Activation of Group Ill Metabotropic Glutamate Receptor Suppresses the Membrane Currents in Newt Photoreceptors
- 145 Downloads
It is well established that ON-type and OFF-type bipolar cells receive glutamatergic inputs from photoreceptors via a subtype of group III metabotropic glutamate receptors (mGluRs), mGluR6, and ionotropic glutamate receptors (non-NMDA receptors), respectively. However, it has been reported recently that a group III mGluR selective agonist L-2-amino-4-phosphonobutyrate (L-AP4) not only suppresses the ON responses but also affects the OFF responses of ganglion cells, and that the group III mGluR (mGluR8) is expressed at the terminal of rat photoreceptors. In the newt retina, we confirmed that L-AP4 not only abolished the ON responses but also reduced the OFF responses of ON-OFF ganglion cells. Furthermore, L-AP4 reduced the amplitude of the light-evoked currents but not the glutamate-induced current in OFF bipolar cells. These data suggest that activation of the group III mGluR by L-AP4 may decrease the amount of glutamate release from photoreceptors. In the present study we investigated how L-AP4 affects the glutamate release from photoreceptors. Using a newt retinal slice preparation, we measured the membrane currents of photoreceptors, namely, the calcium current (ICa) and the hyperpolarization-activated current under the whole-cell voltage clamp. Application of L-AP4 reduced the L-type ICa in cones but not in rods. On the other hand, the current activated by membrane hyper-polarization was suppressed by L-AP4 in both cones and rods. This current was identified as Ih since the current was blocked by external application of Cs+ or ZD7288, a specific blocker of Ih, but not by Ba2+.