DIFFERENTIAL LOCALIZATION OF DELTA-GLUTAMATE RECEPTORS IN THE RAT CEREBELLUM - COEXPRESSION WITH AMPA RECEPTORS IN PARALLEL FIBER-SPINE SYNAPSES AND ABSENCE FROM CLIMBING FIBER-SPINE SYNAPSES

The delta 2 glutamate receptors are prominently expressed in Purkinje cells and are thought to play a key role in the induction of cerebella r long-term depression. The synaptic and subsynaptic localization of d elta receptors in rat cerebellar cortex was investigated with sensitiv e and high-resolution immunogold procedures. After postembedding incub ation with an antibody raised to a C-terminal peptide of delta 2, high gold particle densities occurred in all parallel fiber synapses with Purkinje cell dendritic spines, whereas other synapses were consistent ly devoid of labeling. Among the types of immunonegative synapse were climbing fiber synapses with spines and parallel fiber synapses with d endritic stems of interneurons. At the parallel fiber-spine synapse, g old particles signaling delta receptors were restricted to the postsyn aptic specialization. By the use of double labeling with two different gold particle sizes, it was shown that delta and AMPA GluR2/3 recepto rs were colocalized along the entire extent of the postsynaptic specia lization without forming separate domains. The distribution of gold pa rticles representing delta receptors was consistent with a cytoplasmic localization of the C terminus and an absence of a significant presyn aptic pool of receptor molecules. The present data suggest that the de lta 2 receptors are targeted selectively to a subset of Purkinje cell spines and that they are coexpressed with ionotropic receptors in the postsynaptic specialization. This arrangement could allow for a direct interaction between the two classes of receptor.