Orexin A-mediated AKT signaling in the dentate gyrus contributes to the acquisition, expression and reinstatement of morphine-induced conditioned place preference

Orexin-mediated motivated arousal and reward seeking

The neuromodulator orexin has been identified as a key factor for motivated arousal including recent evidence that sleep deprivation-induced enhancement of reward behavior is modulated by orexin. While orexin is not necessary for either reward or arousal behavior, orexin neurons' broad projections, ability to sense the internal state of the animal, and high plasticity of signaling in response to natural rewards and drugs of abuse may underlie heightened drug seeking, particularly in a subset of highly motivated reward seekers. As such, orexin receptor antagonists have gained deserved attention for putative use in addiction treatments. Ongoing and future clinical trials are expected to identify individuals most likely to benefit from orexin receptor antagonist treatment to promote abstinence, such as those with concurrent sleep disorders or high craving, while attention to methodological considerations will aid interpretation of the numerous preclinical studies investigating disparate aspects of the role of orexin in reward and arousal.

Blocking the dopaminergic receptors within the hippocampal dentate gyrus reduced analgesic responses induced by restraint stress in the formalin test

Previous studies have shown that various receptors, including dopamine receptors, are expressed in the hippocampal dentate gyrus (DG). Besides, indicatively, dopamine receptors play an essential role in the modulation of pain perception. On the other hand, stressful experiences can produce analgesia, termed stress-induced analgesia (SIA). The current study examined the probable role of dopamine receptors within the DG in antinociception induced by restraint stress (RS). Ninety-seven male albino Wistar rats were unilaterally implanted with a cannula in the DG. Animals received intra-DG microinjections of SCH23390 or Sulpiride (0.25, 1, and 4 μg/rat) as D1-and D2-like dopamine receptor antagonists, respectively, five minutes before RS. Ten minutes after the end of the induction of RS for three hours, 50 μl 2.5% formalin was injected subcutaneously into the plantar surface of the hind paw to induce persistent inflammatory pain. Pain scores were evaluated at 5-minute intervals for 60 minutes. These findings showed that; exposure to RS for three hours produced SIA in both phases of the formalin test, while this RS-induced analgesia was attenuated in the early and late phases of the formalin test by intra-DG microinjection of SCH23390 and Sulpiride. The results of the present study suggested that both D1- and D2-like dopamine receptors in the DG have a considerable role in the induced analgesia by RS.

Daily treatment with the dual orexin receptor antagonist DORA-12 during oxycodone abstinence decreases oxycodone conditioned reinstatement

Chronic opioid use disturbs circadian rhythm and sleep, encouraging opioid use and relapse. The orexin (OX) system is recruited by opioids and regulates physiological processes including sleep. Dual OX receptor antagonists (DORAs), developed for insomnia treatment, may relieve withdrawal-associated sleep disturbances. This study investigated whether DORA-12, a recently developed DORA, reduces physiological activity disturbances during oxycodone abstinence and consequently prevents oxycodone-seeking behavior. Male and female Wistar rats were trained to intravenously self-administer oxycodone (0.15 mg/kg, 21 sessions; 8 h/session) in the presence of a contextual/discriminative stimulus (SD). The rats were subsequently housed individually (22 h/day) to monitor activity, food and water intake. They received DORA-12 (0-30 mg/kg, p.o.) after undergoing daily 1-h extinction training (14 days). After extinction, the rats were tested for oxycodone-seeking behavior elicited by the SD. Hypothalamus sections were processed to assess oxycodone- or DORA-12-associated changes to the OX cell number. In males, oxycodone-associated increases in activity during the light-phase, reinstatement, and decreases in the number of OX cells observed in the vehicle-treated group were not observed with DORA-12-treatment. Oxycodone-associated increases in light-phase food and water intake were not observed by day 14 of 3 mg/kg DORA-12-treatment and dark-phase water intake was increased across treatment days. In females, OX cell number was unaffected by oxycodone or DORA-12. Three and 30 mg/kg DORA-12 increased females' day 7 dark-phase activity and decreased reinstatement. Thirty mg/kg DORA-12 reduced oxycodone-associated increases in light-phase food and water intake. The results suggest that DORA-12 improves oxycodone-induced disruptions to physiological activities and reduces relapse.

