Effects of vatinoxan in rats sedated with a combination of medetomidine, midazolam and fentanyl.

BACKGROUND: Alpha2-adrenoceptor agonists (α2-agonists) are widely used in animals as sedatives and for pre-anaesthetic medication. Medetomidine has often been given subcutaneously (SC) to rats, although its absorption rate is slow and the individual variation in serum drug concentrations is high via this route. In addition, α2-agonists have various effects on metabolic and endocrine functions such as hypoinsulinaemia, hyperglycaemia and diuresis. Vatinoxan is a peripherally acting α2-adrenoceptor antagonist that, as a hydrophilic molecule, does not cross the blood-brain barrier in significant quantities and thus alleviates peripheral cardiovascular effects and adverse metabolic effects of α2-agonists. Aim of this study was to evaluate the effects of vatinoxan on sedation, blood glucose concentration, voiding and heart and respiratory rates and arterial oxygen saturation in rats sedated with subcutaneous medetomidine, midazolam and fentanyl. RESULTS: Onset of sedation and loss of righting reflex occurred significantly faster with vatinoxan [5.35 ± 1.08 (mean ± SD) versus 12.97 ± 6.18 min and 6.53 ± 2.18 versus 14.47 ± 7.28 min, respectively]. No significant differences were detected in heart and respiratory rates and arterial oxygen saturation between treatments. Blood glucose concentration (18.3 ± 3.6 versus 11.8 ± 1.2 mmol/L) and spontaneous urinary voiding [35.9 (15.1-41.6), range (median) versus 0.9 (0-8.0) mL /kg/min] were significantly higher without vatinoxan. CONCLUSIONS: Acceleration of induction of sedation, alleviation of hyperglycaemia and prevention of profuse diuresis by vatinoxan may be beneficial when sedating rats for clinical and experimental purposes with subcutaneous medetomidine, midazolam and fentanyl.

Perioperative analgesic effects of the erector spinae plane block with bupivacaine or bupivacaine-dexmedetomidine in dogs undergoing hemilaminectomy: A randomized controlled trial.

Objective: To compare the perioperative opioid requirements among dogs receiving an erector spinae plane (ESP) block with bupivacaine, with or without dexmedetomidine, and a control group. Animals and procedure: Thirty client-owned, healthy adult dogs undergoing hemilaminectomy were included in this randomized, prospective, blinded clinical study. Dogs were randomly assigned to 1 of 3 treatment groups: Group B, ESP block with bupivacaine; Group BD, ESP block with bupivacaine and dexmedetomidine; and Group C, control. Rescue intra- and postoperative analgesia consisted of fentanyl and methadone, respectively. Postoperative pain was evaluated using the short form of the Glasgow Composite Measure Pain Scale (CMPS-SF). Results: In Group BD, 0/10 dogs required intraoperative fentanyl, compared to 9/10 in Group C (P < 0.001), whereas 1/10 required postoperative methadone, compared to 9/10 in Group B (P = 0.003) and 10/10 in Group C (P < 0.001). The total amount of intraoperative fentanyl (µg/kg) was 0 (0 to 4) in Group B and 0 (0 to 0) in BD, compared to 6 (0 to 8) in C (P = 0.004 and P < 0.001, respectively). Postoperative methadone (mg/kg) required during the first 12 h was 0.5 (0 to 1.4) in Group B (P = 0.003) and 0 (0 to 0) in BD (P < 0.001), compared to C (P = 0.003 and P < 0.001, respectively). Conclusion: An ESP block with bupivacaine, with or without dexmedetomidine, was associated with a reduction in perioperative opioid consumption and provided effective acute pain control.

