ATP-EVOKED INCREASES IN INTRACELLULAR CALCIUM IN NEURONS AND GLIA FROM THE DORSAL SPINAL-CORD

ATP has been proposed as a possible chemical mediator of synaptic tran smission in the spinal dorsal horn on the basis that it is released in dorsal horn synaptosomes in a Ca2+-dependent manner and that its effe cts mimic those of synaptic inputs to dorsal horn neurons. In the pres ent study we examined the actions of ATP on neurons and glia in cell c ulture using optical and electrophysiological recording techniques. We found that ATP increased intracellular Ca2+ concentration ([Ca2+](i)) in >99% of astrocytes. In contrast, only 35% of neurons and 20% of ol igodendrocytes responded to ATP. The prevalence of the ATP-evoked resp onse in astrocytes led us to characterize the type of receptor mediati ng the response, the source of Ca2+, and the membrane currents activat ed by ATP. We found that ADP was approximately equipotent with ATP in increasing [Ca2+](i) whereas AMP and adenosine had no effect. In addit ion, responses to ATP were blocked in a concentration-dependent manner by the P-2 purinergic receptor antagonist suramin. Furthermore, as it was found that 2-methylthio-ATP was more potent than ATP and that bet a,gamma-methylene-ATP was ineffective, the responses were mediated via the P-2Y subtype of purinergic receptor. The increase in [Ca2+](i) ev oked by ATP persisted in extracellular medium with no added Ca2+ and c ontaining EGTA, indicating that this increase was due to release of Ca 2+ from intracellular stores. Release of Ca2+ by ATP was blocked by th apsigargin but was unaffected by caffeine. ATP had several effects on membrane current activating inward, outward, and mixed currents despit e uniformly causing increases in [Ca2+](i). These observations indicat e that ATP has diverse electrophysiological effects on astrocytes as w ell as increasing [Ca2+](i) in these cells. We speculate that ATP rele ased from synaptic terminals in the dorsal horn might act not only on postsynaptic neurons but also on perisynaptic astrocytes. Thus, a phys iological role for ATP may be as a neuronal-glial signaling molecule w ithin the spinal dorsal horn.