Motoneurons are the final output pathway for the brain's influence on behavior. Here we identify properties of hypoglossal motor output to the tongue musculature. tongue motor control is critical to the pathogenesis of obstructive sleep apnea, a common and serious sleep-related breathing disorder. Studies were performed on mice expressing a light sensitive cation channel exclusively on cholinergic neurons (ChAT-ChR2(H134R)-EYFP). Discrete photostimulations under isoflurane-induced anesthesia from an optical probe positioned above the medullary surface and hypoglossal motor nucleus elicited discrete increases in tongue motor output, with the magnitude of responses dependent on stimulation power (p < 0.001, n = 7) and frequency (P = 0.002, n = 8, with responses to 10 Hz stimulation greater than for 15-25 Hz, P < 0.022). Stimulations during REM sleep elicited significantly reduced responses at powers 3-20 mW compared to non-rapid eye movement (non-REM) sleep and wakefulness (each p < 0.05, n = 7). Response thresholds were also greater in REM sleep (10 mW) compared to non-REM and waking (3 to 5 mW, P < 0.05), and the slopes of the regressions between input photostimulation powers and output motor responses were specifically reduced in REM sleep (P < 0.001). This study identifies that variations in photostimulation input produce tunable changes in hypoglossal motor output in-vivo and identifies REM sleep specific suppression of net motor excitability and responsivity.Obstructive sleep apnea (OSA) is a common and serious breathing disorder with significant clinical, social and economic consequences 1 . OSA is characterized by repeated episodes of upper airway obstruction that occur only during sleep, with the airway obstructions leading to futile breathing efforts, asphyxia, sleep disturbance and other physiological sequalae of medical relevance 2,3 . The root cause of the upper airway obstruction is reduced activity and reflex compensatory responses of the pharyngeal muscles during sleep, whereas in wakefulness the motor activity and responsivity are sufficient to keep the airspace open 4 . Ultimately, state-dependent pharyngeal muscle activity and responsivity of the brainstem motor pools driving these muscles are pivotal to OSA pathophysiology and the phenotypic traits of OSA patients 5-8 .Such state-dependent activity of the pharyngeal musculature is central to OSA pathogenesis, and the hypoglossal motoneuron pool is the source of motor output to the largest of the pharyngeal muscles, the tongue 2,4 . Single unit recordings of tongue motor activity are an experimentally powerful indicator of the discharge patterns and recruitment characteristics of single hypoglossal motoneurons in the medulla that are otherwise inaccessible for investigation [9][10][11][12][13][14][15][16][17][18][19] . One limitation of such single-unit studies, however, is the inability to impose acute, precise and direct control over hypoglossal motor output to interrogate physiological properties relevant to tongue motor control.In the present...

Thyrotropin-releasing hormone (TRH) is produced by the hypothalamus but most brain TRH is located elsewhere where it acts as a neuromodulator. TRH-positive neurons project to the hypoglossal motoneuron pool where TRH receptor RNA shows a high degree of differential expression compared with the rest of the brain. Strategies to modulate hypoglossal motor activity are of physiological and clinical interest given the potential for pharmacotherapy for obstructive sleep apnea (OSA), a common and serious respiratory disorder. Here, we identified the effects on tongue motor activity of TRH and a specific analog (taltirelin) applied locally to the hypoglossal motoneuron pool and systemically in vivo. Studies were performed under isoflurane anesthesia and across sleep–wake states in rats. In anesthetized rats, microperfusion of TRH (n = 8) or taltirelin (n = 9) into the hypoglossal motoneuron pool caused dose-dependent increases in tonic and phasic tongue motor activity (both p &lt; 0.001). However, the motor responses to TRH were biphasic, being significantly larger “early” in the response versus at the end of the intervention (p ≤ 0.022). In contrast, responses to taltirelin were similar “early” versus “late” (p ≥ 0.107); i.e. once elicited, the motor responses to taltirelin were sustained and maintained. In freely behaving conscious rats (n = 10), microperfusion of 10 μM taltirelin into the hypoglossal motoneuron pool increased tonic and phasic tongue motor activity in non-rapid-eye-movement (REM) sleep (p ≤ 0.038). Intraperitoneal injection of taltirelin (1 mg/kg, n = 16 rats) also increased tonic tongue motor activity across sleep–wake states (p = 0.010). These findings inform the studies in humans to identify the potential beneficial effects of taltirelin for breathing during sleep and OSA.

Pharyngeal muscle activity and responsiveness are key pathophysiological traits in human obstructive sleep apnea (OSA) and strong contributors to improvements with pharmacotherapy. The thyrotropin-releasing hormone (TRH) analog taltirelin is of high pre-clinical interest given its neuronal-stimulant properties, minimal endocrine activity, tongue muscle activation following microperfusion into the hypoglossal motor nucleus (HMN) or systemic delivery, and high TRH receptor expression at the HMN compared to rest of the brain. Here we test the hypothesis that taltirelin increases HMN activity and/or responsivity to excitatory stimuli applied across sleep–wake states in-vivo. To target hypoglossal motoneurons with simultaneous pharmacological and optical stimuli we used customized “opto-dialysis” probes and chronically implanted them in mice expressing a light sensitive cation channel exclusively on cholinergic neurons (ChAT–ChR2, n = 12) and wild-type mice lacking the opsin (n = 10). Both optical stimuli applied across a range of powers (P < 0.001) and microperfusion of taltirelin into the HMN (P < 0.020) increased tongue motor activity in sleeping ChAT–ChR2 mice. Notably, taltirelin increased tonic background tongue motor activity (P < 0.001) but not responsivity to excitatory optical stimuli across sleep–wake states (P > 0.098). This differential effect on tonic motor activity versus responsivity informs human studies of the potential beneficial effects of taltirelin on pharyngeal motor control and OSA pharmacotherapy.