NOP Receptor Signaling Cascades

Spotlight on Nociceptin/Orphanin FQ Receptor in the Treatment of Pain

In our society today, pain has become a main source of strain on most individuals. It is crucial to develop novel treatments against pain while focusing on decreasing their adverse effects. Throughout the extent of development for new pain therapies, the nociceptin/orphanin FQ receptor (NOP receptor) has appeared to be an encouraging focal point. Concentrating on NOP receptor to treat chronic pain with limited range of unwanted effects serves as a suitable alternative to prototypical opioid morphine that could potentially lead to life-threatening effects caused by respiratory depression in overdose, as well as generate abuse and addiction. In addition to these harmful effects, the uprising opioid epidemic is responsible for becoming one of the most disastrous public health issues in the US. In this article, the contributing molecular and cellular structure in controlling the cellular trafficking of NOP receptor and studies that support the role of NOP receptor and its ligands in pain management are reviewed.

Participation of the opioid receptor - nitric oxide - cGMP - K+ channel pathway in the peripheral antinociceptive effect of nalbuphine and buprenorphine in rats

The aim of this study was to examine if the peripheral antinociceptive effects of the opioid agonist/antagonist nalbuphine and buprenorphine involve the sequential participation of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) synthesis followed by K⁺ channel opening in the formalin test. Wistar rats (180-220 g) were injected in the dorsal surface of the right hind paw with formalin (1%). Rats received a subcutaneous (s.c.) injection into the dorsal surface of the paw of vehicles or increasing doses of nalbuphine (50-200 μg/paw) or buprenorphine (1-5 μg/paw) 20 min before formalin injection into the paw. Nalbuphine antinociception was reversed by the s.c. injection into the paw of the inhibitor of NO synthesis (N^G-nitro-l-arginine methyl ester (L-NAME)), by the inhibitor of guanylyl cyclase (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ)), by the K_ir6.1-2, ATP-sensitive K⁺ channel inhibitors (glibenclamide and glipizide), by the K_Ca2.1-3, small conductance Ca²⁺-activated K⁺ channel blocker (apamin), by the K_Ca1.1, large conductance Ca²⁺-activated K⁺ channel blocker (charybdotoxin), and by the K_V, voltage-dependent K⁺ channel inhibitors (4-aminopyridine (4-AP) and tetraethylammonium chloride (TEA)). The antinociceptive effect produced by buprenorphine was blocked by the s.c. injection of 4-AP and TEA but not by L-NAME, ODQ, glibenclamide, glipizide, apamin, or charybdotoxin. The present results provide evidence for differences in peripheral mechanisms of action between these opioid drugs.

Therapeutic potentials of NOP and MOP receptor coactivation for the treatment of pain and opioid abuse

Following the identification of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) as an endogenous ligand for the NOP receptor, ample evidence has revealed unique functional profiles of the N/OFQ-NOP receptor system. NOP receptors are expressed in key neural substrates involved in pain and reward modulation. In nonhuman primates (NHPs), NOP receptor activation effectively exerts antinociception and anti-hypersensitivity at the spinal and supraspinal levels. Moreover, NOP receptor activation inhibits dopaminergic transmission and synergistically enhances mu-opioid peptide (MOP) receptor-mediated analgesia. In this article, we have discussed the functional profiles of ligands with dual NOP and MOP receptor agonist activities and highlight their optimal functional efficacy for pain relief and drug abuse treatment. Through coactivation of NOP and MOP receptors, bifunctional NOP/MOP receptor "partial" agonists (e.g., AT-121, BU08028, and BU10038) reveal a wider therapeutic window with fewer side effects. These newly developed ligands potently induce antinociception without MOP receptor agonist-associated side effects such as abuse potential, respiratory depression, itching sensation, and physical dependence. In addition, in both rodent and NHP models, bifunctional NOP/MOP receptor agonists can attenuate reward processing and/or the reinforcing effects of opioids and other abused drugs. While a mixed NOP/opioid receptor "full" agonist cebranopadol is undergoing clinical trials, bifunctional NOP/MOP "partial" agonists exhibit promising therapeutic profiles in translational NHP models for the treatment of pain and opioid abuse. This class of drugs demonstrates the therapeutic advantage of NOP and MOP receptor coactivation, indicating a greater potential for future development.

The Effects of Opioids on HIV Neuropathogenesis

HIV associated neurocognitive disorders (HAND) are a group of neurological deficits that affect approximately half of people living with HIV (PLWH) despite effective antiretroviral therapy (ART). There are currently no reliable molecular biomarkers or treatments for HAND. Given the national opioid epidemic, as well as illegal and prescription use of opioid drugs among PLWH, it is critical to characterize the molecular interactions between HIV and opioids in cells of the CNS. It is also important to study the role of opioid substitution therapies in the context of HIV and CNS damage in vitro and in vivo. A major mechanism contributing to HIV neuropathogenesis is chronic, low-level inflammation in the CNS. HIV enters the brain within 4–8 days after peripheral infection and establishes CNS reservoirs, even in the context of ART, that are difficult to identify and eliminate. Infected cells, including monocytes, macrophages, and microglia, produce chemokines, cytokines, neurotoxic mediators, and viral proteins that contribute to chronic inflammation and ongoing neuronal damage. Opioids have been shown to impact these immune cells through a variety of molecular mechanisms, including opioid receptor binding and cross desensitization with chemokine receptors. The effects of opioid use on cognitive outcomes in individuals with HAND in clinical studies is variable, and thus multiple biological mechanisms are likely to contribute to the complex relationship between opioids and HIV in the CNS. In this review, we will examine what is known about both HIV and opioid mediated neuropathogenesis, and discuss key molecular processes that may be impacted by HIV and opioids in the context of neuroinflammation and CNS damage. We will also assess what is known about the effects of ART on these processes, and highlight areas of study that should be addressed in the context of ART.