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Abstract
Objective
Potassium channels are ubiquitously expressed in all organisms and constitute the most diverse class of ion channels. Their action is regulated by various factors and is involved in neuronal excitability. 4-aminopyridine (4-AP) is a voltage-sensitive K+ channel inhibitor used in studies of experimental neuronal excitation and in the treatment of demyelinating diseases. It is also known as having a strong convulsant activity on hippocampal slices. We investigated the effect of several K+ channel antagonists on evoked potentials during normoxia and hypoxia.
Methods
Field excitatory postsynaptic potentials (fEPSPs), orthodromic (oPSs) and antidromic population spikes (aPSs) were recorded simultaneously in the cornus ammoni1 (CA1) area of rat hippocampal slices. fEPSP/Spike (E-S) ratio was calculated during the experiments to observe sequential changes of pre- and postsynaptic neuronal excitability, especially epileptiform activities. 4-AP, tetraethylammonium (TEA), dequalinium dichloride, paxilline, tertiapin-Q, glibenclamide and barium chloride were tested.
Results
Most of the tested K+ channel blockers showed increase of mean amplitudes in oPS, fEPSP and aPS compared to control slices. 4-AP showed most strong increase in oPS (188.5-2.1%). During the early period of hypoxia, epileptiform activities were observed more prominently in 4-AP treated slices, which were not produced in other K+ channel blockers. Those epileptiform activities were accompanied by E-S potentiation (mean 426.6%). Common antiepileptic drugs-valproic acid, phenytoin and carbamazepine-effectively controlled epileptiform activities induced by 4-AP during hypoxia (p=0.0035).
Conclusion
Our experimental results thus show that 1) potassium channels significantly increase evoked field potentials, 2) epileptiform activities are increased in 4-AP treated slices during hypoxia and they are attributed to E-S potentiation and 3) 4-AP induced epileptiform activities during hypoxia are effectively controlled by antiepileptic drugs.
Figures and Tables
FIGURE 1
Upper: Schematic diagrams representing locations of stimulating and recording sites in CA1 hippocampal region. Lower: Three stimulating sites (orthodromic control input, paired input and antidromic conditioning) and two recording sites (PS, fEPSP). CA: cornus ammoni, PS: population spike, fEPSP: field ex-citatory postsynaptic potentials.
FIGURE 2
Sample tracing of evoked potentials treated by 4-aminopyridine (4-AP). orthodromic (oPS) are significantly increased after superfusing 4-AP (compare a and b), while fEPSP and antidromic population spike (aPS) does not change during the time course. fEPSP: field excitatory postsynaptic potentials.
FIGURE 3
Mean amplitudes of each evoked potentials after superfusion of various potassium channel blockers. A: oPS: All potassium channel blockers shows significantly increased amplitudes of oPS compared to control slices. 4-AP shows largest increase (188.5±2.1%) in oPS. B: fEPSP: Kv blockers show more increase fEPSP than other antagonists and TEA shows most prominent effect (141.8±1.5%). C: aPS: Compared to oPS and fEPSP, aPS shows rather small changes. Barium chloride has most prominent effect (140±3.7%). Asterisk(*) indicates p<0.05 of mean amplitudes between control and test slices (Student's t test for unpaired observations). oPS: orthodromic, fEPSP: field excitatory postsynaptic potentials, aPS: antidromic population spike, Kv: voltage dependent potassium channel, 4-AP: 4-aminopyridine, TEA: tetraethylammonium.
FIGURE 4
Sequential changes of evoked potentials during hypoxia with A1 adenosine antagonist. A: During hypoxia, both oPS and fEPSP rapidly depresses and recover after reoxygenation and show posthypoxic excitation. B: After A1 receptor blocking, depressed fEPSP and oPS are largely attenuated. Note the differences of recovered amplitudes of fEPSP and oPS. (a): E-S poten-tiation and (b): E-S depression. C: In slices superfused with 4-AP, E-S potentiation (c) is appeared during hypoxia. D: Similar result with more attenuated fEPSP and oPS [with E-S potentiation(d)] during hypoxia in slices with DPCPX. oPS: orthodromic, fEPSP: field excitatory postsynaptic potentials, E-S: EPSP-Spike, 4-AP: 4-aminopyridine.
FIGURE 5
Increase of epileptiform activities (additional PS) during hypoxia in slices with 4-AP. Normal tracing (a) and typical samples showing epileptiform activities appearing after superfusiing 4-AP (b) and exaggerated by additional hypoxia (c). Sequential changes of E-S ratio shows prominent E-S potentiation during hypoxia (filled squares). PS: population spike, 4-AP: 4-aminopyridine, E-S: EPSP-Spike.
FIGURE 6
Comparing E-S ratio between hypoxia (A) and 4-AP with hypoxia (B). Sample tracing and mean shows that E-S depression during hypoxia (A) while E-S potentiation in slices with 4-AP and 4-AP with hypoxia (B). E-S: EPSP-Spike, 4-AP: 4-aminopyridine.
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