Facilitation of transmitter release from rat sympathetic neurons via presynaptic P2Y₁ receptors

P2Y₁ receptor antagonists are being developed as antithrombotics. In the vasculature, P2Y₁ receptors are not only expressed in platelets, but also in the endothelium where they indirectly mediate vasorelaxation. By blockade of these latter effects, P2Y₁ antagonists may cause untoward effects. Here, we investigated whether vascular sympathetic axon terminals also possess P2Y₁ receptors by determining [³H]noradrenaline release from superior cervical ganglion neurons in cell culture. ADP inhibited electrically evoked release, but this effect was reverted into facilitation when either P2Y₁₂ receptors were blocked by cangrelor or their signalling cascade was interrupted by pertussis toxin. The facilitation by ADP was also observed with K⁺ stimulation in the presence of tetrodotoxin, was mimicked by 2-methlythio-ATP, and was prevented by suramin, reactive blue 2, and MRS2179. The facilitation by ADP was abolished by the phospholipase C inhibitor U73122, but was not affected by cholera toxin to down-regulate G_s proteins, by H-7 to block protein kinases, or by thapsigargin to deplete intracellular Ca²⁺ stores. In patch clamp recordings, activation of P2Y₁ receptors led to an inhibition of KCNQ channels, but the KCNQ channel modulators retigabine and XE 991 did not alter electrically evoked noradrenaline release or its facilitation by ADP. These results show that presynaptic P2Y₁ receptors mediate facilitation of transmitter release from sympathetic nerve terminals via phospholipase C, and interference with this mechanism by P2Y₁ antagonists can be expected to counteract the previously described vasoconstrictive action of these drugs.