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) activate the metabotropic EAA receptors that are coupled to the hydrolysis of phosphoinositides (PI). Previous studies of hippocampal cross sections demonstrated that PI hydrolysis stimulated by these agonists can be inhibited by either L-aspartate-beta-hydroxamate (L-AbetaHA) or DL-2-amino-3-phosphonopropionate (DL-AP3). The goal of the present studies was to determine if all metabotropic EAA receptors are sensitive to L-AbetaHA and DL-AP3. Two approaches were used. In the first, using cerebellar cross sections, the effects of these agonists and inhibitors were examined. The EC50 values (the concentrations required to evoke half-maximal stimulation) of quisqualate, ibotenate, and trans-ACPD in cerebellum 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 muM for ibotenate, and 18.1 +/- 3.1 muM for trans-ACPD. All data were best fit to theoretical curves that had Hill slopes of 1. In the second approach, another EAA analogue, D-aspartate, was identified as an agonist that stimulates PI hydrolysis. The EC50 for PI hydrolysis stimulated by D-aspartate was 470 +/- 90 muM in hippocampus. Neither L-AbetaHA nor DL-AP3 blocked PI hydrolysis stimulated by D-aspartate in hippocampus. Furthermore, antagonists of ionotropic EAA receptors, antagonists 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 presence of three pharmacologically distinct metabotropic EAA receptor subtypes.