Influence of vagal afferents on supraspinal and spinal respiratory activity following cervical spinal cord injury in rats

Abstract

C(2) spinal hemisection (C2HS) interrupts ipsilateral bulbospinal pathways and induces compensatory increases in contralateral spinal and possibly supraspinal respiratory output. Our first purpose was to test the hypothesis that after C2HS contralateral respiratory motor outputs become resistant to vagal inhibitory inputs associated with lung inflation. Bilateral phrenic and contralateral hypoglossal (XII) neurograms were recorded in anesthetized and ventilated rats. In uninjured (control) rats, lung inflation induced by positive end-expired pressure (PEEP; 3-9 cmH(2)O) robustly inhibited both phrenic and XII bursting. At 2 wk post-C2HS, PEEP evoked a complex response associated with phrenic bursts of both reduced and augmented amplitude, but with no overall change in the mean burst amplitude. PEEP-induced inhibition of XII bursting was still present but was attenuated relative to controls. However, by 8 wk post-C2HS PEEP-induced inhibition of both phrenic and XII output were similar to that in controls. Our second purpose was to test the hypothesis that vagal afferents inhibit ipsilateral phrenic bursting, thereby limiting the incidence of the spontaneous crossed phrenic phenomenon in vagal-intact rats. Bilateral vagotomy greatly enhanced ipsilateral phrenic bursting, which was either weak or absent in vagal-intact rats at both 2 and 8 wk post-C2HS. We conclude that 1) compensatory increases in contralateral phrenic and XII output after C2HS blunt the inhibitory influence of vagal afferents during lung inflation and 2) vagal afferents robustly inhibit ipsilateral phrenic bursting. These vagotomy data appear to explain the variability in the literature regarding the onset of the spontaneous crossed phrenic phenomenon in spontaneously breathing (vagal intact) vs. ventilated (vagotomized) preparations.

Fig. 1.

Representative neurograms demonstrating the impact of positive end-expired pressure (PEEP) on spinal (phrenic) and supraspinal (XII) respiratory motor output in control (uninjured) and C₂ spinal hemisection (C2HS) rats. Figure depicts integrated (∫) phrenic (∫Phr) and hypoglossal (∫XII) bursting during 3 (left), 6 (center), or 9 (right) cmH₂O PEEP in a control animal and 2 C2HS rats (2 and 8 wk after injury). Application of PEEP (dashed lines in each panel) was always accompanied by an increase in tracheal pressure (Ptr). At 2 wk post-C2HS, PEEP-induced inhibition of phrenic and XII output is less than in the control rat. Arrows indicate an “augmented phrenic burst.” See text for a more complete description. Sections marked with “A” and “B” are shown at an expanded timescale in Fig. 2. CL, contralateral (i.e., uninjured or right side); IL, ipsilateral (i.e., injured or left side); BP, arterial blood pressure (scale bar indicates 50–150 mmHg). Ptr scale bar indicates 0–10 cmH₂O; time scale bar indicates 2 s.

Fig. 2.

Representative neurograms depicting an augmented phrenic discharge accompanied by an augmented XII discharge (A) and an inhibited XII discharge (B) during PEEP application. This figure provides expanded timescale traces of the augmented phrenic discharges indicated by “A” and “B” in Fig. 1. The augmented phrenic discharge was accompanied by a similarly augmented XII discharge in A, but B shows that XII discharge could also be reduced under these conditions. Labels are as in Fig. 1. Ptr scale bar represents 0–10 cmH₂O; time scale bar represents 0.25 s.

Fig. 3.

Effects of PEEP on contralateral phrenic (A) and XII (B) burst amplitude. PEEP of 6–9 cmH₂O reduced peak contralateral ∫Phr in both control and 8 wk post-C2HS animals. In contrast, this response was not observed at 2 wk after injury. PEEP reduced peak ∫XII in all groups; however, the effects of PEEP were attenuated at 2 wk after injury. **Different from baseline condition of PEEP = 0 cmH₂O (P < 0.01); #P < 0.05, ##P < 0.01, significant difference between the 2 wk postinjury group and both the 8 wk postinjury and control groups.

Fig. 4.

Effects of PEEP on inspiratory (Ti, A) and expiratory (Te, B) duration and respiratory frequency (C) in control and C2HS rats. Elevation of PEEP induced a typical lung inflation reflex resulting in reduction of Ti, elongation of Te, and a decline in respiratory frequency (bursts/min) in all groups. However, the extent of inhibition during PEEP was attenuated in the 2 wk post-C2HS animals. *P < 0.05, **P < 0.01 vs. baseline value; #P < 0.05, ##P < 0.01 significant difference between the 2 wk postinjury group and both the 8 wk postinjury and control groups.

Fig. 5.

Representative neurograms demonstrating the impact of bilateral vagotomy on spinal (phrenic) and supraspinal (XII) respiratory motor output in control (uninjured) and C2HS rats. Respiratory bursting is shown with vagus nerves intact (left) and shortly after bilateral vagotomy (right). In control rats (top), vagotomy induced the expected slowing of burst frequency and increase in both phrenic and XII burst amplitude. Note also that the onset of inspiratory bursting is similar between the contralateral and ipsilateral phrenic nerves in the control rat (vertical solid lines). At 2 (middle) and 8 (bottom) wk post-C2HS, ipsilateral phrenic inspiratory bursting was typically absent or very weak when the vagus nerves were intact. After vagotomy, however, very clear ipsilateral phrenic bursting was present, and the onset of bursting was slightly delayed relative to the contralateral phrenic burst (compare solid vs. dashed vertical lines). After vagotomy, ipsilateral phrenic bursting was more robust in rats that were 8 vs. 2 wk post-C2HS. Labels are as in Fig. 1. Time scale bar is 0.5 s.

Fig. 6.

Average changes (Δ) in peak integrated phrenic and XII burst amplitude following bilateral vagotomy. Peak contralateral ∫Phr and ∫XII burst amplitudes showed similar increases after vagotomy in control and C2HS rats. However, vagotomy induced a relatively greater increase in the ipsilateral ∫Phr burst amplitude after C2HS. In addition, the increase in the ipsilateral ∫Phr burst amplitude after vagotomy was greater at 8 compared with 2 wk post-C2HS. **P < 0.01 vs. contralateral phrenic amplitude; ##P < 0.01 vs. control group; ^θP < 0.05, significant difference between 2 and 8 wk post-C2HS groups.

Fig. 7.

Mean ipsilateral phrenic inspiratory burst amplitude at 2 and 8 wk post-C2HS. Peak ipsilateral ∫Phr burst amplitude was quantified as an absolute voltage [i.e., arbitrary units (a.u.), A] and as % of the burst amplitude in the contralateral phrenic nerve (% CL, B). Both analyses suggest a progressive increase in ipsilateral phrenic burst amplitude over 2–8 wk after C2HS. Differences between 2 and 8 wk post-C2HS rats were statistically significant only after vagotomy. *P < 0.05, **P < 0.01 vagal-intact vs. vagotomized; ##P < 0.01, significant different between 2 and 8 wk post-C2HS groups.