Animal models of human pain conditions allow for detailed interrogation of known and hypothesized mechanisms of pain physiology in awake, behaving organisms. The importance of the glycinergic system for pain modulation is well known; however, manipulation of this system to treat and alleviate pain has not yet reached the sophistication required for the clinic. Here, we review the current literature on what animal behavioral studies have allowed us to elucidate about glycinergic pain modulation, and the progress toward clinical treatments so far. First, we outline the animal pain models that have been used, such as nerve injury models for neuropathic pain, chemogenic pain models for acute and inflammatory pain, and other models that mimic painful human pathologies such as diabetic neuropathy. We then discuss the genetic approaches to animal models that have identified the crucial glycinergic machinery involved in neuropathic and inflammatory pain. Specifically, two glycine receptor (GlyR) subtypes, GlyRα1(β) and GlyRα3(β), and the two glycine transporters (GlyT), GlyT1 and GlyT2. Finally, we review the different pharmacological approaches to manipulating the glycinergic system for pain management in animal models, such as partial vs. full agonism, reversibility, and multi-target approaches. We discuss the benefits and pitfalls of using animal models in drug development broadly, as well as the progress of glycinergic treatments from preclinical to clinical trials.

Aberrations in spinal glycinergic signalling are a feature of pain chronification. Normalising these changes by inhibiting glycine transporter-2 (GlyT2) is a promising treatment strategy.However, existing GlyT2 inhibitors e.g. ORG25543 are limited by narrow therapeutic windows and severe dose-limiting side effects such as convulsions, and are therefore poor candidates for clinical development. Here, intraperitoneally administered oleoyl-D -lysine, a lipid-based GlyT2 inhibitor, was characterised in mouse models of acute (hotplate), inflammatory (CFA) and chronic neuropathic (CCI) pain. Side effects were also assessed on a numerical rating score, convulsions score, for motor incoordination (rotarod) and for respiratory depression (whole body plethysmography). Oleoyl-D -lysine produced near complete anti-allodynia for chronic neuropathic pain but no anti-allodynia/analgesia in inflammatory or acute pain. No side effects were seen at the peak analgesic dose, 30 mg/kg. Mild side effects were observed at the highest dose, 100 mg/kg, on the numerical rating score, but no convulsions. These results contrasted markedly with ORG25543, which reached less than 50% reduction in allodynia score only at the lethal/near-lethal dose of 50 mg/kg. At this dose, ORG25543 caused maximal side effects on the numerical rating score and severe convulsions. Oleoyl-D -lysine (30 mg/kg) did not cause any respiratory depression, a problematic side effect of opiates. These results show the safe and effective reversal of neuropathic pain in mice by oleoyl-D -lysine and provide evidence for a distinct role of glycine in chronic pain over acute or short-term pain conditions.

Background and Purpose: Changes to spinal glycinergic signalling are a feature of pain chronification. Normalising those changes by inhibiting glycine transporter-2 (GlyT2) is a promising treatment strategy. However, existing GlyT2 inhibitors e.g. ORG25543 are limited by narrow therapeutic windows and severe dose-limiting side effects such as convulsions, and are therefore poor candidates for clinical development. Experimental Approach: Analgesic and side-effect properties of intraperitoneally administered oleoyl-D-lysine, a lipid-based GlyT2 inhibitor, were characterised in mice. Analgesia was assessed in models of chronic neuropathic and inflammatory pain via the von Frey test, and acute nociception via hotplate. Side effects were scored via numerical rating scale, convulsions score, the Rotarod test and whole-body plethysmography for respiratory depression. Key Results: Oleoyl-D-lysine produced significant analgesia/anti-allodynia in the model for chronic neuropathic pain but not for chronic inflammatory or acute pain. No side effects were seen at the peak analgesic dose, 30 mg kg-1. Mild side effects were observed at the highest dose, 100 mg kg-1, in the numerical rating score, but no convulsions. These results contrasted markedly with ORG25543, which produced significant analgesia only at the lethal or near-lethal dose of 50 mg kg-1. At this dose, ORG25543 caused severe side effects on the numerical rating score, severe convulsions, and Rotarod impairment. Oleoyl-D-lysine (30 mg kg-1) did not cause any respiratory depression, a problematic side effect of opiates. Conclusions and Implications: Oleoyl-D-lysine safely and effectively reverses neuropathic pain in mice. GlyT2 inhibitors may be better suited to treating pain of neuropathic origin over other pain aetiologies.