Role of orexin-1 and orexin-2 receptors in the CA1 region of hippocampus in the forced swim stress- and food deprivation-induced reinstatement of morphine seeking behaviors in rats

The stress-induced antinociceptive responses to the persistent inflammatory pain involve the orexin receptors in the nucleus accumbens

Stress suppresses the sense of pain, a physiological phenomenon known as stress-induced analgesia (SIA). Brain orexin peptides regulate many physiological functions, including wakefulness and nociception. The contribution of the orexinergic system within the nucleus accumbens (NAc) in the modulation of antinociception induced by forced swim stress (FSS) remains unclear. The present study addressed the role of intra-accumbal orexin receptors in the antinociceptive responses induced by FSS during the persistent inflammatory pain model in the rat. Stereotaxic surgery was performed unilaterally on 106 adult male Wistar rats weighing 250-305 g. Different doses (1, 3, 10, and 30 nmol/ 0.5 μl DMSO) of orexin-1 receptor (OX1r) antagonist (SB334867) or OX2 receptor antagonist (TCS OX2 29) were administered into the NAc five minutes before exposure to FSS for a 6-min period. The formalin test was carried out using formalin injection (50 μl; 2.5%) into the rat's hind paw plantar surface, which induces biphasic pain-related responses. The first phase begins immediately after formalin infusion and takes 3-5 min. Subsequently, the late phase begins 15-20 min after formalin injection and takes 20-40 min. The findings demonstrated that intra-accumbal microinjection of SB334867 or TCS OX2 29 attenuated the FSS-induced antinociception in both phases of the formalin test, with the TCS OX2 29 showing higher potency. Moreover, the effect of TCS OX2 29 was more significant during the early phase of the formalin test. The results suggest that OX1 and OX2 receptors in the NAc might modulate the antinociceptive responses induced by the FSS.

Involvement of orexin-2 receptors in the CA1 region of the hippocampus in the extinction and reinstatement of methamphetamine-induced conditioned place preference in the rats

The present study, regarding the orexin receptors having a pivotal role in reward-related psychostimulant use disorder (PUD), aimed to investigate the role of orexin-2 (OX2) receptors in the CA1 region of the hippocampus (HPC) in the extinction and reinstatement of methamphetamine (METH)-induced conditioned place preference (CPP). In the first set of investigations, to determine the role of OX2 receptors in the extinction of METH-induced CPP, rats were daily given (during the extinction) bilaterally intra-CA1 region different doses of TCS OX2 29 (1, 3, 10, and 30 nmol/0.5 μl 12% DMSO) as the selective OX2 receptor antagonist. Then, to demonstrate the role of OX2 receptors in the reinstatement of METH-induced CPP after the extinction was established, each rat bilaterally received TCS OX2 29 at the same doses in the CA1 region before injection of the sub-threshold (priming) dose of METH (0.25 mg/kg, sc) on the reinstatement day. The data revealed that the administration of TCS OX2 29 in the CA1 region reduces the mean extinction latency and suppresses the reinstatement of METH-seeking behavior in extinguished rats. Additionally, the potency of TCS OX2 29 to inhibit the reinstatement phase was higher compared to the potency of this drug to modulate the extinction phase of METH-induced CPP. Accordingly, it could be concluded that the blockade of the OX2 receptors in this area might be an essential application and potential therapeutics in treating METH use disorder.

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.

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.

Manifestations of domination: Assessments of social dominance in rodents

Social hierarchies are ubiquitous features of virtually all animal groups. The varying social ranks of members within these groups have profound effects on both physical and emotional health, with lower-ranked individuals typically being the most adversely affected by their respective ranks. Thus, reliable measures of social dominance in preclinical rodent models are necessary to better understand the effects of an individual's social rank on other behaviors and physiological processes. In this review, we outline the primary methodologies used to assess social dominance in various rodent species: those that are based on analyses of agonistic behaviors, and those that are based on resource competition. In synthesizing this review, we conclude that assays based on resource competition may be better suited to characterize social dominance in a wider variety of rodent species and strains, and in both males and females. Lastly, albeit expectedly, we demonstrate that similarly to many other areas of preclinical research, studies incorporating female subjects are lacking in comparison to those using males. These findings emphasize the need for an increased number of studies assessing social dominance in females to form a more comprehensive understanding of this behavioral phenomenon.

