Characterization and validation of a spontaneous acute and protracted oxycodone withdrawal model in male and female mice.

Opioid use disorder (OUD) is a serious health problem that may lead to physical dependence, in addition to affective disorders. Preclinical models are essential for studying the neurobiology of and developing pharmacotherapies to treat these problems. Historically, chronic morphine injections have most often been used to produce opioid-dependent animals, and withdrawal signs indicative of dependence were precipitated by administering an opioid antagonist. In the present studies, we have developed and validated a model of dependence on oxycodone (a widely prescribed opioid) during spontaneous withdrawal in male and female C57BL/6J mice. Dependence was induced by chronically administering oxycodone through osmotic minipumps at different doses for 7 days. Somatic withdrawal signs were measured after 3, 6, 24, and 48 h following minipump removal. Additionally, sensitivity to mechanical, thermal, and cold stimuli, along with anxiety-like behavior, were also measured. Our results indicated that spontaneous withdrawal following discontinuation of oxycodone produced an increase in total withdrawal signs after 60 and 120 mg/kg/day regimens of oxycodone administration. These signs were reversed by the administration of clinically approved medications for OUD. In general, both female and male mice showed similar profiles of somatic signs of spontaneous withdrawal. Spontaneous withdrawal also resulted in mechanical and cold hypersensitivity lasting for 24 and 14 days, respectively, and produced anxiety-like behaviors after 2 and 3 weeks following oxycodone removal. These results help validate a new model of oxycodone dependence, including the temporally distinct emergence of somatic, hyperalgesic, and anxiety-like behaviors, potentially useful for mechanistic and translational studies of opioid dependence.

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.

Theophylline reverses oxycodone's but not fentanyl's respiratory depression in mice while caffeine is ineffective against both opioids.

RATIONALE: The opioid epidemic remains a pressing public health crisis in the United States. Most of these overdose deaths are a result of lethal respiratory depression. In recent years the increasing incidence of opioid-involved overdose deaths has been driven by fentanyl, which is more resistant to adequate reversal by naloxone (NARCAN ®) than semi-synthetic or classical morphinan predecessors like oxycodone and heroin. For this and other reasons (e.g., precipitating withdrawal) non-opioidergic pharmacotherapies to reverse opioid-depressed respiration are needed. Methylxanthines are a class of stimulant drugs including caffeine and theophylline which exert their effects primarily via adenosine receptor antagonism. Evidence suggests methylxanthines can stimulate respiration by enhancing neural activity in respiratory nuclei in the pons and medulla independent of opioid receptors. This study aimed to determine whether caffeine and theophylline can stimulate respiration in mice when depressed by fentanyl and oxycodone. METHODS: Whole-body plethysmography was used to characterize fentanyl and oxycodone's effects on respiration and their reversal by naloxone in male Swiss Webster mice. Next, caffeine and theophylline were tested for their effects on basal respiration. Finally, each methylxanthine was evaluated for its ability to reverse similar levels of respiratory depression induced by fentanyl or oxycodone. RESULTS AND CONCLUSIONS: Oxycodone and fentanyl dose-dependently reduced respiratory minute volume (ml/min; MVb) that was reversible by naloxone. Caffeine and theophylline each significantly increased basal MVb. Theophylline, but not caffeine, completely reversed oxycodone-depressed respiration. In contrast, neither methylxanthine elevated fentanyl-depressed respiration at the doses tested. Despite their limited efficacy for reversing opioid-depressed respiration when administered alone, the methylxanthines safety, duration, and mechanism of action supports further evaluation in combination with naloxone to augment its reversal of opioid-depressed respiration.

Fentanyl analog structure-activity relationships demonstrate determinants of diverging potencies for antinociception and respiratory depression.

