Differential modulation of ionotropic glutamate receptors by glufosinate ammonium, the glutamate-mimetic

Although glufosinate ammonium (GLA) is known to affect hyperexcitability, neurodevelopment, and psychiatric disorders via ionotropic glutamate receptor (iGluR) activation, its detailed mechanisms remain unclear. This study characterizes the effects of GLA on the activity of N-methyl-d-aspartate receptor (NMDAR) and amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) in rat primary neuron cultures and their membrane expression. GLA activated GluA1 L497Y, a desensitization mutant and a glutamate sensor, but did not activate the γ-aminobutyric acid (GABA) c channel, a GABA sensor. GLA inhibited NMDAR-mediated current in the presence of glutamate/glycine and facilitated AMPAR-mediated current in the presence of glutamate. GLA activated NMDAR-mediated Ca²⁺ transients in the absence of glutamate and inhibited NMDAR-mediated Ca²⁺ transients in the presence of glutamate in HEK 293 cells expressing NMDAR, indicating that GLA acts similarly to glutamate. Moreover, GLA reduced the frequency of excitatory post-synaptic current (EPSC_NMDAR) and EPSC_AMPAR at synapses. Additionally, GLA induced AMPAR-mediated steady-state inward currents. Furthermore, it is noteworthy that GLA treatment caused neuronal death after 24 h but not after 5 h of incubation. After 5 h treatment, NMDAR expression preferentially declined, while AMPAR expression enhanced, as measured by the intensity of GluN1 and GluA1 in somas and processes. These results were consistent with reduced NMDAR-mediated Ca²⁺ transients and increased AMPAR-mediated Ca²⁺ transients. This is the first report to demonstrate the onset mechanism of GLA on NMDAR and AMPAR before neuronal death. These findings provide insight into understanding neurotoxicity and potential therapeutic targets for mitigating GLA-induced damage.