A comparison of non-NMDA receptor channels in type-2 astrocytes and granule cells from rat cerebellum

1. Patch-clamp recording methods have been used to compare the pharmacological properties and single-channel characteristics of non-NMDA receptor channels in cerebellar type-2 astrocytes and granule cells. 2. In type-2 astrocytes whole-cell concentration-response curves for glutamate, quisqualate, AMPA and kainate gave EC50 values of 5.8, 3.8, 7.6 and 160 microM and Hill slopes of 1.65, 1.18, 1.64 and 1.65, respectively, resembling estimates for granule cell receptors. 3. The non-NMDA receptor antagonists CNQX and diCl-HQC (see Methods) inhibited whole-cell kainate currents in both cell types. The IC50 for CNQX antagonism of the kainate response was 536 nM in type-2 astrocytes, and 500 nM in granule cells. The IC50 for diCl-HQC was 3.5 microM in astrocytes and 3.7 microM in granule cells. 4. CNQX acted as a competitive antagonist of whole-cell kainate responses in type-2 astrocytes and granule cells giving Schild plots with a slope near 1. The equilibrium constant, K, for CNQX binding was 524 nM in astrocytes and 489 nM in granule cells. 5. Quisqualate and AMPA responses showed rapid desensitization in type-2 astrocytes with a ratio of steady-state to peak response of 0.09. Concanavalin A reduced this desensitization. 6. Non-NMDA channels in type-2 astrocytes and granule cells showed a low permeability to Ca2+ ions with a reversal potential, for kainate-activated whole-cell currents in isotonic Ca2+, of approximately -25 mV for astrocytes and -45 mV for granule cells. 7. Outside-out patches from type-2 astrocytes exhibited a range of single-channel conductances that were superficially similar to the glutamate-activated conductances in granule cells. However, the type-2 astrocytes were devoid of NMDA receptors, hence all of these conductances originated from non-NMDA channels. Their slope conductances were approximately 11, 21, 32, 42 and 52 pS. Amplitudes were verified with mean low-variance plots and single-channel current-voltage curves, which were linear. 8. There was also evidence of lower conductance kainate-activated channels in astrocyte patches. From noise analysis their estimated mean conductance was 1.9 pS, as described for the 'low-conductance' type kainate responses in cerebellar neurones. 9. Apparent open times, shut times and burst lengths of AMPA-activated (3-10 microM) channels were examined in patches from type-2 astrocytes, and kinetic properties of the 40 and 50 pS levels were compared with the lower levels. 10. Our results indicate some marked pharmacological similarities between non-NMDA receptor channels in type-2 astrocytes and granule cells.(ABSTRACT TRUNCATED AT 400 WORDS)