Current Pharmaceutical Design - Volume 21, Issue 7, 2015

Neuronal injury not only results in severe alteration in the function of primary sensory neurons and their central projection pathway, but is also associated with a robust immune response at almost every level of the somatosensory system. Evidence from animal studies suggests undoubtedly that bi-directional signalling between the immune system and the nervous system contribute to the development and maintenance of chronic neuropathic pain. Non-neuronal cells, including peripheral immune cells, CNS/PNS glial cells and endothelial cells play important roles in the neuroimmune interaction and subsequent persistent hypersensitivity. Various cytokines and chemokines have been identified as key signalling molecules in the crosstalk. However, majority evidence showing inflammation in neuropathic pain was generated from animal models at acute phase. Whether and to what extent inflammation or non-neuronal cells are involved at chronic stage of neuropathic pain needs to be further explored, and evidence of inflammation in chronic pain from human studies is still largely awaited. Therapeutic agents targeting inflammation provide an exciting prospect. Yet, considering the heterogeneous conditions presented in neuropathic pain, no matter the etiologies, or the pathophysiology during different stages of the disease; and the complexity of the immune response to the damage on the nervous system, it appears that finely tuned strategies of modulating inflammation are essential to warrant an effective treatment for neuropathic pain. We want to reduce pain; we also want to promote tissue repair and functional recovery.

Neurotensin (NT) is an endogenous 13 amino acid neuropeptide with profound opioid-independent analgesic effects. This role of NT is thought to be mediated by both neurotensin receptor subtype 1 (NTS1) and neurotensin receptor subtype 2 (NTS2). NT and its receptors are widely distributed in the pain circuits in central nervous system. Thus NT might modulate pain in many structures of pain pathway, such as spinal cord, rostroventral medulla (RVM) and periaqueductal gray (PAG). Actually either intrathecal application of NT or direct injection of NT into RVM or PAG or intracerebroventricular injection of NT showed analgesic effects. NT exerted its antinociceptive effects in both acute pain and chronic pain models. The analgesic effects of NT were originally found in acute pain experiments. In the case of pathological pain, for example, formalin injection induced inflammatory pain and sciatic nerve constriction induced neuropathic pain, NT also shows antinociceptive effects. The effects exist in somatic pain as well as visceral pain induced by noxious colorectal distension (CRD) or writhing test. It should be noted that NT plays an important role in stress-induced antinociception (SIAN), especially in higher intensity stress experiments. However as a neuropeptide, NT is susceptible to degradation by peptidases and cannot cross the blood-brain barrier (BBB). Great efforts have been made to find NT analogues that are more biologically stable and could inhibit pain by systematic administration. The present review focuses on the analgesic role and the underlying mechanisms of NT and its analogues in pain, especially in chronic pain models.

Neuropathic pain is characterized by complicated combination of positive (e.g., hyperalgesia and allodynia) and negative (e.g., hypoesthesia and hypoalgesia) symptoms, and is often refractory to conventional pharmacological agents, including morphine. Although the molecular mechanisms for positive symptoms are extensively studied, those for negative symptoms are poorly understood. There is convincing evidence that altered gene expression within peripheral and central nervous systems is a key mechanism for neuropathic pain; however, its transcriptional mechanisms are poorly understood. Epigenetic modifications, such as DNA methylation and histone modifications (e.g., acetylation, methylation, and phosphorylation), are known to cause stable gene expression via chromatin remodeling. These mechanisms have a role not only in the determination of developmental cell fates, but also in the physiological and pathological processes in nervous system. Moreover, epigenetic therapies using epigenetic modifying compounds are progressively advanced in the treatments of diverse diseases, including cancer and neurological diseases. Importantly, there is emerging evidence that a variety of genes undergo epigenetic regulation via DNA methylation and histone modifications within peripheral and central nervous systems, thereby contributing to the alterations in both pain sensitivity and pharmacological efficacy in neuropathic pain. In this review, we will highlight the epigenetic gene regulation underlying neuropathic pain, especially focusing on the negative symptoms. Moreover, we will discuss whether epigenetic mechanisms can serve as a potential target to treat neuropathic pain.

A neuropeptide nociceptin or orphanin FQ (N/OFQ) is an endogenous ligand for the orphan opioid receptor-like receptor. During studies on the analysis of the precursor of N/OFQ, we identified a novel neuropeptide produced from the same precursor and named it “nocistatin (NST)”. Intrathecal (i.t.) administration of N/OFQ induces pain responses including touch-evoked allodynia and thermal hyperalgesia, and simultaneous administration of NST blocks the allodynia and hyperalgesia induced by N/OFQ. In the years since these discoveries, N/OFQ has been shown to be involved in a wide range of pharmacological activities, such as relaying pain perception in peripheral tissues, to the central nervous system, and NST was shown to have opposite effects on various central functions evoked by N/OFQ. Pharmacological characterization using various neurotransmitter agents, agonists, antagonists and knockout mice in vivo; electrophysiological and immunohistological analysis ex vivo; and molecular cloning using affinity chromatography of high-performance affinity nanobeads; and protein processing measurement using bioluminescence resonance energy transfer (BRET) in vitro have generated new insights into pain transmission regulated by NST and N/OFQ. This review focuses on the molecular and cellular mechanisms of pain transmission regulated by NST.

