Episodic phrenic-inhibitory vagus nerve stimulation paradoxically induces phrenic long-term facilitation in rats

Abstract

All respiratory long-term facilitation (LTF) is induced by inspiratory-excitatory stimulation, suggesting that LTF needs inspiratory augmentation and is the result of a Hebbian mechanism (coincident pre- and post-synaptic activity strengthens synapses). The present study examined the long-term effects of episodic inspiratory-inhibitory vagus nerve stimulation (VNS) on phrenic nerve activity. We hypothesized that episodic VNS would induce phrenic long-term depression. The results are compared with those obtained following serotonin receptor antagonism or episodic carotid sinus nerve stimulation (CSNS). Integrated phrenic neurograms were measured before, during and after three episodes of 5 min VNS (50 Hz, 0.1 ms), each separated by a 5 min interval, at a low (approximately 50 microA), medium (approximately 200 microA) or high (approximately 500 microA) stimulus intensity in anaesthetized, vagotomized, neuromuscularly blocked and artificially ventilated rats. Medium- and high-intensity VNS eliminated rhythmic phrenic activity during VNS, while low-intensity VNS only reduced phrenic burst frequency. At 60 min post-VNS, phrenic amplitude was higher than baseline (35 +/- 5% above baseline, mean +/- S.E.M., P < 0.05) in the high-intensity group but not in the low- (-4 +/- 4%) or medium-intensity groups (-10 +/- 15%), or in the high-intensity with methysergide group (4 mg kg(-1), i.p.) (-11 +/- 5%). These data, which are inconsistent with our hypothesis, indicate that phrenic-inhibitory VNS induces a serotonin-dependent phrenic LTF similar to that induced by phrenic-excitatory CSNS (33 +/- 7%) and may require activation of high-threshold afferent fibres. These data also suggest that the synapses on phrenic motoneurons do not use the Hebbian mechanism in this LTF, as these motoneurons were suppressed during VNS.

Figure 1. Integrated phrenic neurograms recorded before, during and after episodic electrical stimulation of the VN (A) or CSN (B)

A, integrated phrenic nerve (Phr) and mean arterial blood pressure (MAP) responses to three episodes of 5 min electrical stimulation (50 Hz, 0.1 ms duration) delivered to the proximal end of the left cervical VN in three individual rats, at a low (50 μA), medium (200 μA) and high (500 μA) stimulus intensity. B, Phr and MAP responses to five episodes of 2 min CSNS (20 Hz, 0.1 ms duration, 30 μA) in one rat. Notice that data from only one episode during nerve stimulation are presented in these recordings and note that the recording paper speed during the 5 min of stimulation is different from the rest.

Figure 2. The effects of episodic VNS on phrenic nerve activity

A, average changes from baseline in peak amplitude of integrated phrenic nerve activity normalized to a percentage of the baseline (ΔPhr, %baseline). B, phrenic burst frequency (Δf, bursts min⁻¹). C, minute phrenic activity normalized to a percentage of the baseline (Δ(Phr ×f), %baseline). These data were obtained following three episodes of 5 min VNS (50 Hz, 0.1 ms duration), at low (50 ± 5 μA, n = 5, triangles), medium (200 ± 18 μA, n = 6, circles) and high (507 ± 53 μA, n = 8, squares) stimulus intensity. Data are expressed as means ±s.e.m.*Significantly different from baseline; †significantly different from the other groups (P < 0.05). These data indicate that high-intensity, but not low- or medium-intensity, VNS induces phrenic LTF.

Figure 3. LTF of phrenic nerve activity elicited by VNS in untreated control (n = 8, open squares) and methysergide-pre-treated (n = 6, filled squares) rats

A, average changes from baseline in peak amplitude of integrated phrenic nerve activity normalized to a percentage of the baseline (ΔPhr, %baseline). B, phrenic burst frequency (Δf, bursts min⁻¹). C, minute phrenic nerve activity normalized to a percentage of the baseline (Δ(Phr ×f), %baseline). These data were obtained following three episodes of 5 min VNS (50 Hz, 0.1 ms duration, 507 ± 53 μA) in control and methysergide (4 mg kg⁻¹, I.P., 517 ± 6 μA) groups. The same control data are presented in Fig. 2. Data are expressed as means ±s.e.m.*Significantly different from baseline; †significantly different from the methysergide group. These data suggest that VNS-induced phrenic LTF requires the activation of serotonin receptors.

Figure 4. LTF of phrenic nerve activity elicited by CSNS (triangles) and VNS (squares)

A, average changes from baseline in peak amplitude of integrated phrenic nerve activity normalized to a percentage of the baseline (ΔPhr, %baseline). B, phrenic burst frequency (Δf, bursts min⁻¹). C, minute phrenic nerve activity normalized to a percentage of the baseline (Δ(Phr ×f), %baseline). These data were obtained following either five episodes of 2 min CSNS (20 Hz, 0.1 ms duration, 30–100 μA) in the CSNS group (n = 8) or three episodes of 5 min VNS (50 Hz, 0.1 ms, 507 ± 53 μA) in the VNS group (n = 8). The same VNS data are presented in Figs 2 and 3. Data are expressed as means ±s.e.m.*Significantly different from baseline (VNS group); ‡significantly different from baseline (CSNS group). These data suggest that VNS-induced phrenic LTF is similar in magnitude and pattern to CSNS-induced LTF.