The Role of Orexin-1 Receptors Within the Hippocampal CA1 Area in the Extinction and Reinstatement of Methamphetamine-Seeking Behaviors

Psychostimulant addiction is a chronic brain disorder with high relapse rates, requiring new therapeutic strategies. The orexin system is highly implicated in processing reward and addiction through connections with critical areas such as the hippocampus. This study investigated the role of orexin-1 receptors (OX1R) within the CA1 subregion of the hippocampus in the extinction and reinstatement of the methamphetamine-induced conditioned place preference. After cannulae implantation, recovery, and establishing the methamphetamine place preference, 98 male Wistar rats received different doses of bilateral intra-CA1 selective OX1R antagonist, SB334867 (1, 3, 10, and 30 nmol/0.5 μl DMSO per side) during the 10-day extinction period (daily) or after extinction phase, just on the reinstatement day (single dose) in separate experimental and control groups. The findings indicated that bilateral microinjection of SB334867 into the CA1 area during the extinction period could significantly reduce the extinction latency and maintenance of rewarding aspects of methamphetamine dose-dependently (3, 10, and 30 nmol). In another set of experiments, a single dose of bilateral intra-CA1 SB334867 administration on the reinstatement phase prevented the methamphetamine-induced reinstatement of drug-seeking behaviors at the high doses (10, and 30 nmol). The present study provided more evidence for the implication of hippocampal OX1R in the maintenance of rewarding and reinforcing properties of methamphetamine and its role in the relapse of methamphetamine-seeking behavior. Further investigations on the role of the orexin system, including the orexin-2 receptors in treating addiction, are needed to introduce its antagonists as effective therapeutic options for psychostimulant addiction.

Distinct roles for orexin-1 and orexin-2 receptors in the dentate gyrus of the hippocampus in the methamphetamine-seeking behavior in the rats

Because of the relapsing properties of psychostimulants such as methamphetamine (Meth), there is no established pharmacotherapy for Meth addiction. The orexinergic system is a promising target for treating psychostimulant use disorders and relapse. However, to the best of our knowledge, no investigation regarding the role of orexin receptors in the dentate gyrus (DG) region of the hippocampus has been conducted in the extinction and reinstatement of Meth-seeking behavior. Two stainless-steel guide cannulae were bilaterally implanted into the DG of the rats' brains. The unbiased conditioned place preference (CPP) procedure was conducted to induce Meth conditioning. Following the five days Meth injections (1 mg/kg; sc), animals received intra-DG microinjection of SB334867 or TCS OX2 29, as orexin 1 (OX1) or orexin 2 (OX2) receptor antagonists, respectively (without Meth administration) during extinction phase to elucidate the role of orexin receptors in the latency of the extinction period in the Meth-conditioned rats. To evaluate the role of orexin receptors in the DG region in the reinstatement of Meth-seeking behavior, the extinguished rats received SB334867 or TCS OX2 29 before injecting a priming dose of Meth (0.25 mg/kg; sc). The results indicated two distinct roles for the OX1 and OX2 receptors in the DG region. TCS OX2 29 attenuated the extinction latency, and SB334867 considerably reduced the reinstatement of Meth-seeking behavior in this region. Therefore, the DG region's orexinergic system might be a potential therapeutic target for psychostimulant use disorders.

Sleep-mediated regulation of reward circuits: implications in substance use disorders

Our modern society suffers from both pervasive sleep loss and substance abuse-what may be the indications for sleep on substance use disorders (SUDs), and could sleep contribute to the individual variations in SUDs? Decades of research in sleep as well as in motivated behaviors have laid the foundation for us to begin to answer these questions. This review is intended to critically summarize the circuit, cellular, and molecular mechanisms by which sleep influences reward function, and to reveal critical challenges for future studies. The review also suggests that improving sleep quality may serve as complementary therapeutics for treating SUDs, and that formulating sleep metrics may be useful for predicting individual susceptibility to SUDs and other reward-associated psychiatric diseases.

