DELTA-GLUTAMATE RECEPTORS ARE DIFFERENTIALLY DISTRIBUTED AT PARALLEL AAD CLIMBING FIBER SYNAPSES ON PURKINJE-CELLS

Neurons containing multiple excitatory inputs may sort and target glut amate receptor subtypes to subsets of synapses. A good model for testi ng this hypothesis is the Purkinje cell, which expresses significant l evels of ha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, kainate, N -methyl-D-aspartate, delta-, and metabotropic glutamate receptors. Pur kinje cells receive two excitatory inputs, the parallel and climbing f ibers; the combined effect of stimulation of these two inputs is to pr oduce longterm depression of parallel fiber/Purkinje cell neurotransmi ssion. Distribution of glutamate receptors in these two synapse popula tions in rat cerebella was studied using preembedding immunocytochemis try with antibodies to GluR1, GluR2/3, GluR5-7, NR1, delta 1/2, and mG luR1 alpha. Moderate/dense postsynaptic staining was most frequent in postsynaptic densities and spines of both parallel and climbing fiber synapses with mGluR1 alpha antibody, was intermediate in frequency wit h GluR2/3 and GluR5-7 antibodies, and was least frequent with GluR1 an d NR1 antibodies. The most striking finding was the absence of signifi cant postsynaptic staining with delta 1/2 antibody in climbing fiber s ynapses in adult animals, even though postsynaptic staining was preval ent in parallel fiber synapses with this antibody. In contrast to adul ts, moderate/dense postsynaptic immunolabeling of climbing fiber synap ses with delta 1/2 antibody was common in rats at 10 days postnatal. T his study provides direct morphological evidence that delta-glutamate receptors are differentially targeted to synapse populations. Our resu lts support previous suggestions that delta 2 is involved in developme nt of parallel and climbing fiber synapses and in long-term depression of parallel fiber/Purkinje synaptic responses in adults.