Opioid overdoses, particularly those involving fentanyl-related substances (FRS), present a significant public health challenge in the United States. This structure-activity relationship (SAR) study evaluated the relationship between the chemical structure of seventeen FRS and their in vivo mu-opioid-receptor (MOR) mediated effects. SAR evaluations included fluorine substitutions on the aniline or phenethyl ring and variations in N-acyl chain length. Adult male Swiss Webster mice were administered fluorinated regioisomers of fentanyl, butyrylfentantyl and valerylfentanyl, and compared to MOR standards including morphine, buprenorphine, and fentanyl to determine if they would elicit prototypical opioid-like effects including hyperlocomotion (open-field test), antinociception (warm-water tail-withdrawal test), and hypoventilation (whole-body plethysmography test). To determine if the MOR was the pharmacological mechanism responsible for these effects, naltrexone or naloxone pretreatments were administered to evaluate their actions on FRS-induced antinociception and hypoventilation. There were three main findings. First, FRS elicited hyperlocomotion, antinociception, and hypoventilation in mice to varying degrees, similar to prototypical MOR standards. Second, the rank order of potencies for hypoventilatory effects of FRS were different for each series including FRS with increasing N-acyl chain length (i.e., acetylfentanyl, fentanyl, butyrylfentanyl, valerylfentanyl, hexanoylfentanyl), phenethyl-fluorinated regioisomers (e.g., 2'-fluorofentanyl, 3'-fluorofentanyl, 4'-fluorofentanyl), and aniline-fluorinated regioisomers (e.g., ortho-fluorofentanyl, meta-fluorofentanyl, para-fluorofentanyl). Third, the degree of separation in potencies observed for the antinociceptive and hypoventilatory effects of these drugs did not always follow that which was observed for their antinociceptive and hyperlocomotor effects. This study clarifies the in vivo activities for these FRS and elucidates a SAR for MOR-mediated effects among structural isomers.

Amphetamines modulate fentanyl-depressed respiration in a bidirectional manner.

BACKGROUND: The opioid epidemic remains one of the most pressing public health crises facing the United States. Fentanyl and related synthetic opioid agonists have largely driven the rising rates of associated overdose deaths, in part, because of their surreptitious use as substitutes for other opioids and as adulterants in psychostimulants. Deaths involving opioids typically result from lethal respiratory depression, and it is currently unknown how co-use of psychostimulants with opioids affects respiratory toxicity. Considering psychostimulant overdoses have increased over 3-fold since 2013, and half of those co-involved opioids, this is a cardinal question. METHODS: Naloxone, d-amphetamine (AMPH), and (±)-methamphetamine (METH) were evaluated for their effects on basal and fentanyl-depressed respiration. Minute volume (MVb) was measured in awake, freely moving mice via whole-body plethysmography to quantify fentanyl-induced respiratory depression and its modulation by dose ranges of each test drug. RESULTS: Naloxone immediately reversed respiratory depression induced by fentanyl only at the highest dose tested (10 mg/kg). Both AMPH and METH exhibited bidirectional effects on MVb under basal conditions, producing significant (p ≤ 0.05) depressions then elevations of respiration as dose increased. Under depressed conditions the bidirectional effects of AMPH and METH on respiration were exaggerated, exacerbating and then reversing fentanyl-induced depression as dose increased. CONCLUSIONS: These results indicate that co-use of amphetamines with fentanyl may worsen respiratory depression, but conversely, monoaminergic components of the amphetamines may possibly be exploited to mitigate fentanyl overdose.

Structurally diverse fentanyl analogs yield differential locomotor activities in mice.

