Metabotropic and ionotropic glutamate receptors regulate calcium channel currents in salamander retinal ganglion cells

Glutamate suppressed high-voltage-activated barium currents (IBa,HVA) in tiger salamander retinal ganglion cells. Both ionotropic (iGluR) and metabotropic (mGluR) receptors contributed to this calcium channel inhibition. Trans-ACPD (1-aminocyclopentane-trans-1S,3R-dicarboxylic acid), a broad-spectrum metabotropic glutamate receptor agonist, suppressed a dihydropyridine-sensitive barium current. Kainate, an ionotropic glutamate receptor agonist, reduced an ω-conotoxin GVIA-sensitive current. The relative effectiveness of selective agonists indicated that the predominant metabotropic receptor was the L-2-amino-4-phosphonobutyrate (l-AP4)-sensitive, group III receptor. This receptor reversed the action of forskolin, but this was not responsible for calcium channel suppression. l-AP4 raised internal calcium concentration. Antagonists of phospholipase C, inositol trisphosphate (IP3) receptors and ryanodine receptors inhibited the action of metabotropic agonists, indicating that group III receptor transduction was linked to this pathway. The action of kainate was partially suppressed by BAPTA, by calmodulin antagonists and by blockers of calmodulin-dependent phosphatase. Suppression by kainate of the calcium channel current was more rapid when calcium was the charge carrier, instead of barium. The results indicate that calcium influx through kainate-sensitive glutamate receptors can activate calmodulin, which stimulates phosphatases that may directly suppress voltage-sensitive calcium channels. Thus, ionotropic and metabotropic glutamate receptors inhibit distinct calcium channels. They could act synergistically, since both increase internal calcium. These pathways provide negative feedback that can reduce calcium influx when ganglion cells are depolarized. Many neurotransmitters suppress voltage-activated calcium channels, particularly the high-voltage-activated L- and N-types (Hille, 1994). One function of this inhibition is to autoregulate transmitter release by presynaptic negative feedback. A conventional model is that transmitter release from the presynaptic terminal results in activation of postsynaptic ionotropic receptors and concomitant stimulation of presynaptic metabotropic receptors. However, there are many examples of postsynaptic metabotropic receptors. In fact, the first description of a metabotropic glutamate receptor was the retinal ‘on’ bipolar postsynaptic receptor (Slaughter & Miller, 1981; Nawy & Jahr, 1990; Shiells & Falk, 1990). There is also evidence that ionotropic glutamate receptors may suppress voltage-gated calcium channels (Nistri & Cherubini, 1991). Clearly, narrowly defined roles do not adequately detail the physiology of ionotropic and metabotropic glutamate receptors. We chose to examine the glutamatergic regulation of calcium currents in amphibian retinal ganglion cells. These neurons possess a plethora of glutamate receptors, including the AMPA/kainate and NMDA ionotropic receptors (Slaughter & Miller, 1983) and at least one form of metabotropic receptor (Akopian & Witkovsky, 1996). Additionally, these neurons are thought to be purely postsynaptic in the retina and therefore ionotropic and metabotropic receptors may function in a unique manner to control postsynaptic integration rather than regulation of transmitter release. Recordings from ganglion cells revealed sustained, high-voltage-activated calcium channel currents. These currents were suppressed by both ionotropic and metabotropic glutamate agonists. Ganglion cells possessed all three groups of metabotropic receptor, although group III agonists produced the largest effect on the calcium channel. The metabotropic receptors stimulated calcium release from internal stores, acting through both inositol trisphosphate (IP3) and ryanodine receptors, leading to suppression of L-type calcium channels. The kainate-mediated inhibition of N-type calcium channels probably involves two mechanisms, one mediated by calcium-calmodulin and one by phosphatases.