Phrenic motor neuron adenosine 2A receptors elicit phrenic motor facilitation

Although adenosine 2A (A_2A ) receptor activation triggers specific cell signalling cascades, the ensuing physiological outcomes depend on the specific cell type expressing these receptors. Cervical spinal adenosine 2A (A_2A ) receptor activation elicits a prolonged facilitation in phrenic nerve activity, which was nearly abolished following intrapleural A_2A receptor siRNA injections. A_2A receptor siRNA injections selectively knocked down A_2A receptors in cholera toxin B-subunit-identified phrenic motor neurons, sparing cervical non-phrenic motor neurons. Collectively, our results support the hypothesis that phrenic motor neurons express the A_2A receptors relevant to A_2A receptor-induced phrenic motor facilitation. Upregulation of A_2A receptor expression in the phrenic motor neurons per se may potentially be a useful approach to increase phrenic motor neuron excitability in conditions such as spinal cord injury.

Cervical spinal adenosine 2A (A_2A ) receptor activation elicits a prolonged increase in phrenic nerve activity, an effect known as phrenic motor facilitation (pMF). The specific cervical spinal cells expressing the relevant A_2A receptors for pMF are unknown. This is an important question since the physiological outcome of A_2A receptor activation is highly cell type specific. Thus, we tested the hypothesis that the relevant A_2A receptors for pMF are expressed in phrenic motor neurons per se versus non-phrenic neurons of the cervical spinal cord. A_2A receptor immunostaining significantly colocalized with NeuN-positive neurons (89 ± 2%). Intrapleural siRNA injections were used to selectively knock down A_2A receptors in cholera toxin B-subunit-labelled phrenic motor neurons. A_2A receptor knock-down was verified by a ∼45% decrease in A_2A receptor immunoreactivity within phrenic motor neurons versus non-targeting siRNAs (siNT; P < 0.05). There was no evidence for knock-down in cervical non-phrenic motor neurons. In rats that were anaesthetized, subjected to neuromuscular blockade and ventilated, pMF induced by cervical (C3-4) intrathecal injections of the A_2A receptor agonist CGS21680 was greatly attenuated in siA_2A (21%) versus siNT treated rats (147%; P < 0.01). There were no significant effects of siA_2A on phrenic burst frequency. Collectively, our results support the hypothesis that phrenic motor neurons express the A_2A receptors relevant to A_2A receptor-induced pMF.