Suvorexant, an FDA-approved dual orexin receptor antagonist, reduces oxycodone self-administration and conditioned reinstatement in male and female rats

Background: The Department of Health and Human Services reports that prescription pain reliever (e.g., oxycodone) misuse was initiated by 4,400 Americans each day in 2019. Amid the opioid crisis, effective strategies to prevent and treat prescription opioid use disorder (OUD) are pressing. In preclinical models, the orexin system is recruited by drugs of abuse, and blockade of orexin receptors (OX receptors) prevents drug-seeking behavior. The present study sought to determine whether repurposing suvorexant (SUV), a dual OX receptor antagonist marketed for the treatment of insomnia, can treat two features of prescription OUD: exaggerated consumption and relapse. Methods: Male and female Wistar rats were trained to self-administer oxycodone (0.15 mg/kg, i. v., 8 h/day) in the presence of a contextual/discriminative stimulus (S^D) and the ability of SUV (0-20 mg/kg, p. o.) to decrease oxycodone self-administration was tested. After self-administration testing, the rats underwent extinction training, after which we tested the ability of SUV (0 and 20 mg/kg, p. o.) to prevent reinstatement of oxycodone seeking elicited by the S^D. Results: The rats acquired oxycodone self-administration and intake was correlated with the signs of physical opioid withdrawal. Additionally, females self-administered approximately twice as much oxycodone as males. Although SUV had no overall effect on oxycodone self-administration, scrutiny of the 8-h time-course revealed that 20 mg/kg SUV decreased oxycodone self-administration during the first hour in males and females. The oxycodone S^D elicited strong reinstatement of oxycodone-seeking behavior that was significantly more robust in females. Suvorexant blocked oxycodone seeking in males and reduced it in females. Conclusions: These results support the targeting of OX receptors for the treatment for prescription OUD and repurposing SUV as pharmacotherapy for OUD.

Hippocampal orexin receptors: Localization and function

Orexin (hypocretin) is secreted from the perifornical/lateral hypothalamus and is well known for sleep regulation. Orexin has two, orexin A and B, transcripts and two receptors, type 1 and 2 (OX1R and OX2R), located in the plasma membrane of neurons in different brain areas, including the hippocampus involved in learning, memory, seizures, and epilepsy, as physiologic and pathologic phenomena. OX1R is expressed in the dentate gyrus and CA1 and the OX2R in the CA3 areas. Orexin enhances learning and memory as well as reward, stress, seizures, and epilepsy, partly through OX1Rs, while either aggravating or alleviating those phenomena via OX2Rs. OX1Rs activation induces long-term changes of synaptic responses in the hippocampus, an age and concentration-dependent manner. Briefly, we will review the localization and functions of hippocampal orexin receptors, their role in learning, memory, stress, reward, seizures, epilepsy, and hippocampal synaptic plasticity.

Reviewing the role of the orexinergic system and stressors in modulating mood and reward-related behaviors

In this review study, we aimed to introduce the orexinergic system as an important signaling pathway involved in a variety of cognitive functions such as memory, motivation, and reward-related behaviors. This study focused on the role of orexinergic system in modulating reward-related behavior, with or without the presence of stressors. Cross-talk between the reward system and orexinergic signaling was also investigated, especially orexinergic signaling in the ventral tegmental area (VTA), the nucleus accumbens (NAc), and the hippocampus. Furthermore, we discussed the role of the orexinergic system in modulating mood states and mental illnesses such as depression, anxiety, panic, and posttraumatic stress disorder (PTSD). Here, we narrowed down our focus on the orexinergic signaling in three brain regions: the VTA, NAc, and the hippocampus (CA1 region and dentate gyrus) for their prominent role in reward-related behaviors and memory. It was concluded that the orexinergic system is critically involved in reward-related behavior and significantly alters stress responses and stress-related psychiatric and mood disorders.

Pleasure, addiction, and hypocretin (orexin)

The hypocretins/orexins were discovered in 1998. Within 2 years, this led to the discovery of the cause of human narcolepsy, a 90% loss of hypothalamic neurons containing these peptides. Further work demonstrated that these neurons were not simply linked to waking. Rather these neurons were active during pleasurable behaviors in waking and were silenced by aversive stimulation. This was seen in wild-type mice, rats, cats, and dogs. It was also evident in humans, with increased Hcrt release during pleasurable activities and decreased release, to the levels seen in sleep, during pain. We found that human heroin addicts have, on average, an increase of 54% in the number of detectable Hcrt neurons compared to "control" human brains and that these Hcrt neurons are substantially smaller than those in control brains. We found that in mice, chronic morphine administration induced the same changes in Hcrt neuron number and size. Our studies in the mouse allowed us to determine the specificity, dose response relations, time course of the change in the number of Hcrt neurons, and that the increased number of Hcrt neurons after opiates was not due to neurogenesis. Furthermore, we found that it took a month or longer for these anatomical changes in the mouse brain to return to baseline. Human narcoleptics, despite their prescribed use of several commonly addictive drugs, do not show significant evidence of dose escalation or substance use disorder. Similarly, mice in which the peptide has been eliminated are resistant to addiction. These findings are consistent with the concept that an increased number of Hcrt neurons may underlie and maintain opioid or cocaine use disorders.

