Cervical dorsal rhizotomy (CDR) enhances long term facilitation of respiratory motor output in rats

Long term facilitation (LTF) is a long lasting, serotonin-dependent enhancement of respiratory motor output following episodic hypoxia. Because bilateral CDR increases serotonergic innervation of phrenic motoneurons (Prakash et al., ibid), we hypothesized that CDR (C3-C5) increases LTF in phrenic (spinal), but not hypoglossal (brainstem) motor output. 28 days before studies, surgery was performed on two groups of adult male rats (CDR and sham-operated, both N =6). LTF was assessed in anesthetized, vagotomized and pump ventilated rats that were exposed to three, 5 min episodes of isocapnic hypoxia (FIO₂=0.11), separated by 5 min hyperoxic intervals (FIO₂=0.5). LTF in both phrenic (p=0.001) and hypoglossal (p=0.047) respiratory motor output was greater in CDR vs sham operated rats. The results suggest that enhanced serotonergic modulation: 1) increases LTF in both spinal (phrenic) and brainstem (hypoglossal) respiratory motor output following CDR, and 2) overrides decreased excitability due to phrenic motoneuron enlargement following CDR (Zhan et al., ibid). We speculate that CDR enhances serotonin terminal density in all respiratory motor nuclei innervated by the same raphe neurons.