No abstract

The location and the role of gamma-aminobutyric acid type B (GABA(B)) receptors in the central nervous system have recently received considerable attention, whilst relatively little is known regarding the peripheral nervous system. In this regard, here we demonstrate for the first time that GABA(B) receptor isoforms [i.e. GABA(B(1)) and GABA(B(2))] are specifically localized in the rat Schwann cell population of the sciatic nerve. Using the selective GABA(B) agonist [i.e. (-)-baclofen] and the antagonists (i.e. CGP 62349, CGP 56999 A, CGP 55845 A), 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 specific myelin proteins (i.e. glycoprotein Po, peripheral myelin protein 22, myelin-associated glycoprotein, connexin 32), providing evidence for a physiological role of GABA(B) receptors in the glial cells of the peripheral nervous system.

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the adult mammalian central nervous system (CNS), exerts its action via an interaction with specific receptors (e.g., GABAA and GABAB). These receptors are expressed not only in neurons but also on glial cells of the CNS, which might represent a target for the allosteric action of neuroactive steroids. Herein, we have demonstrated first that in the peripheral nervous system (PNS), the sciatic nerve and myelin-producing Schwann cells express both GABAA and GABAB receptors. Specific ligands, muscimol and baclofen, respectively, control Schwann-cell proliferation and expression of some specific myelin proteins (i.e., glycoprotein P0 and peripheral myelin protein 22 [PMP22]). Moreover, the progesterone (P) metabolite allopregnanolone, acting via the GABAA receptor, can influence PMP22 synthesis. In addition, we demonstrate that P, dihydroprogesterone, and allopregnanolone influence the expression of GABAB subunits in Schwann cells. The results suggest, at least in the myelinating cells of the PNS, a cross-interaction within the GABAergic receptor system, via GABAA and GABAB receptors and neuroactive steroids.

No abstract

J. Neurochem. (2009) 112, 980–990. Abstract Recent evidence showed that neurotransmitters are synthesised in glial cells, such as the Schwann cells, which form myelin sheaths in the PNS. While the presence of GABA type A (GABA‐A) receptors has been previously demonstrated in these cells, the evidence of GABA synthesis remained still elusive. In an attempt to demonstrate the presence of GABA in rat Schwann cells, we adopted a strategy, using several integrated neurochemical, molecular as well as immunocytochemical approaches. We first demonstrated the presence of glutamic acid decarboxylase of 67 kDa (GAD67) in Schwann cells, a crucial enzyme of the GABA synthesis mechanism. Second, we demonstrated that GABA is synthesized and localized in Schwann cells. As the third step we showed that allopregnanolone (10 nM), a potent allosteric modulator of GABA‐A receptors, stimulates GABA synthesis through increased levels of GAD67 in Schwann cells. Analysis of intracellular signalling mechanisms revealed that the protein kinase A pathway, through enhanced cAMP levels and cAMP response element binding protein phosphorylation, modulates the allosteric action of allopregnanolone at the GABA‐A receptor in Schwann cells. Our findings are the first to demonstrate that this GABA mechanism is active in Schwann cells thus establishing new potential therapeutic targets to control Schwann cell biology, which may prove useful in the treatment of several neurodegenerative disorders.