Cholinergic premotor modulation of hypoglossal motor activity identified from combined "opto-dialysis" in vivo

A muscarinic acetylcholine receptor (MAchR) mechanism at the hypoglossal motor nucleus (HMN) can suppress tongue motor activity, and anti-MAchR agents are a component of pharmacotherapy for obstructive sleep apnea. However, the source and operation of cholinergic modulation of HMN activity are unknown in vivo. Here, we identify tongue motor responses to optical stimulation of cholinergic neurons at the HMN and intermediate reticular nucleus (IRt) in isoflurane-anesthetized transgenic mice (ChAT-ChR2(H134R)-EYFP) with and without pharmacological manipulation of the HMN. The IRt constitutes a major source of cholinergic input to the HMN and relay station for transmission of respiratory drive. In response to 2-s pulsed (10 ms at 10 Hz, 0-20 mW, n = 15) or tonic ("square-wave", 0-20 mW, n = 9) optical stimulation of the HMN or IRt, there was a power-dependent increase in tongue motor responses with lesser responses from the IRt (all P < 0.05). During tonic IRt stimulation, we identified that MAchR antagonism at the HMN (scopolamine, 2 mM, n = 7) increased endogenous respiratory tongue motor activity at higher stimulation intensities (20 mW, P = 0.030), whereas addition of nicotinic (N) AchR antagonism (mecamylamine, 200 µM) decreased respiratory tongue activity at lower stimulation intensities (5 mW, P = 0.012). These data identify opposing inhibitory and excitatory MAchR- and NAchR-dependent modulation of respiratory hypoglossal motor output. In contrast, MAchR antagonism alone or combined with NAchR antagonism at the HMN decreased evoked tonic motor activity (10 and 20 mW, P < 0.034), identifying reduction of a tonic excitatory effect. Together, these data identify cholinergic modulation of IRt-evoked HMN activity that shapes tonic and respiratory hypoglossal motor outputs in vivo.NEW & NOTEWORTHY The intermediate reticular nucleus (IRt) provides the largest innervation of the hypoglossal motor nucleus (HMN), including its cholinergic input, and serves as the critical relay for transmission of the respiratory input. Here, we combined optical stimulation of light-sensitive cation channels at the cholinergic IRt with pharmacological manipulation at the HMN in anesthetized transgenic mice in vivo. We identify differential opposing inhibitory and excitatory modulation of IRt-evoked HMN activity that shape tonic and respiratory hypoglossal motor outputs.