Synthetic narcotics have been implicated as the single greatest contributor to increases in opioid-related fatalities in recent years. This study evaluated the effects of nine fentanyl-related substances that have emerged in the recreational drug marketplace, and for which there are no existing or only limited in vivo data. Adult male Swiss Webster mice were administered fentanyl-related substances and their effects on locomotion as compared to MOR agonist standards were recorded. In locomotor activity tests, morphine (100, 180 mg/kg), buprenorphine (1, 10 mg/kg), fentanyl (1, 10 mg/kg), cyclopropylfentanyl (1, 10 mg/kg), cyclopentylfentanyl (10 mg/kg), (±)-cis-3-methylbutyrylfentanyl (0.1, 1, 10 mg/kg), ortho-methylacetylfentanyl (10 mg/kg), para-chloroisobutyrylfentanyl (100 mg/kg), ocfentanil (1, 10 mg/kg), and ortho-fluoroacrylfentanyl (0.1, 1, 10 mg/kg) elicited significant (p ≤ 0.05) dose-dependent increases in locomotion. However, 2,2,3,3-tetramethylcyclopropylfentanyl did not have any effects on locomotion, even when tested up to 100 mg/kg, and 4'-methylacetylfentanyl (10, 100 mg/kg) significantly decreased locomotion. The rank order of efficacy for stimulating locomotion (maximum effect as a % of fentanyl's maximum effect) for fentanyl-related substances relative to MOR agonist standards was cyclopropylfentanyl (108.84 ± 20.21) > fentanyl (100 ± 15.3) > ocfentanil (79.27 ± 16.92) > morphine (75.9 ± 14.5) > (±)-cis-3-methylbutyrylfentanyl (68.04 ± 10.08) > ortho-fluoroacrylfentanyl (63.56 ± 19.88) > cyclopentylfentanyl (56.46 ± 8.54) > para-chloroisobutyrylfentanyl (22.44 ± 8.51) > buprenorphine (11.26 ± 2.30) > ortho-methylacetylfentanyl (9.45 ± 2.92) > 2,2,3,3-tetramethylcyclopropylfentanyl (6.75 ± 1.43) > 4'-methylacetylfentanyl (3.47 ± 0.43). These findings extend in vivo results from previous reports documenting additional fentanyl related-related substances that stimulate locomotion similar to known abused opioids while also identifying some anomalies.

Glucocorticoid receptor dysregulation underlies 5-HT2AR-dependent synaptic and behavioral deficits in a mouse neurodevelopmental disorder model.

Prenatal environmental insults increase the risk of neurodevelopmental psychiatric conditions in the offspring. Structural modifications of dendritic spines are central to brain development and plasticity. Using maternal immune activation (MIA) as a rodent model of prenatal environmental insult, previous results have reported dendritic structural deficits in the frontal cortex. However, very little is known about the molecular mechanism underlying MIA-induced synaptic structural alterations in the offspring. Using prenatal (E12.5) injection with polyinosinic-polycytidylic acid potassium salt as a mouse MIA model, we show here that upregulation of the serotonin 5-HT2A receptor (5-HT2AR) is at least in part responsible for some of the effects of prenatal insults on frontal cortex dendritic spine structure and sensorimotor gating processes. Mechanistically, we report that this upregulation of frontal cortex 5-HT2AR expression is associated with MIA-induced reduction of nuclear translocation of the glucocorticoid receptor (GR) and, consequently, a decrease in the enrichment of GR at the 5-HT2AR promoter. The translational significance of these preclinical findings is supported by data in postmortem human brain samples suggesting dysregulation of GR translocation in frontal cortex of schizophrenia subjects. We also found that repeated corticosterone administration augmented frontal cortex 5-HT2AR expression and reduced GR binding to the 5-HT2AR promoter. However, virally (adeno-associated virus) mediated augmentation of GR function reduced frontal cortex 5-HT2AR expression and improved sensorimotor gating processes via 5-HT2AR. Together, these data support a negative regulatory relationship between GR signaling and 5-HT2AR expression in the mouse frontal cortex that may carry implications for the pathophysiology underlying 5-HT2AR dysregulation in neurodevelopmental psychiatric disorders.

Ulotaront, a novel TAAR1 agonist with 5-HT1A agonist activity, lacks abuse liability and attenuates cocaine cue-induced relapse in rats.

BACKGROUND: Ulotaront (SEP-363856) is a trace amine-associated receptor 1 (TAAR1) agonist with 5-hydroxytryptamine type 1A (5-HT1A) agonist activity that is currently in Phase 3 clinical development for the treatment of schizophrenia. Unlike available antipsychotics, the efficacy of ulotaront is not mediated by blockade of dopamine D2 or serotonin 5-HT2A receptors. In a short-term randomized clinical trial, ulotaront has demonstrated significant efficacy in the treatment of adults with an acute exacerbation of schizophrenia. Given ulotaront's novel mechanism of action a series of preclinical studies were performed to evaluate its potential abuse liability. METHODS: A battery of studies were conducted in male and female rats to evaluate whether ulotaront produces behavioral changes suggestive of human abuse potential. In addition, studies were undertaken to probe the potential for ulotaront to block reinstatement of cocaine-seeking behavior in male rats. RESULTS: Ulotaront was not self-administered by rats trained to self-administer amphetamine, cocaine, or heroin. The subjective qualities of ulotaront were distinct from those produced by amphetamine in a drug discrimination procedure. Ulotaront, and buspirone, a non-scheduled anxiolytic with 5-HT1A agonism, partially generalized to the interoceptive cue elicited by 3, 4-methylenedioxymethamphetamine (MDMA). In addition, ulotaront demonstrated a trend to reduce cocaine-primed induced reinstatement, and dose-dependently reduced cue-reinstated responding. CONCLUSION: The current results suggest that the TAAR1/5-HT1A agonist ulotaront is not likely to pose a risk for recreational abuse in humans and may have potential therapeutic utility as a treatment of substance use disorders.