Effets analgésiques périopératoires du bloc des érecteurs du rachis avec de la bupivacaïne ou de la bupivacaïne-dexmédétomidine chez les chiens subissant une hémilaminectomie: un essai contrôlé randomisé. Objectif: Comparer les besoins périopératoires en opioïdes chez les chiens recevant un bloc des érecteurs de la colonne vertébrale (ESP) avec de la bupivacaïne, avec ou sans dexmédétomidine, et un groupe témoin. Animaux et procédure: Trente chiens adultes en bonne santé appartenant à des clients subissant une hémilaminectomie ont été inclus dans cette étude clinique randomisée, prospective et en aveugle. Les chiens ont été répartis au hasard dans 1 des 3 groupes de traitement: groupe B, bloc ESP avec bupivacaïne; groupe BD, bloc ESP avec bupivacaïne et dexmédétomidine; et groupe C, témoin. L'analgésie de secours peropératoire et postopératoire consistait respectivement en fentanyl et en méthadone. La douleur postopératoire a été évaluée à l'aide du formulaire abrégé de l'échelle de mesure de la douleur de Glasgow (CMPS-SF). Résultats: Dans le groupe BD, 0/10 chiens ont eu besoin de fentanyl peropératoire, contre 9/10 dans le groupe C (P < 0,001), tandis que 1/10 ont eu besoin de méthadone postopératoire, contre 9/10 dans le groupe B (P = 0,003) et 10/10 dans le groupe C (P < 0,001). La quantité totale de fentanyl peropératoire (µg/kg) était de 0 (0 à 4) dans le groupe B et de 0 (0 à 0) dans le groupe BD, contre 6 (0 à 8) dans le groupe C (P = 0,004 et P < 0,001, respectivement). La méthadone postopératoire (mg/kg) nécessaire au cours des 12 premières heures était de 0,5 (0 à 1,4) dans le groupe B (P = 0,003) et de 0 (0 à 0) dans le groupe BD (P < 0,001), par rapport au groupe C (P = 0,003). et P < 0,001, respectivement). Conclusion: Un bloc ESP avec de la bupivacaïne, avec ou sans dexmédétomidine, a été associé à une réduction de la consommation peropératoire d'opioïdes et a permis un contrôle efficace de la douleur aiguë.(Traduit par Dr Serge Messier).

Transcriptional signatures of fentanyl use in the mouse ventral tegmental area.

Synthetic opioids such as fentanyl contribute to the vast majority of opioid-related overdose deaths, but fentanyl use remains broadly understudied. Like other substances with misuse potential, opioids cause lasting molecular adaptations to brain reward circuits, including neurons in the ventral tegmental area (VTA). The VTA contains numerous cell types that play diverse roles in opioid use and relapse; however, it is unknown how fentanyl experience alters the transcriptional landscape in specific subtypes. Here, we performed single nuclei RNA sequencing to study transcriptional programs in fentanyl-experienced mice. Male and female C57/BL6 mice self-administered intravenous fentanyl (1.5 µg/kg/infusion) or saline for 10 days. After 24 h abstinence, VTA nuclei were isolated and prepared for sequencing on the 10× platform. We identified different patterns of gene expression across cell types. In dopamine neurons, we found enrichment of genes involved in growth hormone signalling. In dopamine-glutamate-GABA combinatorial neurons, and some GABA neurons, we found enrichment of genes involved in Pi3k-Akt signalling. In glutamate neurons, we found enrichment of genes involved in cholinergic signalling. We identified transcriptional regulators for the differentially expressed genes in each neuron cluster, including downregulated transcriptional repressor Bcl6, and upregulated transcription factor Tcf4. We also compared the fentanyl-induced gene expression changes identified in mouse VTA with a published rat dataset in bulk VTA, and found overlap in genes related to GABAergic signalling and extracellular matrix interaction. Together, we provide a comprehensive picture of how fentanyl self-administration alters the transcriptional landscape of the mouse VTA that serves as the foundation for future mechanistic studies.

Distinct µ-opioid ensembles trigger positive and negative fentanyl reinforcement.

