Excitatory amino acid receptor agonists stimulate membrane inositol phospholipid hydrolysis and increase cytoplasmic free Ca2+ in primary cultures of retinal neurons.
A variety of neurotransmitters are believed to elicit effects through receptor-stimulated inositol phospholipid metabolism. It appears that most major types of retinal neurons receive a direct glutamatergic input. The aim of the present studies was to characterize excitatory amino acid (EAA) receptor-mediated breakdown of inositol phospholipids and changes in Ca2+ homeostasis in primary avian retinal cell cultures. Cell monolayers, prepared from 8-day-old chick embryo neural retina, were labelled with [3H]inositol for 48 h, and used after 7 days in vitro. Kainic acid stimulated the accumulation of inositol phosphates in a time- and dose-dependent manner (ED50 = 30 microM). The EAA receptor agonists glutamate, N-methyl-D-aspartate (NMDA), ibotenate and quisqualate were all active, with the rank order: glutamate greater than kainate greater than NMDA much greater than ibotenate approximately quisqualate. External Ca2+ was required for these effects. Agonist actions were inhibited by type-specific antagonists, and also Mg2+ in the case of glutamate and NMDA. Glutamate, NMDA and kainate also elevated cytosolic free Ca2+ in individual retinal cells loaded with the Ca2(+)-sensitive dye Fura-2, as assessed by digital fluorescence ratio imaging microscopy. The agonist-induced increases in [Ca2+]i were largely dependent on extracellular Ca2+, independent of membrane depolarization and were blocked by Mg2+ for glutamate and NMDA. These results demonstrate that vertebrate retinal cells possess EAA receptors coupled to intracellular signal transduction pathways.
Milani
D
D
Fidia Research Laboratories, Department of CNS Research, Terme (PD), Italy.
Facci
L
L
Buso
M
M
Toffano
G
G
Leon
A
A
Skaper
S D
SD
D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D006152:Cyclic GMP / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D005971:Glutamates / Q000494:pharmacology; D018698:Glutamic Acid; D006868:Hydrolysis; D007608:Kainic Acid / Q000494:pharmacology; D016202:N-Methylaspartate / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D010716:Phosphatidylinositols / Q000378:metabolism*; D017459:Receptors, Amino Acid; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism*; D012160:Retina / Q000187:drug effects / Q000378:metabolism*
D000596:Amino Acids; D005971:Glutamates; D010716:Phosphatidylinositols; D017459:Receptors, Amino Acid; D011956:Receptors, Cell Surface; D018698:Glutamic Acid; D016202:N-Methylaspartate; D006152:Cyclic GMP; D007608:Kainic Acid; D002118:Calcium