Multiple opioid receptors mediate the respiratory depressant effects of fentanyl-like drugs in the rat

Tropine exacerbates the ventilatory depressant actions of fentanyl in freely-moving rats

Our lab is investigating the efficacy profiles of tropine analogs against opioid-induced respiratory depression. The companion manuscript reports that the cell-permeant tropeine, tropine ester (Ibutropin), produces a rapid and sustained reversal of the deleterious actions of fentanyl on breathing, alveolar-arterial (A-a) gradient (i.e., index of alveolar gas exchange), and arterial blood-gas (ABG) chemistry in freely-moving male Sprague Dawley rats, while not compromising fentanyl analgesia. We report here that in contrast to Ibutropin, the injection of the parent molecule, tropine (200 μmol/kg, IV), worsens the adverse actions of fentanyl (75 μg/kg, IV) on ventilatory parameters (e.g., frequency of breathing, tidal volume, minute ventilation, peak inspiratory and expiratory flows, and inspiratory and expiratory drives), A-a gradient, ABG chemistry (e.g., pH, pCO2, pO2, and sO2), and sedation (i.e., the righting reflex), while not affecting fentanyl antinociception (i.e., the tail-flick latency) in freely-moving male Sprague Dawley rats. These data suggest that tropine augments opioid receptor-induced signaling events that mediate the actions of fentanyl on breathing and alveolar gas exchange. The opposite effects of Ibutropin and tropine may result from the ability of Ibutropin to readily enter peripheral and central cells. Of direct relevance is that tropine, resulting from the hydrolysis of Ibutropin, would combat the Ibutropin-induced reversal of the adverse effects of fentanyl. Because numerous drug classes, such as cocaine, atropine, and neuromuscular blocking drugs contain a tropine moiety, it is possible that their hydrolysis to tropine has unexpected/unintended consequences. Indeed, others have found that tropine exerts the same behavioral profile as cocaine upon central administration. Together, these data add valuable information about the pharmacological properties of tropine.

Reinforcing effects of fentanyl and sufentanil aerosol puffs in rats

RATIONALE

Rapidly evolving e-cigarette technology developed for self-administering nicotine aerosol has the potential to be utilized to self-administer other aerosolized drugs of abuse. Rodent models which mirror characteristics of human e-cigarette use are necessary to explore the degree to which this may be a public health concern.

OBJECTIVES

Our goal was to develop a highly translational model of discrete nose-only aerosol puff drug delivery to explore the reinforcing effects of fentanyl and sufentanil aerosols in rats.

METHODS

Male and female Sprague-Dawley rats were trained to perform a multiple schedule FR1 lever-press, 4-s (second) nose hold operant during which the subject's orofacial areas were exposed to drug-free glycerol/propylene glycol aerosol produced by a commercial e-cigarette at a power setting of 18 watts. Each completed 4-s drug-free vehicle aerosol exposure resulted in a 3-s presentation of a 0.1-ml dipper of sweetened milk solution. After training, rats were then allowed to self-administer 4-s nose-only puffs of fentanyl (100-6000 µg/ml) or sufentanil (30-500 µg/ml) aerosol in the absence of paired milk dipper reinforcers.

RESULTS

All 31 rats learned the lever-press/nose-poke multiple schedule for milk dippers alone and 25 accepted exposure to 4 s of 18 watts of drug-free vehicle aerosol when paired with milk dipper presentations. In the absence of paired milk dipper presentations, fentanyl aerosol puffs at concentrations of 1000 and 3000 µg/ml as well as 100 µg/ml puffs of sufentanil served as reinforcers compared to both air puffs and drug-free vehicle aerosol puffs. There were no significant differences between males and females in number of fentanyl or sufentanil puffs self-administered.

CONCLUSIONS

Discrete nose-only puffs of two potent opioids under exposure conditions comparable to puff durations in human e-cigarette users serve as reinforcers in rats. This outcome suggests that under appropriate conditions e-cigarettes might be a potential alternative delivery mechanism for illicit opioids.

