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-Delt a(2)-en-valproate or 2-en-valproate), an unsaturated metabolite of val proic acid (VPA), on intracellularly recorded sodium-dependent action potentials of cultured mouse spinal cord and cortical neurons were com pared with those of the anticonvulsant sodium valproate (NaVP). The ma ximal rate of rise of action potentials triggered by trains of 1-msec or 400-msec pulses declined progressively until failure to fire in bot h cell types during exposure to trans-2-en-NaVP or NaVP was observed. The limitation of firing by both drugs was concentration, voltage, rat e and time dependent. The IC50 of trans-2-en-NaVP was 1.2 x 10(-3) at less than or equal to 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 neur ons. This may reflect slower rates of firing during 400-msec depolariz ations in neurons of this type. In paired-pulse experiments, the absol ute 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 less th an or equal to 1 hr. Coincubation of the spinal cord neurons with tran s-2-en-NaVP and NaVP for 24 hr showed no hyperadditive effect of these two drugs in vitro. Limitation of sustained repetitive firing was rev ersed by hyperpolarization in the continuing presence of either drug a nd incubation in drug-free medium. Limitation of sodium-dependent acti on potential firing rates could contribute, at least in part, to the a nticonvulsant effect of trans-2-en-NaVP.