Morphine and MK-801 administration leads to alternative N-methyl-D-aspartate receptor 1 splicing and associated changes in reward seeking behavior and nociception on an operant orofacial assay

Transcriptomic analysis of differentially alternative splicing patterns in mice with inflammatory and neuropathic pain

Although the molecular mechanisms of chronic pain have been extensively studied, a global picture of alternatively spliced genes and events in the peripheral and central nervous systems of chronic pain is poorly understood. The current study analyzed the changing pattern of alternative splicing (AS) in mouse brain, dorsal root ganglion, and spinal cord tissue under inflammatory and neuropathic pain. In total, we identified 6495 differentially alternatively spliced (DAS) genes. The molecular functions of shared DAS genes between these two models are mainly enriched in calcium signaling pathways, synapse organization, axon regeneration, and neurodegeneration disease. Additionally, we identified 509 DAS in differentially expressed genes (DEGs) shared by these two models, accounting for a small proportion of total DEGs. Our findings supported the hypothesis that the AS has an independent regulation pattern different from transcriptional regulation. Taken together, these findings indicate that AS is one of the important molecular mechanisms of chronic pain in mammals. This study presents a global description of AS profile changes in the full path of neuropathic and inflammatory pain models, providing new insights into the underlying mechanisms of chronic pain and guiding genomic clinical diagnosis methods and rational medication.

Rapastinel accelerates loss of withdrawal signs after repeated morphine and blunts relapse to conditioned place preference

The purpose of the present study was to evaluate the efficacy of rapastinel, an allosteric modulator of NMDA receptor function, to accelerate the loss of opioid withdrawal symptoms and blunt or prevent relapse to morphine conditioned place preference (CPP) in rats. Two studies were conducted. In study 1, adult and adolescent male and female rats were treated with increasing doses of morphine (5 mg/kg, bid to 25 mg/kg bid) for 5 days. On day 6 animals were treated with naloxone (1 mg/kg) and withdrawal was assessed. They were then treated with saline or rapastinel (5 mg/kg) on days 6 and 8, and withdrawal was assessed on day 9. Rapastinel treated animals exhibited significantly lower levels of withdrawal signs on day 9. No sex or age differences were observed. In Study 2, CPP for morphine was established in adult rats (males and females) by 4 daily pairings with saline and morphine (am/pm alternation). They were tested for CPP on day 5, and then treated with rapastinel (5 mg/kg) or saline daily on days 6-10 of extinction. On day 11 they received a final dose of rapastinel or saline followed by extinction trial. On day 12, animals received 1 mg/kg of morphine and were tested for relapse. Rapastinel did not affect extinction of CPP, but rapastinel-treated animals spent significantly less time in the previously morphine-paired side than saline-treated animals during the relapse trial. These findings of accelerated loss of withdrawal signs and blunted relapse to CPP suggest that rapastinel could provide an adjunctive therapy for opioid dependence during initiation of pharmacotherapy for opioid dependence.

Pharmacological Characterization of Orofacial Nociception in Female Rats Following Nitroglycerin Administration

Rodent models of human disease can be valuable for understanding the mechanisms of a disease and for identifying novel therapies. However, it is critical that these models be vetted prior to committing resources to developing novel therapeutics. Failure to confirm the model can lead to significant losses in time and resources. One model used for migraine headache is to administer nitroglycerin to rodents. Nitroglycerin is known to produce migraine-like pain in humans and is presumed to do the same in rodents. It is not known, however, if the mechanism for nitroglycerin headaches involves the same pathological processes as migraine. In the absence of known mechanisms, it becomes imperative that the model not only translates into successful clinical trials but also successfully reverse translates by demonstrating efficacy of current therapeutics. In this study female rats were given nitroglycerin and nociception was evaluated in OPADs. Estrous was not monitored. Based on the ED₅₀ of nitroglycerin a dose of 10 mg/kg was used for experiments. Sumatriptan, caffeine, buprenorphine and morphine were administered to evaluate the reverse translatability of the model. We found that nitroglycerin did not produce mechanical allodynia in the face of the rats, which is reported to be a consequence of migraine in humans. Nitroglycerin reduced the animals’ participation in the assay. The reduced activity was verified using an assay to measure exploratory behavior. Furthermore, the effects of nitroglycerin were not reversed or prevented by agents that are effective acute therapies for migraine. Two interesting findings from this study, however, were that morphine and nitroglycerin interact to increase the rats’ tolerance of mechanical stimuli on their faces, and they work in concert to slow down the central motor pattern generator for licking on the reward bottle. These interactions suggest that nitroglycerin generated nitric oxide and mu opioid receptors interact with the same neuronal circuits in an additive manner. The interaction of nitroglycerin and morphine on sensory and motor circuits deserves additional examination. In conclusion, based on the results of this study the use of nitroglycerin at these doses in naïve female rats is not recommended as a model for migraine headaches.

