Chloride-cotransport blockade desynchronizes neuronal discharge in the "epileptic" hippocampal slice

Antagonism of the chloride-cotransport system in hippocampal slices has bee n shown to block spontaneous epileptiform (i.e., hypersynchronized) dischar ges without diminishing excitatory synaptic transmission. Here we test the hypotheses that chloride-cotransport blockade, with furosemide or low-chlor ide (low-[Cl-](o)) medium, desynchronizes the firing activity of neuronal p opulations and that this desynchronization is mediated through nonsynaptic mechanisms. Spontaneous epileptiform discharges were recorded from the CA1 and CA3 cell body layers of hippocampal slices. Treatment with low-[Cl-](o) medium led to cessation of spontaneous synchronized bursting in CA1 greate r than or equal to 5-10 min before its disappearance from CA3. During the t ime that CA3 continued to burst spontaneously but CA1 was silent, electrica l stimulation of the Schaffer collaterals showed that hyperexcited CA1 syna ptic responses were maintained. Paired intracellular recordings from CA1 py ramidal cells showed that during low-[Cl-](o) treatment, the tinting of act ion potential discharges became desynchronized; desynchronization was ident ified with phase lags in firing times of action potentials between pairs of neurons as well as a with a broadening and diminution of the CA1 field amp litude. Continued exposure to low-[Cl-], medium increased the degree of the firing-time phase shifts between pairs of CA1 pyramidal cells until the ep ileptiform CAI field potential was abolished completely. Intracellular reco rdings during 4-aminopyridine (4-AP) treatment showed that prolonged low-[C l-], exposure did not diminish the frequency or amplitude of spontaneous po stsynaptic potentials. CA3 antidromic responses to Schaffer collateral stim ulation were not significantly affected by prolonged low-[Cl-], exposure. I n contrast to CA1, paired intracellular recordings from CA3 pyramidal cells showed that chloride-cotransport blockade did not cause a significant desy nchronization of action potential firing times in the CA3 subregion at the time that CA1 synchronous discharge was blocked but did reduce the number o f action potentials associated with CA3 burst discharges. These data suppor t our hypothesis that the anti-epileptic effects of chloride-cotransport an tagonism in CAI are mediated through the desynchronization of population ac tivity. We hypothesize that interference with Na+,K+,2Cl(-) cotransport res ults in an increase in extracellular potassium ([K+](o)) that reduces the n umber of action potentials that are able to invade axonal arborizations and varicosities in all hippocampal subregions. This reduced efficacy of presy naptic action potential propagation ultimately leads to a reduction of syna ptic drive and a desynchronization of the firing of CA1 pyramidal cells.