P.Branchereau, D.Cattaert et al. in Scientific Reports

Depolarizing GABA/glycine synaptic events switch from excitation to inhibition during frequency increases.
Branchereau P, Cattaert D, Delpy A, Allain AE, Martin E, Meyrand P.
Sci Rep. 2016 Feb 25;6:21753. doi: 10.1038/srep21753.

By acting on their ionotropic chloride channel receptor, GABA and glycine represent the major inhibitory amino acid transmitters of the central nervous system. These inhibitory transmitters control the input-output (I-O) relationship of excitatory drives impinging on neurons. However, under numerous circumstances, GABAergic/glycinergic postsynaptic potentials are depolarizing (dGPSPs) and exert mixed excitatory (depolarizing) and inhibitory (shunting) effects on the I-O relationship. The relationship between the inhibitory and excitatory components of dGPSPs is therefore a complex phenomenon.

In the present study, we address the relative roles of the excitatory (depolarization) and inhibitory (shunting) components of dGPSPs by a quantitative analysis of their interplay. We also consider the interaction of three parameters (chloride equilibrium ECl, chloride conductance gClp and resting membrane potential ERest) and their modulation by neuron passive properties.

We examined foetal mouse spinal motoneurons (MNs), which express various ECl values and shapes (i.e., size and input resistance Rin) during the course of development. We performed whole-cell patch-clamp recordings from immature E13.5 and more mature E17.5 spinal MNs, in which [Cl-]i is high and low, respectively (Delpy et al, J. Physiol. 2008). The effect of gClp was probed by controlled local applications of the GABAAR agonist isoguvacine (via pressure microejection). To gain a deeper understanding of the mechanisms controlling dGPSP effects, we combined the physiological analysis with computational simulations. These simulations allowed us to measure and model the role of each parameter (Em, ECl, gClp, neuron size and Rin) in the magnitude and time course of inhibition and excitation during single dGPSPs.

Thanks to the combined physiological experiments and computer simulations, our study demonstrates that regardless of the neuron passive properties, increasing gClp may result in a switch from excitatory to inhibitory dGPSP depending on the ECl. We also show that dGPSP trains can be excitatory at low rates and inhibitory at high rates. Finally, we demonstrate that dGPSP trains modulate the outcome of excitatory drives (EPSPs) as follows: 1) time-locked EPSPs that are generated during the early phase of dGPSPs are inhibited and time-locked EPSPs that are generated during the late phase of dGPSPs are facilitated; 2) the transition from inhibition to is more abrupt as gClp increases; and 3) random EPSP barrages are facilitated by low-rate dGPSP trains and inhibited by high-rate dGPSP trains, with the inhibition becoming more abrupt as gClp increases.

These results indicate that compared with classical EPSPs, excitatory dGPSPs switch to potent inhibition as their discharge frequency increases, which precludes the overexcitation of the neuronal network and represents a fundamental property of these dGPSPs.

Switch from excitatory to inhibitory dGPSP barrage with increasing frequency. A1, Whole-cell patch-clamp recording from an E13.5 MN while stimulating the ventral funiculus (VF) at various frequencies. A2, Average I/O frequency relationship calculated from five E13.5 MNs. A3, Spontaneous burst of activity demonstrating that the MN is able to fire at a frequency reaching 20 Hz. B1, Simulation of the E13.5-model MN with the same characteristics as the real MN presented in A-B. B2, Bell-shaped I/O frequency relationship. B3, Curves of peak sodium channel currents (INaPeak) against the imposed membrane potential in the absence (black) and the presence (grey) of a constant gCl.