Abstract
Purpose: :
On bipolar cells are connected to photoreceptors via a sign-inverting synapse. At this synapse, glutamate binds to mGluR6, a metabotropic receptor which couples to the closure of a cation-selective transduction channel. The molecular identity of both the receptor and the G protein are known, but the identity of transduction channel remains elusive. Here we investigate the physiological properties of the transduction channel using mouse rod bipolar cells (RBCs) as a model system.
Methods: :
Retinal slices from mice were prepared using standard techniques. Drugs were delivered to the dendrites of RBCs via either a puffer pipet or a fast flow apparatus. The mGluR6 agonist L-AP4 was added to the bath in all experiments. Transduction currents were generated by voltage-clamping RBCs at +40 mV and applying the mGluR antagonist LY341495 (LY).
Results: :
1) The pharmacological profile of the RBC transduction channel was found to be similar to the vanilloid receptor TRPV1: Transduction currents were as the current was blocked by ruthenium red, potentiated by 2-APB, and antagonized by TRPV1 antagonists SB366791 and capsazepine. 2) Application of the TRPV1 agonists capsaicin and anandamide elicited a current with properties that were similar to the transduction current. The transduction current as well as currents elicited by capsaicin and anandamide were characterized by a reversal potential of 0 mV, and prominent outward rectification. 3) Comparison of separate and simultaneous application of LY and capsaicin revealed mutual occlusion of the capsaicin-evoked and transduction current, further supporting the idea that transduction cascade and capsaicin operate on the same population of channels. 4) We also tested the possibility that TRPV1 might mediate the response to LY as well as to capsaicin and anandamide by recording from RBCs in the TRPV1-/- mice. The mGluR6 pathway appeared to be unperturbed in the TRPV1-/- mouse, as the responses to LY, capsaicin and anandamide were present.
Conclusions: :
Our results suggest that the postsynaptic channel in RBCs is a TRP channel. This is consistent with a recent report in appaloosa horses showing a mutation in the TRP channel TRPM1 results in the loss of the b wave. Further studies will be needed to determine whether TRPM1 and the native RBC channel have similar functional properties.
Keywords: bipolar cells • synapse • electrophysiology: non-clinical