Activation of metabotropic glutamate receptors modulates the voltage-gated sustained calcium current in a teleost horizontal cell

Activation of metabotropic glutamate receptors modulates thevoltage-gated sustained calcium current in a teleost horizontal cell. In the teleost retina, cone horizontal cells contain avoltage-activated sustained calcium current, which has been proposed tobe involved in visual processing. Recently, several studies havedemonstrated that modulation of voltage-gated channels can occurthrough activation of metabotropic glutamate receptors (mGluRs).Because glutamate is the excitatory neurotransmitter in the vertebrateretina, we have used whole cell electrophysiological techniques toexamine the effect of mGluR activation on the sustained voltage-gated calcium current found in isolated cone horizontal cells in the catfishretina. In pharmacological conditions that blocked voltage-gated sodiumand potassium channels, as well asN-methyl-D-aspartate (NMDA) and non-NMDAchannels, application of L-glutamate or1-aminocyclopentane-1,3-dicarboxylic acid(1S,3R-ACPD) to voltage-clamped conehorizontal cells acted to increase the amplitude of the calciumcurrent, expand the activation range of the calcium current by 10 mVinto the cell's physiological operating range, and shift the peakcalcium current by 5 mV. To identify and characterize the mGluRsubtypes found on catfish cone horizontal cells, agonists of group I,group II, or group III mGluRs were applied via perfusion. Group I andgroup III mGluR agonists mimicked the effect of L-glutamateor 1S,3R-ACPD, whereas group II mGluR agonistshad no effect on L-type calcium current activity. Inhibition studiesdemonstrated that group I mGluR antagonists significantly blocked themodulatory effect of the group I mGluR agonist,(S)-3,5-dihydroxyphenylglycine. Similar results were obtained when the group III mGluR agonist,L-2-amino-4-phosphonobutyric acid, was applied in thepresence of a group III mGluR antagonist. These results provideevidence for two groups of mGluR subtypes on catfish cone horizontalcells. Activation of these mGluRs is linked to modulation of thevoltage-gated sustained calcium current.