The acid-sensing ion channel (ASIC) has emerged as a novel type of ion channel that is activated by extracellular protons as well as nonproton ligands. Advances in ASIC research have resolved its multifaceted structural and functional properties, including its widespread distribution, polymodal activation, and activity-dependent regulation of its expression. All of these properties promote a better understanding of the roles played by pH dynamics as well as damage-related signals through activation of ASICs in pain and anxiety. Importantly, even more studies have provided strong evidence supporting the effectiveness of targeting ASICs with pharmacological agents or gene knockdown for treating pain and anxiety. Here we review the contribution of ASICs at the peripheral and central levels to the development of acute pain, inflammatory pain, neuropathic pain, and anxiety-related disorders, as well as their potential underlying mechanisms. Accumulating evidence suggests that ASICs represent a novel class of promising targets for developing effective therapies for pain and anxiety.

Long-term potentiation (LTP), referring to a lasting increase in efficacy of synaptic transmission, is a common mechanism of memory storage in central nervous system (CNS). LTP at C-fiber synapses in spinal dorsal horn is considered as a synaptic model of pathological pain, as the spinal LTP is only induced by noxious electrical and natural stimuli but not by innoxious ones and LTPinducible stimulation is capable of leading to lasting behavioral signs of pathological pain in human and in animals. The molecular mechanisms of spinal LTP at C-fiber synapses are similar to hippocampal LTP in following aspects. Induction of LTP depends on postsynaptic Ca2+ rise resulting from opening of N-methyl-D-aspartate channels (NMDA) and voltage-gated calcium channels (VGCCs), and Ca2+ release from intracellular store; Early-phase LTP (<3h) needs activation of intracellular protein kinase A (PKA), protein kinase C (PKC), calcium/calmodulin-dependent protein kinase II (CaMKII), phospholipase C (PLC) and release of nitric oxide (NO); Late-phase LTP (>3h) is dependent on de novo protein synthesis; Activation of either dopamine D1 receptors or PKA, and extrogenous brain-derived neurotrophic factor (BDNF) or ATP directly induces late-phase LTP. Therefore, the drugs targeting at the above molecules may impair memory function of hippocampus. The striking difference between hippocampal LTP and spinal LTP at C-fiber synapses is that activation of glial cells and the over–expression of proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α) and interleukin– beta (IL-1β), inhibit LTP in hippocampus, but promote LTP in spinal dorsal horn. The drugs targeting at the neuroinflammatory process may not only attenuate pathological pain but also improve memory in hippocampus.

The treatment of chronic pain arising from deep tissues is currently inadequate and there is need for new pharmacological agents to provide analgesia. The endogenous paracrine hormone/neurotransmitter oxytocin is intimately involved in the modulation of multiple physiological and psychological functions. Recent experiments have given clear evidence for a role of oxytocin in the modulation of nociception. The present article reviews the existent human and basic science data related to the direct and indirect effects of oxytocin on pain. Due to its analgesic, anxiolytic, antidepressant and other central nervous system effects, there is strong evidence that oxytocin and other drugs acting through the oxytocin receptor could act as multifunctional analgesics with unique therapeutic value.

Recent investigations of the cellular and molecular mechanisms of pain provide new hopes for more effective treatments for patients with chronic pain. At the molecular and genetic levels, new proteins and genes related to sensory sensation have been identified. However, many of these new discoveries have not resulted in better and more effective treatments for chronic pain. This disconnect between discovery and better treatment options is due, in part to the negative side effects associated with new treatment options, and also as a result of the ineffectiveness of these new drugs for inhibiting chronic pain. In this review, I will explore this disconnect between discovery and treatment, and propose that the failure of previous medicines can be due to their limited effects on injury-related plasticity, and question the common misperception of seeking compounds for high efficacy before understanding basic mechanisms of the target proteins in pain-related plasticity.

Ebselen is a synthetic organoselenium compound that has been considered a potential pharmacological agent with low toxicity, showing antioxidant, anti-inflammatory and neuroprotective effects. It is bioavailable, blood-brain barrier permeant and safe based on cellular toxicity and Phase I-III clinical trials. There is evidence that ebselen inhibits acetylcholinesterase (AChE) activity, an enzyme that plays a key role in the cholinergic system by hydrolyzing acetylcholine (ACh), in vitro and ex vivo. This system has a well-known relationship with cognitive process, and AChE inhibitors, such as donepezil and galantamine, have been used to treat cognitive deficits, mainly in the Alzheimer’s Disease (AD). However, these drugs have poor bioavailability and a number of side effects, including gastrointestinal upsets and hepatotoxicity. In this way, this study aimed to evaluate the effect of ebselen on cerebral AChE activity in vitro and to determine the kinetic profile and the reversibility of inhibition by dialysis. Ebselen inhibited the cerebral AChE activity with an IC50 of 29 µM, similar to IC50 found with pure AChE from electric eel, demonstrating a mixed and reversible inhibition of AChE, since it increased Km and decreased Vmax. The AChE activity was recovered within 60 min of dialysis. Therefore, the use of ebselen as a therapeutic agent for treatment of AD should be considered, although memory behavior tasks are needed to support such hypothesis.

