Activation by N-acetyl-L-aspartate of acutely dissociated hippocampal neurons in rats via metabotropic glutamate receptors.
OBJECTIVE
We previously reported that an increase in the N-acetyl-L-aspartate (NAA) level due to the lack of aspartoacylase gene was found in the brain of the tremor rat (tm/tm), which is a mutant with a causative gene named tm that shows epileptic seizures. Therefore, NAA is suggested to be one of the factors involved in the induction of epileptic seizures. Patch-clamp studies were performed to determine whether NAA produces an excitatory effect on acutely dissociated rat hippocampal neurons.


METHODS
Acutely dissociated hippocampal neurons were prepared from normal Wistar rats aged 3-4 weeks. NAA-induced currents were investigated by using the whole-cell voltage-clamp recording technique.


RESULTS
Application of NAA at concentrations of 100 nM to 1 mM through a U-tube for 2 s produced an inward current in a concentration-dependent manner at a holding potential of -60 mV. When the current-voltage relation was examined, the reversal potential of the NAA-induced current was found to be approximately 0 mV. The NAA-induced current was inhibited by bath application of the metabotropic glutamate receptor (mGluR) antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG) and by intracellular application of guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS), a nonhydrolyzable GDP analogue. However, the NAA-induced current remained unaffected by glutamic acid diethyl ester, a non-N-methyl-D-aspartate (NMDA)-subtype ionotropic glutamate receptor antagonist, or the voltage-dependent ion channel blockers tetrodotoxin, CdCl2, and tetraethylammonium-chloride. Conversely, the mGluR agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD) also induced an inward current, with a reversal potential of 0 mV. The ACPD-induced current also was inhibited by MCPG.


CONCLUSIONS
These results suggest that NAA acts on the G protein-coupled mGluRs to induce an inward current that results in excitation of the neurons, thereby contributing to the occurrence of epileptic seizures.
Yan
Hai-Dun
HD
Department of Pharmacology, Hiroshima University School of Medicine, Hiroshima, Japan.
Ishihara
Kumatoshi
K
Serikawa
Tadao
T
Sasa
Masashi
M
D000200:Action Potentials / Q000187:drug effects* / Q000502:physiology; D000818:Animals; D001224:Aspartic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology* / Q000633:toxicity; D004305:Dose-Response Relationship, Drug; D004827:Epilepsy / Q000139:chemically induced / Q000503:physiopathology; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects* / Q000502:physiology; D066298:In Vitro Techniques; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D018094:Receptors, Metabotropic Glutamate / Q000502:physiology*
D018094:Receptors, Metabotropic Glutamate; D001224:Aspartic Acid; C000179:N-acetylaspartate