Fentanyl is a powerful painkiller that elicits euphoria and positive reinforcement1. Fentanyl also leads to dependence, defined by the aversive withdrawal syndrome, which fuels negative reinforcement2,3 (that is, individuals retake the drug to avoid withdrawal). Positive and negative reinforcement maintain opioid consumption, which leads to addiction in one-fourth of users, the largest fraction for all addictive drugs4. Among the opioid receptors, µ-opioid receptors have a key role5, yet the induction loci of circuit adaptations that eventually lead to addiction remain unknown. Here we injected mice with fentanyl to acutely inhibit γ-aminobutyric acid-expressing neurons in the ventral tegmental area (VTA), causing disinhibition of dopamine neurons, which eventually increased dopamine in the nucleus accumbens. Knockdown of µ-opioid receptors in VTA abolished dopamine transients and positive reinforcement, but withdrawal remained unchanged. We identified neurons expressing µ-opioid receptors in the central amygdala (CeA) whose activity was enhanced during withdrawal. Knockdown of µ-opioid receptors in CeA eliminated aversive symptoms, suggesting that they mediate negative reinforcement. Thus, optogenetic stimulation caused place aversion, and mice readily learned to press a lever to pause optogenetic stimulation of CeA neurons that express µ-opioid receptors. Our study parses the neuronal populations that trigger positive and negative reinforcement in VTA and CeA, respectively. We lay out the circuit organization to develop interventions for reducing fentanyl addiction and facilitating rehabilitation.

Assessment of the antinociceptive, respiratory-depressant, and reinforcing effects of the low pKa fluorinated fentanyl analogs, FF3 and NFEPP.

RATIONALE: Recent studies report that fentanyl analogs with relatively low pKa values produce antinociception in rodents without other mu opioid-typical side effects due to the restriction of their activity to injured tissue with relatively low pH values. However, it is unclear if and to what degree these compounds may produce mu opioid-typical side effects (respiratory depression, reinforcing effects) at doses higher than those required to produce antinociception. OBJECTIVES: The present study compared the inflammatory antinociceptive, respiratory-depressant, and reinforcing effects of fentanyl and two analogs of intermediate (FF3) and low (NFEPP) pKa values in terms of potency and efficacy in male and female Sprague-Dawley rats. METHODS: Nociception was produced by administration of Complete Freund's Adjuvant into the hind paw of subjects, and antinociception was measured using an electronic Von Frey test. Respiratory depression was measured using whole-body plethysmography. Reinforcing effects were measured in self-administration using a progressive-ratio schedule of reinforcement. The dose ranges tested for each drug encompassed no effect to maximal effects. RESULTS: All compounds produced full effects in all measures but varied in potency. FF3 and fentanyl were equipotent in antinociception and self-administration, but FF3 was less potent than fentanyl in respiratory depression. NFEPP was less potent than fentanyl in every measure. The magnitude of potency difference between antinociception and other effects was greater for FF3 than for NFEPP or fentanyl, indicating that FF3 had the widest margin of safety when relating antinociception to respiratory-depressant and reinforcing effects. CONCLUSIONS: Low pKa fentanyl analogs possess potential as safer analgesics, but determining the optimal degree of difference for pKa relative to fentanyl will require further study due to some differences between the current results and findings from prior work with these analogs.

Classics in Chemical Neuroscience: Xylazine.

Xylazine (also known as "tranq") is a potent nonopioid veterinary sedative that has recently experienced a surge in use as a drug adulterant, most often combined with illicitly manufactured fentanyl. This combination may heighten the risk of fatal overdose. Xylazine has no known antidote approved for use in humans, and age-adjusted overdose deaths involving xylazine were 35 times higher in 2021 than 2018. In April 2023, the Biden Administration declared xylazine-laced fentanyl an emerging drug threat in the United States. In 2022, the Drug Enforcement Agency (DEA) reported nearly a quarter of seized fentanyl powder contained xylazine. This dramatic increase in prevalence has solidified the status of xylazine as an emerging drug of abuse and an evolving threat to public health. The following narrative review outlines the synthesis, pharmacokinetics, pharmacodynamics, and adverse effects of xylazine, as well as the role it may play in the ongoing opioid epidemic.

L-NAC and L-NAC methyl ester prevent and overcome physical dependence to fentanyl in male rats.

