The concentration of synaptically released glutamate outside of the climbing fiber-Purkinje cell synaptic cleft

AMPA receptors and glutamate transporters expressed by cerebellar Bergmann glial cells are activated by neurotransmitter released from climbing fibers (Bergles et al., 1997). Based on anatomical evidence, this is most likely the result of glutamate diffusing out of the climbing fiber-Purkinje cell s ynaptic clefts (Palay and Chan-Palay, 1974). We used the change in the EC50 of the Bergmann glia AMPA receptors produced by cyclothiazide (CTZ) to est imate the concentration of glutamate reached at the glial membrane. The dec rease of the EC50 gives rise to a concentration-dependent potentiation of t he AMPA receptor-mediated responses (Patneau et al., 1993). By comparing th e increase in amplitude of the AMPA receptor response in the Bergmann glia (840 +/- 240%; n = 8) with the shift in the glutamate dose-response curve m easured in excised patches (EC50, 1810 mu M in control vs 304 mu M in CTZ), we estimate that the extrasynaptic transmitter concentration reaches 160-1 90 mu M. This contrasts with the concentration in the synaptic cleft, thoug ht to rapidly rise above 1 mM, but is stilt high enough to activate glutama te receptors. These results indicate that the sphere of influence of synapt ically released glutamate can extend beyond the synaptic cleft.