The role of tonic glycinergic conductance in cerebellar granule cells signalling and the effect of gain-of-function mutation.

KEY POINTS: A T258F mutation of the glycine receptor increases the receptor affinity to endogenous agonists, modifies single-channel conductance and shapes response decay kinetics. Glycine receptors of cerebellar granule cells play their functional role not continuously, but when granule cell layer starts receiving high amount of excitatory inputs. Despite their relative scarcity, tonically active glycine receptors of cerebellar granule cells make a significant impact on action potential generation, inter-neuronal crosstalk, and modulate synaptic plasticity in neural networks; extracellular glycine increases probability of postsynaptic response occurrence acting at NMDA receptors and decreases this probability acting at glycine receptors. Tonic conductance through glycine receptors of cerebellar granule cells is a yet undiscovered element of bi-phasic mechanism which regulates processing of sensory inputs in the cerebellum. T258F point mutation disrupts this bi-phasic mechanism, thus illustrating possible role of the gain-of-function mutations of glycine receptor in development of neural pathologies.

ABSTRACT: Functional glycine receptors (GlyRs) were repeatedly detected in cerebellar granule cells (CGCs), where they deliver exclusively tonic inhibitory signalling. The functional role of this signaling, however, remains unclear. Apart from that, there is accumulating evidence of the important role of GlyRs of cerebellar structures in development of neural pathologies such as hyperekplexia, which can be triggered by GlyR gain-of-function mutations. In this research we initially tested functional properties of GlyRs, carrying the yet understudied T258F gain-of-function mutation, to find that this mutation makes significant modifications in GlyR response to endogenous agonists. Next, we clarified the role of tonic GlyR conductance in neuronal signalling generated by single CGC and by neural networks in cell cultures and in living cerebellar tissue of C57Bl-6J mice. We found that GlyRs of CGC deliver a significant amount of tonic inhibition not continuously, but when cerebellar granule layer starts receiving substantial excitatory input. Under these conditions tonically active GlyRs become a part of neural signalling machinery allowing generation of action potentials (APs) bursts of limited length in response to sensory-evoked signals. GlyRs of CGCs support a biphasic modulatory mechanism which enhances AP firing when excitatory input intensity is low, but suppresses it when excitatory input rises to a certain critical level. This enables one of the key functions of the CGC layer: formation of sensory representations and their translation into motor output. Finally, we have demonstrated that T258F mutation in CGC GlyRs modifies single-cell and neural network signalling, and brakes a biphasic modulation of AP-generating machinery. This article is protected by copyright. All rights reserved.