D-Cysteine Ethyl Ester Reverses the Deleterious Effects of Morphine on Breathing and Arterial Blood–Gas Chemistry in Freely-Moving Rats

Abstract

Cell-penetrant thiol esters including the disulfides, D-cystine diethyl ester and D-cystine dimethyl ester, and the monosulfide, L-glutathione ethyl ester, prevent and/or reverse the deleterious effects of opioids, such as morphine and fentanyl, on breathing and gas exchange within the lungs of unanesthetized/unrestrained rats without diminishing the antinociceptive or sedative effects of opioids. We describe here the effects of the monosulfide thiol ester, D-cysteine ethyl ester (D-CYSee), on intravenous morphine-induced changes in ventilatory parameters, arterial blood–gas chemistry, alveolar–arterial (A-a) gradient (i.e., index of gas exchange in the lungs), and sedation and antinociception in freely-moving rats. The bolus injection of morphine (10 mg/kg, IV) elicited deleterious effects on breathing, including depression of tidal volume, minute ventilation, peak inspiratory flow, and inspiratory drive. Subsequent injections of D-CYSee (2 × 500 μmol/kg, IV, given 15 min apart) elicited an immediate and sustained reversal of these effects of morphine. Morphine (10 mg/kg, IV) also A-a gradient, which caused a mismatch in ventilation perfusion within the lungs, and elicited pronounced changes in arterial blood–gas chemistry, including pronounced decreases in arterial blood pH, pO2 and sO2, and equally pronounced increases in pCO2 (all responses indicative of decreased ventilatory drive). These deleterious effects of morphine were immediately reversed by the injection of a single dose of D-CYSee (500 μmol/kg, IV). Importantly, the sedation and antinociception elicited by morphine (10 mg/kg, IV) were minimally affected by D-CYSee (500 μmol/kg, IV). In contrast, none of the effects of morphine were affected by administration of the parent thiol, D-cysteine (1 or 2 doses of 500 μmol/kg, IV). Taken together, these data suggest that D-CYSee may exert its beneficial effects via entry into cells that mediate the deleterious effects of opioids on breathing and gas exchange. Whether D-CYSee acts as a respiratory stimulant or counteracts the inhibitory actions of µ-opioid receptor activation remains to be determined. In conclusion, D-CYSee and related thiol esters may have clinical potential for the reversal of the adverse effects of opioids on breathing and gas exchange, while largely sparing antinociception and sedation.

Heroin- and Fentanyl-Induced Respiratory Depression in a Rat Plethysmography Model: Potency, Tolerance, and Sex Differences

“…Of the eight calculated parameters, we measured, we chose the following to be presented here: minute ventilation, tidal volume, inspiratory time, peak inspiratory flow and apneic pause (calculated as: (expiratory time/relaxation time) -1). These parameters are those that give a fuller understanding of the effects of the opioids on ventilatory timing and mechanics (Getsy, Baby, Gruber, Gaston, et al, 2022;Getsy, Baby, May, Young, et al, 2022;Getsy, Young, Grossfield, Seckler, et al, 2022;Lewis et al, 2022;Seckler et al, 2022). Minute ventilation provides a general assessment of how opioids affect the total volume of air breathed per minute and is the multiplicative index of frequency of breathing and tidal volume.…”

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

“…The changes in Freq and TV resulted in a prompt increase in MV of approximately 25–30 min in duration. Note that the minor degree of variability in baseline (pre-fentanyl) levels of Freq and TV that resulted in a consistent level of MV is to be expected when recording values from freely-moving rats ( Henderson et al, 2013 ; 2014 ; Gaston et al, 2020; Getsy et al, 2014 ; Gaston et al, 2021 ; Jenkins et al, 2021 ; Getsy et al, 2022a ; Getsy et al, 2022b ; Getsy et al, 2022c ; Getsy et al, 2022d ; Getsy et al, 2022e ; Lewis et al, 2022 ; Seckler et al, 2022 ).…”

“…The question arises as to whether this arousal is directly linked to the observed ventilatory responses that will be addressed below. The two processes may not be linked since the injection of the 2.5 mg/kg dose of NLXmi caused pronounced ventilatory responses in fentanyl-treated rats, whereas these ventilatory responses were not associated with increases in either NEBI or NEBI/Freq, which are hallmarks of ventilatory instability ( Getsy et al, 2014 ; Gaston et al, 2021 ; Getsy et al, 2022c ; Getsy et al, 2022d ; Getsy et al, 2022e ; Lewis et al, 2022 ). More specifically, it appeared that the 2.5 mg/kg dose of NLXmi merely restored the behavioral status of the fentanyl-treated rats but caused unexpectedly dramatic changes in ventilation, which suggests the existence of an inhibitory opioid receptor system in ventilatory control pathways.…”

“…The reporting of Ti and Te and respiratory quotient (Ti/Te) is much less frequent, despite their obvious importance to understanding how inspiratory and expiratory networks are affected by drugs. Moreover, the reporting of EIP and EEP is rare, even though these parameters provide key information as to how drugs such as opioids affect predominantly brainstem mechanisms controlling pauses between inspiration and expiration ( Getsy et al, 2014 ; Gaston et al, 2021 ; Getsy et al, 2022c ; d ,e; Lewis et al, 2022 ). Many other key parameters, such as inspiratory and expiratory drives, PIF, PEF, EF 50 , relaxation time, expiratory delay (Te-RT), and NEBI, are similarly underused parameters of value to the understanding of respiratory physiology and the effects of drugs and disease processes on ventilatory processes.…”

“…The administration of fentanyl caused the rats to assume numerous types of postures, including being motionless, sprawled out on their stomach on the chamber floor, lying motionless on their side, and splayed out on their stomach with the head up against the chamber wall. The duration of the fentanyl sedation was taken as the time to full recovery of the righting reflex (i.e., when rats attained and maintained a normal posture on all four legs) following the injections of vehicle or NLXmi ( Gaston et al, 2021 ; Getsy et al, 2022c ; Getsy et al, 2022d ; Getsy et al, 2022e ; Lewis et al, 2022 ). Male rats: one group (304 ± 2 g in body weight, n = 9) received an injection of fentanyl (75 μg/kg, IV), followed by an injection of vehicle (saline) after 15 min.…”