Opioid-environment interaction: Contrasting effects of morphine administered in a novel versus familiar environment on acute and repeated morphine induced behavioral effects and on acute morphine ERK activation in reward associated brain areas

We report that environmental context can have a major impact on morphine locomotor behavior and ERK effects. We manipulated environmental context in terms of an environmental novelty/ familiarity dimension and measured morphine behavioral effects in both acute and chronic morphine treatment protocols. Wistar rats (n=7 per group) were injected with morphine 10 mg/kg or vehicle (s.c.), and immediately placed into an arena for 5 min, and locomotor activity was measured after one or 5 days. The morphine treatments were initiated either when the environment was novel or began after the rats had been familiarized with the arena by being given 5 daily nondrug tests in the arena. The results showed that acute and chronic morphine effects were strongly modified by whether the environment was novel or familiar. Acute morphine administered in a novel environment increased ERK activity more substantially in several brain areas, particularly in reward-associated areas such as the VTA in comparison to when morphine was given in a familiar environment. Repeated morphine treatments initiated in a novel environment induced a strong locomotor sensitization, whereas repeated morphine treatments initiated in a familiar environment did not induce a locomotor stimulant effect but rather a drug discriminative stimulus dis-habituation effect. The marked differential effects of environmental novelty/familiarity and ongoing dopamine activity on acute and chronic morphine treatments may be of potential clinical relevance for opioid drug addiction.

Parental Exposure to Morphine Before Conception Decreases Morphine and Cocaine-Induced Locomotor Sensitization in Male Offspring

Repeated exposure to abused drugs leads to reorganizing synaptic connections in the brain, playing a pivotal role in the relapse process. Additionally, recent research has highlighted the impact of parental drug exposure before gestation on subsequent generations. This study aimed to explore the influence of parental morphine exposure 10 days prior to pregnancy on drug-induced locomotor sensitization. Adult male and female Wistar rats were categorized into morphine-exposed and control groups. Ten days after their last treatment, they were mated, and their male offspring underwent morphine, methamphetamine, cocaine, and nicotine-induced locomotor sensitization tests. The results indicated increased locomotor activity in both groups after drug exposure, although the changes were attenuated in morphine and cocaine sensitization among the offspring of morphine-exposed parents (MEPs). Western blotting analysis revealed altered levels of D2 dopamine receptors (D2DRs) in the prefrontal cortex and nucleus accumbens of the offspring from MEPs. Remarkably, despite not having direct in utero drug exposure, these offspring exhibited molecular alterations affecting morphine and cocaine-induced sensitization. The diminished sensitization to morphine and cocaine suggested the development of a tolerance phenotype in these offspring. The changes in D2DR levels in the brain might play a role in these adaptations.

Insular cortex subregions have distinct roles in cued heroin seeking after extinction learning and prolonged withdrawal in rats

Evidence indicates that the anterior (aIC), but not posterior (pIC), insular cortex promotes cued reinstatement of cocaine seeking after extinction in rats. It is unknown whether these subregions also regulate heroin seeking and whether such involvement depends on prior extinction learning. To address these questions, we used baclofen and muscimol (BM) to inactivate the aIC or pIC bilaterally during a seeking test after extinction or prolonged withdrawal from heroin. Male Sprague-Dawley rats in the extinction groups underwent 10+ days of heroin self-administration, followed by 6+ days of extinction sessions, and subsequent cued or heroin-primed reinstatement. Results indicate that aIC inactivation increased cued reinstatement of heroin seeking after extinction, whereas pIC inactivation prevented cued reinstatement. To determine whether these effects were extinction-dependent, we conducted a subsequent study using both sexes with prolonged withdrawal. Male and female rats in the withdrawal groups underwent 10+ days of heroin self-administration, followed by cued seeking tests after 1 and 14 days of homecage withdrawal to measure incubation of heroin craving. In this case, the findings indicate that aIC inactivation had no effect on incubation of heroin craving after withdrawal in either sex, whereas pIC inactivation decreased heroin craving only in males. These findings suggest that the aIC and pIC have opposing roles in suppressing vs promoting cued heroin seeking after extinction and that these roles are distinct from those in cocaine seeking. Moreover, the incubation of craving results suggest that new contingency learning is necessary to recruit the aIC in cued heroin seeking.

