Differential modulation of AMPA receptor mediated currents by evans blue in postnatal rat hippocampal neurones

1 The modulation of non-N-methyl-D-aspartate (NMDA) receptor-mediated whole cell currents and of glutamatergic synaptic transmission by purified Evans Blue (EB) was investigated in rat cultured postnatal hippocampal neuroses by use of patch clamp recordings and a fast drug application system. 2 Three different groups of neurones could be distinguished with respect to the type of modulation obtained with 10 mu M EB: EB was either a predominant inhibitor of desensitization (13% of the neurones), a predominant inhibitor of current amplitudes (42%) or a mixed inhibitor of both properties (45%). Both effects were not use-dependent and reached maximal levels after 30 s of pre-equilibration with the diazo dye. 3 Dose-response curves obtained from glutamate activated whole cell currents yielded an IC50 value for EB of 13.3 mu M (Hill coefficient: 1.3) for the inhibition of desensitization, and an IC50 value of 10.7 mu M (Hill coefficient: 1.2) for the inhibition of current amplitudes. 4 Chicago acid SS (100 mu M) which is one of the synthesis precursors of EB had no effect on current amplitudes of glutamate activated whole cell currents but was a weak inhibitor of desensitization in all hippocampal neurones investigated, irrespective of the type of modulation obtained with EB in the same neurone. 5 Oxidatively modified EB (the so-called VIMP (10 mu M)) had no effect on the kinetics but was a partial inhibitor of glutamate-activated whole cell currents in ail hippocampal neurones investigated. 6 EB (10 mu M) inhibited the amplitudes of non-NMDA receptor mediated autaptic currents to the same extent (to 39+/-19% of control) as observed for glutamate activated whole cell currents (to 41+/-17% and 56+/-20%). However, the decay of the autaptic responses remained uninfluenced upon EB application, indicating that either receptor desensitization does not dominate the time course of the synaptic response or that the non-NMDA receptors sensitive to modulation of desensitization by EB are not present in the postsynaptic membrane. 7 In conclusion, EB differentially modulates alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor gating in different subsets of neurones. Upon identification of the cellular determinants for the differential modulation (e.g. AMPA receptor subunit composition) EB could become a useful tool to investigate receptor subtypes during electrophysiological recordings.