Respiratory depressant effects of fentanyl analogs are opioid receptor-mediated.

Opioid-related fatalities involving synthetic opioids have reached unprecedented levels. This study evaluated the respiratory depressant effects of seven fentanyl analogs that have either emerged in the illicit drug supply or been identified in toxicological analyses following fatal or non-fatal intoxications. Adult male Swiss Webster mice were administered fentanyl analogs (isobutyrylfentanyl, crotonylfentanyl, para-methoxyfentanyl, para-methoxybutyrylfentanyl, 3-furanylfentanyl, thiophenefentanyl, and benzodioxolefentanyl) and their effects on minute volume as compared to mu-opioid receptor (MOR) agonist standards (fentanyl, morphine, and buprenorphine) were measured using whole body plethysmography (WBP). All drugs elicited significant (p ≤ 0.05) hypoventilation relative to vehicle for at least one dose tested: morphine (1, 3.2, 10, 32 mg/kg), buprenorphine, (0.032, 0.1, 0.32, 1, 3.2 mg/kg), fentanyl (0.0032, 0.01, 0.032, 0.1, 1, 32 mg/kg), isobutyrylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), crotonylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), para-methoxyfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), para-methoxybutyrylfentanyl (0.32, 1, 3.2, 10 mg/kg), 3-furanylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), thiophenefentanyl (1, 3.2, 10, 32, 100 mg/kg), and benzodioxolefentanyl (3.2, 10, 32, 100 mg/kg). The ED50 values for hypoventilation showed a rank order of potency as follows: fentanyl (ED50 = 0.96 mg/kg) > 3-furanylfentanyl (ED50 = 2.60 mg/kg) > crotonylfentanyl (ED50 = 2.72 mg/kg) > para-methoxyfentanyl (ED50 = 3.31 mg/kg) > buprenorphine (ED50 = 10.8 mg/kg) > isobutyrylfentanyl (ED50 = 13.5 mg/kg) > para-methoxybutyrylfentanyl (ED50 = 16.1 mg/kg) > thiophenefentanyl (ED50 = 18.0 mg/kg) > morphine (ED50 = 55.3 mg/kg) > benzodioxolefentanyl (ED50 = 10,168 mg/kg). A naloxone pretreatment (10 mg/kg) attenuated the hypoventilatory effects of all drugs. These results establish that the respiratory depressant effects of these fentanyl analogs are at least in part mediated by the MOR.

Sex-specific effects of psychedelics on prepulse inhibition of startle in 129S6/SvEv mice.

BACKGROUND: Prepulse inhibition (PPI) of startle is a sensorimotor gating phenomenon perturbed in a variety of neuropsychiatric conditions. Psychedelics disrupt PPI in rats and humans, but their effects and involvement of the serotonin 5-HT2A receptor (5-HT2AR) in mice remain unexplored. METHODS: We tested the effect of the psychedelic 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.5 mg/kg, i.p.) on startle amplitude and %PPI in response to acoustic stimuli under up to four different experimental conditions that included changes in background and stimulus intensity, prepulse and pulse duration, and interstimulus interval in male and female 129S6/SvEv mice. We also evaluated the effect of the 5-HT2AR antagonist M100,907 (1 mg/kg, i.p.) on DOI-induced startle amplitude and %PPI, as well as the effect of the psychedelic LSD (0.24 mg/kg, i.p.) and the dopamine agonists apomorphine (5 mg/kg, s.c.) and SKF-82,958 (0.5 mg/kg, i.p.) in male 129S6/SvEv mice. RESULTS: DOI altered startle amplitude with either pulse alone or prepulse + pulse presentations in all PPI conditions, and increased %PPI in three out of four PPI conditions in male mice - an effect that was prevented by M100,907. In female mice, DOI increased %PPI without affecting startle amplitude. %PPI was positively correlated with startle amplitude in males while being negatively correlated in female mice. In male mice, LSD also increased %PPI, although it did not affect startle amplitude, whereas apomorphine and SKF-82,958 induced decreases in %PPI. CONCLUSION: Our findings highlight a distinct effect of the psychedelic DOI on PPI in 129S6/SvEv mice, suggesting 5-HT2AR-dependent PPI improvement in a paradigm-dependent and sex-dependent manner.

