Effects of sub-lethal doses of fentanyl on vital physiologic functions and withdrawal-like behaviors in adult goats

Nitrosyl factors play a vital role in the ventilatory depressant effects of fentanyl in freely moving guinea pigs

An understanding of intracellular mechanisms by which fentanyl and other synthetic opioids exert adverse effects on breathing is needed. Using freely moving adult male guinea pigs, we administered the nitric oxide synthase (NOS) inhibitor, L-NAME (NG-nitro-L-arginine methyl ester), to determine whether nitrosyl factors, such as nitric oxide and S-nitrosothiols, play a role in fentanyl-induced respiratory depression. Ventilatory parameters were recorded by whole body plethysmography to determine the effects of fentanyl (75 μg/kg, IV) in guinea pigs that had received a prior injection of vehicle (saline), L-NAME or the inactive D-isomer, D-NAME (both at 50 μmol/kg, IV), 15 min beforehand. L- and D-NAME elicited minor effects on most parameters, including frequency of breathing, tidal volume and minute ventilation, although L-NAME did decrease end expiratory pause and non-eupneic breathing index (NEBI). Subsequent injection of fentanyl in guinea pigs pre-treated with vehicle produced profound and sustained reductions in frequency, tidal volume, minute ventilation, peak inspiratory flow, and inspiratory and expiratory drives, while increasing inspiratory time, expiratory time, end inspiratory pause, and NEBI. These ventilatory depressant effects of fentanyl seen in guinea pigs pre-treated with vehicle were markedly diminished in guinea pigs pre-treated with L-NAME. Moreover, the adverse effects of fentanyl on many recorded breathing parameters were converted to stimulatory effects. In contrast, D-NAME did not alter any of the effects of fentanyl on breathing. This study is the first to characterize the role nitrosyl factors play in the intracellular mechanisms involved in fentanyl-induced respiratory depression in guinea pigs.

Modeling Insights into Potential Mechanisms of Opioid-Induced Respiratory Depression within Medullary and Pontine Networks

The opioid epidemic is a pervasive health issue and continues to have a drastic impact on the United States. This is primarily because opioids cause respiratory suppression and the leading cause of death in opioid overdose is respiratory failure (i.e., opioid-induced respiratory depression, OIRD). Opioid administration can affect the frequency and magnitude of inspiratory motor drive by activating μ-opioid receptors that are located throughout the respiratory control network in the brainstem. This can significantly affect ventilation and blunt CO2 responsiveness, but the precise neural mechanisms that suppress breathing are not fully understood. Previous research has suggested that opioids affect medullary and pontine inspiratory neuron activity by disrupting upstream elements within this circuit. Inspiratory neurons within this network exhibit synchrony consistent with shared excitation from other neuron populations and recurrent mechanisms. One possible target for opioid suppression of inspiratory drive are excitatory synapses. Reduced excitability of these synaptic elements may result in disfacilitation and reduced synchrony among inspiratory neurons. Downstream effects of disfacilitation may result in abnormal output from phrenic motoneurons resulting in distressed breathing. We tested the plausibility of this hypothesis with a computational model of the respiratory network by targeting the synaptic excitability in fictive medullary and pontine populations. The synaptic conductances were systematically decreased while monitoring the overall respiratory motor pattern and aggregate firing rates of subsets of cell populations. Simulations suggest that highly selective, rather than generalized, actions of opioids on synapses within the inspiratory network may account for different observed breathing mechanics.

The effect of fentanyl on immobility after noxious stimulation in isoflurane-anaesthetized pigs: Exploring the role of the serotonergic system

OBJECTIVE

To investigate if fentanyl induces immobility through activation of the serotonergic 5HT1A receptor, by using the 5HT1A-antagonist robalzotan.

STUDY DESIGN

A prospective, blinded, randomized, two-group study.

ANIMALS

A group of 12 mixed-breed pigs aged 71-79 days.

METHODS

The motor response to clamping a claw was assessed in isoflurane-anaesthetized pigs at baseline, then fentanyl was infused intravenously (IV) for 40 minutes and clamping was repeated. The infusion started at 20 μg kg-1 hour-1 and was increased by 60% until fentanyl produced immobility, defined as no motor response for 60 seconds. Subsequently, either robalzotan (1 mg kg-1) or the same volume of saline was injected IV and clamping was repeated. The change in response was compared with Fisher's exact test. Mean arterial blood pressure (MAP) and heart rate (HR) were extracted for 2 minutes before and after 60 seconds of clamping, and the differences compared with a Wilcoxon signed-rank test. Dynamic respiratory compliance was calculated at baseline and after fentanyl; p < 0.05.

RESULTS

Baseline clamping produced a motor response within 5 seconds. This was abolished by fentanyl. Robalzotan or saline did not alter this (p = 0.45). As a response to clamping, MAP and HR changed with median (range) -0.5 (-4.4 to 22.2) mmHg and -1 (-7 to 1.5), respectively, where HR changed significantly (p = 0.039). The 95% confidence interval for the effect size of fentanyl upon dynamic compliance was -3.25 to -1.65 mL cmH2O-1.

CONCLUSIONS AND CLINICAL RELEVANCE

No indication was found for the 5HT1A receptor to be involved in fentanyl-induced reduction of the motor response to claw clamping. The decreased compliance after fentanyl could suggest onset of chest wall rigidity.

