Trans-2-en-valproic acid limits action potential firing frequency in mouse central neurons in cell culture

Effects of the trans-isomer of 2-en-valproate (trans-2-en-NaVP; E-delta2-en-valproate or 2-en-valproate), an unsaturated metabolite of valproic acid (VPA), on intracellularly recorded sodium-dependent action potentials of cultured mouse spinal cord and cortical neurons were compared with those of the anticonvulsant sodium valproate (NaVP). The maximal rate of rise of action potentials triggered by trains of 1-msec or 400-msec pulses declined progressively until failure to fire in both cell types during exposure to trans-2-en-NaVP or NaVP was observed. The limitation of firing by both drugs was concentration, voltage, rate and time dependent. The IC50 of trans-2-en-NaVP was 1.2 x 10(-3) at < or =1 hr and 4.8 x 10(-5) M at 24 to 48 hr. Trans-2-en-NaVP did not limit sustained repetitive firing in all cortical neurons. This may reflect slower rates of firing during 400-msec depolarizations in neurons of this type. In paired-pulse experiments, the absolute refractory period was 7 msec in control solution and 15 msec (P < .01 vs. control; n = 9) in solution containing 6 x 10(-4) M trans-2-en-NaVP. Firing was limited in all spinal cord neurons after exposure to 0.5 mM NaVP for 24 to 48 hr; 80% were limited by 1 mM NaVP at < or =1 hr. Coincubation of the spinal cord neurons with trans-2-en-NaVP and NaVP for 24 hr showed no hyperadditive effect of these two drugs in vitro. Limitation of sustained repetitive firing was reversed by hyperpolarization in the continuing presence of either drug and incubation in drug-free medium. Limitation of sodium-dependent action potential firing rates could contribute, at least in part, to the anticonvulsant effect of trans-2-en-NaVP.