Diurnal sex differences in morphine withdrawal revealed by continuous assessment of voluntary home cage wheel running in the rat

Animal studies modeling recreational opioid use show more severe withdrawal symptoms in male compared to female rats, whereas our study modeling opioid use for pain showed a greater withdrawal-induced decrease in wheel running in female rats. The objective of this experiment was to determine whether sex differences in spontaneous morphine withdrawal are caused by differences in assessment method (i.e., wheel running vs. somatic symptoms). Twice daily injections of morphine (5 - 20 mg/kg, s.c.) for 5 days produced a dose and time dependent decrease in wheel running that was greater in male compared to female rats. Termination of morphine administration resulted in an overall decrease in running and a decrease in the amount of running during the dark phase of the light cycle from 95 % to approximately 75 %. In male rats, this decrease in the percent of dark running was caused by a large decrease in dark phase running, whereas female rats had a slightly higher increase in light phase running. Withdrawal also reduced maximal running speed and caused a decrease in body weight that was larger in male than female rats. Withdrawal symptoms were greatest on the day following the last morphine injection, but persisted for all 3 days of assessment. Morphine withdrawal produced a greater decrease in dark phase wheel running and body weight in male rats and a greater increase in light phase running in female rats. Voluntary home cage wheel running provides a continuous measure of opioid withdrawal that is consistent with other measures of opioid withdrawal.

Large field-of-view metabolic profiling of murine brain tissue following morphine incubation using label-free multiphoton microscopy

BACKGROUND

Although the effects on neural activation and glucose consumption caused by opiates such as morphine are known, the metabolic machinery underlying opioid use and misuse is not fully explored. Multiphoton microscopy (MPM) techniques have been developed for optical imaging at high spatial resolution. Despite the increased use of MPM for neural imaging, the use of intrinsic optical contrast has seen minimal use in neuroscience.

NEW METHOD

We present a label-free, multimodal microscopy technique for metabolic profiling of murine brain tissue following incubation with morphine sulfate (MSO4). We evaluate two- and three-photon excited autofluorescence, and second and third harmonic generation to determine meaningful intrinsic contrast mechanisms in brain tissue using simultaneous label-free, autofluorescence multi-harmonic (SLAM) microscopy.

RESULTS

Regional differences quantified in the cortex, caudate, and thalamus of the brain demonstrate region-specific changes to metabolic profiles measured from FAD intensity, along with brain-wide quantification. While the overall intensity of FAD signal significantly decreased after morphine incubation, this metabolic molecule accumulated near the nucleus accumbens.

COMPARISON WITH EXISTING METHODS

Histopathology requires tissue fixation and staining to determine cell type and morphology, lacking information about cellular metabolism. Tools such as fMRI or PET imaging have been widely used, but lack cellular resolution. SLAM microscopy obviates the need for tissue preparation, permitting immediate use and imaging of tissue with subcellular resolution in its native environment.

CONCLUSIONS

This study demonstrates the utility of SLAM microscopy for label-free investigations of neural metabolism, especially the intensity changes in FAD autofluorescence and structural morphology from third-harmonic generation.

Blockade of mGluR5 in nucleus accumbens modulates calcium sensor proteins, facilitates extinction, and attenuates reinstated morphine place preference in rats

Numerous findings confirm that the metabotropic glutamate receptors (mGluRs) are involved in the conditioned place preference (CPP) induced by morphine. Here we focused on the role of mGluR5 in the nucleus accumbens (NAc) as a main site of glutamate action on the rewarding effects of morphine. Firstly, we investigated the effects of intra-NAc administrating mGluR5 antagonist 3-((2-Methyl-1,3-thiazol-4-yl) ethynyl) pyridine hydrochloride (MTEP; 1, 3, and 10 μg/μl saline) on the extinction and the reinstatement phase of morphine CPP. Moreover, to determine the downstream signaling cascades of mGluR5 in morphine CPP, the protein levels of stromal interaction molecules (STIM1 and 2) in the NAc and hippocampus (HPC) were measured by western blotting. The behavioral data indicated that the mGluR5 blockade by MTEP at the high doses of 3 and 10 μg facilitated the extinction of morphine-induced CPP and attenuated the reinstatement to morphine in extinguished rats. Molecular results showed that the morphine led to increased levels of STIM proteins in the HPC and increased the level of STIM1 without affecting STIM2 in the NAc. Furthermore, intra-NAc microinjection of MTEP (10 μg) in the reinstatement phase decreased STIM1 in the NAc and HPC and reduced the STIM2 in the HPC. Collectively, our data show that morphine could facilitate brain reward function in part by increasing glutamate-mediated transmission through activation of mGluR5 and modulation of STIM proteins. Therefore, these results highlight the therapeutic potential of mGluR5 antagonists in morphine use disorder.

