Cervical interneuron bursting during hypoxia in anesthetized rats

Abstract

Cervical spinal laminae VII‐X contain interneurons that are synaptically coupled to phrenic motoneurons (PMNs). Neurophysiological studies suggest these cells are not necessary for transmission of inspiratory drive, but their role in modulating PMN output during respiratory stimulation or plasticity is unknown. In ongoing studies, we are exploring the role of cervical interneurons in hypoxia‐induced phrenic plasticity. Since spinal serotonin (5‐HT) can induce phrenic plasticity, we first examined whether pre‐phrenic cervical interneurons are innervated by 5‐HT. 5‐HT immunoreactivity was robust throughout cervical laminae VII‐X and in the immediate vicinity of pre‐phrenic interneurons identified with a transynaptic retrograde tracer (pseudorabies virus, delivered to the diaphragm). Cervical interneuron activity was recorded in anesthetized, paralyzed, vagotomized and ventilated adult rats. Initial results indicate that tonic and phasic bursting interneurons are distributed throughout the C3‐C5 intermediate gray matter. During respiratory stimulation with hypoxia a subset of the cervical interneurons showed an increase in burst frequency, and in some cases switched from tonic to phasic bursting. Cervical interneurons are anatomically positioned to modulate phrenic motor output, but their potential role in initiating and/or maintaining phrenic plasticity remains to be determined.