Drug-Induced Conditioned Place Preference and Its Practical Use in Substance Use Disorder Research

The conditioned place preference (CPP) paradigm is a well-established model utilized to study the role of context associations in reward-related behaviors, including both natural rewards and drugs of abuse. In this review article, we discuss the basic history, various uses, and considerations that are tied to this technique. There are many potential takeaway implications of this model, including negative affective states, conditioned drug effects, memory, and motivation, which are all considered here. We also discuss the neurobiology of CPP including relevant brain regions, molecular signaling cascades, and neuromodulatory systems. We further examine some of our prior findings and how they integrate CPP with self-administration paradigms. Overall, by describing the fundamentals of CPP, findings from the past few decades, and implications of using CPP as a research paradigm, we have endeavored to support the case that the CPP method is specifically advantageous for studying the role of a form of Pavlovian learning that associates drug use with the surrounding environment.

Evaluation of the relationship between the gene expression level of orexin-1 receptor in the rat blood and prefrontal cortex, novelty-seeking, and proneness to methamphetamine dependence: A candidate biomarker

BACKGROUND

previous studies have suggested that methamphetamine (METH) abuse may affect orexin regulation. However, the data regarding the relationship between the current level of orexin and the vulnerability to METH abuse are minimal. Here, we have investigated the correlation between the gene expression level of the orexin-1 receptor (OX1R) in the rat prefrontal cortex (PFC) and blood lymphocytes and susceptibility to METH dependence and its impact on novelty-seeking behavior.

METHODS

male Wistar rats were first examined for novelty-seeking behavior by the novel object recognition test, and the expression level of OX1R in their blood lymphocytes was evaluated by real-time PCR. Then, the susceptibility to METH abuse was investigated by voluntary METH oral consumption test. According to the amounts of METH consumption, the animals were divided into two groups of METH preferring and non-preferring. Half of the rats in each group were sacrificed, and the level of OX1R in their blood lymphocytes and PFC tissue was measured. The other half were sacrificed for the same reason after two weeks of drug abstinence.

RESULTS

The indexes of novelty-seeking behavior were significantly higher in the METH- preferring group compared to the non-preferring animals. Furthermore, the expression level of OX1R in the blood lymphocytes and PFC in the preferring group was considerably higher than the non-preferring group.

CONCLUSION

Up-regulation of the mRNA expression level of OX1R in the lymphocytes and PFC may predict vulnerability to the METH consumption and novelty-seeking, which may serve as a potential biomarker for METH abuse.

Pre-exposure of adolescent mice to morphine results in stronger sensitization and reinstatement of conditioned place preference than pre-exposure to hydrocodone

BACKGROUND

Opioids are commonly prescribed to treat moderate-to-severe pain. However, their use can trigger the development of opioid use disorder. A major problem in treating opioid use disorder remains the high rate of relapse.

AIM

The purpose of this study was to determine whether there are differences among opioids in their ability to trigger relapse after pre-exposure during adolescence.

METHODS

On postnatal day 33, mice were examined for the acute locomotor response to saline, morphine, or hydrocodone (5 mg/kg). They were administered with the corresponding opioid or saline during postnatal days 34-38 (20 mg/kg) and 40-44 (40 mg/kg). On postnatal day 45, they were recorded for the development of locomotor sensitization (5 mg/kg). Starting on postnatal day 55, mice were examined for the acquisition (1, 5, 10, 20, and 40 mg/kg), extinction, and drug-induced reinstatement (1, 2.5, and 5 mg/kg) of conditioned place preference.

RESULTS

There were no significant differences in the acute locomotor response to morphine and hydrocodone. Morphine induced significantly stronger locomotor sensitization as compared to hydrocodone. Pre-exposure to morphine, but not hydrocodone, sensitized the acquisition of conditioned place preference. There were no significant differences in extinction rates. Mice pre-exposed to morphine reinstate conditioned place preference after priming with a 1 mg/kg dose. In contrast, higher priming doses were required for reinstatement in all other experimental groups.

CONCLUSIONS

Adolescent mice administered with morphine develop greater sensitization to its effects and subsequently reinstate conditioned place preference more readily than mice administered with hydrocodone. This suggests higher risk for relapse after pre-exposure to morphine during adolescence as compared to hydrocodone.

