Dendritic and somatic glutamate receptor channels in rat cerebellar Purkinje cells

1 The properties of glutamate receptor (GluR) channels in outside‐out patches from the dendrites and somata of rat cerebellar Purkinje cells in brain slices were studied using fast agonist application techniques. Dendritic patches were isolated 40–130 μ m from the soma. 2 Outside‐out patches from both dendrites and somata of Purkinje cells responded to application of glutamate with a current which desensitized rapidly and nearly completely. Currents evoked by glutamate application were blocked by 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX), were mimicked by l ‐α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA), and were modulated by cyclothiazide. Kainate produced small, non–desensitizing currents. No currents were observed in response to aspartate application. Responses characteristic of NMDA receptor activation were not observed. These findings indicate that glutamate‐activated currents were mediated by the AMPA subtype of GluR. 3 Deactivation of the GluR channels following 1 ms pulses of glutamate occurred with a time constant of 1.23 ± 0.07 ms in dendritic and 1.12 ± 0.04 ms in somatic patches. Desensitization occurred with a time constant of 5.37 ± 0.26 ms in dendritic and 5.29 ± 0.29 ms in somatic patches. The time constant of recovery from desensitization caused by a 1 ms application of 1 m m glutamate was 36 ms in dendritic patches and 33 ms in somatic patches. 4 Half‐maximal activation of the GluR channels was achieved at a glutamate concentration of 432 μ m . Deactivation kinetics were not dependent on the glutamate concentration, while desensitization became slower at lower glutamate concentrations. 5 Pre‐equilibration of patches with low concentrations of glutamate reduced the peak current activated by 1 m m glutamate. The IC 50 for this effect was 8.7 μ m . Equilibrium desensitization did not affect the kinetics of the current activated by 1 m m glutamate. 6 The current–voltage relationship of the peak current was linear in normal Na + ‐rich external solution, with a reversal potential near 0 mV. In Ca 2+ ‐rich external solution, the reversal potentials were −51.4 ± 2.9 and −51.5 ± 2.8 mV for dendritic and somatic patches, respectively, indicating that these glutamate channels have a low permeability to Ca 2+ ( P Ca / P Cs = 0.053). 7 The mean single‐channel conductance of the GluR channels measured using non–stationary fluctuation analysis was ∼8 pS in dendritic and somatic patches, and the maximum open probability was at least 0.7 with 5 m m glutamate. 8 GluR channel kinetics in patches excised from the soma of neonatal (postnatal day 4; P4) Purkinje cells, before the development of the dendritic arborization of the Purkinje cell, were similar to those in patches excised from more mature (P12–18) Purkinje cells. 9 Dendritic and somatic GluR channels in Purkinje cells appear to be functionally identical, are AMPA‐subtype receptors containing the GluR‐B subunit, and have rapid kinetics and low permeability to Ca 2+ . A kinetic model was constructed which faithfully reproduces the gating characteristics of the GluR channels.