S-Nitroso-L-Cysteine Stereoselectively Blunts the Deleterious Effects of Fentanyl on Breathing While Augmenting Antinociception in Freely-Moving Rats

Endogenous and exogenously administered S-nitrosothiols modulate the activities of central and peripheral systems that control breathing. We have unpublished data showing that the deleterious effects of morphine on arterial blood-gas chemistry (i.e., pH, pCO2, pO2, and sO2) and Alveolar-arterial gradient (i.e., index of gas exchange) were markedly diminished in anesthetized Sprague Dawley rats that received a continuous intravenous infusion of the endogenous S-nitrosothiol, S-nitroso-L-cysteine. The present study extends these findings by showing that unanesthetized adult male Sprague Dawley rats receiving an intravenous infusion of S-nitroso-L-cysteine (100 or 200 nmol/kg/min) markedly diminished the ability of intravenous injections of the potent synthetic opioid, fentanyl (10, 25, and 50 μg/kg), to depress the frequency of breathing, tidal volume, and minute ventilation. Our study also found that the ability of intravenously injected fentanyl (10, 25, and 50 μg/kg) to disturb eupneic breathing, which was measured as a marked increase of the non-eupneic breathing index, was substantially reduced in unanesthetized rats receiving intravenous infusions of S-nitroso-L-cysteine (100 or 200 nmol/kg/min). In contrast, the deleterious effects of fentanyl (10, 25, and 50 μg/kg) on frequency of breathing, tidal volume, minute ventilation and non-eupneic breathing index were fully expressed in rats receiving continuous infusions (200 nmol/kg/min) of the parent amino acid, L-cysteine, or the D-isomer, namely, S-nitroso-D-cysteine. In addition, the antinociceptive actions of the above doses of fentanyl as monitored by the tail-flick latency assay, were enhanced by S-nitroso-L-cysteine, but not L-cysteine or S-nitroso-D-cysteine. Taken together, these findings add to existing knowledge that S-nitroso-L-cysteine stereoselectively modulates the detrimental effects of opioids on breathing, and opens the door for mechanistic studies designed to establish whether the pharmacological actions of S-nitroso-L-cysteine involve signaling processes that include 1) the activation of plasma membrane ion channels and receptors, 2) selective intracellular entry of S-nitroso-L-cysteine, and/or 3) S-nitrosylation events. Whether alterations in the bioavailability and bioactivity of endogenous S-nitroso-L-cysteine is a key factor in determining the potency/efficacy of fentanyl on breathing is an intriguing question.

Nitrosyl factors play a vital role in the ventilatory depressant effects of fentanyl in unanesthetized rats

There is an urgent need to understand the intracellular mechanisms by which synthetic opioids, such as fentanyl, depress breathing. We used L-NAME (N^G-nitro-L-arginine methyl ester), a nitric oxide synthase (NOS) inhibitor, to provide evidence for a role of nitric oxide (NO) and nitrosyl factors, including S-nitrosothiols, in fentanyl-induced suppression of breathing in rats. We measured breathing parameters using unrestrained plethysmography to record the changes produced by bolus administration of fentanyl (25 μg/kg, IV) in male Sprague Dawley rats that were pretreated with vehicle (saline), L-NAME (50 μmol/kg, IV) or the inactive D-isomer, D-NAME (50 μmol/kg, IV), 15 min previously. L-NAME produced a series of ventilatory changes that included (i) sustained elevations in breathing frequency, due to the reductions in the durations of inspiration and expiration, (ii) sustained elevations in minute ventilation, accompanied by minimal changes in tidal volume, and (iii) increases in inspiratory drive and expiratory drive, and peak inspiratory flow and peak expiratory flow. Subsequent administration of fentanyl in rats pretreated with vehicle produced negative effects on breathing, including decreases in frequency, tidal volume and therefore minute ventilation. Fentanyl elicited markedly different responses in rats that were pretreated with L-NAME, and conclusively, the negative effects of fentanyl were augmented by the NOS inhibitor. D-NAME did not alter ventilatory parameters or modulate the effects of fentanyl on breathing. Our study fully characterized the effects of L-NAME on ventilation in rats and is the first to suggest a potential role of nitrosyl factors in the ventilatory responses to fentanyl. Our data shows that nitrosyl factors reduce the expression of fentanyl-induced changes in ventilation.

Systemic Administration of Tempol Attenuates the Cardiorespiratory Depressant Effects of Fentanyl

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.

