Modulation of firing activity by ATP in dopamine neurons of the rat substantia nigra pars compacta

ATP acts as a neurotransmitter or co-neurotransmitter in many areas of the CNS and peripheral nervous systems; however, little is known about the expression and functional role of purinoceptors (P2) in midbrain dopaminergic neurons. Therefore, we investigated P2X receptor expression and regulation of spontaneous firing activity in dopaminergic neurons of the substantia nigra pars compacta (SNc) in rats using patch-clamp and Ca2+-imaging techniques. In most neurons, application of ATP (1 μM–1 mM) increased firing rate dose-dependently (EC50=1.26±0.26 μM, n=45). When the P2-receptor agonists such as 2-methylthio-adenosine 5′-triphosphate (2-MeSATP) or ATPγS were applied or pressure-applied to the neuron, the firing activity increased together with a rise in cytosolic Ca2+ concentration ([Ca2+]c), but application of beta,gamma-methylene ATP (P2X1, 3 agonist) or methylthio-adenosine 5′-diphosphate (P2Y1 agonist) had no effect. In many neurons, the effect of ATP was abolished by the application of the P2-receptor antagonists, suramin or pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS). When ATP was applied in a Ca2+-free solution, there was no detectable change in [Ca2+]c, suggesting that ATP does not release Ca2+ from intracellular stores. In the single-cell reverse transcription polymerase chain reaction (RT-PCR), we found that 65% of dopaminergic neurons expressed mRNAs for P2X receptors; positive amplifications of P2X6 (57.1%), P2X2/6 (25.0%), and P2X4 mRNA (17.9%), respectively. From the above results, we could conclude that ATP modulates firing activities in the rat SNc dopaminergic neurons, possibly via P2X2, P2X2/6, and/or P2X4 receptors.