Increased inhibition in dentate gyrus granule cells following exposure to GABA-uptake blockers

Rats anesthetized with urethane had stimulating and recording electrodes placed in the perforant pathway and in the dentate gyrus. They were then exposed to increasing doses of either the vehicle control dimethylsulfoxide (DMSO) or one of two gamma-aminobutyric acid (GABA)-uptake blockers (SKF-100330A or SKF-89976A). Analysis of evoked field potentials from dentate granule cells indicated that the only effect of the GABA uptake blockers was to increase the threshold for evoking the field population spikes (PS). No other measure of excitatory postsynaptic potentials (EPSPs) or PS's were significantly affected. The lack of effect on evoked EPSP by these drugs suggests no direct effect on transmitter release at this synapse, while the increase in PS threshold suggests a slight decrease in granule cell excitability. The effects of the two GABA-uptake blockers on synaptically mediated facilitation and inhibition was tested by using paired-pulse paradigms. Both GABA-uptake blockers increased early GABA-mediated inhibition to a greater extent than they reduced synaptically mediated facilitation. Neither GABA uptake blocker appeared to effect the late inhibition seen at paired-pulse intervals of 400-1000 ms which is presumably associated with calcium-activated increases in potassium conductance. These effects on granule cell responses occurred at doses found previously not to be associated with side effects and yet to be anticonvulsant in unanesthetized rats. These data confirm in vivo that SKF-100330A and SKF-89976A increase GABA-mediated inhibition. The effect on granule cell excitability and late inhibition are minimal. Although facilitation was reduced by exposure to these drugs, the mechanism of this reduction (direct or prolongation of early inhibition) cannot be determined.