Fentanyl activates opposing opioid and non-opioid receptor systems that control breathing

Abstract

Fentanyl elicits profound disturbances in ventilatory control processes in humans and experimental animals. The traditional viewpoint with respect to fentanyl-induced respiratory depression is that once the effects on the frequency of breathing (Freq), tidal volume (TV), and minute ventilation (MV = Freq × TV) are resolved, then depression of breathing is no longer a concern. The results of the present study challenge this concept with findings, as they reveal that while the apparent inhibitory effects of fentanyl (75 μg/kg, IV) on Freq, TV, and MV in adult male rats were fully resolved within 15 min, many other fentanyl-induced responses were in full effect, including opposing effects on respiratory timing parameters. For example, although the effects on Freq were resolved at 15 min, inspiratory duration (Ti) and end inspiratory pause (EIP) were elevated, whereas expiratory duration (Te) and end expiratory pause (EEP) were diminished. Since the effects of fentanyl on TV had subsided fully at 15 min, it would be expected that the administration of an opioid receptor (OR) antagonist would have minimal effects if the effects of fentanyl on this and other parameters had resolved. We now report that the intravenous injection of a 1.0 mg/kg dose of the peripherally restricted OR antagonist, methyl-naloxone (naloxone methiodide, NLXmi), did not elicit arousal but elicited some relatively minor changes in Freq, TV, MV, Te, and EEP but pronounced changes in Ti and EIP. In contrast, the injection of a 2.5 mg/kg dose of NLXmi elicited pronounced arousal and dramatic changes in many variables, including Freq, TV, and MV, which were not associated with increases in non-apneic breathing events such as apneas. The two compelling conclusions from this study are as follows: 1) the blockade of central ORs produced by the 2.5 mg/kg dose of NLXmi elicits pronounced increases in Freq, TV, and MV in rats in which the effects of fentanyl had apparently resolved, and 2) it is apparent that fentanyl had induced the activation of two systems with counter-balancing effects on Freq and TV: one being an opioid receptor inhibitory system and the other being a non-OR excitatory system.

Keywords: breathing; fentanyl; naloxone methiodide; opioid receptors; rats.

FIGURE 1

Time to recovery of the righting reflex following the bolus injection of vehicle (saline) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male and female rats. There were nine rats in each group. The data are presented as the mean ± SEM. *p < 0.05, significant Pre-values. ^† p < 0.05, NLXmi 2.5 versus NLXmi 1.0 or vehicle. ANOVA statistics for males: F_2,24 = 34.5; p < 0.0001. ANOVA statistics for females: F_2,24 = 48.5; p < 0.0001.

FIGURE 2

Frequency of breathing (Panel A), tidal volume (Panel B), and minute ventilation (Panel C) before (Pre) and after the injection of vehicle (saline) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male rats. There were six rats in each group. The data are presented as the mean ± SEM.

FIGURE 3

Frequency of breathing (Panel A), tidal volume (Panel B), and minute ventilation (Panel C) before (Pre) and after the injection of fentanyl (75 μg/kg, IV) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male rats. There were four rats in each group. The data are presented as the mean ± SEM.

FIGURE 4

Inspiratory time (Ti) (Panel A), expiratory time (Te) (Panel B), and Ti/Te (Panel C) before (Pre) and after the injection of fentanyl (75 μg/kg, IV) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male rats. There were four rats in each group. The data are presented as the mean ± SEM.

FIGURE 5

End inspiratory pause (Panel A) and end expiratory pause (Panel B) before (Pre) and after the injection of fentanyl (75 μg/kg, IV) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male rats. There were four rats in each group. The data are presented as the mean ± SEM.

FIGURE 6

Peak inspiratory flow (PIF) (Panel A), peak expiratory flow (PEF) (Panel B), PIF/PEF (Panel C), and expiratory flow at 50% expired tidal volume (EF₅₀) (Panel D) before (Pre) and after the injection of fentanyl (75 μg/kg, IV) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male rats. There were four rats in each group. The data are presented as the mean ± SEM.

FIGURE 7

Relaxation time (Panel A) and expiratory time–relaxation Time (Te-RT) (Panel B) before (Pre) and after the injection of fentanyl (75 μg/kg, IV) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male rats. There were four rats in each group. The data are presented as the mean ± SEM.