Non-nucleoside reverse-transcriptase inhibitors (NNRTIs), major components of highly active antiretroviral therapy (HAART), are effective in suppressing viral replication and preventing the progress of HIV-1 infection to AIDS. However, rapid blood clearance in vivo could significantly impair the efficiency of the anti-HIV-1 activity and result in multiple daily doses which might lead to poor patient compliance. Here we attempted to employ biodegradable organic nanoparticles (NPs) to encapsulate DAAN15h, a derivative of 4-substituted 1, 5-diarylaniline with potent anti-HIV activities. Nanoparticles encapsulating DAAN15h (NP-DAAN15h) displayed a spherical shape with a size of 97.01 ± 3.64 nm and zeta potential of -19.1 ± 3.78 mV, and they exhibited a sustained controlled release behavior in vitro. The cellular uptake of NPs on TZM-b1 cells, MT-2 cells and M7 cells, possibly through lipid raft-mediated and energydependent active transport processes, was significantly enhanced. NP-DAAN15h, which possessed no significant in vitro cytotoxicity, showed improved antiviral activity against laboratory-adapted and primary HIV-1 isolates with different subtypes and tropisms, including RT-resistant variants. NP-DAAN15h exhibited a significantly prolonged blood circulation time, decreased plasma elimination rate, and enhanced AUC(0-t). NP-DAAN15h, a nanoparticle-encapsulated NNRTI, exhibits enhanced cellular uptake, improved anti-HIV-1 efficacy and prolonged in vivo circulation time, suggesting good potential for further development as a new NNRTI formulation for clinical use.

Chronic kidney disease (CKD) is a serious public health problem. Current therapies are designed to slow down progression of the disease and avoid the necessity of dialysis or kidney transplantation. CKD is characterized by chronic inflammation and progressive cell death resulting in fibrotic rebuilding of renal tissue. Melatonin, the primary product of the pineal gland, has been shown to have pluripotent protective effects in many organs and tissues. It exerts anti-hypertensive, anti-inflammatory, anti-apoptotic, and antiremodelling actions. A principal mechanism of these numerous melatonin benefits resides in its extraordinary high efficacy as an antioxidant and scavenger protecting cells both extracellularly and in all subcellular structures. In addition to these receptor-independent actions, the effects of melatonin via specific MT-receptors may be beneficial. In several animal models of CKD, involving experimental hypertension, diabetes mellitus and various models of nephrotoxicity, melatonin reduced the oxidative burden, attenuated the chronic inflammation and limited apoptosis. These effects were associated with the reduction of proteinuria, damage of parenchymal cells and fibrosis. In humans, melatonin’s chronobiological action attenuates sleep disturbances in hemodialyzed patients suffering from a relative melatonin deficiency. Moreover, melatonin reduces the oxidative burden and improves iron metabolism in hemodialyzed patients. In conclusion, the pleiotropic physiological actions of melatonin induce beneficial effects at numerous pathophysiological levels related to CKD both under experimental and clinical conditions. It is hoped that this review will prompt a large clinical trial to determine the efficacy of this nontoxic indoleamine as a potential treatment for this debilitating disease.

Searching for safe and effective treatments for HIV infection is still a great challenge worldwide in spite of the 27 marketed anti-HIV drugs and the powerful highly active antiretroviral therapy (HAART). As a promising prospect for generation of new HIV therapy drugs, multiple ligands (MDLs) were greatly focused on recently due to their lower toxicity, simplified dosing and patient adherence than single-target drugs. Till now, by disrupting two active sites or steps of HIV replications, a number of HIV dual inhibitors, such as CD4-gssucap120 inhibitors, CXCR4-gp20 inhibitors, RT-CXCR4 inhibitors, RT-protease inhibitors, RT-integrase inhibitors, and RTassociated functions inhibitors have been identified. Generally, these dual inhibitors were discovered mainly through screening approaches and design strategies. Of these compounds, the molecules bearing small skeletons exhibited strong anti-HIV activity and aroused great attention recently. Reviewing the progress of the dual small-molecule HIV inhibitors from the point of view of their scaffolds and discovery strategies will provide valuable information for producing more effective anti-HIV drugs. In this regard, novel dual small-molecule HIV inhibitors were illustrated, and their discovery paradigms as the major contents were also summarized in this manuscript.