1. Group III metabotropic glutamate receptors and exocytosed protons inhibit L-type calcium currents in cones but not in rods.
- Author
-
Hosoi N, Arai I, and Tachibana M
- Subjects
- Animals, Calcium metabolism, Drug Interactions, Electric Stimulation methods, Excitatory Amino Acid Agonists pharmacology, Excitatory Amino Acid Antagonists pharmacology, Excitatory Postsynaptic Potentials drug effects, Excitatory Postsynaptic Potentials physiology, Excitatory Postsynaptic Potentials radiation effects, Glutamic Acid pharmacology, Guanosine Diphosphate analogs & derivatives, Guanosine Diphosphate pharmacology, HEPES pharmacology, In Vitro Techniques, Light, Membrane Potentials drug effects, Membrane Potentials physiology, Membrane Potentials radiation effects, Neural Inhibition drug effects, Neural Inhibition radiation effects, Neurons drug effects, Patch-Clamp Techniques methods, Propionates pharmacology, Quinoxalines pharmacology, Retinal Cone Photoreceptor Cells metabolism, Retinal Rod Photoreceptor Cells drug effects, Retinal Rod Photoreceptor Cells metabolism, Salamandridae, Synaptic Transmission drug effects, Synaptic Transmission physiology, Synaptic Transmission radiation effects, Thionucleotides pharmacology, Vision, Ocular drug effects, Vision, Ocular physiology, Vision, Ocular radiation effects, Visual Pathways drug effects, Calcium Channels, L-Type physiology, Neural Inhibition physiology, Protons, Receptors, AMPA physiology, Retina cytology, Retinal Cone Photoreceptor Cells drug effects
- Abstract
Light responses of photoreceptors (rods and cones) are transmitted to the second-order neurons (bipolar cells and horizontal cells) via glutamatergic synapses located in the outer plexiform layer of the retina. Although it has been well established that postsynaptic group III metabotropic glutamate receptors (mGluRs) of ON bipolar cells contribute to generating the ON signal, presynaptic roles of group III mGluRs remain to be elucidated at this synaptic connection. We addressed this issue by applying the slice patch-clamp technique to the newt retina. OFF bipolar cells and horizontal cells generate a steady inward current in the dark and a transient inward current at light offset, both of which are mediated via postsynaptic non-NMDA receptors. A group III mGluR-specific agonist, L-2-amino-4-phosphonobutyric acid (L-AP-4), inhibited both the steady and off-transient inward currents but did not affect the glutamate-induced current in these postsynaptic neurons. L-AP-4 inhibited the presynaptic L-type calcium current (ICa) in cones by shifting the voltage dependence of activation to more positive membrane potentials. The inhibition of ICa was most prominent around the physiological range of cone membrane potentials. In contrast, L-AP-4 did not affect L-type ICa in rods. Paired recordings from photoreceptors and the synaptically connected second-order neurons confirmed that L-AP-4 inhibited both ICa and glutamate release in cones but not in rods. Furthermore, we found that exocytosed protons also inhibited ICa in cones but not in rods. Selective modulation of ICa in cones may help broaden the dynamic range of synaptic transfer by controlling the amount of transmitter release from cones.
- Published
- 2005
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