CHARACTERIZATION OF L-2-AMINO-4-PHOSPHONOBUTANOATE ACTION FOLLOWING SENSITIZATION BY QUISQUALATE IN RAT HIPPOCAMPAL SLICE CULTURES

An excitatory action of L-2-amino-4-phosphonobutanoate (L-AP4), a glutamate analogue, is observed following pre-exposure of tissue to quisqualate. We have studied the mechanism of sensitization Of L-AP4 responses by quisqualate in voltage-clamped CA3 pyramidal cells in rat hippocampal slice cultures in the presence of tetrodotoxin. Prior to quisqualate addition, CA3 cells did not respond to L-AP4 (50 -1000-mu-M). Following brief application of quisqualate (500 nM for 30 S), L-AP4 (50 - 200-mu-M) induced a complex excitatory response which could be obtained for > 1 h. L-AP4 caused an ionotropic inward current associated with a conductance increase. This response was in part sensitive to 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and in part sensitive to D-2-amino-5-phosphonovalerate (D-AP5) and Mg2+ ions. At depolarizing potentials, in the presence of CNQX and D-AP5, L-AP4 caused excitation by depressing K+ currents, mimicking the metabotropic action of glutamate. This indicates that the action Of L-AP4 is mediated by three different receptor types: N-methyl-D-aspartate (NMDA) receptors, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptors, and glutamatergic metabotropic receptors. The L-AP4 response persisted in solutions containing low Ca2+ and high Mg2+ concentrations or 100 - 200-mu-M Cd2+, suggesting that it is independent of extracellular Ca2+. We were unable to identify any substance other than quisqualate capable of sensitizing the L-AP4 action. This effect also occurred when quisqualate was applied in Ca2+-free solution or in solutions containing low concentrations of Na+ or Cl-. Sensitization Of L-AP4 responses by quisqualate was not observed in acutely dissociated pyramidal cells recorded by means of the whole-cell recording mode, although ionotropic quisqualate responses were present. Sensitization was readily reversed by short applications of the endogenous excitatory amino acids glutamate, aspartate and homocysteate at concentrations of 10 - 100-mu-M. Our data are consistent with the hypothesis that the excitatory action Of L-AP4 results from a Ca2+-independent release of endogenous excitatory amino acids from some presynaptic neuronal or glial site.