Prevalence of high impulsivity and its relation to sleep indices in opioid use disorder patients receiving methadone maintenance treatment

BACKGROUND

The relation between impulsivity and sleep indices is not well determined in patients receiving methadone maintenance treatment (MMT).

AIMS

to evaluate high impulsivity prevalence, its risk factors and relation with sleep indices.

METHODS

a random MMT sample (n = 61) plus MMT current cocaine users (n = 20) were assessed for impulsivity (Barratt impulsivity scale [BIS-11] and Balloon Analogue Risk task [BART]), sleep quality (Pittsburg Sleep Quality Index [PSQI]), sleepiness (The Epworth sleepiness scale [ESS]), and substance in urine.

RESULTS

81 patients, aged 56.6 ± 10, 54.3% tested positive to any substance, 53.1% with poor sleep (PSQI>5) and 43.2% with daytime sleepiness (ESS >7) were studied. Impulsivity (BIS-11 ≥ 72) prevalence was 27.9% (of the representative sample), and 30.9% of all participants. These patients characterized with any substance and shorter duration in MMT with no sleep indices or other differences including BART balloon task performance (that was higher only in any substance than non-substance user group). However, impulsive score linearly correlated with daytime sleepiness (R = 0.2, p = 0.05). Impulsivity proportion was lowest among those with no cocaine followed by cocaine use and the highest in those who used cocaine and opiates (20.8%, 33.3% and 60% respectively, p = 0.02), as daily sleep (38.3%, 42.1% and 60%, p = 0.3) although not statistically significant.

CONCLUSION

Daytime sleepiness correlated with impulsivity, but cocaine usage is the robust factor. Further follow-up is warranted to determine whether substance discontinuing will lead to a reduction in impulsivity, and improved vigilance. Sleep quality did not relate to daytime sleepiness and impulsivity and need further research.

Repeated Morphine Exposure Alters Temporoamonic-CA1 Synaptic Plasticity in Male Rat Hippocampus

In this study, the electrophysiological and biochemical consequences of repeated exposure to morphine in male rats on glutamatergic synaptic transmission, synaptic plasticity, the expression of GABA receptors and glutamate receptors at the temporoammonic-CA1 synapse along the longitudinal axis of the hippocampus (dorsal, intermediate, ventral, DH, IH, VH, respectively) were investigated. Slice electrophysiological methods, qRT-PCR, and western blotting techniques were used to characterize synaptic plasticity properties. We showed that repeated morphine exposure (RME) reduced excitatory synaptic transmission and ability for long-term potentiation (LTP) in the VH as well as eliminated the dorsoventral difference in paired-pulse responses. A decreased expression of NR2B subunit in the VH and an increased expression GABAA receptor of α1 and α5 subunits in the DH were observed following RME. Furthermore, RME did not affect the expression of NR2A, AMPA receptor subunits, and γ2GABAA and GABAB receptors in either segment of the hippocampus. In sum, the impact of morphine may differ depending on the region of the hippocampus studied. A distinct change in the short- and long-term synaptic plasticity along the hippocampus long axis due to repeated morphine exposure, partially mediated by a change in the expression profile of glutamatergic receptor subunits. These findings can be useful in further understanding the cellular mechanism underlying deficits in information storage and, more generally, cognitive processes resulting from chronic opioid abuse.

Differential effects of acute and prolonged morphine withdrawal on motivational and goal-directed control over reward-seeking behaviour

Opioid addiction is a relapsing disorder marked by uncontrolled drug use and reduced interest in normally rewarding activities. The current study investigated the impact of spontaneous withdrawal from chronic morphine exposure on emotional, motivational and cognitive processes involved in regulating the pursuit and consumption of food rewards in male rats. In Experiment 1, rats experiencing acute morphine withdrawal lost weight and displayed somatic signs of drug dependence. However, hedonically driven sucrose consumption was significantly elevated, suggesting intact and potentially heightened reward processing. In Experiment 2, rats undergoing acute morphine withdrawal displayed reduced motivation when performing an effortful response for palatable food reward. Subsequent reward devaluation testing revealed that acute withdrawal disrupted their ability to exert flexible goal-directed control over reward seeking. Specifically, morphine-withdrawn rats were impaired in using current reward value to select actions both when relying on prior action-outcome learning and when given direct feedback about the consequences of their actions. In Experiment 3, rats tested after prolonged morphine withdrawal displayed heightened rather than diminished motivation for food rewards and retained their ability to engage in flexible goal-directed action selection. However, brief re-exposure to morphine was sufficient to impair motivation and disrupt goal-directed action selection, though in this case, rats were only impaired in using reward value to select actions in the presence of morphine-paired context cues and in the absence of response-contingent feedback. We suggest that these opioid-withdrawal induced deficits in motivation and goal-directed control may contribute to addiction by interfering with the pursuit of adaptive alternatives to drug use.

