Intrathecal injection of GluR6 antisense oligodeoxynucleotides alleviates acute inflammatory pain of rectum in rats

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

Chronic low back pain: a mini-review on pharmacological management and pathophysiological insights from clinical and pre-clinical data

Globally, low back pain (LBP) is one of the most common health problems affecting humans. The lifetime prevalence of non-specific LBP is approximately 84%, with the chronic prevalence at about 23%. Chronic LBP in humans is defined as LBP that persists for more than 12 weeks without a significant pain improvement. Although there are numerous evidence-based guidelines on the management of acute LBP, this is not the case for chronic LBP, which is regarded as particularly difficult to treat. Research aimed at discovering new drug treatments for alleviation of chronic mechanical LBP is lacking due to the paucity of knowledge on the pathobiology of this condition, despite its high morbidity in the affected adult population. For a debilitating condition such as chronic LBP, it is necessary to assess the sustained effects of pharmacotherapy of various agents spanning months to years. Although many rodent models of mechanical LBP have been developed to mimic the human condition, some of the major shortcomings of many of these models are (1) the presence of a concurrent neuropathic component that develops secondary to posterior intervertebral disc puncture, (2) severe model phenotype, and/or (3) use of behavioural endpoints that have yet to be validated for pain. Hence, there is a great, unmet need for research aimed at discovering new biological targets in rodent models of chronic mechanical LBP for use in drug discovery programs as a means to potentially produce new highly effective and well-tolerated analgesic agents to improve relief of chronic LBP. On a cautionary note, it must be borne in mind that because humans and rats display orthograde and pronograde postures, respectively, the different mechanical forces on their spines add to the difficulty in translation of promising rodent data to humans.

Role of glutamatergic neurotransmission in the enteric nervous system and brain-gut axis in health and disease

Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury.

Pharmacological Modulation of GluK1 and GluK2 by NETO1, NETO2, and PSD95

The association between the kainate receptors (KARs) GluK1 and GluK2 and the modifying proteins neuropilin- and tolloid-like 1 (NETO1), neuropilin- and tolloid-like 2 (NETO2), and postsynaptic density protein 95 (PSD95) is likely to produce distinct GluK1 and GluK2 pharmacology in postsynaptic neurons. However, little is known about their corresponding modulatory effects on GluK1 and GluK2 activity in high-throughput assays for cell-based drug discovery. Using heterologous cells that potentially mimic the response in native cells in a fluorescence imaging plate reader (FLIPR) assay, we have investigated assays that incorporate (1) coexpression of GluK1 or GluK2 with their modulatory proteins (NETO1, NETO2, PSD95) and/or (2) enablement of assays with physiological concentration of native GluK1 and GluK2 agonist (glutamate) in the absence of an artificial potentiator (e.g., concanavalin A [Con A]). We found that in the absence of Con A, both NETO1 and NETO2 accessory proteins are able to potentiate kainate- and glutamate-evoked GluK1-mediated Ca(2+) influx. We also noted the striking ability of PSD95 to enhance glutamate-stimulated potentiation effects of NETO2 on GluK1 without the need for Con A and with a robust signal that could be utilized for high-throughput FLIPR assays. These experiments demonstrate the utility of heterologous cells coexpressing PSD95/NETO2 with GluK1 or GluK2 in native cell-mimicking heterologous cell systems for high-throughput assays and represent new avenues into the discovery of KAR modulating therapies.

Role of principal ionotropic and metabotropic receptors in visceral pain

Visceral pain is the most common form of pain caused by varied diseases and a major reason for patients to seek medical consultation. It also leads to a significant economic burden due to workdays lost and reduced productivity. Further, long-term use of non-specific medications is also associated with side effects affecting the quality of life. Despite years of extensive research and the availability of several therapeutic options, management of patients with chronic visceral pain is often inadequate, resulting in frustration for both patients and physicians. This is, most likely, because the mechanisms associated with chronic visceral pain are different from those of acute pain. Accumulating evidence from years of research implicates several receptors and ion channels in the induction and maintenance of central and peripheral sensitization during chronic pain states. Understanding the specific role of these receptors will facilitate to capitalize on their unique properties to augment the therapeutic efficacy while at the same time minimizing unwanted side effects. The aim of this review is to provide a concise review of the recent literature that reports on the role of principal ionotropic receptors and metabotropic receptors in the modulation visceral pain. We also include an overview of the possibility of these receptors as potential new targets for the treatment of chronic visceral pain conditions.

A systematic review of the evidence for central nervous system plasticity in animal models of inflammatory-mediated gastrointestinal pain

BACKGROUND

Abdominal pain frequently accompanies inflammatory disorders of the gastrointestinal tract (GIT), and animal models of GIT inflammation have been developed to explore the role of the central nervous system (CNS) in this process. Here, we summarize the evidence from animal studies for CNS plasticity following GIT inflammation.

METHODS

A systematic review was conducted to identify studies that: (1) used inflammation of GIT organs, (2) assessed pain or visceral hypersensitivity, and (3) presented evidence of CNS involvement. Two hundred and eight articles were identified, and 79 were eligible for analysis.

RESULTS

Rats were most widely used (76%). Most studies used adult animals (42%) with a bias toward males (74%). Colitis was the most frequently used model (78%) and 2,4,6-trinitrobenzenesulfonic acid the preferred inflammatory agent (33%). Behavioral (58%), anatomical/molecular (44%), and physiological (24%) approaches were used alone or in combination to assess CNS involvement during or after GIT inflammation. Measurement times varied widely (<1 h-> 2 wk after inflammation). Blinded outcomes were used in 42% studies, randomization in 10%, and evidence of visceral inflammation in 54%. Only 3 studies fulfilled our criteria for high methodological quality, and no study reported sample size calculations.

CONCLUSIONS

The included studies provide strong evidence for CNS plasticity following GIT inflammation, specifically in the spinal cord dorsal horn. This evidence includes altered visceromotor responses and indices of referred pain, elevated neural activation and peptide content, and increased neuronal excitability. This evidence supports continued use of this approach for preclinical studies; however, there is substantial scope to improve study design.

The utility of ionotropic glutamate receptor antagonists in the treatment of nociception induced by epidural glutamate infusion in rats

Background:

The authors have previously demonstrated that human herniated disc material contains high concentrations of free glutamate. In an experimental model, elevated epidural glutamate concentrations in the lumbar spine can cause a focal hyperesthetic state.

Methods:

Rats underwent epidural glutamate infusion in the lumbar spine by a miniosmotic pump over a 72-hour period. Some rats underwent coinfusion with glutamate and ionotropic glutamate antagonists. Nociception was assessed by von Frey fibers and by assessment of glutamate receptor expression in the corresponding dorsal horn of the spinal cord.

Results:

The kainic acid antagonist, UBP 301, decreased epidural glutamate-based hyperesthesia in a dose dependent manner. Concordant with these findings, there was significant decrease in kainate receptor expression in the dorsal horn. The N-Methyl-4-isoxazoleproionic acid (NMDA) antagonist Norketamine also significantly diminished hyperesthesia and decreased receptor expression in the dorsal horn.

Conclusions:

Both UBP 301, the kainic acid receptor antagonist and Norketamine, an NMDA receptor antagonist, dampened epidural glutamate-based nociception. Focal epidural injections of Kainate or NMDA receptor antagonists could be effective treatments for disc herniation-based lumbar radiculopathy.