Involvement of orexin receptors within the hippocampal dentate gyrus in morphine-induced reinstatement in food-deprived rats

The orexinergic system is found to cooperate in mediating stress-induced drug relapse. The orexinergic terminals innervate neurons of the hippocampal dentate gyrus (DG) which is a key structure in the maintenance and reinstatement of drug addiction. However, the specific contribution of intra-DG orexin receptors to stress-induced reinstatement has not been completely known. In the current study, the effects of intra-DG administration of SB334867, an orexin-1 receptor (OX1R) antagonist, and TCS OX2 29, an orexin-2 receptor (OX2R) antagonist, were investigated on the reinstatement induced by a sub-threshold dose of morphine and food deprivation (FD) stress. Adult male rats received different doses of SB334867 or TCS OX2 29 (3, 10, and 30 nM/0.5 μl DMSO 12%) bilaterally into the DG in separate groups, following the acquisition and extinction of morphine-induced conditioned place preference (CPP). Then, the reinstatement was evaluated by the 24 h FD stress and/or a sub-threshold dose of morphine (0.5 mg/kg, s.c.). CPP scores and locomotor activities were recorded during the test. The findings indicated that pre-treatment with the highest dose of SB334867 (30 nM) and two higher doses of TCS OX2 29 (10 and 30 nM) blocked the sub-threshold dose and FD stress-induced reinstatement of morphine. The effect of TCS OX2 29 on reduction of reinstatement was more pronounced than that of SB334867. It suggests a role for the orexin receptors, especially OX2R within the DG region in the stress-induced reinstatement of morphine-seeking behaviours in extinguished rats.

Role of Orexin-1 Receptor Within the Ventral Tegmental Area in Mediating Stress- and Morphine Priming-induced Reinstatement of Conditioned Place Preference in Rats

Introduction:

Orexin-containing neurons exist in the lateral hypothalamic region, sending their projections toward mesolimbic regions such as the Ventral Tegmental Area (VTA).

Methods:

In the current study, a Reinstatement model is used to examine the effects of intra-VTA administration of SB334867 as an Orexin-1 Receptor (OX1R) antagonist on drug priming- and Forced Swim Stress (FSS)-induced reinstatement of morphine. Eighty-eight male adult albino Wistar rats, weighing 200–280 g, were bilaterally implanted by cannulas into the VTA. We induced the Conditioned Place Preference (CPP) by Subcutaneous (SC) injection of morphine (5 mg/kg) daily in three days. Then, the CPP score was calculated. After a 24-h “off” period following achievement of extinction criterion, the rats were tested for drug priming-induced reinstatement by a priming dose of morphine (1 mg/kg, SC) and for FSS-induced reinstatement 10 min after FSS. In the next experiments, the animals received different doses of intra-VTA administration of SB334867 (0.3, 3, and 1 nM/0.3 μL 12% DMSO per side) and bilaterally were subsequently tested for FSS- and morphine priming-induced reinstatement.

Results:

Our findings indicated that the FSS could induce the reinstatement of seeking behaviors. Furthermore, intra-VTA administration of OX1R antagonists suppressed FSS- and drug priming-induced reinstatement dose-dependently.

Conclusion:

It is concluded that FSS and drug priming-induced reinstatement might be mediated, at least in part, by stimulation of orexin receptors in the VTA.

Sex Differences in Neuroplasticity- and Stress-Related Gene Expression and Protein Levels in the Rat Hippocampus Following Oxycodone Conditioned Place Preference

Prescription opioid abuse is a serious public health issue. Recently, we showed that female and male Sprague-Dawley rats acquire conditioned place preference (CPP) to the mu opioid receptor agonist oxycodone. Anatomical analysis of the hippocampus from these rats unveiled sex differences in the opioid system in a way that would support excitation and opiate associative learning processes especially in females. In this study, we examined the expression and protein densities of opioid, plasticity, stress and related kinase and signaling molecules in the hippocampus of female and male rats following oxycodone CPP. Oxycodone CPP females have: a) increases in ARC (activity regulated cytoskeletal-associated protein)-immunoreactivity (ir) in CA3 pyramidal cells; b) decreases in Npy (neuropeptide Y) gene expression in the medial hippocampus but higher numbers of NPY-containing hilar interneurons compared to males; c) increases in Crhr2 (corticotropin releasing factor receptor 2) expression in CA2/3; d) increases in Akt1 (AKT serine/threonine kinase 1) expression in medial hippocampus; and e) decreases in phosphorylated MAPK (mitogen activated protein kinase)-ir in CA1 and dentate gyrus. Oxycodone CPP males have: a) increases in Bdnf (brain derived-neurotrophic factor) expression, which is known to be produced in granule cells, relative to females; b) elevated Mapk1 expression and pMAPK-ir in the dentate hilus which harbors newly generated granule cells; and c) increases in CRHR1-ir in CA3 pyramidal cell soma. These sex-specific changes in plasticity, stress and kinase markers in hippocampal circuitry parallel previously observed sex differences in the opioid system after oxycodone CPP.