Adductor Canal Nerve Block versus Intra-articular Anesthetic in Knee Arthroscopy: A Single-Blinded Prospective Randomized Trial

Effective perioperative pain control following knee arthroscopy allows patients to reduce narcotic intake, avoid side effects of these medications, and recover more quickly. Adductor canal nerve blockade (ACB) and intra-articular injection of local anesthetic have been described as adjuvant treatments for postoperative pain control following surgery of the knee. This study directly compares the effect of each of these treatment modalities. Patients undergoing knee arthroscopy were blinded and randomized to receive either an ACB (n = 60) or intra-articular injection of local anesthetic (IAB, n = 64). Outcome measures included patient reported visual analog scale (VAS) scores at 1, 2, 4, 8, 16, 24, 36, 48 hours and 1 week and total narcotic consumption at 12, 24, and 48 hours postoperatively. Student's t-tests were used to compare unadjusted VAS scores at each time point and use of postoperative pain medication between treatment groups. Adjusted VAS scores were estimated in a multivariable general linear model with interaction of time and treatment group and other relevant covariates. There were no statistically significant differences between the two groups in terms of gender, age, body mass index, and insurance type. ACB patients had significantly higher pain scores than IAB patients at hours 1 and 2 (hour 1: 4.02 [2.99] vs. 2.59 [3.00], p = 0.009; hour 2: 3.12 [2.44] vs. 2.17 [2.62], p = 0.040). ACB patients had higher pain scores than IAB patients up to hour 16, though hours 4 to 16 were not significantly different. Adjusted covariate analyses demonstrate an additional statistically significant reduction in pain score in the IAB group at hour 4. There were no differences in narcotic consumption. Intraoperative local anesthetic and regional ACB each provides adequate pain control following knee arthroscopy, and intraoperative local anesthetic may provide enhanced pain control for up to 4 hours postoperatively. LEVEL OF EVIDENCE: : Level 1 evidence, randomized control trial.

Decreased myelin-related gene expression in the nucleus accumbens during spontaneous neonatal opioid withdrawal in the absence of long-term behavioral effects in adult outbred CFW mice

Prenatal opioid exposure is a major health concern in the United States, with the incidence of neonatal opioid withdrawal syndrome (NOWS) escalating in recent years. NOWS occurs upon cessation of in utero opioid exposure and is characterized by increased irritability, disrupted sleep patterns, high-pitched crying, and dysregulated feeding. The main pharmacological strategy for alleviating symptoms is treatment with replacement opioids. The neural mechanisms mediating NOWS and the long-term neurobehavioral effects are poorly understood. We used a third trimester-approximate model in which neonatal outbred pups (Carworth Farms White; CFW) were administered once-daily morphine (15 mg/kg, s.c.) from postnatal day (P) day 1 through P14 and were then assessed for behavioral and transcriptomic adaptations within the nucleus accumbens (NAc) on P15. We also investigated the long-term effects of perinatal morphine exposure on adult learning and reward sensitivity. We observed significant weight deficits, spontaneous thermal hyperalgesia, and altered ultrasonic vocalization (USV) profiles following repeated morphine and during spontaneous withdrawal. Transcriptome analysis of NAc from opioid-withdrawn P15 neonates via bulk mRNA sequencing identified an enrichment profile consistent with downregulation of myelin-associated transcripts. Despite the neonatal behavioral and molecular effects, there were no significant long-term effects of perinatal morphine exposure on adult spatial memory function in the Barnes Maze, emotional learning in fear conditioning, or in baseline or methamphetamine-potentiated reward sensitivity as measured via intracranial self-stimulation. Thus, the once daily third trimester-approximate exposure regimen, while inducing NOWS model traits and significant transcriptomic effects in neonates, had no significant long-term effects on adult behaviors.

