title: The action of valproate on spontaneous epileptiform activity in the absence of synaptic transmission and on evoked changes in [Ca2+]o and [K+]o in the hippocampal slice.
abstract: The effects of valproate (VPA) on neuronal excitability and on changes in extracellular potassium ([K+]o) and calcium ([Ca2+]o) were investigated with ion selective-reference electrode pairs in area CA1 of rat hippocampal slices. Field potential responses to single ortho- and antidromic stimuli were unaltered by VPA (1-5 mM). The afferent volley evoked in the Schaffer-commissural fibers was also unaffected. In contrast, VPA (1 mM) depressed frequency potentiation and paired pulse facilitation markedly. Decreases in [Ca2+]o induced either by repetitive stimulation or by application of the excitatory amino acids N-methyl-D-aspartate and quisqualate were reduced, and the latter results suggest that VPA interferes with postsynaptic Ca2+ entry. When synaptic transmission was blocked by lowering [Ca2+]o (0.2 mM) and elevating [Mg2+]o (7 mM), prolonged afterdischarges elicited by antidromic stimulation were blocked by VPA. VPA also suppressed the spontaneous epileptiform activity seen when [Ca2+]o was lowered to 0.2 mM, without elevating [Mg2+]o. The amplitudes of the rises in [K+]o induced by repetitive orthodromic stimulation were only slightly depressed and those elicited by antidromic stimulation were generally unaltered by VPA, as were laminar profiles of stimulus-evoked [K+]o signals. These results indicate that VPA has membrane actions in addition to known effects on excitatory and inhibitory transmitter pools.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002463:Cell Membrane Permeability / Q000187:drug effects; D004558:Electric Stimulation; D004827:Epilepsy / Q000188:drug therapy*; D006624:Hippocampus / Q000187:drug effects*; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects; D011188:Potassium / Q000378:metabolism*; D013569:Synapses / Q000187:drug effects; D009435:Synaptic Transmission / Q000187:drug effects*; D014635:Valproic Acid / Q000494:pharmacology*
chemical_list: D007473:Ion Channels; D014635:Valproic Acid; D011188:Potassium; D002118:Calcium