Prolonged epigenomic and synaptic plasticity alterations following single exposure to a psychedelic in mice.

Clinical evidence suggests that rapid and sustained antidepressant action can be attained with a single exposure to psychedelics. However, the biological substrates and key mediators of psychedelics' enduring action remain unknown. Here, we show that a single administration of the psychedelic DOI produces fast-acting effects on frontal cortex dendritic spine structure and acceleration of fear extinction via the 5-HT2A receptor. Additionally, a single dose of DOI leads to changes in chromatin organization, particularly at enhancer regions of genes involved in synaptic assembly that stretch for days after the psychedelic exposure. These DOI-induced alterations in the neuronal epigenome overlap with genetic loci associated with schizophrenia, depression, and attention deficit hyperactivity disorder. Together, these data support that epigenomic-driven changes in synaptic plasticity sustain psychedelics' long-lasting antidepressant action but also warn about potential substrate overlap with genetic risks for certain psychiatric conditions.

Fentanyl-related substances elicit antinociception and hyperlocomotion in mice via opioid receptors.

Synthetic opioids have been implicated as the single greatest contributor to rising drug-related fatalities in recent years. This study evaluated mu-opioid receptor (MOR) mediated effects of seven fentanyl-related substances that have emerged in the recreational drug marketplace, and for which there are no existing or only limited in vivo data. Adult male Swiss Webster mice were administered fentanyl-related substances and their effects on nociception and locomotion as compared to MOR agonist standards were observed. In locomotor activity tests, morphine (100, 180 mg/kg), fentanyl (1, 10 mg/kg), beta-methylfentanyl (10 mg/kg), para-methoxyfentanyl (10 mg/kg), fentanyl carbamate (100 mg/kg), and 3-furanylfentanyl (10 mg/kg), elicited significant (p ≤ 0.05) dose-dependent increases in locomotion. However, para-methylfentanyl and beta'-phenylfentanyl did not produce significant effects on locomotion at doses up to 100 mg/kg and phenylfentanyl (100 mg/kg) significantly decreased locomotion. In warm-water tail-withdrawal tests, all substances produced significant dose-dependent increases in antinociception with increasing ED50 values (95% CI) of fentanyl [0.08 mg/kg (0.04-0.16)] > para-methoxyfentanyl [0.43 mg/kg (0.23-0.77)] > 3-furanylfentanyl [0.51 mg/kg (0.36-0.74)] > beta-methylfentanyl [0.74 mg/kg (0.64-0.85)] > para-methylfentanyl [1.92 mg/kg (1.48-2.45)] > fentanyl carbamate [5.59 mg/kg (4.11-7.54)] > morphine [7.82 mg/kg (5.42-11.0)] > beta'-phenylfentanyl [19.4 mg/kg (11.0-34.4)] > phenylfentanyl [55.2 mg/kg (33.5-93.0)]. Naltrexone (1 mg/kg) increased ED50 values several fold with decreasing magnitudes of para-methylfentanyl (63.1×) > para-methoxyfentanyl (22.5×) > beta'-phenylfentanyl (21.0×) > 3-furanylfentanyl (20.6×) > beta-methylfentanyl (19.2×) > phenylfentanyl (5.23×) > fentanyl (3.95×) > fentanyl carbamate (2.21×) > morphine (1.48×). These findings expand upon in vivo results from previous studies and establish that the effects of these fentanyl related-related substances are at least in part mediated by the MOR.