Orexin receptors in the CA1 region of hippocampus modulate the stress-induced antinociceptive responses in an animal model of persistent inflammatory pain

Stress activates multiple neural pathways and neurotransmitters that often suppress pain perception, the phenomenon called stress-induced analgesia (SIA). Orexin neurons from the lateral hypothalamus project to entire brain structures such as the hippocampus. The present study examined this hypothesis that orexinergic receptors in the CA1 region of the hippocampus may play a modulatory role in the development of SIA in formalin test as an animal model of persistent inflammatory pain. One hundred-two adult male Wistar rats were administered with intra-CA1 orexin-1 receptor (OX1r) antagonist, SB334867, at the doses of 3, 10, 30, and 100 nmol or TCS OX2 29 as orexin-2 receptor (OX2r) antagonist at the doses of 1, 3, 10, and 30 nmol. Five min later, rats were exposed to forced swim stress (FSS) for a 6-min period. Then, pain-related behaviors induced by formalin injection were measured at the 5-min blocks during a 60-min period of formalin test. The current study indicated that solely stress exposure elicits antinociception in the early and late phases of the formalin test. The FSS-induced analgesia was prevented by intra-CA1 administration of SB334867 or TCS OX2 29 during either phase of the formalin test. Moreover, the contribution of the OX2r in the mediation of analgesic effect of stress was more prominent than that of the OX1r during both phases of the formalin test. It is suggested that OX1r and OX2r in the CA1 region of the hippocampus are involved in stress-induced analgesia in the animal model of persistent inflammatory pain.

The effect of orexin-2 and endocannabinoid-1 antagonists on neuronal activity of hippocampal CA1 pyramidal neurons in response to tramadol in rats

Background:

CA1, as a major structure involved in learning and memory, has been shown to be affected by tramadol addiction. Both orexin and endocannabinoid receptors express in CA1 and play an important role in drug dependency. The aim of this study was to evaluate the modulatory effects of orexin-2 (OX2R) and endocannabinoid-1 (CB1R) receptors on neuronal activity in CA1, in response to tramadol in rats.

Materials and Methods:

Male Wistar rats were divided into 8 groups (n = 6–7); saline-dimethyl sulfoxide (DMSO), tramadol-DMSO, saline-TCS-OX2-29, saline-AM251, tramadol-TCS-OX2-29, tramadol-AM251, saline-TCS-OX2-29-AM251, tramadol-TCS-OX2-29-AM251. Tramadol was injected intraperitoneally, and then, AM251 (1 nmol/0.3 μL), CB1R antagonist and TCS-OX2-29 (1 nmol/0.3 μL), OX2R antagonist, were microinjected individually or concurrently into the CA1. Using in vivo extracellular single-unit recording, the firing of CA1 pyramidal neurons was investigated.

Results:

Tramadol decreased neuronal activity in CA1 (P < 0.01) but increased it after micro-injection of DMSO. TCS-OX2-29 increased neuronal activity in saline group (P < 0.05) but decreased it in tramadol group. AM251 had no effect on saline group but decreased neuronal activity in tramadol group (P < 0.05). Concurrent micro-injection of TCS-OX2-29 and AM251 had no effect on saline group but decreased neuronal activity in tramadol group (P < 0.05).

Conclusions:

Our findings suggest that neural activity in CA1 is rapidly affected by acute use of tramadol, and some of these effects may be induced through the endocannabinoid and orexin systems. Thus, the function of endocannabinoid and orexin systems in CA1 may play a role in tramadol addiction.

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.

Modulatory role of the orexin system in stress-induced analgesia: Involvement of the ventral tegmental area

BACKGROUND

Exposure to stressful experiences is often accompanied by suppressing pain perception, referred to as stress-induced analgesia. The neuropeptides orexins are essential in regulating the mechanism that responds to stressful and painful stimuli. Meanwhile, the ventral tegmental area (VTA), as a part of descending pain inhibitory system, responds to noxious stimuli. This study aimed to investigate the role of intra-VTA administration of orexin receptor antagonists on stress-induced antinociceptive responses in the animal model of acute pain.

