Cross-talk inhibition between 5-HT and 5-HT receptors in phrenic motor facilitation via NADPH oxidase and PKA.

Intermittent spinal serotonin receptor activation elicits phrenic motor facilitation (pMF), a form of spinal respiratory motor plasticity. Episodic activation of either serotonin type 2 (5-HT) or type 7 (5-HT) receptors elicits pMF, although they do so via distinct cellular mechanisms known as the Q (5-HT) and S (5-HT) pathways to pMF. When coactivated, these pathways interact via mutual cross-talk inhibition. Although we have a rudimentary understanding of mechanisms mediating cross-talk interactions between spinal 5-HT subtype A (5-HT) and 5-HT receptor activation, we do not know if similar interactions exist between 5-HT subtype B (5-HT) and 5-HT receptors. We confirmed that either spinal 5-HT or 5-HT receptor activation alone elicits pMF and tested the hypotheses that 1) concurrent activation of both receptors suppresses pMF due to cross-talk inhibition; 2) 5-HT receptor inhibition of 5-HT receptor-induced pMF requires protein kinase A (PKA) activity; and 3) 5-HT receptor inhibition of 5-HT receptor-induced pMF requires NADPH oxidase (NOX) activity. Selective 5-HT and 5-HT receptor agonists were administered intrathecally at C4 (3 injections, 5-min intervals) to anesthetized, paralyzed, and ventilated rats. Whereas integrated phrenic nerve burst amplitude increased after selective spinal 5-HT or 5-HT receptor activation alone (i.e., pMF), pMF was no longer observed with concurrent 5-HT and 5-HT receptor agonist administration. With concurrent receptor activation, pMF was rescued by inhibiting either NOX or PKA activity, demonstrating their roles in cross-talk inhibition between these pathways to pMF. This report demonstrates cross-talk inhibition between 5-HT- and 5-HT receptor-induced pMF and that NOX and PKA activity are necessary for that cross-talk inhibition.