Sex-differences in anxiety, neuroinflammatory markers, and enhanced fear learning following chronic heroin withdrawal

Post-traumatic stress disorder (PTSD) and opioid use disorder (OUD) are comorbid in clinical populations. However, both pre-clinical and clinical studies of these co-occurring disorders have disproportionately represented male subjects, limiting the applicability of these findings. Our previous work has identified chronic escalating heroin administration and withdrawal can produce enhanced fear learning. This behavior is associated with an increase in dorsal hippocampal (DH) interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and glial fibrillary acidic protein (GFAP) immunoreactivity. Further, we have shown that these increases in IL-1β and TNF-α are mechanistically necessary for the development of enhanced fear learning. Although these are exciting findings, this paradigm has only been studied in males. The current studies aim to examine sex differences in the behavioral and neuroimmune effects of chronic heroin withdrawal and future enhanced fear learning. In turn, we determined that chronic escalating heroin administration can produce withdrawal in female rats comparable to male rats. Subsequently, we examined the consequence of heroin withdrawal on future enhanced fear learning and IL-1β, TNF-α, and GFAP immunoreactivity. Strikingly, we identified sex differences in these neuroimmune measures, as chronic heroin administration and withdrawal does not produce enhanced fear learning or immunoreactivity changes in females. Moreover, we determined whether heroin withdrawal produces short-term and long-term anxiety behaviors in both female and males. Collectively, these novel experiments are the first to test whether heroin withdrawal can sensitize future fear learning, produce neurobiological changes, and cause short-term and long-term anxiety behaviors in female rats.

NMDA Receptors in the Rat Paraventricular Thalamic Nucleus Reduce the Naloxone-induced Morphine Withdrawal

BACKGROUND

NMDA receptors have a significant role in the development of opioid physical dependence. Evidence demonstrated that a drug of abuse enhances neuronal excitability in the Paraventricular Nucleus (PVT). The current research studied whether blocking NMDA receptors through the administration of MK801 in the PVT nucleus could affect the development of Morphine (Mor) dependence and hence the behavioral indices induced by morphine withdrawal in rats.

METHODS

Male Wistar rats weighing 250-300 g were used. For induction of drug dependence, we injected Mor subcutaneously (s.c.) (6, 16, 26, 36, 46, 56, and 66 mg/kg, 2 ml/kg) at an interval of 24 hours for 7 days. Animals were divided into two groups in which the NMDA receptor antagonist, MK801 (20 mM in 0.1 ml), or its vehicle were applied into the PVT nucleus for 7 days before each Mor administration. On day 8, after injection of naloxone (Nal, 2.5 mg/kg, i.p.), withdrawal behaviors were checked for 25 min.

RESULTS

The current results demonstrated that the blockade of the NMDA receptor in the PVT nucleus significantly increased withdrawal behaviors provoked by the application of Nal in morphinedependent (Mor-d) rats.

CONCLUSION

We concluded that the NMDA receptor in the PVT nucleus changes the development of Mor dependence.

Sex differences in the impact of pain, morphine administration and morphine withdrawal on quality of life in rats

The disruptive effects of pain on quality of life are greater in men than in women, but the disruptive effects of opioid administration and withdrawal tend to be greater in women. These sex differences in pain, acute opioid effects, and opioid withdrawal tend to be opposite to sex differences reported in laboratory rats. We hypothesized that sex differences in humans and rats would more closely align if animal research measured quality of life as opposed to traditional evoked behaviors of pain (e.g., nociceptive reflexes) and opioid withdrawal (e.g., wet dog shakes). The present study assessed quality of life in adult female and male rats by measuring voluntary wheel running in the rat's home cage. Hindpaw inflammation induced by administration of Complete Freund's Adjuvant (CFA) into the right hindpaw caused a greater depression of wheel running in male compared to female rats. Twice daily injections of high morphine doses (5-20 mg/kg) and the subsequent morphine withdrawal caused a greater depression of wheel running in female compared to male rats. These sex differences are consistent with human data that shows the impact of pain on quality of life is greater in men than women, but the negative effects of opioid administration and withdrawal are greater in women. The present data indicate that the clinical significance of animal research would be enhanced by shifting the endpoint from pain and opioid evoked behaviors to measures of quality of life such as voluntary wheel running.