METHOD

Ninety-three adult Wistar rats weighing 230-250 g were unilaterally implanted by a cannulae above the VTA. Animals were pretreated with different doses (1, 3, 10 and 30 nM/0.3 μl) of SB334867 as the orexin-1 receptor antagonist and TCS OX2 29 as the orexin-2 receptor antagonist into the VTA, just 5 min before 6 min exposure to forced swim stress (FSS). Nociceptive threshold was measured using the tail-flick test as a model of acute pain.

RESULTS

The results showed that exposure to FSS could significantly increase analgesic responses. Moreover, intra-VTA administration of SB334768 and TCS OX2 29 blocked the antinociceptive effect of FSS in the tail-flick test.

CONCLUSION

The findings suggest that OX1 and OX2 receptors in the VTA might modulate the antinociceptive behaviours induced by FSS in part.

SIGNIFICANCE

Acute exposure to physical stress suppresses pain-related behaviors in the animal model of acute pain. Blockade of the OX1 and OX2 receptors in the VTA attenuates antinociceptive responses induced by FSS. The contribution of the OX2 receptors in the VTA is more predominant than OX1 receptors in stress-induced analgesia.

Cannabidiol attenuated the maintenance and reinstatement of extinguished methylphenidate-induced conditioned place preference in rats

Methylphenidate (MPH) is a mild CNS stimulant that has been used in hyperactive children, and patients with neurodegenerative and major depressive disorders. Exposure to MPH-associated cues enhances craving and arousal in drug users. On the other hand, cannabidiol (CBD) has antipsychotic potential that might be useful in alleviating symptoms of drug addiction. The aim of this study was to investigate the effect of CBD administration on extinction and reinstatement of MPH-induced conditioning place preference (CPP) in rats. Male rats received MPH (1, 2.5 or 5 mg/kg, i.p) or morphine (5 or 10 mg/kg, s.c.) during the conditioning phase. Following the establishment of CPP, during extinction training, 60 min prior to every CPP session, animals were given daily ICV CBD (10 or 50 μg/5 μL), vehicle alone (DMSO) 10 % or were treatment-naïve. On the reinstatement day animals after receiving the initial dose of MPH, 0.5 mg/kg, and were placed into the CPP box to evaluate the CPP scoring for 10-min. Our findings indicated that morphine (5 and 10 mg/kg; s.c.) and MPH (1 and 2.5 mg/kg; i.p.) induced a CPP. The ICV administration of both doses of CBD (10 and 50 μg/5 μL) prevented the reinstatement of MPH-induced CPP, which displayed shorter extinction latency compared to treatment-naïve or DMSO 10 % groups. Therefore, CBD's site of action is a potential target for reducing the risk of MPH relapse; however, more investigation is required.

Non-Opioid Treatments for Opioid Use Disorder: Rationales and Data to Date

Opioid use disorder (OUD) represents a major public health problem that affects millions of people in the USA and worldwide. The relapsing and recurring aspect of OUD, driven by lasting neurobiological adaptations at different reward centres in the brain, represents a major obstacle towards successful long-term remission from opioid use. Currently, three drugs that modulate the function of the opioidergic receptors, methadone, buprenorphine and naltrexone have been approved by the US Food and Drug Administration (FDA) to treat OUD. In this review, we discuss the limitations and challenges associated with the current maintenance and medication-assisted withdrawal strategies commonly used to treat OUD. We further explore the involvement of glutamatergic, endocannabinoid and orexin signaling systems in the development, maintenance and expression of addiction-like behaviours in animal models of opioid addiction, and as potential and novel targets to expand therapeutic options to treat OUD. Despite a growing preclinical literature highlighting the role of these potential targets in animal models of opioid addiction, clinical and translational studies for novel treatments of OUD remain limited and inconclusive. Further preclinical and clinical investigations are needed to expand the arsenal of primary treatment options and adjuncts to maximise efficacy and prevent relapse.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

Targeting the orexin system for prescription opioid use disorder: Orexin-1 receptor blockade prevents oxycodone taking and seeking in rats

