Anales RANF

S1-03 MOLECULAR MECHANISMS FOR A 3 ADENOSINE RECEPTOR- MEDIATED PAIN CONTROL: AN IN VITRO STUDY ON DORSAL ROOT GANGLION NEURON EXCITABILITY 1 E. Coppi, 1 F. Cherchi, 1 I. Fusco, 1 P. Failli, 1 A. Vona, 1 I. Dettori, 1 L. Gaviano, 1 E. Lucarini, 2 K.A. Jacobson, 2 D.K. Tosh, 3 D. Salvemini, 1 C. Ghelardini, 1 F. Pedata, 1 L. Di Cesare Mannelli and 1 A.M. Pugliese. 1 University of Florence, Florence, Italy. 2 National Institutes of Health, Bethesda, Maryland 20892-0810, United States. 3 Saint Louis University, St. Louis, Missouri 63104, United States . Background. Interest has been focused in recent years on the analgesic effects exerted by adenosine and its receptors, A 1 , A 2A , A 2B and A 3 subtypes, in different in vivo models of acute and chronic pain. In particular, recent studies have focused on the anti- hyperalgesic activity of the A 3 adenosine receptor (A 3 AR) in several chronic pain models, but the cellular and molecular basis of this pain control is still unknown. Material and methods. Here, we investigated the expression (by RT-PCR and immunocytochemical analysis) and functional effects (by patch-clamp technique) of A 3 AR on ionic currents and cell excitability of small-medium sized, capsaicin-sensitive, dorsal root ganglion (DRG) neurons isolated from 3-4 week-old rats of both sexes. Results. We demonstrated that A 3 AR are expressed in rat DRG neurons and modulate different kinds of ionic currents. Two distinct A 3 AR agonists were tested: Cl-IB-MECA and the highly selective MRS5980. Both compounds concentration-dependently (0.1- 100 nM) inhibited Ca 2+ currents evoked by a voltage step depolarization protocol. This effect was sensitive to the A 3 AR antagonist MRS1523 (100 nM) and to the N-type blocker PD173212 but not to the L-type blocker, lacidipine. The endogenous agonist adenosine also reduced N-type Ca 2+ currents, and its effect was inhibited by 56% in the presence of A 3 AR antagonist MRS1523, demonstrating that the majority of adenosine’s effect is mediated by this receptor subtype. Current-clamp recordings demonstrated that neuronal firing of rat DRG neurons was significantly reduced by A 3 AR activation in a MRS1523-sensitive but PD173212-insensitive manner. Conclusion. We conclude that pain-relieving effects observed upon A 3 AR activation could be mediated through N-type Ca 2+ channel block and action potential inhibition in isolated rat DRG neurons. These findings indicate that A 3 AR agonists utilized in the therapy of pain act at a first station reducing dorsal root ganglion neuron excitability.