Lipopolysaccharide attenuates phrenic long-term facilitation following acute intermittent hypoxia

Abstract

Lipopolysaccharide (LPS) induces inflammatory responses, including microglial activation in the central nervous system. Since LPS impairs certain forms of hippocampal and spinal neuroplasticity, we hypothesized that LPS would impair phrenic long-term facilitation (pLTF) following acute intermittent hypoxia (AIH) in outbred Sprague-Dawley (SD) and inbred Lewis (L) rats. Approximately 3h following a single LPS injection (i.p.), the phrenic response during hypoxic episodes is reduced in both rat strains versus vehicle treated, control rats (SD: 84 ± 7% vs. 128 ± 14% baseline for control, p < 0.05; L: 62 ± 10% vs. 90 ± 9% baseline for control, p < 0.05). At 60 min post-AIH, pLTF is also diminished by LPS in both strains: (SD: 22 ± 5% vs. 73.5 ± 14% baseline for control, p < 0.05; L: 18 ± 15% vs. 56 ± 8% baseline for control, p < 0.05). LPS alone does not affect phrenic burst frequency in either rat strain, suggesting that acute LPS injection has minimal effect on brainstem respiratory rhythm generation. Thus, systemic LPS injections and (presumptive) inflammation impair pLTF, a form of spinal neuroplasticity in respiratory motor control. These results suggest that ongoing infection or inflammation must be carefully considered in studies of respiratory plasticity, or during attempts to harness spinal plasticity as a therapeutic tool in the treatment of respiratory insufficiency, such as spinal cord injury.

Figure 1

Representative integrated neurogram from the phrenic nerve activity of an anaesthetized rat before, during and after AIH (Hypoxia 1 to 3) with 3 mg/kg of LPS i.p. (bottom trace) or vehicle treated rats (top trace). Also indicated are magnified bins (5 s duration) showing the time points for data analysis and blood sampling throughout the protocol of pLTF. White dashed line indicates the baseline. Note after the last hypoxia (Hx3) the gradual increase in phrenic nerve burst amplitude above baseline amplitude (i.e. LTF, top trace) compared to LPS treated rats. Hx = Hypoxia.

Figure 2. Phrenic nerve burst amplitude, frequency during baseline and first hypoxia from Sprague-Dawley and Lewis rats treated with 3 mg/kg LPS i.p. or vehicle

A. Phrenic nerve baseline burst amplitude for Sprague Dawley and Lewis rats with LPS or Vehicle. Note that the treatment has no effect on the baseline amplitude in both rat strains. There is a difference in amplitude between the rat strains. B. Phrenic nerve burst amplitude during the first hypoxic episode. Note that the LPS reduces in both rat strains the hypoxic response, and Lewis rat have a lower hypoxic response compared to Sprague Dawley rats. C. Phrenic nerve burst frequency during normoxia and the first hypoxia for LPS treated or Vehicle injected Sprague Dawley and Lewis rats. Note for the Lewis rats the absence of increase in burst frequency during hypoxia. LPS does not have any effect on the ventilatory response to hypoxia on both rat strains. a; p<0.05 vs. strain-matched baseline; b; p<0.05 vs. strain-matched vehicle; *; p<0.05 vs. Sprague treatment-matched.

Figure 3. Phrenic long term facilitation from Sprague-Dawley (SD) and Lewis rats treated with 3 mg/kg LPS i.p. or vehicle

A. Phrenic burst frequency during, baseline, 15 min, 30 min and 60 min post-hypoxia. Note the absence of statistical differences between all the treated/strain groups. B. Phrenic long-term facilitation time course in different treated groups. Note that LPS treated rats have a reduced pLTF expression at 60 min post-hypoxia compared to the vehicle treated rats. C. 60 min post-hypoxia pLTF for time control (which did not receive any hypoxia), Sprague Dawley and Lewis rats with/without LPS. Note that LPS induces a reduction of the pLTF magnitude at 60 min in Sprague Dawley, and Lewis rat with LPS do not express pLTF (compared to a time control and baseline). a; p<0.05 vs. strain-matched baseline; b; p<0.05 vs. strain-matched LPS; *; p<0.05 vs. time control.