Prescription opioids, such as oxycodone, are potent analgesics that are used to treat and manage pain. However, oxycodone is one of the most commonly abused prescription drugs. Finding an effective strategy to prevent prescription opioid use disorder is urgent. Orexin receptors (OrxR1 and OrxR2) have been implicated in the regulation of motivation, arousal, and stress, making them possible targets for the treatment of substance use disorder. To study the significance of environmental stimuli in maintaining the vulnerability to relapse to oxycodone use, resistance to the extinction of oxycodone-seeking behavior that was elicited by an oxycodone-related stimulus was examined. Rats were trained to self-administer oxycodone in the presence of a contextual/discriminative stimulus (S^D). Using this procedure, the rats readily acquired oxycodone self-administration and exhibited increases in physical signs of opioid withdrawal. Following extinction, response-reinstating effects of re-exposure to the S^D perseverated. We then tested whether OrxR blockade prevents oxycodone intake and relapse. The effects of the OrxR1 antagonist SB334867 and OrxR2 antagonist TCSOX229 on oxycodone self-administration were tested. SB334867 significantly decreased oxycodone self-administration, whereas TCSOX229 did not produce any effect. To investigate whether OrxR1 and OrxR2 blockade prevents oxycodone seeking, the rats were tested for the ability of SB334867 and TCSOX229 to prevent the S^D-induced conditioned reinstatement of oxycodone-seeking behavior. SB334867 decreased oxycodone-seeking behavior, whereas TCSOX229 was ineffective. These results suggest that OrxR1 antagonism prevents excessive prescription opioid use and relapse and might be beneficial for the treatment of prescription opioid use disorder.

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.

Effect of acute and chronic restraint stress on electrical activity of prefrontal cortex neurons in the reinstatement of extinguished methamphetamine-induced conditioned place preference: An electrophysiological study

Increased vulnerability to drug abuse has been observed after exposure to stress and the prefrontal cortex (PFC) plays a major role in the control of the stress response and reward pathway. The current study was conducted to clarify the effects of acute and chronic restraint stress on PFC neural activity during the reinstatement of methamphetamine (METH)-induced conditioned place preference (CPP) in rats. Following the establishment of CPP (METH 0.5 mg/kg; s.c. for 3 days) and the extinction phase, male Wistar rats were divided into threshold (0.25 mg/kg; s.c.) and sub-threshold (0.125 mg/kg; s.c.) METH-treated super groups to induce reinstatement. Each super group contained control (non-stressed), acute restraint stress (ARS) and chronic restraint stress (CRS) groups. in vivo single unit recordings were performed on the urethane-anesthetized rats in these groups. After baseline recordings (10-min period) of the neurons in the PFC, their firing activity was recorded for 50 min during the reinstatement phase after injection of METH. The results showed that the threshold dose, but not the sub-threshold dose, of METH significantly increased PFC neural activity in the non-stressed animals. The sub-threshold dose of METH notably changed this activity in both the ARS and CRS groups. These changes in the excited neurons after the sub-threshold dose in the ARS and CRS groups were significantly higher than those in the non-stressed group. It appears that the PFC is implicated in the associated reward pathway and stress functions. METH affected the firing rate of PFC neurons and stress amplified the effect of METH on changes in the neuronal firing rate in the PFC.

Intra-hippocampal administration of orexin receptor antagonists dose-dependently attenuates reinstatement of morphine seeking behavior in extinguished rats

It has been shown that the hippocampus plays an essential role in the regulation of reward and memory as indicated by the conditioned place preference (CPP) paradigm. Morphine-induced CPP is a common method to consider motivational properties of morphine in animals. Recently, this model has been used in many laboratories to investigate neuronal mechanisms underlying reinstatement of morphine seeking induced by drug re-exposure. Our previous studies indicate that the hippocampus especially CA1 region is involved in reinstatement of drug-seeking behaviors. Also, several studies have shown that orexin attenuates key functional and behavioral effects of its co-transmitter dynorphin. The present study evaluates the role of orexinergic receptors within the CA1 region of the hippocampus in the reinstatement of morphine-induced CPP. Therefore, after the extinction period, the different doses (SB 334867; 0.3, 3, and 30 nM/0.5 μl DMSO) of either orexin-1 or -2 receptor antagonists were bilaterally microinjected into the CA1, 15 min before receiving an effective priming dose of morphine (1 mg/kg). The results revealed that administration of both SB 334867 and TCS OX2 29 prior to injection of the priming dose of morphine significantly reduced the reinstatement of morphine-induced CPP without altering the animal's locomotor activity. Also, the 50% effective dose value of SB 334867 on the reinstatement of morphine seeking behavior was close three times more than that in TCS OX2 29 treatment group. Therefore, the consequences suggested that both orexin receptors in the CA1 play a considerable role in the reinstatement of morphine-induced CPP.