N-acetyl-L-cysteine (L-NAC) is a proposed therapeutic for opioid use disorder. This study determined whether co-injections of L-NAC (500 µmol/kg, IV) or its highly cell-penetrant analogue, L-NAC methyl ester (L-NACme, 500 µmol/kg, IV), prevent acquisition of acute physical dependence induced by twice-daily injections of fentanyl (125 µg/kg, IV), and overcome acquired dependence to these injections in freely-moving male Sprague Dawley rats. The injection of the opioid receptor antagonist, naloxone HCl (NLX; 1.5 mg/kg, IV), elicited a series of withdrawal phenomena (i.e. behavioral and cardiorespiratory responses, hypothermia and body weight loss) in rats that received 5 or 10 injections of fentanyl and similar numbers of vehicle co-injections. With respect to the development of dependence, the NLX-precipitated withdrawal phenomena were reduced in rats that received had co-injections of L-NAC, and more greatly reduced in rats that received co-injections of L-NACme. In regard to overcoming established dependence, the NLX-precipitated withdrawal phenomena in rats that had received 10 injections of fentanyl (125 µg/kg, IV) were reduced in rats that had received co-injections of L-NAC, and more greatly reduced in rats that received co-injections of L-NACme beginning with injection 6 of fentanyl. This study provides compelling evidence that co-injections of L-NAC and L-NACme prevent the acquisition of physical dependence and overcome acquired dependence to fentanyl in male rats. The higher efficacy of L-NACme is likely due to its greater cell penetrability in brain regions mediating dependence to fentanyl and interaction with intracellular signaling cascades, including redox-dependent processes, responsible for the acquisition of physical dependence to fentanyl.

Fentanyl Analogs Exert Antinociceptive Effects via Sodium Channel Blockade in Mice.

The formalin test is one approach to studying acute pain in rodents. Similar to formalin, injection with glutamate and veratrine can also produce a nociceptive response. This study investigated whether opioid-related compounds could suppress glutamate- and veratrine-induced nociceptive responses in mice at the same dose. The administration of morphine (3 mg/kg), hydromorphone (0.4 mg/kg), or fentanyl (0.03 mg/kg) suppressed glutamate-induced nociceptive response, but not veratrine-induced nociceptive response at the same doses. However, high doses of morphine (10 mg/kg), hydromorphone (2 mg/kg), or fentanyl (0.1 mg/kg) produced a significant reduction in the veratrine-induced nociceptive response. These results indicate that high doses are required when using morphine, hydromorphone, or fentanyl for sodium channel-related neuropathic pain, such as ectopic activity. As a result, concerns have arisen about overdose and abuse if the dose of opioids is steadily increased to relieve pain. In contrast, trimebutine (100 mg/kg) and fentanyl analog isobutyrylfentanyl (iBF; 0.1 mg/kg) suppressed both glutamate- and veratrine-induced nociceptive response. Furthermore, nor-isobutyrylfentanyl (nor-iBF; 1 mg/kg), which is a metabolite of iBF, suppressed veratrine-induced nociceptive response. Besides, the optimal antinociceptive dose of iBF, unlike fentanyl, only slightly increased locomotor activity and did not slow gastrointestinal transit. Cancer pain is a complex condition driven by inflammatory, neuropathic, and cancer-specific mechanisms. Thus, iBF may have the potential to be a superior analgesic than fentanyl.

The role of morphine- and fentanyl-induced impairment of intestinal epithelial antibacterial activity in dysbiosis and its impact on the microbiota-gut-brain axis.

Recent evidence suggests that chronic exposure to opioid analgesics such as morphine disrupts the intestinal epithelial layer and causes intestinal dysbiosis. Depleting gut bacteria can preclude the development of tolerance to opioid-induced antinociception, suggesting an important role of the gut-brain axis in mediating opioid effects. The mechanism underlying opioid-induced dysbiosis, however, remains unclear. Host-produced antimicrobial peptides (AMPs) are critical for the integrity of the intestinal epithelial barrier as they prevent the pathogenesis of the enteric microbiota. Here, we report that chronic morphine or fentanyl exposure reduces the antimicrobial activity in the ileum, resulting in changes in the composition of bacteria. Fecal samples from morphine-treated mice had increased levels of Akkermansia muciniphila with a shift in the abundance ratio of Firmicutes and Bacteroidetes. Fecal microbial transplant (FMT) from morphine-naïve mice or oral supplementation with butyrate restored (a) the antimicrobial activity, (b) the expression of the antimicrobial peptide, Reg3γ, (c) prevented the increase in intestinal permeability and (d) prevented the development of antinociceptive tolerance in morphine-dependent mice. Improved epithelial barrier function with FMT or butyrate prevented the enrichment of the mucin-degrading A. muciniphila in morphine-dependent mice. These data implicate impairment of the antimicrobial activity of the intestinal epithelium as a mechanism by which opioids disrupt the microbiota-gut-brain axis.