Image-guided cranial irradiation-induced ablation of dentate gyrus neurogenesis impairs extinction of recent morphine reward memories

Dentate gyrus adult neurogenesis is implicated in the formation of hippocampal-dependent contextual associations. However, the role of adult neurogenesis during reward-based context-dependent paradigms-such as conditioned place preference (CPP)-is understudied. Therefore, we used image-guided, hippocampal-targeted X-ray irradiation (IG-IR) and morphine CPP to explore whether dentate gyrus adult neurogenesis plays a role in reward memories created in adult C57BL/6J male mice. In addition, as adult neurogenesis appears to participate to a greater extent in retrieval and extinction of recent (<48 hr posttraining) versus remote (>1 week posttraining) memories, we specifically examined the role of adult neurogenesis in reward-associated contextual memories probed at recent and remote timepoints. Six weeks post-IG-IR or Sham treatment, mice underwent morphine CPP. Using separate groups, retrieval of recent and remote reward memories was found to be similar between IG-IR and Sham treatments. Interestingly, IG-IR mice showed impaired extinction-or increased persistence-of the morphine-associated reward memory when it was probed 24-hr (recent) but not 3-weeks (remote) postconditioning relative to Sham mice. Taken together, these data show that hippocampal-directed irradiation and the associated decrease in dentate gyrus adult neurogenesis affect the persistence of recently-but not remotely-probed reward memory. These data indicate a novel role for adult neurogenesis in reward-based memories and particularly the extinction rate of these memories. Consideration of this work may lead to better understanding of extinction-based behavioral interventions for psychiatric conditions characterized by dysregulated reward processing.

Recent advances in understanding the roles of hypocretin/orexin in arousal, affect, and motivation

The hypocretins (Hcrts) are two alternatively spliced neuropeptides (Hcrt1/Ox-A and Hcrt2/Ox-B) that are synthesized exclusively in the hypothalamus. Data collected in the 20 years since their discovery have supported the view that the Hcrts play a broad role in the control of arousal with a particularly important role in the maintenance of wakefulness and sleep-to-wake transitions. While this latter point has received an overwhelming amount of research attention, a growing literature has begun to broaden our understanding of the many diverse roles that the Hcrts play in physiology and behavior. Here, we review recent advances in the neurobiology of Hcrt in three sections. We begin by surveying findings on Hcrt function within normal sleep/wake states as well as situations of aberrant sleep (that is, narcolepsy). In the second section, we discuss research establishing a role for Hcrt in mood and affect (that is, anxiety, stress, and motivation). Finally, in the third section, we briefly discuss future directions for the field and place an emphasis on analytical modeling of Hcrt neural activity. We hope that the data discussed here provide a broad overview of recent progress in the field and make clear the diversity of roles played by these neuromodulators.

Generation of silent synapses in dentate gyrus correlates with development of alcohol addiction

The brain circuits and synaptic processes that underlie alcohol addiction are currently the subject of intensive research. Here we focus on hippocampal circuitry and show that chemogenetic inhibition of dentate gyrus (DG) during presentation of alcohol-associated cues has long-lasting effects on mice behavior. DG inhibition enhances alcohol seeking and drinking, suggesting that DG regulates addiction-related behaviors. To test this hypothesis, we perform whole-cell patch-clamp recordings from the granule cells of DG and look for electrophysiological correlates of alcohol addiction. We observe that presentation of alcohol-associated cue light that induces relapse to alcohol-seeking results in generation of silent synapses, that lack functional AMPA receptors. Furthermore, using human criteria of addiction, we differentiate mice controlling their alcohol consumption from those that undergo transition to addiction to discover that the levels of silent synapses induced by alcohol cues are specifically increased in the addicted mice. As the total level of dendritic spines that harbor synapses is constant at this time point, our data indicate that synapses of perforant path to DG are weakened during cue relapse. Finally we demonstrate that, acamprosate, a drug that limits alcohol drinking and seeking in addicts, prevents generation of silent synapses in DG upon presentation of alcohol-associated cues. Altogether, our data suggest that weakening of DG synapses upon cue relapse contributes to persistent alcohol addiction-related behaviors.

