Coactivation of purinergic (P 2Y) receptors reduces agonist efficacy at serotonin 1B (5-HT 1B), but not 5-HT 1A receptors. Herein, we report that pretreatment for 5 min with the P 2Y receptor agonist ATP reduced agonist responsiveness at the 5-HT 1A, but not at the 5-HT 1B, receptor. The effect of ATP pretreatment on the 5-HT 1A receptor response rapidly reversed within a 10 min time frame betw...