Negative cross talk between anionic GABAa and cationic P2X ionotropic receptors of rat dorsal root ganglion neurons

Abstract

Using whole-cell patch-clamp recording and intracellular Ca2+ imaging of rat cultured DRG neurons, we studied the cross talk between GABAA and P2X receptors. A rapidly fading current was the main response to ATP, whereas GABA elicited slowly desensitizing inward currents. Coapplication of these agonists produced a total current much smaller than the linear summation of individual responses (68 ± 5% with 10 μM ATP plus 100 μM GABA). Occlusion was observed regardless of ATP response type. Neurons without functional P2X receptors manifested no effect of ATP on GABA currents (and vice versa). Occlusion was also absent in the presence of the P2X blocker trinitrophenyl-ATP (TNP-ATP) or of the GABA blocker picrotoxin, indicating a lack of involvement by metabotropic ATP or GABA receptors. Less occlusion was obtained when ATP was applied 2 sec after GABA than when GABA was applied after ATP. Changing the polarity of GABA currents by using intracellular SO42- instead of Cl- significantly reduced the occlusion of ATP currents by GABA, suggesting an important role for Cl- efflux in this phenomenon. Occlusion was enhanced whenever intracellular Ca2+ ([Ca2+]i) was not buffered, indicating the cross talk-facilitating role of this divalent cation. Ca2+ imaging showed that ATP (but not GABA) increased [Ca2+]i in voltageclamped or intact neurons. Our data demonstrated a novel Cl- and Ca2+-dependent interaction between cationic P2X and anionic GABAA receptors of DRG neurons. Such negative cross talk might represent a model for a new mechanism to inhibit afferent excitation to the spinal cord as GABA and ATP are coreleased within the dorsal horn.