Morphine regulates adult neurogenesis and contextual memory extinction via the PKCε/Prox1 pathway

We have previously reported that the miR-181a/Prox1/Notch1 pathway mediates the effect of morphine on modulating lineage-specific differentiation of adult neural stem/progenitor cells (NSPCs) via a PKCε-dependent pathway, whereas fentanyl shows no such effect. However, the role of the PKCε/Prox1 pathway in mediating drug-associated contextual memory remains unknown. The current study investigated the effect of PKCε/Prox1 on morphine-induced inhibition of adult neurogenesis and drug-associated contextual memory in mice, while the effect of fentanyl was tested simultaneously. By using BrdU labeling, we were able to examine the lineages of differentiated NSPCs in adult DG. PKCε knockout blocked morphine's effects on inducing in vivo astrocyte-preferential differentiation of NSPCs, but did not alter NSPC lineages upon fentanyl treatment. Inhibited adult neurogenesis further resulted in prolonged extinction and enhanced reinstatement of morphine-induced CPP, as well as prolonged extinction of space reference memory indicated by the Morris water maze paradigm. However, after fentanyl administration, no significant changes were found between wild-type and PKCε knockout mice, during either CPP or water maze tasks. When the lentivirus encoding Nestin-promoter-controlled Prox1 cDNA was injected into hippocampi of wildtype and PKCε knockout adult mice to modulate PKCε/Prox1 activity, similar effects were discovered in adult mice injected with lentivirus encoding Prox1, and more dramatic effects were found in PKCε knockout mice with concurrent Prox1 overexpression. In conclusion, morphine mediates lineage-specific NSPC differentiation, inhibits adult neurogenesis and regulates contextual memory retention via the PKCε/Prox1 pathway, which are implicated in the eventual context-associated relapse.

Orexin A in men with heroin use disorder undergoing methadone maintenance treatment

Orexins have played a role in reward-seeking and addiction-related behavior. There are few reports in the literature on serum levels of orexins in patients with heroin use disorder (HUD) undergoing methadone maintenance treatment (MMT). The aim of this study was to investigate the serum levels of orexin A in HUD patients undergoing MMT. Fifty male HUD patients undergoing MMT and 25 healthy males were enrolled for this study. Serum orexin A were measured with assay kits. Using analysis of covariance (ANCOVA) with body mass index (BMI) adjustments, the serum levels of orexin A in HUD men undergoing MMT were found to be significantly higher than in healthy controls. In conclusion, our results suggest that MMT might increase orexin A levels in HUD patients.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

The role of CA3-LS-VTA loop in the formation of conditioned place preference induced by context-associated reward memory for morphine

Addiction-related behaviors, such as conditioned place preference (CPP), require animals to remember an association between environmental cue and drug treatment, and exposure to environmental cue is one of the key contributing factors to relapse. However, how central neural circuit participates in the formation of CPP induced by stimulus of morphine-paired environment remains unknown. In the present study, we found that reexposure to morphine-paired environment significantly increased the activity of hippocampal CA3 neurons, increased the excitability of GABAergic neurons and expression of glutamic acid decarboxylase 65/67 in the caudal lateral septum (LSc) and decreased the activity of GABAergic neurons and GAD65/67 expression in ventral tegmental area (VTA), leading to activation (disinhibition) of dopaminergic neurons. Inactivation of CA3 neurons attenuated GABAergic neurons activity and decreased the upregulation of GAD65/67 in LSc, prevented the dopaminergic neurons activation,and GAD65/67 downregulation in VTA and ameliorated the CPP behavior following exposure to morphine-paired context. Blockade of NMDA receptor in LSc also prevented the upregulation of GAD65/67 in LSc and formation of CPP induced by stimulus of morphine-paired environment. Suppression of GAD activity in LSc also remarkably attenuated the dopaminergic neurons activation and the GAD65/67 downregulation in VTA and prevented the formation of CPP induced by reexposure to morphine-associated context. Collectively, these results, for the first time, illustrated the involvement of neural circuitry of CA3-LSc-VTA, through integration of the contexts and reward information, participated in the reinstatement of CPP induced by exposure to morphine-associated context, which advanced our understanding on neurobiological basis for the context-associated memory and rewarding behavior.

