Steven F. Stasheff

Induction of epileptiform activity in hippocampal slices by trains of electrical stimuli

In this paper we present an in vitro model of epileptogenesis based on electrical stimulation rather than pharmacological or ionic manipulations. Hippocampal slices given a series of stimulus trains similar to those used in kindling exhibited 3 types of epileptiform activity in CA3: afterdischarges immediately following the trains; spontaneous bursts of multiple population spikes; and bursts triggered by single stimuli. The afterdischarges and spontaneous bursts may be comparable to those seen in vivo during kindling; also, the progression of these features in this model was similar to their progression during kindling. All epileptiform activities were long-lasting, persisting for up to 3.5 h following the last train. This stimulus train-induced population bursting should be valuable as an acute model of hippocampal epileptogenesis, and may also help elucidate hippocampal participation in the kindling process.

Regenerative, all-or-none electrographic seizures in the rat hippocampal slice in Mg-free and physiological medium.

All-or-none electrographic seizures (EGSs) were studied in hippocampal slices from young (21- to 38-day-old) rats in medium containing low (0 mM) or physiological (0.9 mM) levels of magnesium, with and without the GABAB agonist baclofen. Extracellular recording and stimulation were performed in stratum pyramidale and stratum radiatum of CA3, respectively. EGS activity was induced by exposure to low-Mg medium or by delivering repetitive stimulus trains in physiological Mg medium. After EGS activity had stabilized, the EGSs were tested for all-or-none behavior by varying the number of pulses in a train. An EGS was considered all-or-none if subthreshold stimulation produced no afterdischarge bursts, and if the EGS duration was largely independent of the number of suprathreshold stimulus pulses. According to this measure, EGSs in Mg-free + baclofen medium were all-or-none. EGSs evoked in physiological Mg medium were also all-or-none, although the threshold was higher, and the EGS duration lower, than in Mg-free medium. This all-or-none characteristic was observed whether the EGSs were induced by prior exposure to Mg-free medium or by repetitive stimulation, and in the presence and absence of baclofen. The all-or-none characteristic suggests that while the triggering mechanism for EGSs is strongly dependent on stimulus intensity, regenerative mechanisms--independent of stimulus intensity--are responsible for the maintenance of EGSs. EGSs are also terminated by mechanisms not dependent on stimulus intensity.

Selective suppression of in vitro electrographic seizures by low-dose tetrodotoxin: a novel anticonvulsant effect.

Localized injections of 50 microM tetrodotoxin (TTX) in rat hippocampal slices blocked stimulus train-evoked electrographic seizures (EGSs) for several hours. Responses to single stimuli were minimally altered during TTX block of the EGSs. This selective reduction of epileptiform activity could result from general blockade of action potentials in an anatomically distinct group of neurons in the slice. To test this hypothesis, we systematically mapped TTX injection sites in the hippocampal slice, and found that TTX injections that blocked EGSs were nearly always located in or invaded CA2/3 stratum radiatum and/or stratum lacunosum-moleculare. A high degree of recurrent activity in this region contributes to both epileptiform activity and responses to single stimuli; hence our selective inhibition of EGSs suggests a more pharmacologically specific anticonvulsant effect of TTX. Consistent with this hypothesis, we found that low concentrations of TTX (5, 10, or 20 nM) in the perfusion medium blocked EGSs without decreasing the amplitude of extracellular responses to single stimuli. Polysynaptic activity and/or antidromic firing may be particularly vulnerable to TTX action on voltage-gated sodium channels, due to their lower the safety factor for action potential propagation. Selective reduction of this activity may disrupt the abnormal neuronal activity underlying EGSs.

AXON Terminal Hyperexcitability Seen in Epileptogenesis In Vitro

In the study of epilepsy, a great deal of attention has been devoted to synaptic mechanisms of seizure induction and expression. Many efforts have focused on understanding changes in the balance between synpatic excitation and inhibition. For example, the application of a number of excitatory amino acids can evoke epileptiform activity (Lehman et al., 1987; McCaslin and Morgan, 1986;. Meldrum, 1986; Piredda and Gale, 1986; Turski et al., 1987a,b), and, conversely, antagonists of excitatory amino acid receptors can suppress seizures or seizurelike activity in some models (Czuczwar et al., 1985; Czuczwar and Meldrum, 1982; De Sarro et al., 1985; Heinemann et al., 1985; Lehman et al., 1987; McNamara et al., 1988; Meldrum, 1986; Sagratella et al., 1987; Traynelis and Dingledine, 1988; Turski et al., 1987a; Walther et al., 1986). Changes in responses to excitatory amino acids are associated with pathological neuroplasticity and accompany both in vivo kindling and similar in vitro stimulation models of epileptogenesis (Mody et al., 1988; Stelzer et al., ss1987). In addition to altering the receptor mediation of synaptic responses, excitation within a neural network may be increased by altering the frequency and probability of synaptic transmission. Thus, in the hippocampal slice, the application of convulsant drugs, disturbances in extracellular ion concentrations, and tetanic stimulation which leads to epileptiform activity can all result in a greater number of excitatory synaptic interactions between cells.