CELL-TYPE-SPECIFIC ORGANIZATION OF GLYCINE RECEPTOR CLUSTERS IN THE MAMMALIAN SPINAL-CORD

Glycinergic synapses play a major role in shaping the activity of spin al 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 neuro ns. In the present study we have investigated the organization of geph yrin expression along the dendritic membranes of alpha- and gamma-moto neurons, Ia inhibitory interneurons, and Renshaw cells. Gephyrin is a protein responsible for the postsynaptic clustering of glycine recepto rs, and the features of gephyrin and glycine receptor alpha(1)-subunit immunofluorescent clusters displayed similar characteristics on ventr al horn spinal neurons. However, the density of clusters and their top ographical organization and architecture varied widely in different ne urons and in different dendritic regions. For motoneurons and Ia inhib itory interneurons, cluster size and complexity increased with distanc e fr om the soma, perhaps as a mechanism to enhance the influence of d istal synapses. Renshaw cells were special in that they displayed an a bundant complement of large and morphologically complex clusters conce ntrated in their somas and proximal dendrites. Serial electron microsc opy confirmed that the various immunoreactivity patterns observed with immunofluorescence accurately parallel the variable organization of p re- and postsynaptic active zones of glycinergic synapses. Finally, sy naptic boutons from single-labeled axons of glycinergic neurons (Ia in hibitory 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 de ndritic arbor. (C) 1997 Wiley-Liss, Inc.