4-Aminopyridine produces epileptiform activity in hippocampus and enhances synaptic excitation and inhibition

Abstract

Using extra- and intracellular recording techniques, we investigated the epileptiform activity induced by low concentrations (5 and 10 microM) of bath-applied 4-aminopyridine (4-AP) in the CA3 subfield of rat hippocampal slices. We also studied the effects of 4-AP on the excitatory and inhibitory synaptic conductance changes in CA3 neurons produced by mossy fiber stimulation. Low concentrations of 4-AP induced spontaneously occurring epileptiform discharges at extracellular potassium concentrations between 1 and 10 mM. In contrast, picrotoxin and bicuculline produced spontaneous epileptiform discharges at extracellular potassium concentrations between 5 and 10 mM. The paroxysmal depolarizing shift (PDS) induced by 4-AP was also investigated. At potentials between -40 and -10 mV, the waveform of the PDS consisted of a depolarizing component enveloped by a hyperpolarizing component. The amplitude of the depolarizing component of the PDS was a monotonic function of the membrane potential, and the mean measured reversal potential was -25.7 mV. Under voltage-clamp conditions, the measured conductance associated with the depolarizing component of the PDS averaged 110 nS, with a reversal potential of -14.1 mV. Application of 5 microM 4-AP produced an increase in the inhibitory synaptic conductance change calculated from currents measured 15 ms following mossy fiber stimulation. The mean value increased from 35.2 to 58.1 nS (P less than 0.05) without a significant change in reversal potential. A concentration of 10 microM 4-AP also produced an increase in this inhibitory synaptic conductance change (from 53.3 to 66.3 nS, P less than 0.05) but caused a significant depolarization of the reversal potential (from -66.5 to -61.6 mV, P less than 0.05). This change in reversal potential may reflect a prolongation of the excitatory synaptic currents produced by 4-AP that contributes to the current measured 15 ms from the stimulus. Following application of either 5 or 10 microM 4-AP, there were no significant changes in the resting potential or input resistance of the neurons studied. Application of 5 microM 4-AP also significantly increased the amplitude of the measured excitatory synaptic conductance change produced by mossy fiber stimulation (from 27.9 to 44.1 nS, P less than 0.05) without producing a change in the reversal potential. In 5 of 21 neurons studied, a long-lasting outward synaptic current was present at holding potentials near rest following mossy fiber stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Models of drug-induced epileptiform synchronization in vitro

“…Shortly after, Voskuyl and Albus (1985) identified 2 types of spontaneous, interictal-like field potentials in isolated hippocampal slices; the first type resembled the spontaneous epileptiform activity induced by GABA A receptor antagonists and originated in the CA3 subfield, while the second lasted longer, spread at slower velocity and was resistant to glutamatergic receptor antagonism. These results were later confirmed by other investigators (Rutecki et al, 1987;Michelson and Wong, 1991;Perreault and Avoli, 1992;Watts and Jefferys, 1993).…”

“…It has, in fact, been reported that during blockade of GABA A receptor signaling, spontaneous interictal events in hippocampal slices occur only when the extracellular [K + ] is set to levels higher than 5 mM (Tancredi and Avoli, 1987;Rutecki et al, 1987). …”

Mechanisms of physiological and epileptic HFO generation

“…Many of them rely on blocking GABA A receptor mediated inhibition with drugs such as picrotoxin, bicuculline or penicillin (Dingledine and Gjerstad, 1979;Schwartzkroin and Prince, 1978). Convulsant treatments which do not block inhibition include 4-aminopyridine (Avoli et al, 1996;D'Antuono et al, 2002;Rutecki et al, 1987), elevated K + (Jiruska et al, 2010a;Rutecki et al, 1985;Traub and Dingledine, 1990) and low Mg 2+ (Anderson et al, 1986;Derchansky et al, 2008). Another acute and realistic model in vivo is brain trauma, which leads to electrographic seizures in the majority of animals (Topolnik et al, 2003).…”

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

“…However, the pioneering studies made in Johnston's laboratory with single-electrode voltage-clamp recordings have demonstrated that the amplitude of the interictal responses recorded during application of medium containing 4AP, tetraethylammonium, or elevated [K + ] are characterized by reversal potentials that are more negative than those associated with reversal potentials of paroxysmal depolarization shifts induced by GABA A receptor antagonists (Rutecki et al, 1985(Rutecki et al, , 1987(Rutecki et al, , 1990. Hence, synaptic inhibition is operative in these in vitro models of interictal discharge.…”