Motor effects of fentanyl in isoflurane-anaesthetized pigs and the subsequent effect of ketanserin or naloxone

OBJECTIVE

To examine the effect of ketanserin and naloxone on fentanyl-induced motor activity in isoflurane-anaesthetized pigs.

STUDY DESIGN

Randomized, blinded, prospective two-group study.

ANIMALS

A group of 12 crossbred pigs weighing 22-31 kg.

METHODS

Fentanyl was administered to isoflurane-anaesthetized pigs at 7.5 μg kg-1 hour-1 for 40 minutes intravenously, followed by an intravenous injection of naloxone 0.1 mg kg-1 or ketanserin 1 mg kg-1. Electromyography (EMG) and accelerometry were used to record motor unit activity and tremors, respectively. To test the effect of drug administration on motor activity, data from a 5 minute period at baseline, immediately before and after antagonist injection were compared in a mixed model; p < 0.05.

RESULTS

Results are reported with the median difference, 95% confidence intervals and corresponding p-values in brackets. Fentanyl significantly increased EMG activity [30.51 (1.84-81.02) μV, p = 0.004] and induced tremors [0.09 (0.02-0.18) m s-2, p < 0.001] in 10 of 12 pigs. Ketanserin significantly reduced EMG [32.22 (6.29-136.80) μV, p = 0.001] and tremor [0.10 (0.03-0.15) m s-2, p = 0.007] activity. No significant effect was found for naloxone on EMG [26.76 (-13.28-91.17) μV, p = 0.4] or tremors [0.08 (-0.01-0.19) m s-2, p = 0.08].

CONCLUSIONS AND CLINICAL RELEVANCE

Fentanyl can induce motor activity in anaesthetized pigs, with a suggested link to the serotonergic system. This study shows that ketanserin can antagonize this activity, which supports the role of serotonin. This knowledge contributes to the general understanding of the motor effects of fentanyl and especially the problem of tremors in anaesthetized pigs.

Endocannabinoid agonist 2-arachidonoylglycerol differentially alters diurnal activity and sleep during fentanyl withdrawal in male and female mice

Fentanyl has become the leading driver of opioid overdoses in the United States. Cessation of opioid use represents a challenge as the experience of withdrawal drives subsequent relapse. One of the most prominent withdrawal symptoms that can contribute to opioid craving and vulnerability to relapse is sleep disruption. The endocannabinoid agonist, 2-Arachidonoylglycerol (2-AG), may promote sleep and reduce withdrawal severity; however, the effects of 2-AG on sleep disruption during opioid withdrawal have yet to be assessed. Here, we investigated the effects of 2-AG administration on sleep-wake behavior and diurnal activity in mice during withdrawal from fentanyl. Sleep-wake activity measured via actigraphy was continuously recorded before and after chronic fentanyl administration in both male and female C57BL/6J mice. Immediately following cessation of fentanyl administration, 2-AG was administered intraperitoneally to investigate the impact of endocannabinoid agonism on opioid-induced sleep disruption. We found that female mice maintained higher activity levels in response to chronic fentanyl than male mice. Furthermore, fentanyl administration increased wake and decreased sleep during the light period and inversely increased sleep and decreased wake in the dark period in both sexes. 2-AG treatment increased arousal and decreased sleep in both sexes during first 24-h of withdrawal. On withdrawal day 2, only females showed increased wakefulness with no changes in males, but by withdrawal day 3 male mice displayed decreased rapid-eye movement sleep during the dark period with no changes in female mice. Overall, repeated administration of fentanyl altered sleep and diurnal activity and administration of the endocannabinoid agonist, 2-AG, had sex-specific effects on fentanyl-induced sleep and diurnal changes.

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

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.

Sex-specific Effects of the Endocannabinoid Agonist 2-Arachidonoylglycerol on Sleep and Circadian Disruptions during Fentanyl Withdrawal

Fentanyl has become the leading driver of opioid overdoses. Cessation of opioid use represents a challenge as the experience of withdrawal drives subsequent relapse. One of the most prominent withdrawal symptoms that can contribute to opioid craving and vulnerability to relapse is sleep disruption. The endocannabinoid agonist, 2-Arachidonoylglycerol (2-AG), may promote sleep and reduce withdrawal severity; however, the effects of 2-AG on sleep disruption during opioid withdrawal have yet to be assessed. Here, we investigate the effects of 2-AG administration on sleep-wake behavior and diurnal activity in mice during withdrawal from fentanyl. Sleep-wake activity was continuously recorded before and after chronic fentanyl administration in both male and female C57BL/6J mice. Immediately following cessation of fentanyl administration, 2-AG was administered intraperitoneally to investigate the impact of endocannabinoid agonism on opioid-induced sleep disruption. Female mice maintained higher activity levels in response to chronic fentanyl than male mice. Furthermore, fentanyl increased wake and decreased sleep during the light period and inversely increased sleep and decreased wake in the dark period in both sexes. 2-AG treatment increased arousal and decreased sleep in both sexes during first 24 hrs of withdrawal. On withdrawal day 2, only female showed increased wakefulness with no changes in males, but by withdrawal day 3 male mice displayed decreased rapid-eye movement sleep during the dark period with no changes in female mice. Overall, repeated administration of fentanyl altered sleep and diurnal activity and administration of the endocannabinoid agonist, 2-AG, had sex-specific effects on fentanyl-induced sleep and diurnal changes.