Co-administration of naloxone and dexmedetomidine to simultaneously reverse acute effects of fentanyl and methamphetamine in rats.

BACKGROUND: The incidence of combination methamphetamine (METH)-opioid overdose has substantially increased in recent years. While agitation is uncommon after the naloxone (NLX) reversal of opioids, it is a major clinical concern in acute METH intoxication and can be physiologically antagonized by opioid-induced sedation. This study aimed to perform initial preclinical analysis of the safety and efficacy of dexmedetomidine (DEXMED) co-administered with NLX to attenuate METH-induced locomotor activity, as a rat model of agitation, after the reversal of fentanyl (FENT)-induced sedation. METHODS: Male Sprague Dawley rats were administered subcutaneous (SC) 0.1mg/kg FENT ± 1mg/kg METH. Fifteen min later, SC 0.1mg/kg NLX ± an increasing (0, 0.032, 0.056, and 0.1mg/kg) DEXMED dose was administered prior to the measurement of locomotor activity. After a washout period, the FENT ± METH and NLX ± DEXMED administration with the highest dose of DEXMED was administered for measurement of blood oxygen saturation and heart rate. RESULTS: After the NLX reversal of FENT-induced sedation, adjunct DEXMED substantially and significantly reduced METH-induced locomotor activity (p<0.05) at all doses tested. While the addition of DEXMED did not significantly reduce blood oxygenation in METH treated rats, it did so in the absence of METH. Also, DEXMED significantly reduced heart rate compared to non-DEXMED treated groups and resulted in further significant reductions in the animals not exposed to METH (p<0.05). CONCLUSIONS: These data provide preclinical evidence that DEXMED may be a safe and effective chemical restraint for METH-induced agitation after NLX opioid reversal.

Maternal and neonatal effects of epidural levobupivacaine combined with fentanyl or sufentanil for elective cesarean-section in brachycephalic breeds.

The aim of this study was to compare the safety and clinical efficacy of epidural levobupivacaine combined with fentanyl or sufentanil for bitches undergoing elective cesarean-section and the impact of these anesthetic protocols on neonatal viability. The anesthetic protocol consisted of intramuscular morphine (0.2 mg/kg), followed by an intravenous bolus of propofol, in a dose sufficient to allowed the puncture of the lumbosacral space. The dogs were randomly allocated to receive 0.5 % levobupivacaine plus fentanyl (2.5 µg/kg; LF: n = 9) or sufentanil (1 µg/kg; LS; n = 11). Maternal cardiorespiratory parameters were monitored at specific time points during surgery. Intraoperative propofol supplementation was based on the presence of head and/or thoracic limb movements. Neonatal reflex responses and the Apgar score (range 0-10 points) were assessed at 5 and 60 minutes after birth. Puppy mortality rate was recorded until 24 hours after birth. Data were analyzed using two-way ANOVA, Tukey's test, Wilcoxon signed rank test, and Fisher's exact test (P < 0.05). Intraoperatively, maternal cardiorespiratory variables and propofol requirements were similar between groups, with no detection of anesthetic complications. The puppy reflex responses did not differ between groups at any time point. The medians (range) of Apgar scores were lower (P = 0.016) in the LF [5 (1-9)] at 5 minutes in comparison with LS [6 (2-9)], while no intergroup differences were recorded at 60 minutes [LF = 8 (2-10); LS = 9 (6-10]. The total mortality rate was 4.1 %. In the LS group, no puppies died, while in the LF 8 % of the puppies died in the first 24 hours after birth (P = 0.11). Epidural levobupivacaine combined with fentanyl or sufentanil provided minimal maternal and neonatal adverse effects, but neither protocol enabled the performance of a C-section in 100 % of the French and English bulldogs, without propofol supplementation.

