INVOLVEMENT OF METABOTROPIC AND IONOTROPIC GLUTAMATE RECEPTORS IN INOSITOL POLYPHOSPHATE FORMATION IN CARP RETINAL SLICES

The contribution of ionotropic and metabotropic glutamate receptors to inositol polyphosphate accumulation in carp retinal slices was invest igated using myo-[2-H-3]inositol prelabelling. in the presence of the glutamate agonists quisqualate, (RS)-alpha-amino-3-hydroxy-5-methyl-4- isoxazole propionic acid (AMPA) and trans-(+/-)-1-amino-1,3-cyclopenta ne-dicarboxylic acid (t-ACPD), formation of [H-3]inositol phosphate wa s significantly increased in a dose-dependent manner, with EC(50), val ues of 350 nM, 1.5 mu M and 10 mu M respectively. The complete AMPA-in duced response and a large component of the quisqualate-induced respon se were inhibited in a competitive manner when the ionotropic antagoni st 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was present. Furthermor e, the remaining level of quisqualate-induced [H-3]inositol phosphate formation closely matched that produced by ACPD alone, and coincubatio n of AMPA and ACPD showed additive effects, suggesting that the quisqu alate-induced response resulted from coactivation of metabotropic and ionotropic glutamate receptors. The ionotropic component was partially reduced in the presence of cobalt, suggesting indirect effects result ing from synaptic interactions. We could exclude indirect effects thro ugh depolarization-induced release of other neurotransmitters. Only se rotonin (EC(50) 1 mu M) and carbachol (at a concentration of 1 mM) sti mulated [H-3]inositol phosphate formation, but their antagonists did n ot affect the quisqualate response and coactivation with quisqualate a nd serotonin or carbachol resulted in additive effects. The ionotropic component was completely suppressed when Ca2+ was omitted from the me dium and cobalt was present. This makes it likely that the ionotropic component resulted from Ca2+ entry through AMPA-gated channels and sub sequent Ca2+-dependent activation of phospholipase C.