Characterization of TRPM8-Like Channels Activated by the Cooling Agent Icilin in the Macrophage Cell Line RAW 264.7

Icilin is recognized as a chemical agonist of nociceptors and can activate TRPM8 channels. However, whether this agent has any effects on immune cells remains unknown. In this study, the effects of icilin on ion currents were investigated in RAW 264.7 murine macrophage-like cells. Icilin (1-100 mu M) increased the amplitude of nonselective (NS) cation current (I(NS)) in a concentration-dependent manner with an EC(50) value of 8.6 mu M. LaCl(3) (100 mu M) or capsazepine (30 mu M) reversed icilin-induced I(NS); however, neither apamin (200 nM) nor iberiotoxin (200 nM) had any effects on it. In cell-attached configuration, when the electrode was filled with icilin (30 mu M), a unique population of NS cation channels were activated with single-channel conductance of 158 pS. With the use of a long-lasting ramp pulse protocol, increasing icilin concentration produced a left shift in the activation curve of NS channels, with no change in the slope factor of the curve. The probability of channel opening enhanced by icilin was increased by either elevated extracellular Ca(2+) or application of ionomycin (10 mu M), while it was reduced by BAPTA-AM (10 mu M). Icilin-stimulated activity is associated with an increase in mean open time and a reduction in mean closed time. Under current-clamp conditions, icilin caused membrane depolarization. Therefore, icilin interacts with the TRPM8-like channel to increase I(NS) and depolarizes the membrane in these cells.