Glycine transporter 1 (GlyT1) inhibitors exhibit anticonvulsant properties in the rat maximal electroshock threshold (MEST) test

Glycine can act as either an inhibitory neurotransmitter or as a potentiator of NMDA-dependent excitatory neurotransmission. There is some evidence that glycine can have both pro- and anticonvulsant properties in various rodent models of epilepsy. In the present study we tested several glycine transporter 1 (GlyT1) inhibitors including NFPS, SSR 504734, Lu AA21279, Org 25935, SB-710622, GSK931145, as well as the glycine agonist d-serine, in the maximal electroshock threshold (MEST) test in the rat. In a series of experiments, male Sprague–Dawley rats (n = 12/group) were pre-treated with a compound of interest and then received an electric shock delivered via corneal electrodes. A cohort of satellite animals (n = 3/group) was also used to measure blood and brain levels of Org 25935. All GlyT1 inhibitors increased seizure thresholds dose-dependently, indicative of anticonvulsant activity. SB-710622 and GSK931145 had lower minimum effective doses (MEDs) in the MEST test than other GlyT1 inhibitors. At estimated tmax, increases in dose administered were paralleled by increases in blood and brain concentrations of Org 25935. Thus, increasing extracellular concentration of glycine via inhibition of its uptake protects from electroshock-induced seizures in the rat. Whether strychnine-sensitive or strychnine-insensitive glycine binding sites are involved in this effect remains to be determined.