Repeated intravenous doxapram induces phrenic motor facilitation

Abstract

Doxapram is a respiratory stimulant used to treat hypoventilation. Here we investigated whether doxapram could also trigger respiratory neuroplasticity. Specifically, we hypothesized that intermittent delivery of doxapram at low doses would lead to long-lasting increases (i.e., facilitation) of phrenic motor output in anesthetized, vagotomized, and mechanically-ventilated rats. Doxapram was delivered intravenously in a single bolus (2 or 6mg/kg) or as a series of 3 injections (2mg/kg) at 5min intervals. Control groups received pH-matched saline injections (vehicle) or no treatment (anesthesia time control). Doxapram evoked an immediate increase in phrenic output in all groups, but a persistent increase in burst amplitude only occurred after repeated dosing with 2mg/kg. At 60min following the last injection, phrenic burst amplitude was 168±24% of baseline (%BL) in the group receiving 3 injections (P<0.05 vs. controls), but was 103±8%BL and 112±4%BL in the groups receiving a single dose of 2 or 6mg/kg, respectively. Following bilateral section of the carotid sinus nerves, the acute phrenic response to doxapram (2mg/kg) was reduced by 68% suggesting that at low doses the drug was acting primarily via the carotid chemoreceptors. We conclude that intermittent application of doxapram can trigger phrenic neuroplasticity, and this approach might be of use in the context of respiratory rehabilitation following neurologic injury.

Keywords: Doxapram; Phrenic motor facilitation; Respiratory neuroplasticity.

Figure 1

Representative example illustrating the acute impact of intravenously delivered doxapram (2 mg/kg) on phrenic nerve activity. The top panel shows the moving averaged or “integrated” phrenic signal (∫Phr, mV). The bottom panel shows the expanded traces for the time points indicated by (i) and (ii) as wells as the unprocessed or “raw” signal (Phr) and corresponding blood pressure trace (mmHg). Delivery of doxapram (indicated by an arrow) caused an immediate increase in phrenic burst amplitude, frequency and MAP lasting several minutes.

Figure 2

Mean ∫Phr amplitude and burst frequency immediately following doxapram injection. Data are presented from five experimental groups: intermittent doxapram (2 mg/kg), single doxapram (2 or 6 mg/kg), vehicle injection and time control. The plots are labeled “1, 2 or 3” to correspond to the intermittent doxapram injections. In the four groups that did not receive intermittent injections, measures of phrenic activity were taken at a comparable time point. All of the doxapram treated groups showed a similar increase in phrenic output during period 1. A progressive augmentation of ∫Phr amplitude occurred in rats receiving intermittent doxapram injections. *, significantly higher vs. single doxapram (2mg/kg and 6mg/kg), vehicle injections and time control; #, significantly different vs. doxapram injections; ‡, significantly different than injections 2 and 3, or comparable time points.

Figure 3

Representative examples showing the phrenic motor response to three successive injections of doxapram at 2 mg/kg (A) or a single bolus of doxapram at 6 mg/kg (B). The panels depict the moving averaged or “integrated” phrenic signal (∫Phr, mV) and the breath-by-breath inspiratory burst frequency (breaths per minute). Data are shown during the baseline period, doxapram injections and an approximately 1 hour period following the final injection. The bottom panels show expanded traces for the time points indicated by i - iv. The calibration bars for blood pressure (BP) represent mmHg.

Figure 4

Mean phrenic nerve activity at 30 and 60 min following doxapram. Data are presented from five experimental groups: intermittent doxapram (2 mg/kg), single doxapram (2 or 6 mg/kg), vehicle injection and time control. Mean ∫Phr amplitude and burst frequency are shown relative to baseline (% baseline). #, significantly higher than vehicle injection and time control groups; *, significantly higher than all other groups.

Figure 5

Mean phrenic response to doxapram in animals that received bilateral carotid sinus nerve sectioning (CSNX). ∫Phr amplitude and burst frequency are shown relative to baseline (% baseline). *, significantly higher vs. 2mg/kg and 4mg/kg doxapram doses; #, significantly different vs. all groups with CSNX.