Temporal regulation of agonist efficacy at 5-hydroxytryptamine (5-HT)_1A and 5-HT_1B receptors

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 between P_2Y receptor and 5-HT_1A receptor activation. ATP pretreatment effects on 5-HT_1A agonist responsiveness were blocked by the protein kinase inhibitors staurosporine and bisindolylmaleimide, suggesting that the ATP-mediated temporal regulation involves activation of protein kinase C (PKC). Moreover, the temporal effect of ATP was blocked by incubation with 1% ethanol, suggesting that consequences of phospholipase D (PLD) activation play a role. ATP pretreatment blocked the inhibitory effect produced by 5-HT_2C receptor activation on the 5-HT_1A but not the 5-HT_1B, receptor response, suggesting that the 5-HT_1A receptor itself was the target for PLD/PKC action. Finally, ethanol did not block the reduction in responsiveness of the 5-HT_1A receptor system produced by activation of PKC with phorbol ester treatment, suggesting that PKC activation lies downstream of PLD. Taken together, these data suggest that activation of P_2Y receptors can reduce responsiveness of the 5-HT_1A receptor system via a PLD/PKC-dependent mechanism that is highly dependent upon the temporal pattern of receptor activation. Moreover, this work underscores the importance of time as a variable in receptor signaling cross talk and serves to further illustrate differences between the 5-HT_1A and 5-HT_1B receptor systems.