Effects of phenytoin on the amygdala neurons in vitro

Screening for the effects of antiepileptic drugs on short term plasticity using a time efficient bioassay

Screening for changes in the short-term plasticity (STP) characteristics induced by antiepileptic drugs (AEDs) can be accelerated using a novel in vitro bioassay. The bioassay is based on the analysis of varying population spike (PS) amplitudes recorded in the CA1 region of the hippocampal slice in response to Poisson distributed random electrical stimuli. Three antiepileptic drugs (phenytoin 100 microM, carbamazepine 100 microM, and valproate 700 microM) were tested at maximal effective therapeutic concentrations. The data were analyzed using an advanced nonlinear approach that is more specific and time-efficient than the conventional paired pulse and fixed frequency train methods. STP was quantified by the first and the second order Volterra kernels. The first order kernel (k1) represented the mean PS amplitude while the second order kernel (k2) quantified the effect on the current PS amplitude of the interaction between the current stimulus impulse and each past stimulus impulse within a time (memory) window mu. The mean PS (k1 decreased by 15%, 10%, and 7% when phenytoin, carbamazepine, and valproate were added respectively. Phenytoin caused an increase in the k2 peak facilitation in the high frequency domain. Carbamazepine impaired frequency facilitation in the theta frequency range by causing a left shift in the second order kernel.

The antiepileptic drug phenytoin affects sodium transport in toad epithelium

The effects of phenytoin on isolated Pleurodema thaul toad skin were investigated. Low (micromolar) concentrations of the antiepileptic agent applied to the outside surface of the toad epithelium increased the electrical parameters (short-circuit current and potential difference) by over 40%, reflecting stimulation of Na(+) transport, whereas higher (millimolar concentrations, outside and inside surface) decreased both electric parameters, the effect being greater at the inside surface (40% and 80% decrease, respectively). The amiloride test showed that the stimulatory effect was accompanied by an increase and the inhibitory effect by a decrease in the sodium electromotive force (ENa). It is concluded that the drug interaction with membrane lipid bilayers might result in a distortion of the lipid-protein interface contributing to disturbance of Na(+) epithelial channel activity. After applying the Na(+)-K(+)-ATPase blocker ouabain and replacing the Na(+) ions in the outer Ringer's solution by choline, it was concluded that both active and passive transport are involved in sodium absorption, although active transport predominates.

Frequency-dependent inhibition of neuronal activity by topiramate in rat hippocampal slices

1. Topiramate is a structurally novel anticonvulsant which was recently approved for adjunctive therapy in partial and secondarily generalized seizures. The present study was aimed at elucidating the mechanisms underlying the anticonvulsant efficacy of topiramate using intra- and extracellular recording techniques in the in vitro hippocampal slices. 2. When stimuli were delivered every 20 s, topiramate had no measurable effect on both field excitatory postsynaptic potentials (fEPSPs) and population spikes (PSs). However, increasing the stimulation frequency from 0.05-0.2 Hz, topiramate significantly decreased the slope of fEPSP and the amplitude of PS in a concentration-dependent manner. The amplitude of presynaptic fiber volley was also reduced. 3. Topiramate did not affect the magnitude of paired-pulse inhibition and monosynaptically evoked inhibitory postsynaptic potentials (IPSPs). 4. Sustained repetitive firing was elicited by injection of long duration (500 ms) depolarizing current pulses (500-800 pA). Superfusion with topiramate significantly reduced the number of action potentials evoked by a given current pulse. 5. After blockade of GABA receptors by bicuculline, burst firing which consisted of a train of several spikes riding on a large depolarizing wave termed paroxysmal depolarizing shift (PDS) was recorded. Application of topiramate reduced the duration of PDS and later spikes with less effect on the initial action potential. 6. These results suggest that frequency-dependent inhibition of neuronal activity due to blockade of Na+ channels may account largely for the anticonvulsant efficacy of topiramate.