Fast non-genomic effects of progesterone-derived neurosteroids on nociceptive thresholds and pain symptoms Alexandre Charlet, Franc ßois Lasbennes, Pascal Darbon, Pierrick Poisbeau * Institut des Neurosciences Cellulaires et Inte ´gratives, UMR 7168 Centre national de la Recherche Scientifique – Universite ´ Louis Pasteur, Department Nociception and Pain, 21 rue Rene ´ Descartes, F-67084 Strasbourg Cedex, Strasbourg, France Received 2 April 2008; received in revised form 10 June 2008; accepted 10 June 2008 Abstract Fast Inhibitory controls mediated by glycine (GlyRs) and GABAA receptors (GABAARs) play an important role to prevent the apparition of pathological pain symptoms of allodynia and hyperalgesia. The use of positive allosteric modulators of these receptors, specifically expressed in the spinal cord, may represent an interesting strategy to limit or block pain expression. In this study, we have used stereoisomers of progesterone metabolites, acting only via non-genomic effects, in order to evaluate the contribution of GlyRs and GABAARs for the reduction of mechanical and thermal heat hypernociception. We show that 3a neurosteroids were particularly efficient to elevate nociceptive thresholds in naive animal. It also reduced mechanical allodynia and thermal heat hyperalgesia in the carrageenan model of inflammatory pain. This effect is likely to be mediated by GABAA receptors since 3b isomer was inefficient. More interestingly, 3a5b neurosteroid was only efficient on mechanical allodynia while having no effect on thermal heat hyperalgesia. We characterized these paradoxical effects of 3a5b neurosteroid using the strychnine and bicuculline models of allodynia. We clearly show that 3a5b neurosteroid exerts an antinociceptive effect via a positive allosteric modulation of GABAARs but, at the same time, is pronociceptive by reducing GlyR function. This illustrates the importance of the inhibitory amino acid receptor channels and their allosteric modulators in spinal pain processing. Moreover, our results indicate that neurosteroids, which are synthesized in the dorsal horn of the spinal cord and have limited side effects, may be of significant interest in order to treat pathological pain symptoms. Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. Keywords: Spinal cord; Neurosteroid; Progesterone; Allopregnanolone; Mechanical allodynia; Thermal heat hyperalgesia; GABAA receptor; Glycine receptor; Carrageenan; Inflammatory pain; Spinal inhibition 1. Introduction Neuroactive steroids might be of significant interest to treat severe neuropathologies [10,11,30] associated with neuronal hyperexcitability. Non-genomic rapid effects of neurosteroids are particularly efficient while using allopregnanolone (3a5aTHP: 3a,5a-tetrahydro-proges- terone) or pregnanolone (3a5bTHP: 3a,5b-tetrahydro- progesterone), two metabolites of progesterone which acts at nanomolar concentration on GABAA receptors (GABAARs) and potentiate their inhibitory function in the central nervous system [16,35]. Interestingly, changes in the level of endogenous 3a-neurosteroids have been reported in many physiological [3,21,24,26] and patho- logical situations [2,7,8,11,12,30,35]. Recently, the neu- rosteroid binding sites on GABAARs have been identified and characterized [17]. Depending on the sub- unit composition of GABAARs [16], a positive allosteric modulation is achieved by 3a-neurosteroids binding in a cavity within in the transmembrane domains of the a subunit, whereas a direct activation of the receptor is possible if the neurosteroids bind to interfacial residues between a and b subunits. This indicates that neuroster- oids preferentially access the receptor from the 0304-3959/$34.00 Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pain.2008.06.016 * Corresponding author. Tel.: +33 3 90 24 14 76; fax: +33 3 88 61 33 47. E-mail address: poisbeau@neurochem.u-strasbg.fr (P. Poisbeau). www.elsevier.com/locate/pain Pain 139 (2008) 603–609