Suppression of the glutamate receptor δ2 subunit produces a specific impairment in cerebellar long-term depression

1. The role of the glutamate receptor subunit δ2 in the induction of cerebellar long-term depression (LTD) was investigated by application of antisense oligonucleotides. The δ2 subunit is selectively localized to Purkinje cells (PCs), with the highest levels being in the PC dendritic spines, where parallel fibers are received and where cerebellar LTD is expressed. 2. Immunocytochemical analysis of calbindin-positive PCs revealed that both the dendritic and somatic expression of δ2 was reduced in antisense- but not in sense-treated cultures. An antisense oligonucleotide directed against the related subunit δ1 did not affect the expression of δ2 in PCs. 3. Cerebellar LTD may be reliably induced in a preparation of cultured embryonic cerebellar neurons from the mouse when parallel and climbing fiber stimulation are replaced by brief glutamate pulses and strong, direct depolarization of the PC, respectively. Application of an antisense oligonucleotide directed against δ2 completely blocked the induction of LTD produced by glutamate/depolarization conjunctive stimulation. A δ2 sense oligonucleotide or an antisense oligonucleotide directed against the related δ1 subunit had no effect. 4. The effect of the δ2 antisense oligonucleotide was not related to attenuation of calcium influx via voltage-gated channels or calcium mobilization via metabotropic glutamate receptors, as assessed with fura-2 microfluorimetry. Current flow through α-amino-3-hydroxy-5- methyl-4-isoxazolepropionic acid-receptor-associated ion channels also appeared unaltered. All three of these processes have previously been shown to be required for cerebellar LTD induction. The observation that δ2 is involved in a metabotropic glutamate receptor-independent signaling pathway that is required for LTD induction supports the view that δ2 participates in the formation of a novel postsynaptic receptor complex.