CAFFEINE AND RELATED-COMPOUNDS BLOCK INHIBITORY AMINO ACID-GATED CL- CURRENTS IN FRESHLY DISSOCIATED RAT HIPPOCAMPAL-NEURONS

1 The effects of caffeine and related compounds on responses mediated by inhibitory amino acids were investigated in freshly dissociated rat hippocampal pyramidal neurones by conventional and nystatin perforate d patch-clamp techniques. 2 Glycine and gamma-aminobutyric acid (GABA) evoked Cl- currents in hippocampal neurones. The half-maximum effecti ve concentrations (EC50) of glycine and GABA were 8.5 x 10(-5) and 5 x 10(-6) M, respectively. 3 Caffeine reversibly inhibited both 10(-4) M glycine- and 10(-5) M GABA-induced Cl- currents in a concentration-de pendent manner. The half-maximum inhibitory concentrations (IC50) of c affeine were 4.5 x 10(-4) M for the glycine response and 3.6 x 10(-3) M for the GABA response. 4 Caffeine shifted the concentration-response curve of I(Gly) to the right without affecting the maximum response. 5 The inhibitory action of caffeine did not show voltage-dependency. 6 The blocking action of caffeine was not affected by intracellular per fusion with 5 mM BAPTA or by pretreatment with the protein kinase A in hibitor, H-8. This excludes the participation of Ca2+ or cyclic AMP in the inhibitory action of caffeine. 7 Caffeine failed to inhibit the a ugmentations of aspartate- and N-methyl-D-aspartate (NMDA) -gated curr ent by glycine, suggesting that caffeine has no effect on the alloster ic glycine binding site on the NMDA receptor. 8 The inhibitory effects of some xanthine derivatives on I(Gly) were compared. The inhibitory potency of those compounds on I(Gly) was in the order of pentoxifyllin e > theophylline greater-than-or-equal-to caffeine > paraxanthine > IB MX greater-than-or-equal-to theobromine > dyphylline. Xanthine had no effect. 9 The results indicate that methylxanthines including caffeine may act directly on the glycine receptor Cl- channel complex in rat h ippocampal pyramidal neurones. The blockade of the inhibitory amino ac id response by methylxanthines may be involved in the excitatory side effects of methylxanthines in the mammalian central nervous system.