Xylazine does not enhance fentanyl reinforcement in rats: A behavioral economic analysis.

The adulteration of illicit fentanyl with the alpha-2 agonist xylazine has been designated an emerging public health threat. The clinical rationale for combining fentanyl with xylazine is currently unclear, and the inability to study fentanyl/xylazine interactions in humans warrants the need for preclinical research. We studied fentanyl and xylazine pharmacodynamic and pharmacokinetic interactions in male and female rats using drug self-administration behavioral economic methods. Fentanyl, but not xylazine, functioned as a reinforcer under both fixed-ratio and progressive-ratio drug self-administration procedures. Xylazine combined with fentanyl at three fixed dose-proportion mixtures did not significantly alter fentanyl reinforcement as measured using behavioral economic analyses. Xylazine produced a proportion-dependent decrease in the behavioral economic Q0 endpoint compared to fentanyl alone. However, xylazine did not significantly alter fentanyl self-administration at FR1. Fentanyl and xylazine co-administration did not result in changes to pharmacokinetic endpoints. The present results demonstrate that xylazine does not enhance the addictive effects of fentanyl or alter fentanyl plasma concentrations. The premise for why illicitly manufacture fentanyl has been adulterated with xylazine remains to be determined.

The impact of chronic fentanyl administration on the cerebral cortex in mice: Molecular and histological effects.

PURPOSE: Fentanyl, a fully synthetic opioid, is widely used for severe pain management and has a huge abuse potential for its psychostimulant effects. Unlike other opioids, the neurotoxic effects of chronic fentanyl administration are still unclear. In particular, little is known about its effect on the cerebral cortex. The current study aims to test the chronic toxicity of fentanyl in the mice model. METHODS: Adult male Balb/c mice were chronically treated with low (0.05 mg/kg, i.p) and high (0.1 mg/kg, i.p) doses of fentanyl for 5 consecutive weeks, and various neurotoxic parameters, including apoptosis, oxidative stress, and neuroinflammatory response were assessed in the cortex. Potential histological as well as neurochemical changes were also evaluated. RESULTS: The results of this study show that chronic fentanyl administration induced intense levels of apoptosis, oxidative stress, and neuroinflammation in the cerebral cortex. These findings were found to be correlated with histopathological characteristics of neural degeneration and white matter injury. Moreover, fentanyl administration was found to reduce the expression of both NMDA receptor subunits and dopamine receptors and elevate the level of epidermal growth factor (EGF). CONCLUSION: Fentanyl administration induced neurotoxic effects in the mouse cerebral cortex that could be primarily mediated by the evoked oxidative-inflammatory response. The altered expression of NMDA receptors, dopamine receptors, and EGF suggests the pernicious effects of fentanyl addiction that may end in the development of toxic psychosis.

Melatonin attenuates fentanyl - induced behavioral sensitization and circadian rhythm disorders in mice.

Melatonin is a neurohormone synthesized by the pineal gland to regulate the circadian rhythms and has proven to be effective in treating drug addiction and dependence. However, the effects of melatonin to modulate the drug-seeking behavior of fentanyl and its underlying molecular mechanism is elusive. This study was designed to investigate the effects of melatonin on fentanyl - induced behavioral sensitization and circadian rhythm disorders in mice. The accompanying changes in the expression of Brain and Muscle Arnt-Like (BMAL1), tyrosine hydroxylase (TH), and monoamine oxidase A (MAO-A) in relevant brain regions including the suprachiasmatic nucleus (SCN), nucleus accumbens (NAc), prefrontal cortex (PFC), and hippocampus (Hip) were investigated by western blot assays to dissect the mechanism by which melatonin modulates fentanyl - induced behavioral sensitization and circadian rhythm disorders. The present study suggest that fentanyl (0.05, 0.1 and 0.2 mg/kg) could induce behavioral sensitization and melatonin (30.0 mg/kg) could attenuate the behavioral sensitization and circadian rhythm disorders in mice. Fentanyl treatment reduced the expression of BMAL1 and MAO-A and increased that of TH in relevant brain regions. Furthermore, melatonin treatment could reverse the expression levels of BMAL1, MAO-A, and TH. In conclusion, our study demonstrate for the first time that melatonin has therapeutic potential for fentanyl addiction.