Role of the orexin receptors within the nucleus accumbens in the drug priming-induced reinstatement of morphine seeking in the food deprived rats

Orexin plays a key role in mediating stress-induced drug relapse. However, the role of different types of orexinergic receptors that modulate stress-induced drug seeking remains unknown. The nucleus accumbens (NAc) has an important role in the reward system and receives orexinergic projections of the lateral hypothalamus. In addition, orexin interacts with other receptors that are involved in drug reinstatement. Therefore, in the present study, the role of orexin receptors in the NAc in morphine priming- induced reinstatement and the effect of food deprivation (FD) on drug reinstatement were examined. The extinguished morphine preference rats were tested for reinstatement following the 24-h FD condition after conditioning was induced. In the other groups, the animals were given intra-accumbal administration of SB334867 (01, 1 and 10 nM/0.5 μl DMSO) as an orexin-1 receptor antagonist and TCSOX229 (1, 5 and 25 nM/0.5 μl DMSO), as an orexin-2 receptor antagonist. The results showed that the blockade of two types of orexin receptors in the NAc remarkably attenuated the effect of FD on the drug reinstatement; however, they were more effective in FD condition. These findings indicate that the NAc is a brain area within which orexin has a fundamental role in the effect of stress on morphine-induced reinstatement and the effect of food deprivation- on the reinstatement of morphine.

Nortriptyline Enhances Morphine-Conditioned Place Preference in Neuropathic Rats: Role of the Central Noradrenergic System

BACKGROUND

Combination drug therapy is commonly used to treat chronic pain conditions such as neuropathic pain, and antidepressant is often used together with opioid analgesics. While rewarding is an intrinsic property of opioid analgesics, it is unknown whether the use of an antidepressant would influence opioid reward, which may contribute to opioid addiction. In this study, we examined whether nortriptyline (a tricyclic antidepressant and a first-line medication for neuropathic pain) would enhance the morphine rewarding property in both naive and chronic constriction sciatic nerve injury (CCI) rats.

METHODS

The rewarding effect of these drugs was assessed using conditioned place preference (CPP). The real-time polymerase chain reaction, western blot, and enzyme-linked immunosorbent assay analysis were used to investigate the function of central noradrenergic system.

RESULTS

In naive rats, coadministration of nortriptyline with morphine did not change the acquisition of morphine-induced CPP. However, nortriptyline enhanced the acquisition, delayed the extinction, and augmented the reinstatement of morphine-induced CPP in CCI rats. In CCI rats treated with both nortriptyline and morphine, the expression of α2A-adrenergic receptors, norepinephrine transporter, and tyrosine hydroxylase was markedly decreased in the locus coeruleus, whereas the norepinephrine concentration in the nucleus accumbens was remarkably increased.

CONCLUSIONS

These results demonstrate that nortriptyline enhanced morphine reward when both drugs were used to treat neuropathic pain in rats and that this behavioral phenotype is likely to be mediated by upregulation of the central noradrenergic system. These findings may have implications in opioid therapy commonly used for chronic pain management.

GluR2-3Y Inhibits the Acquisition and Reinstatement of Morphine-Induced Conditioned Place Preference in Rats

Accumulating evidence indicates that α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs) are involved in the relapse to abused drugs. However, the role of AMPARs containing the GluR2 subunit in opiate addiction is still unclear. GluR2-3Y, an interfering peptide, prevents the endocytosis of AMPARs containing the GluR2 subunit. In this study, we explored the effect of intravenous injection of GluR2-3Y on the acquisition, expression, and reinstatement of morphine-induced conditioned place preference (mCPP) in rats. We found that infusion of GluR2-3Y (1.5 nmol/g) one hour before morphine during the conditioning phase inhibited the acquisition of mCPP, while an identical injection one hour before the post-conditioning test had no influence on the expression of mCPP. Injection of GluR2-3Y (1.5 nmol/g) after mCPP extinction blocked the morphine-induced reinstatement of mCPP. Our results strongly support the hypothesis that inhibition of AMPAR endocytosis provides a new target for the treatment of opiate addiction.