miR-219-5p targets CaMKIIγ to attenuate morphine tolerance in rats

Morphine tolerance is a clinical challenge in pain management. Emerging evidence suggests that microRNA (miRNA) plays a regulatory role in the development of morphine tolerance. miR-219-5p (miR-219) targets calmodulin-dependent protein kinase II γ (CaMKIIγ) to activate central pain sensitization via N-methyl-D-aspartate (NMDA) receptor. Therefore, we hypothesized that miR-219-5p attenuates morphine tolerance by targeting CaMKIIγ. We found that the expression of miR-219-5p was decreased significantly after chronic morphine treatment. Overexpression of miR-219-5p by lentivirus injection prevents the development of morphine tolerance. CaMKIIγ, the target gene of miR-219-5p was downregulated by overexpression of miR-219-5p both in vivo and in vitro. Furthermore, we found that lentiviral-mediated miR-219-5p decreased the expression of NMDA receptor subunit 1 (NR1), leading to attenuation of morphine tolerance. Overall, the data demonstrate that miR-219-5p plays a crucial role in alleviating morphine tolerance by inhibiting the CaMKII/NMDA receptor pathway. Overexpression of miR-219-5p may be a potential strategy to ameliorate morphine tolerance.

Phosphorylation of the N-methyl-d-aspartate receptor is increased in the nucleus accumbens during both acute and extended morphine withdrawal

Opioid withdrawal causes a dysphoric state that can lead to complications in pain patients and can propagate use in drug abusers and addicts. Opioid withdrawal changes the activity of neurons in the nucleus accumbens, an area rich in both opioid-binding mu opioid receptors and glutamate-binding NMDA receptors. Because the accumbens is an area important for reward and aversion, plastic changes in this area during withdrawal could alter future behaviors in animals. We discovered an increase in phosphorylation of serine 897 in the NR1 subunit of the NMDA receptor (pNR1) during acute morphine withdrawal. This serine can be phosphorylated by protein kinase A (PKA) and dephosphorylated by calcineurin. We next demonstrated that this increased pNR1 change is associated with an increase in NR1 surface expression. NR1 surface expression and pNR1 levels during acute withdrawal were both reduced by the NMDA receptor antagonist MK-801 (dizocilpine hydrogen maleate) and the PKA inhibitor H-89(N-[2-[[3-(4-bromophenyl)-2-propenyl]amino]ethyl]-5-isoquinolinesulfonamide dihydrochloride hydrate). We also found that pNR1 levels remained high after an extended morphine withdrawal period of 2 months, correlated with reward-seeking behavior for palatable food, and were associated with a decrease in accumbal calcineurin levels. These data suggest that NR1 phosphorylation changes during the acute withdrawal phase can be long lasting and may reflect a permanent change in NMDA receptors in the accumbens. These altered NMDA receptors in the accumbens could play a role in long-lasting behaviors associated with reward and opioid use.

The effects of a co-application of menthol and capsaicin on nociceptive behaviors of the rat on the operant orofacial pain assessment device

Background

Transient receptor potential (TRP) cation channels are involved in the perception of hot and cold pain and are targets for pain relief in humans. We hypothesized that agonists of TRPV1 and TRPM8/TRPA1, capsaicin and menthol, would alter nociceptive behaviors in the rat, but their opposite effects on temperature detection would attenuate one another if combined.

Methods

Rats were tested on the Orofacial Pain Assessment Device (OPAD, Stoelting Co.) at three temperatures within a 17 min behavioral session (33°C, 21°C, 45°C).

