The inhibitory effect of ADP is enhanced in the absence of P2Y1 signaling. The magnitudes of depolarization-evoked Ca++ transients in sensory neurons were measured before, and 5, 10 and 15 minutes after agonist application. Ca++ transients were not affected by application of buffer alone (dashed lines), but were significantly reduced after application of ADP (A-B), IDP (C), or UDPG (D; solid lines). Inhibition was prolonged by application of the selective P2Y1 antagonist MRS2179 (A) and in neurons from P2Y1-/- mice (B). There was no effect of MRS2179 in P2Y1-/- neurons (B). Values are mean ± SEM; n = 10 mice/timepoint each treatment. *p < 0.02 versus control for each treatment.