FIGURE 8

Inspiratory drive (tidal volume/inspiratory time, TV/Ti) (Panel A) and expiratory drive (tidal volume/expiratory time, TV/Te) (Panel B) before (Pre) and after the injection of fentanyl (75 μg/kg, IV) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male rats. There were four rats in each group. The data are presented as the mean ± SEM.

FIGURE 9

Non-eupneic breathing index (Panel A) and NEBI/frequency of breathing (Panel B) before (Pre) and after the injection of fentanyl (75 μg/kg, IV) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male rats. There were four rats in each group. The data are presented as the mean ± SEM.

FIGURE 10

Ventilatory parameters (expressed as % change from Pre-values) [panels (A,B)] at the 15-min time-point following the injection of fentanyl (75 μg/kg, IV) in male rats that subsequently received an injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV). There were four rats in each group. The data are presented as the mean ± SEM. *p < 0.05, significant change from Pre-values. There were no between-group differences for any parameter (p > 0.05 for all comparisons). ANOVA statistics: Freq: F_(2,9) = 0.29 and p = 0.756; TV: F_(2,9) = 0.49 and p = 0.628; MV: F_(2,9) = 0.14 and p = 0.875; Ti: F_(2,9) = 0.21 and p = 0.817; Te: F_(2,9) = 0.01 and p = 0.989; Ti/Te: F_(2,9) = 0.12 and p = 0.890; EIP: F_(2,9) = 4.68 and p = 0.040; EEP: F_(2,9) = 0.05 and p = 0.953; PIF: F_(2,9) = 0.79 and p = 0.484; PEF: F_(2,9) = 0.23 and p = 0.796; PIF/PEF: F_(2,9) = 0.28 and p = 0.760; EF₅₀: F_(2,9) = 0.20 and p = 0.827; RT: F_(2,9) = 5.18 and p = 0.032; Te-RT: F_(2,9) = 0.07 and p = 0.935; InspD: F_(2,9) = 0.65 and p = 0.546; ExpD: F_(2,9) = 0.16 and p = 0.852; NEBI: F_(2,9) = 2.56 and p = 0.132; NEBI/F: F_(2,9) = 2.12 and p = 0.177.

FIGURE 11

Cumulative responses (expressed as % change from Pre-values) [panels (A,B)] recorded over the 10-min period following the injection vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in male rats that received a prior injection of fentanyl (75 μg/kg, IV). There were four rats in each group. The data are presented as the mean ± SEM. *p < 0.05, significant change from Pre-values. ^† p < 0.05, NLXmi 1.0 or 2.5 versus vehicle. ^‡ p < 0.05, NLXmi 2.5 versus NLXmi 1.0. ANOVA statistics: Freq: F_(2,9) = 63.9 and p = 0.000; TV: F_(2,9) = 4.6 and p = 0.042; MV: F_(2,9) = 65.0 and p = 0.000; Ti: F_(2,9) = 9.3 and p = 0.007; Te: F_(2,9) = 15.6 and p = 0.001; Ti/Te: F_(2,9) = 1.9 and p = 0.203; EIP: F_(2,9) = 23.8 and p = 0.000; EEP: F_(2,9) = 1.9 and p = 0.207; PIF: F_(2,9) = 27.4 and p = 0.000; PEF: F_(2,9) = 11.0 and p = 0.004; PIF/PEF: F_(2,9) = 1.0 and p = 0.398; EF₅₀: F_(2,9) = 9.2 and p = 0.007; RT: F_(2,9) = 6.3 and p = 0.020; Te-RT: F_(2,9) = 1.2 and p = 0.358; InspD: F_(2,9) = 34.4 and p = 0.000; ExpD: F_(2,9) = 11.2 and p = 0.004; NEBI: F_(2,9) = 7.8 and p = 0.011; NEBI/F: F_(2,9) = 4.1 and p = 0.550.

FIGURE 12

Frequency of breathing (Panel A), tidal volume (Panel B), and minute ventilation (Panel C) before (Pre) and after the injection of fentanyl (75 μg/kg, IV) and subsequent injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in female rats. The data in Panel D show the cumulative responses (expressed as % change from Pre-values) recorded over the 10-min period following the injection of vehicle or NLXmi (1.0 or 2.5 mg/kg, IV) in female rats that received a prior injection of fentanyl (75 μg/kg, IV). There were four rats in each group. The data are presented as the mean ± SEM. *p < 0.05, significant change from Pre-values. ^† p < 0.05, NLXmi 2.5 versus NLXmi 1.0 or vehicle.