Systemic Administration of Tempol Attenuates the Cardiorespiratory Depressant Effects of Fentanyl

Abstract

Fentanyl is a high-potency opioid receptor agonist that elicits profound analgesia and suppression of breathing in humans and animals. To date, there is limited evidence as to whether changes in oxidant stress are important factors in any of the actions of acutely administered fentanyl. This study determined whether the clinically approved superoxide dismutase mimetic, Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl), or a potent antioxidant, N-acetyl-L-cysteine methyl ester (L-NACme), modify the cardiorespiratory and analgesic actions of fentanyl. We examined whether the prior systemic injection of Tempol or L-NACme affects the cardiorespiratory and/or analgesic responses elicited by the subsequent injection of fentanyl in isoflurane-anesthetized and/or freely moving male Sprague-Dawley rats. Bolus injections of Tempol (25, 50 or 100 mg/kg, IV) elicited minor increases in frequency of breathing, tidal volume and minute ventilation. The ventilatory-depressant effects of fentanyl (5 μg/kg, IV) given 15 min later were dose-dependently inhibited by prior injections of Tempol. Tempol elicited dose-dependent and transient hypotension that had (except for the highest dose) resolved when fentanyl was injected. The hypotensive responses elicited by fentanyl were markedly blunted after Tempol pretreatment. The analgesic actions of fentanyl (25 μg/kg, IV) were not affected by Tempol (100 mg/kg, IV). L-NACme did not modify any of the effects of fentanyl. We conclude that prior administration of Tempol attenuates the cardiorespiratory actions of fentanyl without affecting the analgesic effects of this potent opioid. As such, Tempol may not directly affect opioid-receptors that elicit the effects of fentanyl. Whether, the effects of Tempol are solely due to alterations in oxidative stress is in doubt since the powerful antioxidant, L-NACme, did not affect fentanyl-induced suppression of breathing.

Tempol Reverses the Negative Effects of Morphine on Arterial Blood-Gas Chemistry and Tissue Oxygen Saturation in Freely-Moving Rats

“…A major finding of the present study was that the intravenous injection of Tempol and especially at a 100 mg/kg dose was able to immediately reverse the pronounced negative effects of a 10 mg/kg intravenous dose of morphine on ABG chemistry, A-a gradient and SpO 2 in the freely-moving rats. These findings complement our earlier report that pretreatment with a 100 mg/kg dose of Tempol markedly blunted the deleterious changes in ABG chemistry, A-a gradient and ventilatory parameters (e.g., frequency of breathing, tidal volume and minute ventilation) elicited by the subsequent injection of fentanyl in isoflurane-anesthetized rats (Baby et al, 2021). The present study did not directly address the mechanisms underlying the ability of Tempol to reverse the negative effects of morphine on ventilatory control processes pertinent to our study, however, the rapid and sustained effects of Tempol on ABG chemistry and SpO 2 along with the relatively transient effects on A-a gradient (improved gas exchange in the lungs of morphine-treated rats) certainly suggests that the primary reason for the improved status of the ABG chemistry is due to the ability of Tempol to increase ventilatory performance (e.g., enhanced minute ventilation and inspiratory drive) in the morphine-treated rats.…”

“…In summary, the present study extends our findings regarding the beneficial effects of Tempol against the negative effects of fentanyl on breathing, ABG chemistry and gas-exchange in the lungs, and the potential problems regarding the ability of Tempol to reduce arterial blood pressure (Baby et al, 2021). Previous studies have demonstrated the beneficial effects of Tempol in cell and animal models of neurodegenerative diseases, shock, hypertension, diabetes, ischemia-reperfusion injury, traumatic brain injury, tumororigenesis, chemotherapy-induced neuropathic pain and alopecia (Wilcox and Pearlman, 2008;Wilcox, 2010;Kim et al, 2016;Bernardy et al, 2017;Wang et al, 2018;Afjal et al, 2019;Chiarotto et al, 2019).…”

“…The findings that Tempol reverses the negative effects of morphine on ABG chemistry and SpO 2 in freely-moving Sprague Dawley rats is an important addition to the development of drugs to combat opioid-induced depression of respiratory and cardiovascular systems. From this and our earlier study (Baby et al, 2021) we now know that 1) Tempol is efficacious as a pretreatment against fentanyl (Baby et al, 2021), 2) Tempol is a reversal agent against morphine (present study), 3) Tempol is effective in isoflurane-anesthetized rats (Baby et al, 2021) and in freely-moving rats (present study), and 4) Tempol is efficacious against both fentanyl and morphine.…”

“…the efficacy profile of Tempol. Additionally in our Tempol studies we have not established any of the molecular mechanisms by which readily cell-penetrant Tempol prevents and reverses the negative effects of fentanyl (Baby et al, 2021) and morphine (present study) on cardiorespiratory function, although published data suggests that it is unlikely to involve modulation of BK Ca channels (Baby et al, 2021), or direct blockade of opioid receptors on the basis that Tempol spares morphine analgesia (Baby et al, 2021). With respect to potential plasma membrane and intracellular targets for Tempol, our collaborator Dr. Christopher Ellis (DEVCOM Chemical Biological Center, US Army) is employing phase (Public Health Assessment via Structural Evaluation) technology (Ellis et al, 2019) and molecular docking methods (Ellis et al, 2018) to identify protein sites (e.g., G-protein-coupled receptors, ionchannels, and membrane and intracellular enzymes) to which Tempol, morphine and fentanyl bind to, with emphasis on the signaling proteins/pathways most relevant to opioid-induced cardiorespiratory depression (Ellis et al, 2018).…”

“…We recently reported that l -cysteine ethyl ester ( Mendoza et al, 2013 ), glutathione ethyl ester ( Jenkins et al, 2021 ), and d -cystine methyl and ethyl esters ( Gaston et al, 2021 ) rapidly reverse the negative effects of opioids, such as morphine and fentanyl, on breathing, Alveolar-arterial gradient (i.e., the index of gas exchange within the lungs) and arterial blood-gas (ABG) chemistry while not reducing opioid analgesia. Since these reduced and oxidized thiol esters block the negative effects of opioids on breathing and because the potent reducing agent N-acetyl- l -cysteine methyl ester does not affect opioid-induced depression of breathing ( Baby et al, 2021 , it is unlikely that the thiol esters exert their effects by altering the redox state of cells involved in the effects of opioids on breathing. In support of this, there is no real consensus as to whether opioids induce oxidative stress.…”