Adenosine signaling though A1 receptors inhibits chemosensitive neurons in the retrotrapezoid nucleus

Abstract

A subset of neurons in the retrotrapezoid nucleus (RTN) function as respiratory chemoreceptors by regulating depth and frequency of breathing in response to changes in tissue CO₂/H⁺. The activity of chemosensitive RTN neurons is also subject to modulation by CO₂/H⁺-dependent purinergic signaling. However, mechanisms contributing to purinergic regulation of RTN chemoreceptors are not entirely clear. Recent evidence suggests adenosine inhibits RTN chemoreception in vivo by activation of A1 receptors. The goal of this study was to characterize effects of adenosine on chemosensitive RTN neurons and identify intrinsic and synaptic mechanisms underlying this response. Cell-attached recordings from RTN chemoreceptors in slices from rat or wild-type mouse pups (mixed sex) show that exposure to adenosine (1 µM) inhibits chemoreceptor activity by an A1 receptor-dependent mechanism. However, exposure to a selective A1 receptor antagonist (DPCPX, 30 nM) alone did not potentiate CO₂/H⁺-stimulated activity, suggesting activation of A1 receptors does not limit chemoreceptor activity under these reduced conditions. Whole-cell voltage-clamp from chemosensitive RTN neurons show that exposure to adenosine activated an inward rectifying K⁺ conductance, and at the network level, adenosine preferentially decreased frequency of excitatory but not inhibitory postsynaptic currents. These results show that adenosine activation of A1 receptors inhibits chemosensitive RTN neurons by direct activation of a GIRK-like conductance, and presynaptically, by suppression of excitatory synaptic input to chemoreceptors.

Significance Statement Adenosine is a potent modulator of all aspects of breathing including chemoreception at the level of the retrotrapezoid nucleus (RTN); however, mechanisms by which adenosine regulates activity of RTN chemoreceptors is not known. Here, we show that adenosine activation of A1 receptors inhibits RTN neurons by activation of an inward rectifying K⁺ conductance, and by selective suppression of excitatory synaptic input to chemoreceptors. These results identify a GIRK-like conductance as the first target of purinergic signaling in chemosensitive RTN neurons. This work may also have clinical relevance since A1 receptor antagonists like caffeine are used to treat respiratory problems in premature infancy.