cGMP-Dependent Kinase Regulates Response Sensitivity of the Mouse On Bipolar Cell

Abstract

The visual system can adjust its sensitivity over a wide range of light intensities. Photoreceptors account for some of this adjustment, but there is evidence that postreceptoral processes also exist. To investigate the latter, we pharmacologically mimicked the effects of light stimulation on mouse On bipolar cells, thus avoiding confounding effects of receptoral mechanisms. Here, we report that cGMP selectively enhances responses to dim, but not bright, stimuli through a purely postsynaptic mechanism. This action of cGMP was completely blocked by inhibitors of cGMP-dependent kinase. We propose that cGMP-dependent kinase decreases coupling of the On bipolar cell glutamate receptor to the downstream cascade, thus amplifying small decreases in photoreceptor transmitter levels that would otherwise go undetected by the visual system.

Regulation of ON bipolar cell activity

“…Working in the mammalian retina, our lab used pressure ejection of the mGluR6 antagonist (RS)-a-Cyclopropyl-4-phosphonophenylglycine (CPPG) while flooding the retina with L-AP4 to mimic light flashes, varying the simulated flash intensity by changing the duration of CPPG application. As reported in the dogfish retina, addition of cGMP also caused a leftward shift in the pharmacologically simulated intensity-response function without significantly changing the maximum response (Snellman and Nawy, 2004). Similarly, puffs of CPPG that were too brief to evoke a measurable response in the absence of cGMP could be reliably detected after cGMP application.…”

“…Similarly, puffs of CPPG that were too brief to evoke a measurable response in the absence of cGMP could be reliably detected after cGMP application. Similar results were obtained using activators of guanylate cyclase, as well as inhibitors of cGMP phosphodiesterases (PDE), indicating that rod bipolar cells can generate cGMP at concentration that is sufficient to induce potentiation (Snellman and Nawy, 2004). These studies suggest that cGMP may play a role in setting the threshold for the postsynaptic detection of photons that are absorbed within the rod network.…”

“…Modulation of the activated component of the current by cGMP was, at that time, interpreted as a recruitment of closed channels, and led to the hypothesis that cGMP might play a role in channel gating. As noted in the previous section, cGMP has no effect on the membrane current or voltage of ON bipolar cells when the cation current is completely suppressed in the presence of a saturating concentration of mGluR6 agonist (Snellman and Nawy, 2004). Thus, cGMP appears to act as a true modulator of the current (but see Shiells and Falk, 2002).…”

“…First, dialysis of ON bipolar cells with poorly hydrolyzed analogs of cGMP, does not inhibit or alter the kinetics of the glutamate response (Nawy, 1999). Furthermore, when exogenous glutamate is present in the bath solution to ensure that all the cation channels are closed, cGMP application has no effect on holding current, indicating that it cannot gate the opening of channels on its own (Snellman and Nawy, 2004). These findings call into question the notion that hydrolysis of cGMP is required in order for the cation channel to open.…”

G-protein–mediated inhibition of the Trp channel TRPM1 requires the Gβγ dimer

“…A high concentration of the mGluR6 antagonist LY34149 (100 μM) was then "puffed" onto the dendrites of an ON bipolar cell, resulting in a transient displacement of L-AP4 from mGluR6, and the opening of TRPM1 channels. At physiological membrane potentials, the opening of TRPM1 produces an inward current, but we typically voltage clamp cells at positive voltages, as this tends to reduce run down of the response (18,19), and so the current generated by the opening of TRPM1 is outward. An example of the TRPM1 current generated using this approach is shown in Fig.…”