GABA and glycine co-release optimizes functional inhibition in rat brainstem motoneurons in vitro

Whole-cell patch clamp recordings of miniature inhibitory postsynaptic currents (mIPSCs) were obtained in identified abducens motoneurons (aMns) from young rats (P5-P13). Three types of mIPSC were distinguished according to their kinetics and their sensitivity to receptor antagonists: faster decaying events mediated by glycine receptors (glyRs), slower decaying events mediated by GABA(A) receptors (GABA(A)Rs), and mIPSCs displaying two components corresponding to GABA and glycine co-release. Dual component events accounted for approximately 30 % of mIPSCs, independently of the rat's age and were also identified during evoked transmitter release. In contrast, the kinetics of glyR- and GABA(A)R-mediated mIPSCs became faster during development. Monosynaptic inhibitory postsynaptic potentials (IPSPs) were able to fully inhibit motoneuron discharge elicited by current pulses. When the GABA(A)R-mediated component or the glyR-mediated component of the IPSP was blocked, the inhibition of motoneuron firing was reduced. The 20-80 % rise time and duration of GABA(A)R-mediated IPSPs were significantly longer than those mediated by glyRs. The time window of inhibition for each component was determined using single postsynaptic action potentials elicited with various delays from the onset of the IPSP. GlyR-mediated IPSPs induced fast transient inhibition whereas GABA(A)R-mediated IPSPs induced slow sustained suppression of firing. Using a modelling approach, we found that the two components summated non-linearly. We conclude that in developing aMns, co-release of GABA and glycine determines the strength and timing of inhibition through non-linear interactions between the two components, thus optimizing inhibition of motoneuron function.