DIFFERENTIAL MODULATION OF AMPA RECEPTOR-MEDIATED CURRENTS BY EVANS BLUE IN POSTNATAL RAT HIPPOCAMPAL-NEURONS

1 The modulation of non-N-methyl-D-aspartate (NMDA) receptor-mediated whole cell currents and of glutamatergic synaptic transmission by puri fied Evans Blue (EB) was investigated in rat cultured postnatal hippoc ampal neuroses by use of patch clamp recordings and a fast drug applic ation system. 2 Three different groups of neurones could be distinguis hed with respect to the type of modulation obtained with 10 mu M EB: E B was either a predominant inhibitor of desensitization (13% of the ne urones), a predominant inhibitor of current amplitudes (42%) or a mixe d inhibitor of both properties (45%). Both effects were not use-depend ent and reached maximal levels after 30 s of pre-equilibration with th e 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 co efficient: 1.3) for the inhibition of desensitization, and an IC50 val ue 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 act ivated 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 modifi ed 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 a mplitudes of non-NMDA receptor mediated autaptic currents to the same extent (to 39+/-19% of control) as observed for glutamate activated wh ole cell currents (to 41+/-17% and 56+/-20%). However, the decay of th e autaptic responses remained uninfluenced upon EB application, indica ting that either receptor desensitization does not dominate the time c ourse of the synaptic response or that the non-NMDA receptors sensitiv e to modulation of desensitization by EB are not present in the postsy naptic membrane. 7 In conclusion, EB differentially modulates alpha-am ino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor gati ng in different subsets of neurones. Upon identification of the cellul ar determinants for the differential modulation (e.g. AMPA receptor su bunit composition) EB could become a useful tool to investigate recept or subtypes during electrophysiological recordings.