Erector spinae plane block in dogs undergoing hemilaminectomy: A prospective randomized clinical trial.

OBJECTIVE: To compare the perioperative cumulative opioid consumption and the incidence of cardiovascular complications in dogs undergoing hemilaminectomy in which either an erector spinae plane (ESP) block or systemic opioids were administered. STUDY DESIGN: Prospective randomized clinical trial. ANIMALS: A total of 60 client-owned dogs. METHODS: Dogs were randomized to one of three groups: an ESP block (group ESP), a constant rate infusion of fentanyl (group FNT, positive control) or a single dose of methadone as premedication (group MTD, negative control). Intraoperative nociceptive response was treated with fentanyl [1 µg kg-1, intravenously (IV)] boli. Before closure of the surgical site, morphine (0.1 mg kg-1) was applied to the dura mater. The cumulative dose of opioids was recorded and compared between groups. The incidence of intraoperative bradycardia and/or hypotension and the time to extubation were compared between groups. The short form of the Glasgow Composite Pain Scale (SF-GCPS) was used to score nociception before anaesthetic induction and 1, 2, 6, 12,18 and 24 hours postoperatively. Methadone 0.2 mg kg-1 was administered IV if the SF-GCPS score was ≥ 5. RESULTS: Group MTD required more intraoperative rescue analgesia than groups ESP (p = 0.008) and FNT (p = 0.001). The total cumulative intraoperative dose of fentanyl was higher in groups FNT (p < 0.0001) and MTD (p = 0.002) than in group ESP. The incidence of cardiovascular complications was similar between groups. Extubation time was longer in group MTD (p = 0.03). Postoperatively, the time to first rescue analgesia was longer in group ESP than in group MTD (p = 0.03). The cumulative postoperative opioid consumption and pain scores were similar between groups. CONCLUSIONS AND CLINICAL RELEVANCE: The ESP block resulted in a reduced intraoperative opioid consumption compared with the control positive and negative groups.

Pharmacologic Characterization of Substituted Nitazenes at µ, κ, and Δ Opioid Receptors Suggests High Potential for Toxicity.

The benzimidazole opioids (substituted nitazenes) are highly potent µ opiod receptor (MOR) agonists with heroin- or fentanyl-like effects. These compounds have caused hospitalizations and fatal overdoses. We characterized the in vitro pharmacology and structure-activity relationships of 19 nitazenes with substitutions at three positions of the benzimidazole core. Affinities were assessed using agonist radioligand binding assays at human µ, κ, and Δ opioid receptors (MOR, KOR, and DOR, respectively) heterologously expressed in CHO cells. Notably, for MOR binding, nine substituted nitazenes had significantly higher affinities than fentanyl including N-pyrrolidino etonitazene, N-pyrrilidino isonitazene, and N-desethyl isotonitazene; 13 had subnanomolar affinities. Only metodesnitazene and flunitazene had significantly lower affinities than fentanyl. Affinities for the substituted nitazenes at KOR and DOR relative to MOR were 46- to 2580-fold and 180- to 1280-fold lower, respectively. Functional activities were assessed using [35S]GTPγS binding assays. Four nitazenes had subnanomolar potencies at MOR: N-pyrrolidino etonitazene, N-pyrrilidino isonitazene, N-pyrrilidino protonitazene and N-desethyl isotonitazene. Ten substituted nitazenes had significantly higher potencies than fentanyl. All tested nitazenes were full MOR agonists. Potencies at KOR and DOR relative to MOR were 7.3- to 7920-fold and 24- to 9400-fold lower, respectively. Thus, many of these compounds are high affinity/high potency MOR agonists with elevated potential to elicit toxicity and overdose at low doses. SIGNIFICANCE STATEMENT: Substituted nitazenes are a growing public health threat. Although the 19 nitazenes tested vary in their opioid receptor pharmacology, a number are very high affinity, high potency, and high efficacy compounds- higher than fentanyl. Their pharmacology suggests high potential for harm.

