δ‐Glutamate Receptors Are Differentially Distributed at Parallel and Climbing Fiber Synapses on Purkinje Cells

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Abstract
Neurons containing multiple excitatory inputs may sort and target glutamate receptor subtypes to subsets of synapses. A good model for testing this hypothesis is the Purkinje cell, which expresses significant levels of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate, kainate, N‐methyl‐d‐aspartate, δ‐, and metabotropic glutamate receptors. Purkinje cells receive two excitatory inputs, the parallel and climbing fibers; the combined effect of stimulation of these two inputs is to produce long‐term depression of parallel fiber/Purkinje cell neurotransmission. Distribution of glutamate receptors in these two synapse populations in rat cerebella was studied using preembedding immunocytochemistry with antibodies to GluR1, GluR2/3, GluR5‐7, NR1, δ1/2, and mGluR1α. Moderate/dense postsynaptic staining was most frequent in postsynaptic densities and spines of both parallel and climbing fiber synapses with mGluR1α antibody, was intermediate in frequency with GluR2/3 and GluR5‐7 antibodies, and was least frequent with GluR1 and NR1 antibodies. The most striking finding was the absence of significant postsynaptic staining with δ1/2 antibody in climbing fiber synapses in adult animals, even though postsynaptic staining was prevalent in parallel fiber synapses with this antibody. In contrast to adults, moderate/dense postsynaptic immunolabeling of climbing fiber synapses with δ1/2 antibody was common in rats at 10 days postnatal. This study provides direct morphological evidence that δ‐glutamate receptors are differentially targeted to synapse populations. Our results support previous suggestions that δ2 is involved in development of parallel and climbing fiber synapses and in long‐term depression of parallel fiber/Purkinje synaptic responses in adults.

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