Covalently Binding Adenosine A3 Receptor Agonist ICBM Irreversibly Reduces Voltage-Gated Ca2+ Currents in Dorsal Root Ganglion Neurons

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) adenosine receptor (AR) subtypes, in different in vivo models of chronic pain. In particular, it was demonstrated that selective A(3)AR agonists reduced pro-nociceptive N-type Ca2+ channels in dorsal root ganglion (DRG) neurons isolated from rats and, by this mechanism, inhibit post inflammatory visceral hypersensitivity. In the present study, we investigate the effect of a previously reported irreversibly binding A(3)AR agonist, ICBM, on Ca2+ currents (I-Ca) in rat DRG neurons. Present data demonstrate that ICBM, an isothiocyanate derivative designed for covalent binding to the receptor, concentration-dependently inhibits I-Ca. This effect is irreversible, since it persists after drug removal, differently from the prototypical A(3)AR agonist, Cl-IB-MECA. ICBM pre-exposure inhibits the effect of a subsequent Cl-IB-MECA application. Thus, covalent A(3)AR agonists such as ICBM may represent an innovative, beneficial, and longer-lasting strategy to achieve efficacious chronic pain control versus commonly used, reversible, A(3)AR agonists. However, the possible limitations of this drug and other covalent drugs may be, for example, a characteristic adverse effect profile, suggesting that more pre-clinical studies are needed.