Validation and characterization of oxycodone physical dependence in C57BL/6J mice.

Opioid use disorder is a growing concern in the United States. Mice were used to investigate the mechanisms involving opioid physical dependence and for evaluating medications for treating opioid use disorders. While there are many preclinical reports describing protocols for inducing physical dependence upon morphine, there are fewer preclinical reports describing more contemporary abused prescription opiates. The goal of this study was to characterize and validate a mouse model of oxycodone dependence. Male C57BL/6J mice were injected with saline or increasing doses of oxycodone (9-33 mg/kg) twice daily for 8 days. On the 9th day, mice were challenged with 1 mg/kg naloxone and observed for somatic signs. Mice were pretreated with oxycodone (17, 33, or 75 mg/kg) prior to withdrawal to determine if it could attenuate somatic withdrawal signs. Additional mouse groups were pretreated with 1 mg/kg clonidine. Lastly, we measured somatic signs for 6, 24, and 48 h post-withdrawal during spontaneous and precipitated withdrawal. Pretreating with oxycodone or clonidine dose-dependently prevented the emergence of withdrawal signs. Mice chronically treated with oxycodone exhibited more withdrawal signs than vehicle at 24 h after the final injection during spontaneous withdrawal. In contrast, mice that received repeated naloxone challenges showed peak withdrawal signs at 6 h, and withdrawal signs were significantly greater at all time points compared to vehicle. Reversal of withdrawal effects by positive controls, and establishing spontaneous and precipitated withdrawal paradigms, serve as validation of this model and provide a means to examine novel therapeutics to treat opioid withdrawal.

Methylnaltrexone crosses the blood-brain barrier and attenuates centrally-mediated behavioral effects of morphine and oxycodone in mice.

BACKGROUND: Antagonism of peripheral opioid receptors by methylnaltrexone (MNTX) was recently proposed as a potential mechanism to attenuate the development of opioid analgesic tolerance based on experiments conducted in mice. However, reports indicate that MNTX is demethylated to naltrexone (NTX) in mice, and NTX may subsequently cross the blood-brain barrier to antagonize centrally-mediated opioid effects. The goal of this study was to determine whether MNTX alters centrally-mediated behaviors elicited by the opioid analgesics, morphine and oxycodone, and to quantify concentrations of MNTX and NTX in blood and brain following their administration in mice. METHODS: Combinations of MNTX and morphine were tested under acute and chronic conditions in thermal nociceptive assays. Effects of MNTX and NTX pretreatment were assessed in an oxycodone discrimination operant procedure. Blood and brain concentrations of these antagonists were quantified after their administration using liquid chromatography-mass spectrometry. RESULTS: MNTX dose-dependently attenuated acute and chronic morphine antinociception. MNTX and NTX dose-dependently antagonized the discriminative stimulus effects of oxycodone. MNTX and NTX were detected in both blood and brain after administration of MNTX, confirming its demethylation and demonstrating that MNTX itself can cross the blood-brain barrier. CONCLUSIONS: These results provide converging behavioral and analytical evidence that MNTX administration in mice attenuates centrally-mediated effects produced by opioid analgesics and results in functional concentrations of MNTX and NTX in blood and brain. Collectively, these findings indicate that MNTX cannot be administered systemically in mice for making inferences that its effects are peripherally restricted.

Pharmacological characterization of 17-cyclopropylmethyl-3,14-dihydroxy-4,5-epoxy-6-[(3'-fluoro-4'-pyridyl)acetamido]morphinan (NFP) as a dual selective MOR/KOR ligand with potential applications in treating opioid use disorder.

For thousands of years opioids have been the first-line treatment option for pain management. However, the tolerance and addiction potential of opioids limit their applications in clinic. NFP, a MOR/KOR dual-selective opioid antagonist, was identified as a ligand that significantly antagonized the antinociceptive effects of morphine with lesser withdrawal effects than naloxone at similar doses. To validate the potential application of NFP in opioid addiction treatment, a series of in vitro and in vivo assays were conducted to further characterize its pharmacological profile. In calcium mobilization assays and MOR internalization studies, NFP showed the apparent capacity to antagonize DAMGO-induced calcium flux and etorphine-induced MOR internalization. In contrast to the opioid agonists DAMGO and morphine, cells pretreated with NFP did not show apparent desensitization and down regulation of the MOR. Though in vitro bidirectional transport studies showed that NFP might be a P-gp substrate, in warm-water tail-withdrawal assays it was able to antagonize the antinociceptive effects of morphine indicating its potential central nervous system activity. Overall these results suggest that NFP is a promising dual selective opioid antagonist that may have the potential to be used therapeutically in opioid use disorder treatment.