Glutathione ethyl ester reverses the deleterious effects of fentanyl on ventilation and arterial blood-gas chemistry while prolonging fentanyl-induced analgesia

There is an urgent need to develop novel compounds that prevent the deleterious effects of opioids such as fentanyl on minute ventilation while, if possible, preserving the analgesic actions of the opioids. We report that L-glutathione ethyl ester (GSHee) may be such a novel compound. In this study, we measured tail flick latency (TFL), arterial blood gas (ABG) chemistry, Alveolar-arterial gradient, and ventilatory parameters by whole body plethysmography to determine the responses elicited by bolus injections of fentanyl (75 μg/kg, IV) in male adult Sprague-Dawley rats that had received a bolus injection of GSHee (100 μmol/kg, IV) 15 min previously. GSHee given alone had minimal effects on TFL, ABG chemistry and A-a gradient whereas it elicited changes in some ventilatory parameters such as an increase in breathing frequency. In vehicle-treated rats, fentanyl elicited (1) an increase in TFL, (2) decreases in pH, pO₂ and sO₂ and increases in pCO₂ (all indicative of ventilatory depression), (3) an increase in Alveolar-arterial gradient (indicative of a mismatch in ventilation-perfusion in the lungs), and (4) changes in ventilatory parameters such as a reduction in tidal volume, that were indicative of pronounced ventilatory depression. In GSHee-pretreated rats, fentanyl elicited a more prolonged analgesia, relatively minor changes in ABG chemistry and Alveolar-arterial gradient, and a substantially milder depression of ventilation. GSHee may represent an effective member of a novel class of thiolester drugs that are able to prevent the ventilatory depressant effects elicited by powerful opioids such as fentanyl and their deleterious effects on gas-exchange in the lungs without compromising opioid analgesia.

Respiratory depression and brain hypoxia induced by opioid drugs: Morphine, oxycodone, heroin, and fentanyl

Opioid drugs are important tools to alleviate pain of different origins, but they have strong addictive potential and their abuse at higher doses often results in serious health complications. Respiratory depression that leads to brain hypoxia is perhaps the most dangerous symptom of acute intoxication with opioids, and it could result in lethality. The development of substrate-specific sensors coupled with amperometry made it possible to directly evaluate physiological and drug-induced fluctuations in brain oxygen levels in awake, freely-moving rats. The goal of this review paper is to consider changes in brain oxygen levels induced by several opioid drugs (heroin, fentanyl, oxycodone, morphine). While some of these drugs are widely used in clinical practice, they all are abused, often at doses exceeding the clinical range and often resulting in serious health complications. First, we consider some basic knowledge regarding brain oxygen, its physiological fluctuations, and mechanisms involved in regulating its entry into brain tissue. Then, we present and discuss data on brain oxygen changes induced by each opioid drug within a wide range of doses, from low, behaviorally relevant, to high, likely to be self-administered by drug users. These data allowed us to compare the effects of these drugs on brain oxygen in terms of their potency, time-course, and their potential danger when used at high doses via rapid-onset administration routes. While most data discussed in this work were obtained in rats, we believe that these data have clear human relevance in addressing the alarming rise in lethality associated with the opioid abuse.

Central and Peripheral Mechanisms Underlying Physiological and Drug-Induced Fluctuations in Brain Oxygen in Freely-Moving Rats

The goal of this work is to consider physiological fluctuations in brain oxygen levels and its changes induced by opioid drugs. This review article presents, as a comprehensive story, the most important findings obtained in our laboratory by using high-speed amperometry with oxygen sensors in awake, freely moving rats; most of these findings were separately published elsewhere. First, we show that oxygen levels in the nucleus accumbens (NAc) phasically increase following exposure to natural arousing stimuli. Since accumbal neurons are excited by arousing stimuli and NAc oxygen levels increase following glutamate (GLU) microinjections in the NAc, local neural activation with subsequent cerebral vasodilation appears to mediate the rapid oxygen increases induced by arousing stimuli. While it is established that intra-cerebral entry of oxygen depends on brain metabolism, physiological increases in NAc oxygen occurred more rapidly than increases in metabolic activity as assessed by intra-brain heat production. Therefore, due to neural activation and the subsequent rise in local cerebral blood flow (CBF), the brain receives more oxygen in advance of its metabolic requirement, thus preventing potential metabolic deficits. In contrast to arousing stimuli, three opioid drugs tested (heroin, fentanyl and oxycodone) decrease oxygen levels. As confirmed by our recordings in the subcutaneous space, a densely vascularized location with no metabolic activity of its own, these decreases result from respiratory depression with subsequent fall in blood oxygen levels. While respiratory depression was evident for all tested drugs, heroin was ~6-fold more potent than oxycodone, and fentanyl was 10-20-fold more potent than heroin. Changes in brain oxygen induced by respiratory depression appear to be independent of local vascular and blood flow responses, which are triggered, via neuro-vascular coupling, by the neuronal effects of opioid drugs.