Results

The lick/face ratio (L/F: reward licking events divided by the number of stimulus contacts. Each time there is a licking event a contact is being made.) is a measure of nociception on the OPAD and this was equally reduced at 45°C and 21°C suggesting they are both nociceptive and/or aversive to rats. However, rats consumed (licks) equal amounts at 33°C and 21°C but less at 45°C suggesting that heat is more nociceptive than cold at these temperatures in the orofacial pain model. When menthol and capsaicin were applied alone they both induced nociceptive behaviors like lower L/F ratios and licks. When applied together though, the licks at 21°C were equal to those at 33°C and both were significantly higher than at 45°C.

Conclusions

This suggests that the cool temperature is less nociceptive when TRPM8/TRPA1 and TRPV1 are co-activated. These results suggest that co-activation of TRP channels can reduce certain nociceptive behaviors. These data demonstrate that the motivational aspects of nociception can be influenced selectively by TRP channel modulation and that certain aspects of pain can be dissociated and therefore targeted selectively in the clinic.

Operant assays for assessing pain in preclinical rodent models: highlights from an orofacial assay

Despite an immense investment of resources, pain remains at epidemic proportions. Given this, there has been an increased effort toward appraising the process by which new painkillers are developed, focusing specifically on why so few analgesics make it from the benchside to the bedside. The use of behavioral assays and animal modeling for the preclinical stages of analgesic development is being reexamined to determine whether they are truly relevant, meaningful, and predictive. Consequently, there is a strengthening consensus that the traditional reflex-based assays upon which several decades of preclinical pain research has been based are inadequate. Thus, investigators have recently turned to the development of new preclinical assays with improved face, content, and predictive validity. In this regard, operant pain assays show considerable promise, as they are more sensitive, present better validity, and, importantly, better encompass the psychological and affective dimensions of pain that trouble human pain sufferers. Here, we briefly compare and contrast reflex assays with operant assays, and we introduce a particular operant orofacial pain assay used in a variety of experiments to emphasize how operant pain assays can be applied to preclinical studies of pain.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Use of the Operant Orofacial Pain Assessment Device (OPAD) to measure changes in nociceptive behavior

We present an operant system for the detection of pain in awake, conscious rodents. The Orofacial Pain Assessment Device (OPAD) assesses pain behaviors in a more clinically relevant way by not relying on reflex-based measures of nociception. Food fasted, hairless (or shaved) rodents are placed into a Plexiglas chamber which has two Peltier-based thermodes that can be programmed to any temperature between 7 °C and 60 °C. The rodent is trained to make contact with these in order to access a reward bottle. During a session, a number of behavioral pain outcomes are automatically recorded and saved. These measures include the number of reward bottle activations (licks) and facial contact stimuli (face contacts), but custom measures like the lick/face ratio (total number of licks per session/total number of contacts) can also be created. The stimulus temperature can be set to a single temperature or multiple temperatures within a session. The OPAD is a high-throughput, easy to use operant assay which will lead to better translation of pain research in the future as it includes cortical input instead of relying on spinal reflex-based nociceptive assays.

Long-term changes in reward-seeking following morphine withdrawal are associated with altered N-methyl-D-aspartate receptor 1 splice variants in the amygdala

The NR1 subunit of the NMDA receptor can be alternatively spliced by the insertion or removal of the N1, C1, C2, or C2' regions. Morphine dependence and withdrawal were previously demonstrated to lower N1 and C2' in the accumbens and lower N1, C1, and C2' in the amygdala (AMY). Withdrawal has also been demonstrated to increase motivational and anxiety/stress behaviors in rats. We tested the hypothesis that NR1 splicing would be associated with these behaviors during an extended withdrawal period of 2 months. Motivation was measured using an operant orofacial assay at non-aversive temperatures (37°C) while anxiety and stress were measured by examining this behavior at aversive temperatures (46°C). Lower C1 and C2 expression levels were observed in the AMY in a subset of the population of withdrawn rats even after 2 months of morphine withdrawal. These subsets were associated with a hypersensitivity to adverse conditions which may reflect long-term alterations in the withdrawn population.