The ANKS1B gene and its associated phenotypes: focus on CNS drug response.

The ANKS1B gene was a top finding in genome-wide association studies (GWAS) of antipsychotic drug response. Subsequent GWAS findings for ANKS1B include cognitive ability, educational attainment, body mass index, response to corticosteroids and drug dependence. We review current human association evidence for ANKS1B, in addition to functional studies that include two published mouse knockouts. The several GWAS findings in humans indicate that phenotypically relevant variation is segregating at the ANKS1B locus. ANKS1B shows strong plausibility for involvement in CNS drug response because it encodes a postsynaptic effector protein that mediates long-term changes to neuronal biology. Forthcoming data from large biobanks should further delineate the role of ANKS1B in CNS drug response.

Opioid-like antinociceptive and locomotor effects of emerging fentanyl-related substances.

The emergence of several fentanyl-related substances in the recreational drug marketplace has resulted in a surge of opioid overdose deaths in the United States. Many of these substances have never been examined in living organisms under controlled conditions. In the present study, seven fentanyl-related substances were tested in adult male Swiss Webster mice for their effects on locomotion and antinociception and compared to those of fentanyl and morphine. In locomotor activity tests, fentanyl (1, 10 mg/kg), morphine (100, 180 mg/kg), isobutyrylfentanyl (10 mg/kg), crotonylfentanyl (10 mg/kg), para-fluorobutyrylfentanyl (10, 100 mg/kg), para-methoxybutyrylfentanyl (10 mg/kg), thiophenefentanyl (100 mg/kg), and benzodioxolefentanyl (0.1 mg/kg) produced significant (p ≤ 0.05) dose-dependent increases in locomotion. Valerylfentanyl, however, was without effects on locomotion up to 100 mg/kg. In warm-water tail-withdrawal tests, all substances produced significant (p ≤ 0.05) dose-dependent increases in antinociception with increasing ED50 values (CI) of isobutyrylfentanyl [0.0768 mg/kg (0.044-0.128)] > fentanyl [0.0800 mg/kg (0.0403-0.164)] > para-methoxybutyrylfentanyl [0.106 mg/kg (0.0516-0.195)] > crotonylfentanyl [0.226 mg/kg (0.176-0.292)] > para-fluorobutyrylfentanyl [0.908 mg/kg (0.459-1.58)] > thiophenefentanyl [4.66 mg/kg (3.65-5.95)] > valerylfentanyl [6.43 mg/kg (3.91-10.5)] > morphine [7.82 mg/kg (5.42-11.0)] > benzodioxolefentanyl [46.3 mg/kg (25.8-83.4)]. Naltrexone (1 mg/kg) increased antinociceptive ED50 values several fold in decreasing magnitudes of isobutyrylfentanyl (233x) > para-methoxybutyrylfentanyl (37.7x) > thiophenefentanyl (34.6x) > valerylfentanyl (11.9x) > para-fluorobutyrylfentanyl (10.9x) > benzodioxolefentanyl (8.42x) > crotonylfentanyl (6.27x) > fentanyl (3.95x) > morphine (1.48x). These findings establish that locomotor and antinociceptive effects of several fentanyl-related substances are similar to those of morphine and fentanyl and are mediated by opioid receptors.

Oxycodone-like discriminative stimulus effects of fentanyl-related emerging drugs of abuse in mice.