Chronic Opioid Treatment Arrests Neurodevelopment and Alters Synaptic Activity in Human Midbrain Organoids.

Understanding the impact of long-term opioid exposure on the embryonic brain is critical due to the surging number of pregnant mothers with opioid dependency. However, this has been limited by human brain inaccessibility and cross-species differences in animal models. Here, a human midbrain model is established that uses hiPSC-derived midbrain organoids to assess cell-type-specific responses to acute and chronic fentanyl treatment and fentanyl withdrawal. Single-cell mRNA sequencing of 25,510 cells from organoids in different treatment groups reveals that chronic fentanyl treatment arrests neuronal subtype specification during early midbrain development and alters synaptic activity and neuron projection. In contrast, acute fentanyl treatment increases dopamine release but does not significantly alter gene expression related to cell lineage development. These results provide the first examination of the effects of opioid exposure on human midbrain development at the single-cell level.

Apneic response to fentanyl in adult rats: Role of laryngeal afferents.

Intravenous (systemic) bolus injection of fentanyl (FNT) reportedly induces an immediate vagal-mediated apnea; however, the precise origin of vagal afferents responsible for this apnea remains unknown. We tested whether intralaryngeal (local) application of FNT would also trigger an apnea and whether the apneic response to both local and systemic administration of FNT was laryngeal afferent-mediated. Cardiorespiratory responses to FNT were recorded in anesthetized male adult rats with and without bilateral sectioning of the superior laryngeal nerve (SLNx) or peri-SLN capsaicin treatment (SLNcap) to block local C-fiber signal conduction. Opioid mu-receptor (MOR)-immunoreactivity was detected in laryngeal C- and myelinated neurons. We found that local and systemic administration of FNT elicited an immediate apnea. SLNx, rather than SLNcap, abolished the apneic response to local FNT application though MORs were abundantly expressed in both laryngeal C- and myelinated neurons. Importantly, SLNx failed to affect the apneic response to systemic FNT administration. These results lead to the conclusion that laryngeal afferents' MORs are responsible for the apneic response to local, but not systemic, administration of FNT.

Enantiomeric contributions to methamphetamine's bidirectional effects on basal and fentanyl-depressed respiration in mice.

RATIONALE: Fentanyl remains the primary cause of fatal overdoses, and its co-use with methamphetamine (METH) is a growing concern. We previously demonstrated that racemic METH can either enhance or mitigate opioid-induced respiratory depression (OIRD) dependent upon whether a low or high dose is administered. The optical isomers of METH, dextromethamphetamine (d-METH) and levomethamphetamine (l-METH), differ substantially in their selectivity and potency to activate various monoamine (MA) receptors, and these pharmacological differences may underlie the bidirectional effects of the racemate. Since it is unknown which of METH's MA receptor mechanisms mediate these respiratory effects, examination of METH's pharmacologically distinct enantiomers may provide insight into treatment targets for OIRD. METHODS: The two optical isomers of METH, d-METH and l-METH, were tested in adult male mice to determine their effects on basal and fentanyl-depressed respiratory frequency, tidal volume, and minute ventilation (MVb; i.e., respiratory frequency x tidal volume) using whole-body plethysmography. RESULTS: When tested at dose ranges of 1.0-10 mg/kg, d-METH elevated MVb and l-METH decreased basal MVb. A dose of 30 mg/kg l-METH increased basal MVb. Under fentanyl-depressed conditions, the bidirectional effects of racemic METH were observed with d-METH treatment while l-METH significantly exacerbated OIRD at 1.0 and 3.0 mg/kg. CONCLUSIONS: d-METH and l-METH differentially contribute to the bidirectional respiratory modulation observed by the racemate, with d-METH exhibiting predominantly stimulatory effects and l-METH exhibiting primarily depressant effects depending on dose.