Cell-type specific organization of glycine receptor clusters in the mammalian spinal cord

Abstract

Glycinergic synapses play a major role in shaping the activity of spinal cord neurons. The spatial organization of postsynaptic receptors is likely to determine many functional parameters at these synapses and is probably related to the integrative capabilities of different neurons. In the present study, we have investigated the organization of gephyrin expression along the dendritic membranes of alpha- and gamma-motoneurons, Ia inhibitory interneurons, and Renshaw cells. Gephyrin is a protein responsible for the postsynaptic clustering of glycine receptors, and the features of gephyrin and glycine receptor alpha(1)-subunit immunofluorescent clusters displayed similar characteristics on ventral horn spinal neurons. However, the density of clusters and their topographical organization and architecture varied widely in different neurons and in different dendritic regions. For motoneurons and Ia inhibitory interneurons, cluster size and complexity increased with distance from the soma, perhaps as a mechanism to enhance the influence of distal synapses. Renshaw cells were special in that they displayed an abundant complement of large and morphologically complex clusters concentrated in their somas and proximal dendrites. Serial electron microscopy confirmed that the various immunoreactivity patterns observed with immunofluorescence accurately parallel the variable organization of pre- and postsynaptic active zones of glycinergic synapses. Finally, synaptic boutons from single-labeled axons of glycinergic neurons (Ia inhibitory interneurons) were also associated with postsynaptic receptor clusters of variable shapes and configurations. Our results indicate that mechanisms regulating receptor clustering do so primarily in the context of the postsynaptic neuron identity and localization in the dendritic arbor.

Target selection of proprioceptive and motor axon synapses on neonatal V1‐derived Ia inhibitory interneurons and Renshaw cells

“…We confirmed here that CB and VGLUT1 axons converging on YFP-labeled neonatal V1-INs originate from V1-derived RCs and proprioceptive afferents. Convergent RC and Ia afferent inputs are hallmark criteria for identification of interneurons mediating reciprocal inhibition in lumbar spinal cord (Hultborn et al, 1971;Jankowska and Lindstrom, 1972; Rastad et al, 1990;Alvarez et al, 1997). In our study V1-IaINs defined anatomically by these convergent inputs can be recognized in neonates at P1 and a proportion upregulate PV postnatally with a time course similar to other spinal cord interneurons.…”

“…Criteria for IaIN identification in histological sections was designed to parallel electrophysiological criteria (Alvarez et al, 1997. Ia inhibitory interneurons are the only known ventral interneurons strongly modulated by RC inputs (Hultborn et al, 1971;Wang et al, 2008) and therefore convergence of RC and Ia sensory inputs reliably defines IaINs involved in reciprocal inhibition, at least in lumbar segments (Jankowska, 1992).…”

Distribution of cholinergic contacts on Renshaw cells in the rat spinal cord: a light microscopic study

“…The purpose of the immunolabelling procedure was to reveal VAChT-immunoreactive (VAChT-IR) terminals on Renshaw cells identified by their gephyrin or calbindin D28k immunolabelling characteristics (Alvarez et al 1997;Carr et al 1998). Preparations that could be analysed under bright-field light microscopy conditions were sought in order to obtain accurate morphometric data on the location of contacts in the somatodendritic tree and dimensions of the labelled structures (see below).…”

“…The present study used VAChT immunoreactivity in combination with two immunohistochemical criteria to identify Renshaw cells. One approach was based on the characteristic membrane covering of gephyrin-immunoreactive patches on Renshaw cells (Alvarez et al 1997). Gephyrin is a protein that is required for clustering glycine receptors in the postsynaptic region (Kirsch et al 1993) and in Renshaw cells maps the size and shape of postsynaptic glycine receptor clusters (Alvarez et al 1997).…”

“…One approach was based on the characteristic membrane covering of gephyrin-immunoreactive patches on Renshaw cells (Alvarez et al 1997). Gephyrin is a protein that is required for clustering glycine receptors in the postsynaptic region (Kirsch et al 1993) and in Renshaw cells maps the size and shape of postsynaptic glycine receptor clusters (Alvarez et al 1997). A second approach identified Renshaw cells by their expression of calbindin D28k immunoreactivity (Arvidsson et al 1992;Carr et al 1998).…”