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 ne onatal rat brain slice preparation, we investigated short-term changes in s ynaptic transmission mediated by GABA or glycine. In 1.5 mM extracellular C a2+ [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 GAB Aergic ones. GABAergic currents were strongly facilitated by pulse trains a t 5 or 10 Hz without apparent depression. This phenomenon persisted after p harmacological block of GABA(B) receptors. Glycinergic currents were compar atively 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 stimu lation, at 5 Hz strong facilitation developed with minimal summation of GAB A-mediated currents. Glycinergic currents decayed so fast that summation wa s minimal. As [Ca2+](o) is known to shape short-term synaptic changes, we e xamined if varying [Ca2+](o) could differentially affect facilitation of GA BA-or glycine-operated synapses. With 5 mM [Ca2+](o), the frequency of spon taneous GABAergic or glycinergic currents appeared much higher but GABAergi c current facilitation was blocked (and replaced by depression), whereas gl ycinergic currents remained slightly facilitated. [Ca2+](o) manipulation th us brought about distinct processes responsible for facilitation of GABAerg ic or glycinergic transmission. Our data therefore demonstrate an unexpecte dly robust, short-term increase in the efficiency of GABAergic synapses tha t can become at least as effective as glycinergic synapses.