Beyond rodent models of pain: non-human primate models for evaluating novel analgesic therapeutics and elaborating pain mechanisms.

Evaluation of potential analgesic therapeutics and the elaboration of the neurobiology of pain have heavily relied on pain models developed in rodents. However, a limitation of rodents is their phylogenetic distance from humans, which could in part account for the failure of some preclinical findings to translate to clinical utility. By contrast, given their genetic closeness and phenotypic similarities to humans, it is suggested that there be greater utilization of non-human primates (NHP) in preclinical pain studies. Methods to induce chronic pain-like states and quantify changes in nociception that have been developed in rodents could be adapted to the NHP. Similarly, human experimental injury-induced sensitization, which attempts to temporarily mimic the neuropathology and symptoms observed in the chronic pain state, could be adapted to the NHP. The NHP could then serve as a platform to validate human experimental models as well as proof-of-concept studies. Beyond experimentally modeled pain states, a number of naturally occurring disease states, such as osteoarthritis, are expressed by NHP, which could be utilized for both hypothesis testing and proof-of-concept studies. While NHP studies are logistically cumbersome, it is envisioned that NHP pain models will add value to current preclinical data and greatly facilitate the discovery of novel analgesic treatments.

Nrf2 deficiency leads to behavioral, neurochemical and transcriptional changes in mice.

Nrf2 is a transcription factor that regulates the antioxidant and detoxification enzyme genes and provides defense against oxidative and electrophilic stresses in various tissues. In brain, while neuroprotective functions of Nrf2 have been well documented, Nrf2 contribution to the brain function remains to be elucidated. To address this issue, we investigated whether Nrf2 deficiency affects psychological behaviors, neurotransmitter systems and gene expressions in mice. We conducted four behavioral tests, social interaction, open-field, rotarod and forced swimming tests and found that Nrf2 knockout mice exhibited reduced immobility in the forced swimming test. Neurochemical analyses revealed that the dopamine and serotonin metabolites increased in the brains of Nrf2 knockout mice. We also present a catalog of genes whose expression is Nrf2-dependent in brain under unstressed conditions, which includes a number of xenobiotic-metabolizing enzyme genes. These results thus support our contention that Nrf2 regulates its target genes in brain under unstressed conditions and loss of Nrf2 affects various brain functions.

Studies on somnolence in the daytime caused by drugs used for neuropathic pain.

In the present study, the characteristics of the sleep features of amitriptyline, mexiletine, and N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl)tetrahydropyradine-1(2H)-carbox-amide (BCTC) were studied. Electrodes were chronically implanted into the frontal cortex and the dorsal neck muscles of rats for electroencephalogram (EEG) and electromyogram (EMG) measurements, respectively. EEG and EMG were recorded with an electroencephalograph, and SleepSign ver. 2.0. was used for sleep-wake state analysis. Recordings were performed from 11:00 to 17:00. Amitriptyline caused significant decreases in sleep latency and total wake time and an increase in total non-rapid eye movement (non-REM) sleep time. Mexiletine caused a significant decrease in sleep latency, but no significant effect was observed in total wake time and total non-REM sleep time. On the other hand, BCTC, which is an antagonist of transient receptor potential vaniloid 1 (TRPV1), showed no significant effect on sleep latency, total wake time, total non-REM sleep time, and total REM sleep time. From these results, it can be concluded that a TRPV1 antagonist may become a useful drug for neuropathic pain without sedative side effects in the daytime, different from amitriptyline and mexiletine.

Synergy between a NR2B receptor antagonist DHQ and 3-methyl-gabapentin in mice with neuropathic pain.

The synergistic effect of a selective NR2B NMDA receptor antagonist, (-)-(R)-6-{2-[4-(3-Fluorophenyl)-4-hydroxy-1-piperidinyl]-1-hydroxyethyl-3,4-dihydro-2(1H)-quinolinone (DHQ), and a alpha 2 delta ligand, 3-methyl-gabapentin (3M-GBP), was investigated in the mouse partial sciatic nerve model. The interaction was observed after administration of DHQ and 3M-GBP combination at fixed dose ratios of 1:10 and 1:30 and the dose-response curves shifted approximately 13- and 17-fold leftward, respectively, from the theoretical additive values. However, a fixed dose ratio of 1:50 resulted only in an additive effect. These results indicate the synergistic interaction between DHQ and 3M-GBP in this animal model of neuropathic pain.

Pharmacological characterization of the newly synthesized nociceptin/orphanin FQ-receptor agonist 1-[1-(1-methylcyclooctyl)-4-piperidinyl]-2-[(3R)-3-piperidinyl]-1H-benzimidazole as an anxiolytic agent.