LncRNA MRAK159688 facilitates morphine tolerance by promoting REST-mediated inhibition of mu opioid receptor in rats

Morphine tolerance (MT) caused by the long-term use of morphine is a major medical problem. The molecular mechanism of morphine tolerance remains elusive. Here, we established a morphine tolerance model in rats and verified whether the long noncoding RNA (lncRNA) MRAK159688 is involved in morphine tolerance and its specific molecular mechanism. We show the significant upregulation of MRAK159688 expression in the spinal cord of morphine-tolerant rats. Overexpression of MRAK159688 by a lentivirus reduces the analgesic efficacy of morphine and induces pain behavior. Downregulation of MRAK159688 using a small interfering RNA (siRNA) attenuates the formation of morphine tolerance, partially reverses the development of morphine tolerance and alleviates morphine-induced hyperalgesia. MRAK159688 is located in the nucleus and cytoplasm of neurons, and it colocalizes with repressor element-1 silencing transcription factor (REST) in the nucleus. MRAK159688 potentiates the expression and function of REST, thereby inhibiting the expression of mu opioid receptor (MOR) and subsequently inducing morphine tolerance. Moreover, REST overexpression blocks the effects of MRAK159688 siRNA on relieving morphine tolerance. In general, chronic morphine administration-mediated upregulation of MRAK159688 in the spinal cord contributes to morphine tolerance and hyperalgesia by promoting REST-mediated inhibition of MOR. MRAK159688 downregulation may represent a novel RNA-based therapy for morphine tolerance.

Current Best Practices for Acute and Chronic Management of Patients with Opioid Use Disorder

This comprehensive review on opioids summarizes the scope of the current opioid epidemic, the diagnosis and treatment of opioid use disorder, and the medical and psychiatric complications of opioid use.

Comparative benefits of social housing and buprenorphine on wheel running depressed by morphine withdrawal in rats

RATIONALE

Social support and opioid replacement therapy are commonly used to treat opioid withdrawal.

OBJECTIVE

The present study tested the hypothesis that social housing and buprenorphine administration can restore wheel running depressed by morphine withdrawal in rats.

RESULTS

Experiment 1 assessed disruptive side effects of buprenorphine and found that administration of low doses (3.2, 10, & 32 µg/kg, s.c.) had no impact on voluntary wheel running. Experiment 2 assessed the impact of social housing and acute buprenorphine administration (10 µg/kg) on morphine withdrawal. Two 75 mg morphine pellets were implanted for 3 days to induce dependence. Removal of the morphine pellets caused a decrease in body weight, increase in wet dog shakes, and depression of wheel running during the normally active dark phase of the circadian cycle. Social housing restored wheel running and reduced the number of wet dog shakes but did not affect body weight. Administration of buprenorphine restored wheel running depressed by morphine withdrawal for 2 days in individually housed rats and produced time-dependent changes in socially housed rats: Depression of wheel running in the 3 h following administration and restoration of running subsequently compared to saline-treated controls.

CONCLUSIONS

The impact of buprenorphine and social housing to reduce the effect of morphine withdrawal in rats is consistent with the use of opioid substitution therapy and psychotherapy/social support to treat opioid withdrawal in humans. These data provide further validation for the clinical relevance for the use of wheel running to assess spontaneous opioid withdrawal.

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.

Glucagon-like peptide-1 receptor agonist, exendin-4, reduces reinstatement of heroin-seeking behavior in rats

Opioid use disorder (OUD) causes the death of nearly 130 Americans daily. It is evident that new avenues for treatment are needed. To this end, studies have reported that 'satiety' agents such as the glucagon-like peptide-1 receptor (GLP-1R) agonist, exendin-4 (Ex-4), decreases responding for addictive drugs such as cocaine, nicotine, alcohol, and oxycodone, but no work has been done with heroin. In this study, we used a reward devaluation model in which rats avoid ingesting a saccharin solution that predicts drug availability to test the effects of 2.4 μg/kg Ex-4 on responding for a natural reward cue (i.e., saccharin) and on cue- and drug-induced heroin seeking. The results showed that treatment with Ex-4 during the 16-day abstinence period and on the test day decreased cue-induced heroin seeking. Drug-induced heroin seeking also was reduced by Ex-4, but only when using a 1 h, but not a 6 h, pretreatment time. Treatment with Ex-4 did not alter intake of the saccharin cue when the drug was on board, but a history of treatment with Ex-4 increased acceptance of the saccharin cue in later extinction trials. Finally, treatment with Ex-4 did not alter body weight, but was associated with increased Orexin 1 receptor (OX1) mRNA expression in the nucleus accumbens shell. Taken together, these findings are the first to show that treatment with a GLP-1R agonist can reduce both cue-induced seeking and drug-induced reinstatement of heroin seeking. As such, a GLP-1R agonist may serve as an effective treatment for OUD in humans.

