Adenosine‐driven P38 mitogen‐activated protein kinase and protein kinase A signalling constrains 5HT 2a receptor‐induced phrenic motor facilitation

Abstract Episodic activation of spinal serotonin receptors elicits phrenic motor facilitation (pMF), a form of respiratory motor plasticity. Although 5‐HT 2 and 5‐HT 7 receptors elicit pMF, they do so via distinct cellular mechanisms known as the Q (5‐HT 2A/B ) and S (5‐HT 7 ) pathways. These pathways interact via mutual crosstalk inhibition; thus, concurrent activation of both serotonin receptor subtypes diminishes/abolishes pMF. Although episodic spinal adenosine 2A (A 2a ) receptor activation also elicits pMF via the S pathway, it remains unclear whether 5‐HT 2 and A 2a receptors interact via similar mechanisms. Although some molecules mediating crosstalk inhibition between 5‐HT 2 and 5‐HT 7 receptors have been identified, little is known concerning 5‐HT 2 and A 2a receptor interactions. Thus, we tested the hypotheses that: (1) concurrent 5‐HT 2 and A 2a receptor activation diminishes pMF via mutual crosstalk inhibition, and (2) A 2a receptors inhibit 5‐HT 2a ‐induced pMF via mechanisms that require p38 mitogen‐activated protein kinase (MAPK) and/or protein kinase A (PKA) enzymatic activity. Selective 5‐HT 2a and A 2a receptor agonists were delivered intrathecally at C4 (3, 6 µL injections, 5 min intervals) in urethane‐anaesthetized, ventilated, vagotomized and paralysed Sprague Dawley rats. Spinal 5‐HT 2a (73 ± 12%) or A 2a (90 ± 17%) receptor activation increased integrated phrenic burst amplitude (i.e. pMF). Concurrent receptor activation abolished pMF (15 ± 7.7%), demonstrating crosstalk inhibition. Spinal p38 MAP kinase or protein kinase A inhibition restored pMF, implicating both kinases as key mediators of crosstalk inhibition. Although the functional significance of these complex interactions is not yet fully understood, greater mechanistic understanding will accelerate progress towards optimizing the therapeutic efficacy of acute intermittent hypoxia‐induced respiratory motor plasticity. image Key points Concurrent 5‐HT 2a (Q pathway) and A 2a (S pathway) receptor activation cancels phrenic motor facilitation via mutual crosstalk inhibition. Spinal p38 MAPK or protein kinase A inhibition restores phrenic motor facilitation with concurrent Q and S pathway activation, demonstrating a key role for these kinases in crosstalk inhibition.