Role of metabotropic glutamate (ACPD) receptors at the parallel fiber- Purkinje cell synapse

1. The role of metabotropic glutamate receptors at the parallel fiber (PF)-Purkinje cell synapse in cerebellum was studied by examining the actions of the active stereoisomer (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [1S,3R-ACPD (25-50 μM)] on fura-2-loaded, patch-clamped rat Purkinje cells in thin slices. 2. The bath application of 1S,3R-ACPD evoked a direct postsynaptic depolarization that readily desensitized during prolonged (>1 min) applications of the drug. This depolarizing response to 1S,3R-ACPD differed from the slow depolarization to 1S,3R-ACPD observed in cortical neurons mediated via closure of potassium channels in that it was not associated with an obvious change in membrane conductance and was not blocked by external barium. Similarly, slow inward rectifier currents were not affected during the 1S,3R-ACPD-induced depolarization. 3. The direct depolarization induced by 1S,3R-ACPD was not mediated by N-methyl-D-aspartate (NMDA) or (RS)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid kainate (AMPA)-KA excitatory amino acid (EAA) receptor subtypes, because the response was not blocked in the presence of antagonists of these receptors. 4. The EAA antagonist L-2-amino-3-phosphonopropionic acid, which blocks 1S,3R-ACPD- induced inositide synthesis in other cell types, had no effect on the depolarizing response. 5. Fura-2 measurements of somatic [Ca²⁺](i) revealed that [Ca²⁺](i) was not elevated during the 1S,3R-ACPD-induced depolarization unless the cell fired calcium-dependent action potentials. 6. In addition to the direct depolarization induced by 1S,3R-ACPD, the amplitude of PF-evoked excitatory postsynaptic potentials (EPSPs) was profoundly and reversibly reduced. This effect was observed in all cells regardless of whether a direct depolarization was produced by 1S,3R-ACPD. This reduction of the PF EPSP generally preceded the onset of depolarizing responses, did not desensitize during prolonged applications of 1S,3R-ACPD, and was reversible. 7. The reversible reduction of the PF EPSP by 1S,3R-ACPD was not related to a postsynaptic blocking action of the drug, because responses of Purkinje cells to AMPA, an agonist of the EAA receptor subtype mediating the EPSP, were reversibly potentiated in the presence of 1S,3R-ACPD. 8. The nitric oxide synthesis promoter sodium nitroprusside (1-3 mM) had no effect on the amplitude of PF EPSP or the membrane properties of Purkinje cells. 9. In experiments using the perforated patch recording technique employing pipettes filled with nystatin and fura-2 AM to avoid dialysis of the cell soma, a reversible reduction of the PF EPSP was also observed, which was associated in some cases with an inward current, but no change in [Ca²⁺](i). 10. The results provide evidence for three distinct sites of action of the metabotropic EAA agonist 1S,3R-ACPD: a reduction of the PF EPSP mediated via activation of presynaptic autoreceptors, a potentiation of postsynaptic sensitivity, and a direct depolarizing action. Activation of metabotropic receptors at the PF-Purkinje cell synapse may thus play a significant role in modulating the efficacy of transmission at this synapse during periods of high-frequency transmission.