Involvement of NMDA receptors in tremor expression in Aspa/Hcn1 double-knockout rats

We recently demonstrated that aspartoacylase (Aspa) and hyperpolarization-activated cyclic nucleotide-gated potassium channel 1 (Hcn1) genes were causative of essential tremor (ET) in rats. This finding was obtained using Aspa^em34Kyo/Hcn1^A354V double-mutant rats, but they were bred on a heterogeneous genetic background of two strains, F344 and WTC. Here, we developed an Aspa^em34Kyo/Hcn1^em1Kyo double-knockout rat strain with a homogenous F344 genetic background and studied the ability of glutamate receptor antagonists to suppress ET. The F344-Aspa/Hcn1 double-knockout rats exhibited spontaneous, intense body tremor equivalent to that in the double-mutant rats. N-acetyl-aspartate (NAA), a substrate of ASPA, showed accumulation in all brain regions and in the spinal cord. However, N-acetyl-aspartyl-glutamate (NAAG), which is derived from NAA and interacts with glutamatergic receptors, was decreased in the medulla oblongata of the double-knockout rats. The tremor was suppressed by 3-[(R)-2-carboxypiperazin-4-yl]-prop-2-enyl-1-phosphonic acid, an N-methyl-D-aspartate (NMDA) receptor antagonist, in F344-Aspa/Hcn1 double-knockout rats. The non-NMDA glutamate receptor antagonist NBQX weakly inhibited the tremor, while the metabotropic glutamate receptor antagonist LY341495 showed no effect. In addition, both NR2B subunit-specific (Ro 25-6981) and NR2C/NR2D subunit-specific (cis-piperidine dicarboxylic acid) NMDA receptor antagonists suppressed the tremor. These data indicated that the pathogenesis of tremor in Aspa/Hcn1 double-knockout rats involved ionotropic glutamate receptors, particularly NMDA receptors.