Regulation of Gephyrin Cluster Size and Inhibitory Synaptic Currents on Renshaw Cells by Motor Axon Excitatory Inputs

Abstract

Renshaw cells receive a high density of inhibitory synapses characterized by large postsynaptic gephyrin clusters and mixed glycinergic/ GABAergic inhibitory currents with large peak amplitudes and long decays. These properties appear adapted to increase inhibitory efficacy over Renshaw cells and mature postnatally by mechanisms that are unknown. We tested the hypothesis that heterosynaptic influences from excitatory motor axon inputs modulate the development of inhibitory synapses on Renshaw cells. Thus, tetanus (TeNT) and botulinum neurotoxin A (BoNT-A) were injected intramuscularly at postnatal day 5 (P5) to, respectively, elevate or reduce motor axon firing activity for ϳ2 weeks. After TeNT injections, the average gephyrin cluster areas on Renshaw cells increased by 18.4% at P15 and 28.4% at P20 and decreased after BoNT-A injections by 17.7% at P15 and 19.9% at P20. The average size differences resulted from changes in the proportions of small and large gephyrin clusters. Whole-cell recordings in P9 -P15 Renshaw cells after P5 TeNT injections showed increases in the peak amplitude of glycinergic miniature postsynaptic currents (mPSCs) and the fast component of mixed (glycinergic/GABAergic) mPSCs compared with controls (60.9% and 78.9%, respectively). GABAergic mPSCs increased in peak amplitude to a smaller extent (45.8%). However, because of the comparatively longer decays of synaptic GABAergic currents, total current transfer changes after TeNT were similar for synaptic glycine and GABA A receptors (56 vs 48.9% increases, respectively). We concluded that motor axon excitatory synaptic activity modulates the development of inhibitory synapse properties on Renshaw cells, influencing recruitment of postsynaptic gephyrin and glycine receptors and, to lesser extent, GABA A receptors.

Activity-Dependent Movements of Postsynaptic Scaffolds at Inhibitory Synapses

“…However, our quantitative data favor the notion that the lateral dynamics of inhibitory synapses are directly controlled by the activity of excitatory synapses and, to a lesser extent, by that of inhibitory ones. This is consistent with a recent study showing that excitatory cholinergic inputs affect the morphological and functional properties of inhibitory synapses in Renshaw cells of the spinal cord (Gonzalez-Forero et al, 2005). Regulation of postsynaptic structure motility by synaptic activity is well documented for dendritic spines.…”

Cytoskeleton Regulation of Glycine Receptor Number at Synapses and Diffusion in the Plasma Membrane

“…Excitatory neurotransmission has been shown to regulate inhibitory neurotransmission, gephyrin cluster size and dynamics (Gonzalez-Forero et al, 2005;Hanus et al, 2006), and, through an action on the cytoskeleton, diffusion in the plasma membrane (Richards et al, 2004). At the cellular level, cytoskeletonregulating proteins, such as profilin, Mena/VASP, or GDP/GTP exchange factors, which also regulate microtubules dynamics (Gundersen et al, 2004), are components of both inhibitory (Kins et al, 2000;Giesemann et al, 2003) and excitatory (Dillon and Goda, 2005) postsynaptic differentiations and ubiquitous components of the submembrane cytoskeleton cortex.…”

Mixed Inhibitory Synaptic Balance Correlates with Glutamatergic Synaptic Phenotype in Cerebellar Unipolar Brush Cells

“…The properties of excitatory and inhibitory synapses are, therefore, likely to be regulated in coordination. Quantitative modulation has been demonstrated (Turrigiano and Nelson, 2004;Gonzalez-Forero et al, 2005), but a qualitative regulation of IPSC properties is not established. The phenotypical diversity of mixed inhibition may offer a substrate for this tuning.…”