Immunoelectron microscopy of AMPA receptor subunits reveals three types ofputative glutamatergic synapse in the rat vestibular end organs

To characterize the synapses between hair cells and afferent nerve endings in the rat vestibular end organs, the ultrastructural localization of AMPA receptor subunits (GluR1-4) was examined by postembedding immunogold cytoch emistry. Immunoreactivities for GluR2/3 and GluR4 were associated with the synapses between type I hair cells and the surrounding chaliceal nerve endi ngs and with the bouton type nerve endings contacting type II hair cells. T here was no detectable immunoreactivity for GluR1. A third type of immunore active synapse was found between the outer face of chalices and type Il hai r cells. While the linear densities of gold particles (particles per microm eter postsynaptic specialization) of bouton type endings and chaliceal nerv e endings were the same, the former type of ending showed larger postsynapt ic specializations and, hence, a higher number of receptor molecules. These data indicate that there are three types of putative glutamatergic synapse in the vestibular end organ. (C) 1999 Elsevier Science B.V. All rights res erved.