Rapid Physiological Fluctuations in Nucleus Accumbens Oxygen Levels Induced by Arousing Stimuli: Relationships with Changes in Brain Glucose and Metabolic Neural Activation

Proper entry of oxygen from arterial blood into the brain is essential for maintaining brain metabolism under normal conditions and during functional neural activation. However, little is known about physiological fluctuations in brain oxygen and their underlying mechanisms. To address this issue, we employed high-speed amperometry with platinum oxygen sensors in freely moving male rats. Recordings were conducted in the nucleus accumbens (NAc), a critical structure for sensorimotor integration. Rats were exposed to arousing stimuli of different nature (brief auditory tone, a 1-min novel object presentation, a 3-min social interaction with a conspecific, and a 3-min tail-pinch). We found that all arousing stimuli increased NAc oxygen levels. Increases were rapid (4–10-s onset latencies), modest in magnitude (1–3 μM or 5%–15% over baseline) and duration (5–20 min), and generally correlated with the arousing potential of each stimulus. Two strategies were used to determine the mechanisms underlying the observed increases in NAc oxygen levels. First, we showed that NAc oxygen levels phasically increase following intra-NAc microinjections of glutamate (GLU) that excite accumbal neurons. Therefore, local neural activation with subsequent local vasodilation is involved in mediating physiological increases in NAc oxygen induced by arousing stimuli. Second, by employing oxygen monitoring in the subcutaneous space, a highly-vascularized area with no metabolic activity, we determined that physiological increases in NAc oxygen also depend on the rise in blood oxygen levels caused by respiratory activation. Due to the co-existence of different mechanisms governing oxygen entry into brain tissue, NAc oxygen responses differ from fluctuations in NAc glucose, which, within a normal behavioral continuum, are regulated exclusively by neuro-vascular coupling due to glucose’s highly stable levels in the blood. Finally, we discuss the relationships between physiological fluctuations in NAc oxygen, glucose and metabolic brain activation assessed by intra-brain heat production.

Role of central and peripheral opiate receptors in the effects of fentanyl on analgesia, ventilation and arterial blood-gas chemistry in conscious rats

This study determined the effects of the peripherally restricted μ-opiate receptor (μ-OR) antagonist, naloxone methiodide (NLXmi) on fentanyl (25μg/kg, i.v.)-induced changes in (1) analgesia, (2) arterial blood gas chemistry (ABG) and alveolar-arterial gradient (A-a gradient), and (3) ventilatory parameters, in conscious rats. The fentanyl-induced increase in analgesia was minimally affected by a 1.5mg/kg of NLXmi but was attenuated by a 5.0mg/kg dose. Fentanyl decreased arterial blood pH, pO2 and sO2 and increased pCO2 and A-a gradient. These responses were markedly diminished in NLXmi (1.5mg/kg)-pretreated rats. Fentanyl caused ventilatory depression (e.g., decreases in tidal volume and peak inspiratory flow). Pretreatment with NLXmi (1.5mg/kg, i.v.) antagonized the fentanyl decrease in tidal volume but minimally affected the other responses. These findings suggest that (1) the analgesia and ventilatory depression caused by fentanyl involve peripheral μ-ORs and (2) NLXmi prevents the fentanyl effects on ABG by blocking the negative actions of the opioid on tidal volume and A-a gradient.

