Altered expression and functions of serotonin 5-HT 1A and 5-HT 1B receptors in knock-out mice lacking the 5-HT transporter V. Fabre, 1 C. Beaufour, 1 A. Evrard, 1 A. Rioux, 1 N. Hanoun, 1 K. P. Lesch, 2 D. L. Murphy, 3 L. Lanfumey, 1 M. Hamon, 1 and M.-P. Martres 1 * 1 INSERM U288, Neuropsychopharmacologie Mole Âculaire, Cellulaire et Fonctionnelle, Faculte  de Me Âdecine Pitie Â-Salpe Ãtrie Áre, 91 Boulevard de l'Ho Ãpital, 75634 Paris Cedex 13, France 2 Department of Psychiatry, University of Wu Èrzburg, 97080 Wu Èrzburg, Germany 3 Laboratory of Clinical Science, NIMH-NIH, 9000 Rockville Pike, Bethesda, MD 20892, USA Keywords: 5-HT neurotransmission, adaptive changes, desensitization, down-regulation, G protein coupling Abstract By taking up serotonin (5-hydroxytryptamine, 5-HT) released in the extracellular space, the 5-HT transporter (5-HTT) regulates central 5-HT neurotransmission. Possible adaptive changes in 5-HT neurotransmission in knock-out mice that do not express the 5- HT transporter were investigated with special focus on 5-HT 1A and 5-HT 1B receptors. Speci®c labelling with radioligands and antibodies, and competitive RT-PCR, showed that 5-HT 1A receptor protein and mRNA levels were signi®cantly decreased in the dorsalraphenucleus(DRN),increasedinthehippocampusandunchangedinotherforebrainareasof5-HTT±/±vs.5-HTT+/+mice. Such regional differences also concerned 5-HT 1B receptors because a decrease in their density was found in the substantia nigra (±30%)butnottheglobuspallidusofmutantmice.Intermediatechangeswerenotedin5-HTT+/±micecomparedwith5-HTT+/+and 5-HTT±/± animals. Quanti®cation of [ 35 S]GTP-g-S binding evoked by potent 5-HT 1 receptor agonists con®rmed such changes as a decreaseinthisparameterwasnotedintheDRN(±66%)andthesubstantianigra(±30%)butnototherbrainareasin5-HTT±/±vs.5- HTT+/+mice.Asexpectedfromactionsmediatedbyfunctional5-HT 1A and5-HT 1B autoreceptors,adecreaseinbrain5-HTturnover rate after i.p. administration of ipsapirone (a 5-HT 1A agonist), and an increased 5-HT out¯ow in the substantia nigra upon local application of GR 127935 (a 5-HT 1B/1D antagonist) were observed in 5-HTT+/+ mice. Such effects were not detected in 5-HTT±/± mice, further con®rming the occurrence of marked alterations of 5-HT 1A and 5-HT 1B autoreceptors in these animals. Introduction The serotonin (5-hydroxytryptamine, 5-HT) transporter (5-HTT), a member of the Na + /Cl ± -dependent transporter family, plays a key role in central serotonergic neurotransmission (Barker & Blakely, 1995; Lesch, 1997). By taking up 5-HT that has been released in the extracellular space, the 5-HTT, exclusively expressed by serotonergic neurons (Fujita etal., 1993; Qian etal., 1995; Bengel etal., 1997), controls the concentration of the monoamine neurotransmitter within the close vicinity of its receptors. Selective serotonin reuptake inhibitors (SSRIs), such as ¯uoxetine (Fuller etal., 1975), paroxetine (Dechant & Clissold, 1991) or citalopram (Hyttel, 1977), have established antidepressant properties. By inhibiting 5-HT reuptake, they enhance 5-HT neurotransmission which might be diminished in depressed patients (Auerbach & Hjorth, 1995; Invernizzi etal., 1995). The delay in the therapeutic effect of SSRIs, usually 2±3 weeks, could be explained by adaptive changes in 5-HT autoreceptors (Briley & Chopin, 1993; Blier & De Montigny, 1994). Whereas 5-HT 1A autoreceptors are located on somas and dendrites of 5-HT neurons of which they inhibit the ®ring (Aghajanian & Lakoski, 1984; Haj-Dahmane etal., 1991), 5-HT 1B autoreceptors are located on terminals of these neurons and control negatively the release and the synthesis of 5-HT (Moret & Briley, 1990; Hjorth & Tao, 1991; Boschert etal., 1994; Pineyro etal., 1995). In rodents, chronic treatment with SSRIs induces a progressive functional desensitization of 5-HT 1A autoreceptors (Blier etal., 1990; Le Poul etal., 1995; 2000) and some data support the idea that 5- HT 1B autoreceptors also desensitize under these conditions (Moret & Briley, 1990; O'Connor & Kruk, 1994). However, no changes in the density of either 5-HT 1A or 5-HT 1B receptors were detected (Jolas etal., 1994; Le Poul etal. 2000), leading to the hypothesis that such desensitizations do not result from a downregulation of these receptors. Recently, Bengel etal. (1998) have generated knock-out mice that do not express the 5-HTT. Because these mutants can be considered as a model for whole-life treatment with SSRI, we investigated whether the expression and functions of 5-HT 1A and 5-HT 1B receptors, which both exhibit adaptive changes after long-term treatment with SSRIs in normal rodents (see above), could be altered in 5-HTT±/± and/or 5-HTT+/± mutants compared with littermate 5- HT+/+ control mice. Using brain sections, quanti®cation of 5-HT 1A and 5-HT 1B receptor binding sites was made with selective radioligands and speci®c antibodies, and their G protein coupling Correspondence: Dr V. Fabre at INSERM U288 1 above. E-mail: vfabre@scripps.edu *Present address: INSERM U513, Neurobiologie et Psychiatrie, Faculte  de Me Âdecine, 8 rue du Ge Âne Âral Sarrail, 94010 Cre Âteil Cedex, France Received 28 February 2000, accepted 28 March 2000 European Journal of Neuroscience, Vol. 12, pp. 2299±2310, 2000 ã Federation of European Neuroscience Societies