Modulation of respiratory output by cervical epidural stimulation in the anesthetized mouse

Respiration is produced and controlled by well-characterized brain stem nuclei, but the contributions of spinal circuits to respiratory control and modulation remain under investigation. Many respiratory studies are conducted in in vitro preparations (e.g., brain stem slice) obtained from neonatal rodents. While informative, these studies do not fully recapitulate the complex afferent and efferent neural circuits that are likely to be involved in eupnea (i.e., quiet breathing). To begin to investigate spinal contributions to respiration, we electrically stimulated the cervical spinal cord during unassisted respiration in anesthetized, intact mice. Specifically, we used epidermal electrical stimulation at 20 Hz and varied current intensity to map changes in respiration. Stimulating at 1.5 mA at cervical level 3 (C3) consistently caused a significant increase in respiratory frequency compared with prestimulation baseline and when compared with sham stimulations. The increase in respiratory frequency persisted for several minutes after epidural stimulation ceased. There was no change in tidal volume, and the estimated minute ventilation was increased as a consequence of the increase in respiratory frequency. Sigh frequency also increased during epidural stimulation at C3. Neither the increase in respiratory frequency nor the increase in sighing were observed after stimulation at other dorsal cervical levels. These findings suggest that the spinal circuits involved in the modulation of eupnea and sighing may be preferentially activated by specific endogenous inputs. Moreover, the cervical spinal cord may play a role in respiratory modulation that affects both eupneic respiration and sigh production in intact, adult mice.