Characteristics and comparative severity of respiratory response to toxic doses of fentanyl, methadone, morphine, and buprenorphine in rats

Opioids are known to induce respiratory depression. We aimed to characterize in rats the effects of four opioids on arterial blood gases and plethysmography after intraperitoneal administration at 80% of their LD(50) in order to identify opioid molecule-specific patterns and classify response severity. Opioid-receptor (OR) antagonists, including intravenous 10 mg kg(-1)-naloxonazine at 5 min [mu-OR antagonist], subcutaneous 30 mg kg(-1)-naloxonazine at 24 h [mu1-OR antagonist], subcutaneous 3 mg kg(-1)-naltrindole at 45 min [delta-OR antagonist], and subcutaneous 5 mg kg(-1)-Nor-binaltorphimine at 6 h [kappa-OR antagonist] were pre-administered to test the role of each OR. Methadone, morphine, and fentanyl significantly decreased PaO(2) (P<0.001) and increased PaCO(2) (P<0.05), while buprenorphine only decreased PaO(2) (P<0.05). While all opioids significantly increased inspiratory time (T(I), P<0.001), methadone and fentanyl also increased expiratory time (T(E), P<0.05). Intravenous 10 mg kg(-1)-naloxonazine at 5 min completely reversed opioid-related effects on PaO(2) (P<0.05), PaCO(2) (P<0.001), T(I) (P<0.05), and T(E) (P<0.01) except in buprenorphine. Subcutaneous 30 mg kg(-1)-naloxonazine at 24 h completely reversed effects on PaCO(2) (P<0.01) and T(E) (P<0.001), partially reversed effects on T(I) (P<0.001), and did not reverse effects on PaO(2). Naltrindole reversed methadone-induced T(E) increases (P<0.01) but worsened fentanyl's effect on PaCO(2) (P<0.05) and T(I) (P<0.05). Nor-binaltorphimine reversed morphine- and buprenorphine-induced T(I) increases (P<0.001) but worsened methadone's effect on PaO(2) (P<0.05) and morphine (P<0.001) and buprenorphine's (P<0.01) effects on pH. In conclusion, opioid-related respiratory patterns are not uniform. Opioid-induced hypoxemia as well as increases in T(I) and T(E) are caused by mu-OR, while delta and kappa-OR roles appear limited, depending on the specific opioid. Regarding severity of opioid-induced respiratory effects at 80% of their LD(50), all drugs increased T(I). Methadone and fentanyl induced hypoxemia, hypercapnia, and T(E) increases, morphine caused both hypoxemia and hypercapnia while buprenorphine caused only hypoxemia.

Actions of opioids on respiratory activity via activation of brainstem mu-, delta- and kappa-receptors; an in vitro study

Opioid-induced respiratory depression is well documented. However, exact sites of action and mechanisms for opioid-induced effects on respiration have not yet been elucidated. The present study was carried out on isolated brainstem-spinal cord preparations from newborn rats in order to explore the opioid activity on brainstem mu-, delta- and kappa-receptors. The brainstem-spinal cord was isolated from 0- to 4-day-old Sprague-Dawley rats. The preparation was perfused with artificial cerebrospinal fluid (28.5 degrees C) equilibrated with 95% O2 and 5% CO2 at a pH of 7.4. Neuronal respiratory activity was recorded from the ventrolateral part of the medulla oblongata and efferent impulses from C4 or C5 ventral roots. Effects of the mu-receptor agonist DAGO, the delta-receptor agonist DPDPE and the kappa-receptor agonist U50,488 on respiratory frequency (fR), inspiratory time (Ti) and peak integrated C4 amplitude (Int[C4]) were measured. In addition, the effect of pre-treatment with the mu1 receptor antagonist naloxanazine (35 mg/kg, subcutaneous injection) was evaluated. DAGO reduced fR and Ti in a concentration-dependent manner and caused a reduction of Int(C4) at high concentrations (10 microM). The mu1 receptor antagonist naloxanazine shifted the fR concentration-response curve for DAGO to the right (P < 0.05). DPDPE had no effect on respiratory activities whereas U50,488, like DAGO, reduced fR and Int(C4) in a concentration-dependent manner. It was concluded that mu-opioid receptors, including the mu1 were involved in fR reduction whereas kappa-opioid receptors were involved in reduction of both fR and respiratory amplitude. Delta-opioid receptors do not seem to participate in respiratory modulation at this age.