BACKGROUND: Fentanyl and its structurally related compounds have emerged as the most significant contributors to opioid overdose fatalities in recent years. While there is abundant information about the pharmacological effects of fentanyl, far less is known of its more recently abused analogs. The objective of this study was to determine whether fentanyl and several fentanyl-related substances would engender oxycodone-like responding in a mouse model of oxycodone discrimination. Oxycodone was selected as the training drug due to its high selectivity for mu opioid receptors. Compounds that elicited oxycodone-like responding in this procedure would likely evoke overlapping subjective experiences. METHODS: Adult male C57BL/6 mice were trained to discriminate 1.3 mg/kg oxycodone from vehicle in a food-reinforced, two-lever choice procedure. Generalization tests were conducted with fentanyl and the following fentanyl-related compounds: ocfentanil, 3-furanyl fentanyl, crotonylfentanyl, and valerylfentanyl. RESULTS: Fentanyl and each of its analogs completely generalized to the 1.3 mg/kg oxycodone discriminative stimulus and naltrexone pretreatment significantly decreased oxycodone-like responding for each compound. Rank order potency for engendering oxycodone-appropriate responding was ocfentanil > fentanyl > 3-furanyl fentanyl ≈ crotonylfentanyl > oxycodone > valerylfentanyl. Drug doses that evoked full substitution also significantly suppressed response rates compared to vehicle. CONCLUSIONS: These results indicate that the discriminative stimulus, and by extension, the interoceptive and subjective effects of the tested fentanyl analogs, overlap with those of oxycodone. These observations consequentially support the prediction that they would also engender the likelihood for abuse similar to oxycodone. This article is part of the Special Issue entitled 'Opioid Neuropharmacology: Advances in treating pain and opioid addiction'.

Synthetic Opioids.

Opioid abuse has been a global menace for centuries, but the proliferation of synthetic opioids and their use within this current decade have created epidemic-level harms in some countries. According to the United Nations Office on Drugs and Crime, almost 12 million years were estimated loss of "healthy" life resulting in premature death and disability attributable to global opioid abuse just in 2015. Law enforcement and regulatory authorities have been particularly challenged abating the spread of synthetic opioids because soon after controlling the currently recognized abused opioids, their structures are tweaked, and new entities replace them. Drug racketeers have most often exploited the fentanyl phenylpiperidine structure in this regard, but non-fentanyl-like and classical morphinan-like structures have been pirated as well. A growing number of anecdotal reports identify respiratory depression induced by these newer synthetic opioids to be especially refractive to reversal by antagonists, with consequently high levels of lethality. This review examines three of these synthetic opioids representing three chemical classes (U-47700, MT-45, and acetylfentanyl) that have emerged to be of such menace that they have been brought under international control in recent years and addresses factors that could make synthetic opioids especially untreatable by opioid antagonists.

Nociceptin/orphanin FQ receptors modulate the discriminative stimulus effects of oxycodone in C57BL/6 mice.

BACKGROUND: Nociceptin/orphanin FQ (NOP) receptor ligands have shown efficacy as putative analgesics and can modulate the abuse-related effects of opioids, suggesting therapeutic applications. The discriminative stimulus effects of a drug are related to their subjective effects, a predictor of abuse potential. To determine whether activation of NOP receptors could alter the subjective effects of an abused opioid analgesic, a novel oxycodone discrimination was established in mice, characterized with positive and negative controls, and its expression evaluated with a NOP receptor agonist. METHODS: Adult male C57BL/6 mice were trained to discriminate 1.3 mg/kg oxycodone from vehicle in a two-lever operant procedure. The discrimination was characterized with naloxone challenge, and generalization tests with the µ-opioid receptor agonists, heroin and morphine, and the κ-opioid receptor selective agonist, U50488. Subsequently, effects of the NOP agonist Ro64-6198 were evaluated with and without oxycodone. RESULTS: Oxycodone generalization occurred in a dose-dependent manner and was reversed by naloxone pretreatment. Heroin and morphine, but not U50488, substituted for oxycodone. Co-treatment of 1 mg/kg Ro64-6198 with the oxycodone training dose reduced % oxycodone lever responding (%OLR) and restored response rates to vehicle control levels. J-113397, a NOP antagonist, reversed these effects. Co-administration of 1 mg/kg Ro64-6198 with a range of oxycodone doses resulted in rightward dose-effect curve shifts in %OLR and response rates compared to oxycodone alone. CONCLUSIONS: These results provide additional evidence that NOP receptor activation can modulate the subjective effects of opioid analgesics and represent the first characterization of oxycodone's discriminative stimulus effects in mice.