Cholinergic pre-motor modulation of hypoglossal motor activity identified from combined ‘opto-dialysis’ <i>in-vivo</i>

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 sec pulsed (10ms at 10Hz, 0-20mW, n=15) or tonic (‘square-wave’, 0-20mW, 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&lt;0.05). During tonic IRt stimulation we identified that MAchR antagonism at the HMN (scopolamine, 2mM, n=7) increased endogenous respiratory tongue motor activity at higher stimulation intensities (20mW, P=0.030) whereas addition of nicotinic (N) AchR antagonism (mecamylamine, 200µM) decreased respiratory tongue activity at lower stimulation intensities (5mW, 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 20mW, P&lt;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.