AMPA receptor is involved in transmission of cone signal to ON bipolar cells in carp retina

The present work focuses on characterization of glutamate receptor subtypes mediating cone signal transmission to ON bipolar cells (BCs) in the carp retina, using intracellular recording techniques. Glutamate (5 mM) hyperpolarized cone-dominant ON BCs, which was associated with a suppression of light responses, whereas Co²⁺ (1 mM) depolarized these cells and suppressed their light responses. On the other hand, α-amino-3-hydroxy-5- methyl-4-isoxazolepropionic acid (AMPA) of 20 μM caused a membrane depolarization and blocked depolarizing light responses, L-2-amino-4- phosphonobutryic acid (L-AP4) was without effect. The effects of AMPA were reversed by coapplication of GYKI 52466, an AMPA receptor selective non-competitive antagonist, but persisted in the presence of picrotoxin and strychnine. For rod-dominant ON BCs, both L-AP4 and AMPA reversibly blocked depolarizing light responses, but with membrane potential changes of opposite polarities (hyperpolarization for L-AP4 and depolarization for AMPA). In the inner retina, AMPA depolarized transient ON-OFF amacrine cells and blocked both ON and OFF cone-driven depolarizing responses, but L-AP4 did not. These results suggest that AMPA receptors, but not L-AP4 receptors, are involved in synaptic transmission of cone signal to ON bipolar cells in carp retina.