Lateral habenula electrical stimulation with different intensities in combination with GABAB receptor antagonist reduces acquisition and expression phases of morphine-induced CPP

The lateral habenula (LHb) plays a principal role in response to aversive stimuli and negative emotional states. In this study, we have evaluated the effects of unilateral electrical stimulation (e-stim) of the LHb on morphine-conditioned place preference (CPP), before or after bilateral injections of Gamma-aminobutyric acid-B receptor (GABA_BR) antagonist, phaclofen, in male rats. Morphine (5 mg/kg; s.c.) induced a significant CPP, using a 5-day CPP paradigm. Intra-LHb microinjection of phaclofen or the LHb e-stim decreased only the acquisition of CPP. The 150 μA stimulation plus phaclofen significantly suppressed the expression phase but induced aversion in the acquisition of CPP, and an e-stim of 25 μA in combination with the antagonist, significantly prevented only the acquisition phase. The findings of this study confirm the possible role of GABA_BRs in the LHb on the acquisition and the expression of CPP. These results show that e-stim of LHb alone or plus phaclofen may change the GABA transmission, involving into CPP. Therefore, the GABAergic system, especially through GABA_BRs, may play a prominent role in the behavioral responses to morphine-induced CPP by LHb stimulation.

Blockade of orexin receptors in the hippocampal dentate gyrus reduced the extinction latency of morphine-induced place preference in male rats

Relapse to drugs such as opioids is a major challenge in addiction therapy. It has been known that the orexinergic system has a significant role in mediating reward processing and addiction, as shown by the conditioned place preference (CPP). The dentate gyrus (DG) of the hippocampus receives orexinergic projections from the lateral hypothalamus that has been approved as a critical area arbitrating the maintenance of drug-seeking behavior following the extinction. The present study aimed to investigate the effects of intra-DG administration of the orexin-1 receptor (OX1R) and orexin-2 receptor (OX2R) antagonists on the extinction of morphine-induced CPP in male rats. Animals received different doses of SB334867 (as OX1R antagonist) or TCS OX2 29 (as OX2R antagonist) (0.5, 2.5, and 12.5 nM/0.5 μl DMSO 12 %) bilaterally into the DG during the extinction phase, after CPP had been induced by subcutaneous injection of morphine (5 mg/kg) during a 3-day conditioning phase. The conditioning scores were recorded during the test. The results demonstrated that intra-DG administration of the highest dose of OX1R antagonist (12.5 nM/0.5 μl DMSO 12 %) shortened the extinction latency of morphine-CPP compared to the DMSO group, while the OX2R antagonist did not significantly alter the latency. Findings imply that the blockade of OX1R, but not OX2R, within the DG facilitates the extinction of morphine-induced reward. In conclusion, the OX1R antagonist might be kept in mind as a convenient therapeutic factor in repressing drug-seeking behaviors in an optimum amount of treatment considering the low dose-treatments applied.

Transgenerational effects on anxiety-like behavior following adolescent morphine exposure in female rats

Global opioid use and misuse remains high, despite efforts to decrease rates of prescribing and diversion. Chronic exposure to opioids, particularly during critical periods of development, can lead to long-lasting effects, including effects that may extend to future generations. Using a rodent model, we have demonstrated significant transgenerational effects of female adolescent morphine exposure, despite the absence of in utero drug exposure. While these effects have been observed in both sexes, effects on anxiety-like behavior were only observed in F1 females. The current study was designed to examine both inter- and transgenerational effects of adolescent morphine exposure on anxiety-like behavior. Female Sprague Dawley rats were administered increasing doses of morphine (5-25 mg/kg s.c.) or saline for 10 days during adolescence (PND30-39). Adult diestrous female offspring (MORF1 or SALF1) and grand offspring (F2) were tested for anxiety-like behavior using the elevated plus maze (EPM). F1 females cross-fostered to donor mothers were also examined. The results show that MORF1 and MORF2 females spend significantly more time on the open arms of the EPM compared to SALF1 controls, an effect that persisted in cross-fostered females. Additional studies demonstrate that this effect is estrous cycle dependent, as decreased anxiety-like behavior was observed in diestrus, while increased anxiety-like behavior was observed in estrus. These behavioral effects were not associated with any differences in circulating corticosterone either at baseline or following EPM testing. Thus, female adolescent morphine exposure alters the regulation of anxiety-like behavior in an estrous-dependent manner and this effect persists in the F2 generation.

