Effects of GABA on presumed presynaptic Ca²⁺ entry in hippocampal slices

Evoked field potentials and changes in [Ca²⁺]_o were measured in the 'in vitro' hippocampal slice of the rat. When [Ca] in the perfusion medium was lowered to 0.2 mM synaptic transmission from Schaffer collateral/comissural fibers was blocked. Nevertheless, repetitive stimulation of afferent fibers still resulted in detectable decreases of [Ca²⁺]_o. In contrast to findings in normal medium these decreases in [Ca²⁺]_o could be larger in stratum radiatum than in stratum pyramidale, so mimicking the spatial distribution of activated afferent fibers. These findings suggest, that the loss of extracellular Ca²⁺ in low Ca²⁺ media is predominantly due to entry into presynaptic terminals. This permits to study effects of drugs on presynaptic endings. We found that iontophoretic application of GABA is capable to block this presumed presynaptic Ca²⁺ entry without affecting the electrical activity of the afferent fibers. This suggests, that presynaptic GABA receptors occur also in the Schaffer collateral/commissural fiber system.