Modulation of photoreceptor signalling
Sensory systems continually adjust their sensitivity to transmit stimuli under a wide variety of natural conditions. For the visual system, this process starts in the retina where neurons adapt to changes in visual scene contrast and ambient light level. The bipolar cells (BCs) represent the sole direct excitatory connection between photoreceptors and ganglion cells. However, specific role of neuropeptides mediating calcium signalling within the BC microcircuits is still poorly understood. The EU-funded 'Fast and slow endocytosis at the synapse' (FSES) Marie Curie Fellowship aimed to study the impact of the neuromodulatory regulation at the level of bipolar terminals (BPs) of BCs. Most of the experiments were performed using transgenic zebrafish. Through pharmacological manipulations, researchers observed that disinhibition of calcium signalling at BPs increases response to visual stimulus such as luminance and frequency. The enhancement of the synaptic calcium responses was clear across the activation BPs, but not through the inhibiting pathway. An increase in the spike rate could amplify visual signal at BPs and increase the sensitivity to luminance and frequency. Interestingly, inhibiting terminals presented a faster and transient response to light offset and a shift in the frequency with no changes in the spike rate. Luminance and frequency modulations were eliminated when pharmacological manipulations inhibited depolarisation of activating terminals at light on. This finding suggests that inhibiting terminals are being modulated by the increased excitability at the activating pathway. Overall, the project demonstrated that changes in the intrinsic properties of BPs through disinhibition of calcium signalling can alter the sensitivity of the inner retina.
