In the search for P2-receptors modulating the stimulation-evoked entry of calcium at processes of PC12 cells differentiated in the presence of nerve growth factor and neurotrophin-3, electrically evoked increases in free calcium were assessed by fura-2 microfluorimetry. Omission of calcium and addition of cadmium (100 M) or the N-type calcium channel blocker -conotoxin GVIA (0.5 M) abolished or markedly reduced the evoked responses. The P2Y-receptor agonists 2-methylthio adenosine 5Ј-diphosphate (2-methylthio-ADP), ADP, and adenosine 5Ј-O-(2-thiodiphosphate) (ADP␤S) inhibited the electrically evoked entry of calcium without any changes in basal calcium concentrations. 2-Methylthio-ADP was the most potent agonist. Adenosine, P -methyl-2Ј-deoxyadenosine 3Ј,5Ј-bisphosphate; 10 M), as well as the adenosine A 1 -receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine; 100 nM), caused no change. Pretreatment with pertussis toxin abolished the effect of ADP␤S. Reverse transcriptase-polymerase chain reaction revealed the presence of mRNA for P2Y12-receptors in nondifferentiated and differentiated PC12 cells. The results indicate that processes of differentiated PC12 cells possess P2Y12-receptors coupling to pertussis toxin-sensitive G-proteins and mediating an inhibition of the stimulation-evoked entry of calcium through -conotoxin GVIA-sensitive calcium channels. This suggests a role of P2Y12-receptors in neuromodulation in addition to their involvement in platelet aggregation.The operation of P2-receptors inhibiting the release of neurotransmitters has been demonstrated in the peripheral and central nervous system (for reviews see Silinsky et al., 1998;Inoue et al., 1999;von Kü gelgen et al., 1999a). At postganglionic sympathetic axons the receptors are activated by endogenous adenine nucleotides released as cotransmitters of norepinephrine and, hence, mediate a negative feedback inhibition of sympathetic transmitter release (Fuder and Muth, 1993;von Kü gelgen et al., 1993). Release-inhibiting P2-receptors have also been found in adrenal chromaffin cells and the cell bodies of rat PC12 cells (Gandia et al., 1993;Currie and Fox, 1996;Powell et al., 2000;Vartian and Boehm, 2001;Unterberger et al., 2002). The release-inhibiting P2-receptors share some properties with cloned and expressed P2Y1-receptors (for references see above), but their molecular identity is yet not known. In addition to inhibitory P2-receptors, the cell bodies of postganglionic sympathetic neurons, adrenal chromaffin cells, and PC12 cells possess excitatory P2-receptors of distinct subtypes and with distinct signaling transduction pathways (for reviews see Silinsky et al., 1998;von Kü gelgen et al., 1999a; for PC12 cells see, for example, Arslan et al., 2000;Unterberger et al., 2002).The inhibition of calcium channels at axon terminals is a key element in the modulation of transmitter release by inhibitory G-protein-coupled receptors (see Przywara et al., 1993;Mirotznik et al., 2000;Jarvis and Zamponi, 2001). Therefore, we searched for...