Involvement of Metabotropic and lonotropic 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 investigated using myo-[2-³H]inositol prelabelling. In the presence of the glutamate agonists quisqualate, (RS)-α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and trans-(±)-1-amino-1, 3-cyclopentane-dicarboxylic acid (t-ACPD), formation of [³H]inositol phosphate was significantly increased in a dose-dependent manner, with EC₅₀ values of 350 nM, 1.5 μM and 10 μM respectively. The complete AMPA-induced response and a large component of the quisqualate-induced response were inhibited in a competitive manner when the ionotropic antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) was present. Furthermore, the remaining level of quisqualate-induced [³H]inositol phosphate formation closely matched that produced by ACPD alone, and coincubation of AMPA and ACPD showed additive effects, suggesting that the quisqualate-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 resulting from synaptic interactions. We could exclude indirect effects through depolarization-induced release of other neurotransmitters. Only serotonin (EC₅₀ 1 μM) and carbachol (at a concentration of 1 mM) stimulated [³H]inositol phosphate formation, but their antagonists did not affect the quisqualate response and coactivation with quisqualate and serotonin or carbachol resulted in additive effects. The ionotropic component was completely suppressed when Ca²⁺ was omitted from the medium and cobalt was present. This makes it likely that the ionotropic component resulted from Ca²⁺ entry through AMPA-gated channels and subsequent Ca²⁺-dependent activation of phospholipase C.