Enduring consequences of perinatal fentanyl exposure in mice

Opioid use by pregnant women is an understudied consequence associated with the opioid epidemic, resulting in a rise in the incidence of neonatal opioid withdrawal syndrome (NOWS) and lifelong neurobehavioral deficits that result from perinatal opioid exposure. There are few preclinical models that accurately recapitulate human perinatal drug exposure and few focus on fentanyl, a potent synthetic opioid that is a leading driver of the opioid epidemic. To investigate the consequences of perinatal opioid exposure, we administered fentanyl to mouse dams in their drinking water throughout gestation and until litters were weaned at postnatal day (PD) 21. Fentanyl-exposed dams delivered smaller litters and had higher litter mortality rates compared with controls. Metrics of maternal care behavior were not affected by the treatment, nor were there differences in dams' weight or liquid consumption throughout gestation and 21 days postpartum. Twenty-four hours after weaning and drug cessation, perinatal fentanyl-exposed mice exhibited signs of spontaneous somatic withdrawal behavior and sex-specific weight fluctuations that normalized in adulthood. At adolescence (PD 35), they displayed elevated anxiety-like behaviors and decreased grooming, assayed in the elevated plus maze and sucrose splash tests. Finally, by adulthood (PD 55), they displayed impaired performance in a two-tone auditory discrimination task. Collectively, our findings suggest that perinatal fentanyl-exposed mice exhibit somatic withdrawal behavior and change into early adulthood reminiscent of humans born with NOWS.

A Delta-Opioid Receptor Gene Polymorphism Moderates the Therapeutic Response to Extended-Release Buprenorphine in Opioid Use Disorder

BACKGROUND

Buprenorphine treatment is not equally effective in all patients with opioid use disorder (OUD). Two retrospective studies showed that, among African Americans (AAs), rs678849, a polymorphism in the delta-opioid receptor gene, moderated the therapeutic effect of sublingual buprenorphine.

METHODS

We examined rs678849 as a moderator of the response to an extended-release subcutaneous buprenorphine formulation (BUP-XR) in a 24-week OUD treatment study of 127 AAs and 327 European Americans (EAs). Participants were randomly assigned to receive: (1) BUP-XR as 2 monthly injections of 300 mg followed by either 300 mg monthly or 100 mg monthly for 4 months, or (2) monthly volume-matched placebo injections. Generalized estimating equations logistic regression analyses tested, per population group, the main and interaction effects of treatment (BUP-XR vs placebo) and genotype group (rs678849*CC vs CT/TT) on weekly urine drug screens (UDS).

RESULTS

Among AAs, the placebo group had higher rates of opioid-positive UDS than the BUP-XR group (log odds ratio = 1.67, 95% CI = 0.36, 2.98), but no genotype by treatment effect (P = .80). Among EAs, the placebo group also showed higher rates of opioid-positive UDS than the BUP-XR group (log odds ratio = 1.97, 95% CI = 1.14, 2.79) but a significant genotype by treatment interaction (χ 2(1) = 4.33, P = .04).

CONCLUSION

We found a moderating effect of rs678849 on the response to buprenorphine treatment of OUD in EAs, but not AAs. These findings require replication in well-powered, prospective studies of both AA and EA OUD patients treated with BUP-XR and stratified on rs678849 genotype.

Early life maternal deprivation attenuates morphine induced inhibition in lateral paragigantocellularis neurons in adult rats

Early life stress can serve as one of the principle sources leading to individual differences in confronting challenges throughout the lifetime. Maternal deprivation (MD), a model of early life stress, can cause persistent alterations in brain function, and it may constitute a risk factor for later incidence of drug addiction. It is becoming more apparent that early life MD predisposes opiate abuse in adulthood. Although several behavioral and molecular studies have addressed this issue, changes in electrophysiological features of the neurons are yet to be understood. The lateral paragigantocellularis (LPGi) nucleus, which participates in the mediation of opiate dependence and withdrawal, may be susceptible to modifications following MD. This study sought to find whether early life MD can alter the discharge activity of LPGi neurons and their response to acute morphine administration in adult rats. Male Wistar rats experienced MD on postnatal days (PNDs) 1-14 for three h per day. Afterward, they were left undisturbed until PND 70, during which the extracellular activities of LPGi neurons were recorded in anesthetized animals at baseline and in response to acute morphine. In both MD and control groups, acute morphine administration induced heterogeneous (excitatory, inhibitory, and no effect) responses in LPGi neurons. At baseline recording, the interspike interval variability of the LPGi neurons was attenuated in both inhibitory and excitatory responses in animals with the history of MD. The extent of morphine-induced discharge inhibition was also lower in deprived animals compared to the control group. These findings suggest that early life MD induces long-term alterations in LPGi neuronal activity in response to acute administration of morphine. Therefore, the MD may alter the vulnerability to develop opiate abuse in adulthood.