An important factor in the development and main¬tenance of neuropathic pain is a loss of glycinergic inhibition in the spinal cord. Here glycine is the major inhibitory neuro¬transmitter, but it also has excitatory functions as an obligate co-agonist of glutamate at the NMDA re¬ceptor. Glycine concentration in the synaptic cleft is controlled by glycine transporter 1 and glycine transporter 2. Inhibition of these two glycine transporters can influence glycinergic neuro¬transmission.
The goal of this study was to determine whether the intrathecal application of glycine transporter inhibitors ALX 5407 and ALX 1393, which would raise glycine levels in the synaptic cleft, can influence the nociception of rats. As gly¬cine acts on both the inhibitory glycine receptor and the excitatory NMDA receptor, we ex¬pected to see differential effects.
Bennett and Xie´s chronic constriction injury model was used as a model for neuro¬pathic pain. Hyperalgesia and allodynia develop ten to twelve days after loose ligation of the left sciatic nerve and can be measured to quantify neuropathic pain. An intrathe¬cal PE10 catheter was inserted into the vertebral canal to enable application of both gly¬cine transporter in¬hibitors (three dos¬ages respectively: 10, 50 and 100 μg dis¬solved in 10 μl 100 % DMSO; n = 8) in the vicinity of the dorsal horn of the spinal cord. Following the treatment, me¬chanical paw withdrawal threshold was measured using a von Frey-filament and thermal paw withdrawal latency was measured using a modified Har¬greaves method. Both measurements were made for 240 minutes.
The glycine transporter 1 inhibitor ALX 5407 had dose dependent differential effects on the noci¬cep¬tion of rats. Antinociceptive effects were obtained after application of the low and the high concentrations and pronociceptive effects were obtained after applica¬tion of the intermediate concentration. The pronociceptive actions were most prominent in the un¬injured right paw. There were no side effects. The glycine transporter 2 inhibitor ALX 1393 exerted exclusively antinociceptive ef¬fects that were significant only after appli¬cation of the high concentration. Additionally, at the high concentration, we observed severe neurological side effects in four of eight animals (pareses, paralysis, res¬piratory depres¬sion and analgesia).
The pronociceptive effects of glycine transporter 1 inhibitor ALX 5407 can be ex¬plained by an in¬creased activation of NMDA receptor through un¬hampered spill over of synaptically released glycine. The antinociceptive effects of both glycine transporter inhibitors and the neurological side effects of the glycine transporter 2 inhibitor ALX 1393 are likely to be mediated by the gly¬cine receptor.
This study shows, that neuropathic pain behaviour can be significantly in¬fluenced in vivo by manipulating glycine concentration through a singular intra¬the¬cal injection of spe¬cific glycine transporter inhibitors. The study of glycinergic neurotransmission with the aid of glycine transporter inhibi¬tors promises future development of potent and versatile therapeutics. These may have potential to treat neuropathic pain, act as muscle relaxants and analgesics and also function as cognitive enhancers and treatments for schizo¬phrenia.