MULTIPLE SUBTYPES OF EXCITATORY AMINO-ACID RECEPTORS COUPLED TO THE HYDROLYSIS OF PHOSPHOINOSITIDES IN RAT-BRAIN

The excitatory amino acid (EAA) analogues quisqualate, ibotenate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) activat e the metabotropic EAA receptors that are coupled to the hydrolysis of phosphoinositides (PI). Previous studies of hippocampal cross section s demonstrated that PI hydrolysis stimulated by these agonists can be inhibited by either L-aspartate-beta-hydroxamate (L-AbetaHA) or DL-2-a mino-3-phosphonopropionate (DL-AP3). The goal of the present studies w as to determine if all metabotropic EAA receptors are sensitive to L-A betaHA and DL-AP3. Two approaches were used. In the first, using cereb ellar cross sections, the effects of these agonists and inhibitors wer e examined. The EC50 values (the concentrations required to evoke half -maximal stimulation) of quisqualate, ibotenate, and trans-ACPD in cer ebellum were similar to the EC50 values that we observed previously in hippocampus, but neither L-AbetaHA nor DL-AP3 blocked PI hydrolysis. The EC50 values were 0.65 +/- 0.17 muM for quisqualate, 12.8 +/- 2.5 m uM for ibotenate, and 18.1 +/- 3.1 muM for trans-ACPD. All data were b est fit to theoretical curves that had Hill slopes of 1. In the second approach, another EAA analogue, D-aspartate, was identified as an ago nist that stimulates PI hydrolysis. The EC50 for PI hydrolysis stimula ted by D-aspartate was 470 +/- 90 muM in hippocampus. Neither L-AbetaH A nor DL-AP3 blocked PI hydrolysis stimulated by D-aspartate in hippoc ampus. Furthermore, antagonists of ionotropic EAA receptors, antagonis ts of other receptor systems coupled to PI hydrolysis, and inhibitors of the Na+-dependent L-glutamate transport process also did not block PI hydrolysis stimulated by D-aspartate. These data support the presen ce of three pharmacologically distinct metabotropic EAA receptor subty pes.