Differential short-term changes in GABAergic or glycinergic synaptic efficacy on rat hypoglossal motoneurons

Using whole cell patch-clamp recording from hypoglossal motoneurons of a neonatal rat brain slice preparation, we investigated short-term changes in synaptic transmission mediated by GABA or glycine. In 1.5 mM extracellular Ca2+ [Ca2+](o),pharmacologically isolated GABAergic or glycinergic currents were elicited by electrical stimulation of the reticular formation. At low stimulation frequency, glycinergic currents were larger and faster than GABAergic ones. GABAergic currents were strongly facilitated by pulse trains at 5 or 10 Hz without apparent depression. This phenomenon persisted after pharmacological block of GABA(B) receptors. Glycinergic currents were comparatively much less enhanced than GABAergic currents. One possible mechanism to account for this difference is that GABAergic currents decayed so slowly that consecutive responses summated over an incrementing baseline. However, while synaptic summation appeared at greater than or equal to 10-Hz stimulation, at 5 Hz strong facilitation developed with minimal summation of GABA-mediated currents. Glycinergic currents decayed so fast that summation was minimal. As [Ca2+](o) is known to shape short-term synaptic changes, we examined if varying [Ca2+](o) could differentially affect facilitation of GABA-or glycine-operated synapses. With 5 mM [Ca2+](o), the frequency of spontaneous GABAergic or glycinergic currents appeared much higher but GABAergic current facilitation was blocked (and replaced by depression), whereas glycinergic currents remained slightly facilitated. [Ca2+](o) manipulation thus brought about distinct processes responsible for facilitation of GABAergic or glycinergic transmission. Our data therefore demonstrate an unexpectedly robust, short-term increase in the efficiency of GABAergic synapses that can become at least as effective as glycinergic synapses.