L-cysteine methyl ester overcomes the deleterious effects of morphine on ventilatory parameters and arterial blood-gas chemistry in unanesthetized rats

Abstract

We are developing a series of thiolesters that produce an immediate and sustained reversal of the deleterious effects of opioids, such as morphine and fentanyl, on ventilation without diminishing the antinociceptive effects of these opioids. We report here the effects of systemic injections of L-cysteine methyl ester (L-CYSme) on morphine-induced changes in ventilatory parameters, arterial-blood gas (ABG) chemistry (pH, pCO2, pO2, sO2), Alveolar-arterial (A-a) gradient (i.e., the index of alveolar gas-exchange within the lungs), and antinociception in unanesthetized Sprague Dawley rats. The administration of morphine (10 mg/kg, IV) produced a series of deleterious effects on ventilatory parameters, including sustained decreases in tidal volume, minute ventilation, inspiratory drive and peak inspiratory flow that were accompanied by a sustained increase in end inspiratory pause. A single injection of L-CYSme (500 μmol/kg, IV) produced a rapid and long-lasting reversal of the deleterious effects of morphine on ventilatory parameters, and a second injection of L-CYSme (500 μmol/kg, IV) elicited pronounced increases in ventilatory parameters, such as minute ventilation, to values well above pre-morphine levels. L-CYSme (250 or 500 μmol/kg, IV) also produced an immediate and sustained reversal of the deleterious effects of morphine (10 mg/kg, IV) on arterial blood pH, pCO2, pO2, sO2 and A-a gradient, whereas L-cysteine (500 μmol/kg, IV) itself was inactive. L-CYSme (500 μmol/kg, IV) did not appear to modulate the sedative effects of morphine as measured by righting reflex times, but did diminish the duration, however, not the magnitude of the antinociceptive actions of morphine (5 or 10 mg/kg, IV) as determined in tail-flick latency and hindpaw-withdrawal latency assays. These findings provide evidence that L-CYSme can powerfully overcome the deleterious effects of morphine on breathing and gas-exchange in Sprague Dawley rats while not affecting the sedative or early stage antinociceptive effects of the opioid. The mechanisms by which L-CYSme interferes with the OR-induced signaling pathways that mediate the deleterious effects of morphine on ventilatory performance, and by which L-CYSme diminishes the late stage antinociceptive action of morphine remain to be determined.

The ventilatory depressant actions but not the antinociceptive effects of morphine are blunted in rats receiving intravenous infusion of L-cysteine ethyl ester

“…Adult male Sprague Dawley rats were purchased from Harlan Industries (Madison, WI, USA). After five days of recovery from transportation, the rats received femoral artery catheters and/or two jugular vein catheters under 2–3% isoflurane anesthesia [ 10 – 14 , 23 ]. One of the jugular catheters was to allow for continuous infusion of L-CYSee or L-SERee and the other was to allow bolus injections of morphine (10 mg/kg, IV).…”

“…Factoring in chamber humidity and temperature, cycle analyzers filtered the acquired signals, and FinePointe algorithms generated an array of box flow data that identified waveform segments as acceptable breaths, and minimum and maximum values were determined. Flows at this point were box-flow signals and from this array, minimum and maximum box flow values were then determined and multiplied by a compensation factor from the algorithms [ 24 , 25 ] thus producing TV, PIF, PEF, and EF 50 values used to determine non-eupneic breathing events expressed as NEBI, reported as the percentage of non-eupneic breathing events per each individual epoch [ 10 – 14 , 23 , 26 ]. Apneic pause was determined by the formula, (Te/RT) - 1 [ 10 – 14 , 23 ].…”

“…Flows at this point were box-flow signals and from this array, minimum and maximum box flow values were then determined and multiplied by a compensation factor from the algorithms [ 24 , 25 ] thus producing TV, PIF, PEF, and EF 50 values used to determine non-eupneic breathing events expressed as NEBI, reported as the percentage of non-eupneic breathing events per each individual epoch [ 10 – 14 , 23 , 26 ]. Apneic pause was determined by the formula, (Te/RT) - 1 [ 10 – 14 , 23 ].…”

“…The above samples of arterial blood were injected into a Radiometer blood-gas analyzer (ABL800 FLEX) to obtain ABG values. The A-a gradient defines the differences between alveolar O 2 and arterial blood O 2 concentrations (see 10 – 14 , 23 ). A fall in PaO 2 , without change in A-a gradient, is due to hypoventilation, whereas a decrease in PaO 2 with a concomitant elevation in A-a gradient indicates an ongoing mismatch in ventilation-perfusion in alveoli [ 10 – 14 , 23 ].…”

“…Membrane-permeable thioesters such as L-cysteine ethyl ester (L-CYSee), D-cysteine ethyl ester (D-CYSee), L-cysteine methyl ester (L-CYSme), L-glutathione ethyl ester (L-GSHee), D-cystine dimethyl ester (D-CYSdime) and D-cystine diethyl ester (D-CYSdiee) readily gain access to central and peripheral tissues and have diverse pharmacological actions [ 9 – 14 ]. We have reported that these L-, D-thioesters prevent and/or reverse the adverse effects of opioids such as morphine and fentanyl on ventilation, arterial blood-gas (ABG) chemistry, and Alveolar-arterial (A-a) gradient (index of alveolar gas exchange in the lungs) in unanesthetized freely-moving rats whereas L- and D-thioesters minimally impact the sedative or analgesic effects of the opioids [ 10 – 14 ]. In contrast, the parent thiols, L- or D-cysteine, L-glutathione, and D-cystine, only minimally altered the effects of the opioids [ 9 – 14 ].…”