Characteristics of AMPA receptor-mediated responses of cultured cortical and spinal cord neurones and their correlation to the expression of glutamate receptor subunits, GluR1-4

1 Electrophysiological recordings have been used to characterize responses mediated by AMPA receptors expressed by cultured rat cortical and spinal co rd neurones. The EC50 values for AMPA were 17 and 11 muM, respectively. 2 Responses of cortical neurones to AMPA were inhibited competitively by NB QX (pK(i) = 6.6). Lower concentrations of NBQX (less than or equal to 1 muM ) also potentiated the plateau responses of spinal cord neurones to AMPA, w hich could be attributed to a-depression of desensitization to AMPA. 3 GYKI 52466 inhibited responses of spinal cord neurones to AMPA to about t wice the extent of responses of cortical neurones. 4 Blockade of AMPA receptor desensitization by cyclothiazide (CTZ) potentia ted responses of spinal cord neurones (6.8 fold) significantly more than re sponses of cortical neurones (4.8 fold). Responses of cortical neurones to KA were potentiated 3.5 fold by CTZ, while responses of spinal cord neurone s were unaffected. 5 Ultra-fast applications of AMPA to outside-out patches showed responses o f spinal cord neurones desensitized by 97.5% and exhibit marked inward rect ification, whereas cortical neurones desensitized by 91% and exhibited slig ht outward rectification. The time constants of deactivation and desensitiz ation were about twice as fast in spinal cord than cortical neurones. 6 In cortical neurones, single-cell RT-PCR showed GluR2 and GluR1 accounted for 91% of all subunits and were expressed together in 67% of neurones, pr edominantly as the flip variants (78%). GluR2 was detected alone in 24% of neurones. GluR3 and GluR4 were present in only 14 and 29% of neurones, resp ectively. For spinal cord neurones, GluR4, was detected in 81% of neurones, whereas predominantly flop versions of GluR1, 2 and 3 were detected in 38, 13 and 13% of neurones, respectively. These expression patterns are relate d to the respective pharmacological and mechanistic properties.