of 5-HT receptor subpopulations.  J Neural Transm Gen Sect. 1992;  88 143-156
• 40 Marek G J, Carpenter L L, McDougle C J, Price L H. Synergistic action of 5-HT2A antagonists and selective serotonin reuptake inhibitors in neuropsychiatric disorders.  Neuropsychopharmacology. 2003;  28 402-412
• 41 Miner L A, Backstrom J R, Sanders-Bush E, Sesack S R. Ultrastructural localization of serotonin2A receptors in the middle layers of the rat prelimbic prefrontal cortex.  Neuroscience. 2003;  116 107-117
• 42 Nocjar C, Roth B L, Pehek E A. Localization of 5-HT(2A) receptors on dopamine cells in subnuclei of the midbrain A10 cell group.  Neuroscience. 2002;  111 163-176
• 43 Okazawa H, Yamane F, Blier P, Diksic M. Effects of acute and chronic administration of the serotonin1A agonist buspirone on serotonin synthesis in the rat brain.  J Neurochem. 1999;  72 2022-2031
• 44 Onder E, Tural U. Faster response in depressive patients treated with fluoxetine alone than in combination with buspirone.  J Affect Disord. 2003;  76 223-227
• 45 Paxinos G, Watson C. The rat brain in stereotaxic coordinates (4th edition). San Diego, USA; Academic Press 1998
• 46 Pazos A, Cortes R, Palacios J M. Quantitative autoradiographic mapping of serotonin receptors in the rat brain. II. Serotonin-2 receptors.  Brain Res. 1985;  346 231-249
• 47 Peroutka S J, Howell T A. The molecular evolution of G protein-coupled receptors: focus on 5-hydroxytryptamine receptors.  Neuropharmacology. 1994;  33 319-324
• 48 Petty F, Davis L L, Kabel D, Kramer G L. Serotonin dysfunction disorders: a behavioral neurochemistry perspective.  J Clin Psychiatry. 1996;  57 Suppl 8 11-16
• 49 Pranzatelli M R. Regulation of 5-HT2 receptors in rat cortex. Studies with a putative selective agonist and an antagonist.  Biochem Pharmacol. 1991;  42 1099-1105
• 50 Pälvimäki E P, Roth B L, Majasuo H, Laakso A, Kuoppamäki M, Syvälahti E, Hietala J. Interactions of selective serotonin reuptake inhibitors with the serotonin 5-HT2c receptor.  Psychopharmacology (Berl). 1996;  126 234-240
• 51 Rickels K. Buspirone in clinical practice.  J Clin Psychiatry. 1990;  51 51-54
• 52 Roth B L, Hamblin M, Ciaranello R D. Regulation of 5-HT2 and 5-HT1C serotonin receptor levels. Methodology and mechanisms.  Neuropsychopharmacology. 1990;  3 427-433
• 53 Rowan M J, Anwyl R. Neurophysiological effects of buspirone and isapirone in the hippocampus: comparison with 5-hydroxytryptamine.  Eur J Pharmacol. 1986;  132 93-96
• 54 Sanders-Bush E, Breeding M. Serotonin1c receptor reserve in choroid plexus masks receptor subsensitivity.  J Pharmacol Exp Ther. 1990;  252 984-988
• 55 Saxena P R. Serotonin receptors: subtypes, functional responses and therapeutic relevance.  Pharmacol Ther. 1995;  66 339-368
• 56 Seeman P. Receptor Tables, vol. 2: Drug dissociation constants for neuroreceptors and transporters. Toronto, Canada; SZ Research 1993
• 57 Skrebuhhova T, Allikmets L, Matto V. [3H]-ketanserin binding and elevated plus-maze behavior after chronic antidepressant treatment in DSP-4 and P-CPA pretreated rats: evidence for partial involvement of 5-HT2A receptors.  Methods Find Exp Clin Pharmacol. 1999;  21 483-490
• 58 Stadel J M, De Lean A, Lefkowitz R J. Molecular mechanisms of coupling in hormone reseptor - adenylate cyclase systems.  Adv Enzymol Relat Areas Mol Biol. 1982;  53 1-43
• 59 Van Oekelen D, Luyten W H, Leysen J E. 5-HT2A and 5-HT2C receptors and their atypical regulation properties.  Life Sci. 2003;  72 2429-2449
• 60 VanderMaelen C P, Matheson G K, Wilderman R C, Patterson L A. Inhibition of serotonergic dorsal raphe neurons by systemic and iontophoretic administration of buspirone, a non-benzodiazepine anxiolytic drug.  Eur J Pharmacol. 1986;  129 123-130
• 61 Willins D L, Deutch A Y, Roth B L. Serotonin 5-HT2A receptors are expressed on pyramidal cells and interneurons in the rat cortex.  Synapse. 1997;  27 79-82