(l)-Phenylglycine, but not necessarily other α₂δ subunit voltage-gated calcium channel ligands, attenuates neuropathic pain in rats

Gabapentin and pregabalin have been demonstrated, both in animal pain models and clinically, to be effective analgesics particularly for the treatment of neuropathic pain. The precise mechanism of action for these two drugs is unknown, but they are generally believed to function via initially binding to the α₂δ subunit of voltage-gated Ca²⁺ channels. In this study, we used a pharmacological approach to test the hypothesis whether high affinity interactions with the α₂δ subunit alone could lead to attenuation of neuropathic pain in rats. The anti-allodynic effects of gabapentin and pregabalin, along with three other compounds - (l)-phenylglycine, m-chlorophenylglycine and 3-exo-aminobicyclo[2.2.1]heptane-2-exo-carboxylic acid (ABHCA) - discovered to be potent α₂δ ligands, were tested in the rat spinal nerve ligation model of neuropathic pain. Gabapentin (K_i = 120 nM), pregabalin (180 nM) and (l)-phenylglycine (180 nM) were shown to be anti-allodynic, with respective ED₅₀ values of 230, 90 and 80 μmol/kg (p.o.). (l)-Phenylglycine was as potent as pregabalin and equi-efficacious in reversing mechanical allodynia. In contrast, two ligands with comparable or superior α₂δ binding affinities, m-chlorophenylglycine (K_i = 54 nM) and ABHCA (150 nM), exhibited no anti-allodynic effects at doses of 30-300 μmol/kg (p.o.), although these compounds achieved substantial brain levels. The data demonstrate that, at least in the rat spinal nerve ligation model of neuropathic pain, (l)-phenylglycine has an anti-allodynic effect, but two equally potent α₂δ subunit ligands do not. These results suggest that additional mechanisms, besides α₂δ interactions, may contribute to the effects of compounds like gabapentin, pregabalin and (l)-phenylglycine in neuropathic pain.