Nociceptin/orphanin FQ peptide (NOP)-receptor agonists have been shown to produce anxiolytic-like effects in rodents subjected to various behavioral assays. Recently, we developed a new nonpeptide agonist of the NOP receptor, 1-[1-(1-methylcyclooctyl)-4-piperidinyl]-2-[(3R)-3-piperidinyl]-1H-benzimidazole (MCOPPB), as an anxiolytic agent. MCOPPB has a high affinity for the human NOP receptor (pKi = 10.07 +/- 0.01) and selectivity for the NOP receptor over other members of the opioid receptor family: 12-, 270- and >1000-fold more selective for the NOP receptor than for the micro-, kappa-, and delta-receptor, respectively. In an ex vivo binding study, MCOPPB (10 mg/kg, p.o.) inhibited signaling through the NOP receptor in the mouse brain, suggesting that it penetrated into the brain after it was orally administered. In the mouse Vogel conflict test, MCOPPB (10 mg/kg, p.o.) and diazepam (3 mg/kg, p.o.) elicited anxiolytic-like effects, although MCOPPB produced a bell-shaped response curve. In addition, MCOPPB (10 mg/kg, p.o.) was still effective as an anxiolytic agent even after repeated administration for 5 days. MCOPPB at an oral dose of 10 mg/kg did not affect locomotor activity or memory, nor did it contribute to ethanol-induced hypnosis. On the other hand, the benzodiazepine-type anxiolytic agent diazepam caused memory deficits and enhanced ethanol-induced hypnosis. These findings suggest that MCOPPB - a compound with few adverse effects on the central nervous system - is a potential therapeutic agent for the treatment of anxiety.

Pharmacologic investigation of the mechanism underlying cold allodynia using a new cold plate procedure in rats with chronic constriction injuries.

Cold allodynia is a frequent clinical symptom of patients with neuropathic pain. Despite numerous studies of cold allodynia, using animal models of neuropathic pain, little is known about its underlying mechanisms. This study was performed to establish a method for the pharmacologic evaluation of cold allodynia using several analgesics in a chronic constriction injury (CCI) rat model of neuropathic pain. Compared with the results obtained before the CCI operation, the CCI rats placed on a cork plate at 20 degrees C exhibited a slight change in the paw withdrawal latency because of the mechanical stimulus mediated by the injured paw touching the plate. By contrast, there was a significant reduction in the paw withdrawal latency on a cold metal plate compared with that on the cork plate after the CCI surgery, with the maximum decrease occurring on postoperative day 7. This reduction is thought to specifically reflect cold-induced pain behavior. In addition, both naïve and CCI rats showed behavioral changes at 5 and 0 degrees C, but not at 10 degrees C or higher. Interestingly, a subcutaneous morphine dose of 6 mg/kg completely inhibited cold allodynia induced at 10 degrees C on postoperative day 7. Under this condition, both the sodium channel blocker mexiletine (10 and 30 mg/kg, subcutaneously) and the calcium channel alpha2delta subunit blocker pregabalin (30 and 100 mg/kg, orally) significantly suppressed cold allodynia. Additionally, both resiniferatoxin (0.3 mg/kg, subcutaneously), an ultrapotent analog of capsaicin that desensitizes C fibers, and the VR1 channel antagonist N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl) tetrahydropyrazine-1(2H)-carboxamide (10 and 30 mg/kg, orally) significantly prolonged the paw withdrawal latency. In conclusion, our data suggest that the activation of C fibers mediates cold allodynia.

Pharmacological properties of a novel nociceptin/orphanin FQ receptor agonist, 2-(3,5-dimethylpiperazin-1-yl)-1-[1-(1-methylcyclooctyl)piperidin-4-yl]-1H-benzimidazole, with anxiolytic potential.

We have characterized the pharmacological properties of the novel nociceptin/orphanin FQ peptide receptor (NOP receptor) agonist, 2-(3,5-dimethylpiperazin-1-yl)-1-[1-(1-methylcyclooctyl)piperidin-4-yl]-1H-benzimidazole (PCPB). PCPB bound to the NOP receptor in mouse brain membranes (Ki=0.12 nM) and to recombinant human NOP receptor (Ki=2.1 nM). PCPB showed full agonism for the NOP receptor in isolated mouse vas deferens with a maximal effect and high potency that were similar to the pharmacological profile observed for nociceptin/orphanin FQ (N/OFQ) (pD(2): 6.9+/-0.2; 95+/-2% activity). Orally administered PCPB (30 mg/kg) penetrated well into the brains of the mice. PCPB exhibited an anxiolytic activity in mice subjected to the Vogel conflict test that was comparable to the maximal response induced by diazepam, a representative anxiolytic agent. The anxiolytic effect of PCPB was dose-dependently blocked by the NOP receptor antagonist, J-113397, demonstrating that this effect was mediated by the NOP receptor agonist activity. Behavioral studies in mice also showed that PCPB prolonged the pentobarbital-induced sleeping time but did not cause muscle relaxation at the oral anxiolytic dose of 30 mg/kg. Unlike diazepam, however, these central effects of PCPB were weak. Our results indicate that PCPB is a potent anxiolytic agent with agonistic activities for the NOP receptor.

Changes of sleep patterns in rats with chronic constriction injury under aversive conditions.

In the present study, we investigated the changes of sleep parameters in rats with chronic constriction injury (CCI) under aversive conditions. The electroencephalogram (EEG) in the frontal cortex of CCI rats and electromyogram (EMG) were measured over 6 h by placing rats on sandpaper as an aversive condition, to compare with rats placed on sawdust. Six days after CCI surgery, the rats exhibited significant mechanical allodynia, and also had neuropathic pain. When rats were placed on sawdust, no significant difference was observed between the CCI group and sham-operated control group in sleep latency, total waking time, total non-REM sleep time and total REM sleep time. On the other hand, when CCI rats were placed on sandpaper, a significant increase was observed in sleep latency and total waking time compared with the sham group; however, no significant difference was observed in the total non-REM sleep time and total REM sleep time between these two groups. These results indicate that an important factor of sleep disturbance in CCI rats is not only damage to the nerves but also being under aversive conditions. In addition, it was found that CCI rats placed on sandpaper as an aversive condition can serve as a new sleep disturbance model.