GABA B receptors in Schwann cells in¯uence proliferation and myelin protein expression Valerio Magnaghi, 1 Marinella Ballabio, 1 Ilaria T. R. Cavarretta, 1 Wolfgang Froestl, 2 Jeremy J. Lambert, 3 Ileana Zucchi 4 and Roberto C. Melcangi 1 1 Department of Endocrinology and Center of Excellence on Neurodegenerative Diseases, University of Milan, Via Balzaretti 9, 20133 Milan, Italy 2 Novartis Pharma AG, Therapeutic Area Nervous System, WKL-136.65.25, CH-4002 Basel, Switzerland 3 Department of Pharmacology and Neuroscience, Neuroscience Institute, University of Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY, UK 4 I.T.B. Institute of Biomedical Technologies, National Research Council, Via Fratelli Cervi 93, 20090 Segrate, Milan, Italy Keywords: GABA B receptors, myelin, peripheral glia, rat, sciatic nerve Abstract The location and the role of g-aminobutyric acid type B (GABA B ) receptors in the central nervous system have recently received considerableattention,whilstrelativelylittleisknownregardingtheperipheralnervoussystem.Inthisregard,herewedemonstratefor the®rsttimethatGABA B receptorisoforms[i.e.GABA B(1) andGABA B(2) ]arespeci®callylocalizedintheratSchwanncellpopulationof the sciatic nerve. Using the selective GABA B agonist [i.e. (±)-baclofen] and the antagonists (i.e. CGP 62349, CGP 56999A, CGP 55845A), such receptors are shown to be functionally active and negatively coupled to the adenylate cyclase system. Furthermore, exposure of cultured Schwann cells to (±)-baclofen inhibits their proliferation and reduces the synthesis of speci®c myelin proteins (i.e. glycoproteinPo,peripheralmyelinprotein22,myelin-associatedglycoprotein,connexin32),providingevidenceforaphysiologicalrole of GABA B receptors in the glial cells of the peripheral nervous system. Introduction g-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS), where it interacts with both ionotropic (GABA A and GABA C ) and metabotropic (GABA B ) recep- tors (Bettler et al., 1998; Bowery & Enna, 2000). The GABA B receptor was ®rst identi®ed approximately 20 years ago as a metabotropic receptor with a pharmacological pro®le distinct from that of the GABA A receptor, being insensitive to the GABA A receptor antagonist bicuculline, but activated by certain GABA analogues, for example (±)-baclofen, which is inert at the GABA A receptor (Hill & Bowery, 1981). GABA B receptors are members of the seven transmembrane G- protein-coupled receptor super-family (Bowery et al., 2002), which may in¯uence presynaptic neurotransmitter release and cause post- synaptic `silencing' of excitatory neurotransmission via the activation of second messenger systems, mainly by adenylate cyclase and by modulation of calcium and potassium channel activity (Marshall et al., 1999;Bowery&Enna,2000).Themolecularstructureofmetabotropic GABA B receptors remained elusive until 1997, when the ®rst cDNAs encoding two GABA B receptor proteins initially named GABA B(1a) and GABA B(1b) (Kaupmann et al., 1997) were identi®ed. Subse- quently, novel GABA B receptor isoforms were cloned (Isomoto et al., 1998; Pfaff et al., 1999). Of particular interest, a number of laboratories independently identi®ed a cDNA coding for the GABA B receptor isoform 2 [GABA B(2) ] (Jones et al., 1998; Kaupmann et al., 1998; White et al., 1998; Kuner et al.,1999).TheGABA B(1) is retained in the endoplasmic reticulum and is transported to the cell surface only in the presence of the GABA B(2) , allowing the formation of a func- tional heterodimeric complex (Jones et al., 1998; Kaupmann et al., 1998; White et al., 1998; Kuner et al., 1999; Ng et al., 1999; Calver et al., 2000). Detailed analysis has indicated that GABA B heterodimer component proteins [i.e. GABA B(1) and GABA B(2) ] are expressed widely throughout the neuronal compartment of the brain, spinal cord as well as in dorsal root ganglia (Margeta-Mitrovic et al.,1999;Towers et al., 2000; Charles et al., 2001). However, recent observations suggest that certain types of glial cells (i.e. astrocytes and activated microglia) from the CNS exhibit GABA B receptor immunoreactivity (Charles et al., 2003) and might be considered a possible target for the action of GABA B receptor agonists (Kang et al., 1998; Clark et al., 2000). Although GABA B receptors were ®rst identi®ed in the periph- eral nervous system (PNS), and in particular at autonomic nerve terminals (Bowery et al., 1981), information regarding their putative presence and function in the glial cells of PNS (i.e. Schwann cells) are still lacking. Our previous studies had suggested Schwann cells to be a target for the action of GABA, as they express mRNAs coding for a number of GABA A receptor subunits (i.e.a 23 , b 13 ) (Melcangi et al., 1999; Magnaghi et al., 2001). The main aim of the present study was to investigate GABA B receptor localization in the glial cells of a peripheral nerve, e.g. the rat sciatic nerve, and to determine the functional consequences of receptor activation, which may clarify their putative physiological role. European Journal of Neuroscience, Vol. 19, pp. 2641±2649, 2004 ß Federation of European Neuroscience Societies doi:10.1111/j.1460-9568.2004.03368.x Correspondence: Dr Valerio Magnaghi, as above. E-mail: valerio.magnaghi@unimi.it Received 27 November 2003, revised 3 March 2004, accepted 5 March 2004