Nonlinearity and noise at the rod-rod bipolar cell synapse

Regional variation in the organization and connectivity of the first synapse in the primate night vision pathway

Sensitivity of primate daylight vision varies across the visual field. This is attributed to regional variations in cone photoreceptor density and synaptic connectivity of the underlying circuitry. In contrast, we have limited understanding of how synapse organization of the primate night vision pathway changes across space. Using serial electron microscopy, we reconstructed the first synapse of the night vision pathway between rod photoreceptors and second-order neurons, at multiple locations from the central part of the primate retina, fovea, to the periphery. We find that most facets of the rod synapse connectivity vary across retinal regions. However, rod synaptic divergence and convergence patterns do not change in the same manner across locations. Moreover, patterns of rod synapse organization are tightly correlated with photoreceptor density. Such regional heterogeneities revise the connectivity diagram of the primate rod synapse which will shape synapse function and sensitivity of the night vision pathway across visual space.

Properties of multivesicular release from mouse rod photoreceptors support transmission of single-photon responses

Vision under starlight requires rod photoreceptors to transduce and transmit single-photon responses to the visual system. Small single-photon voltage changes must therefore cause detectable reductions in glutamate release. We found that rods achieve this by employing mechanisms that enhance release regularity and its sensitivity to small voltage changes. At the resting membrane potential in darkness, mouse rods exhibit coordinated and regularly timed multivesicular release events, each consisting of ~17 vesicles and occurring two to three times more regularly than predicted by Poisson statistics. Hyperpolarizing rods to mimic the voltage change produced by a single photon abruptly reduced the probability of multivesicular release nearly to zero with a rebound increase at stimulus offset. Simulations of these release dynamics indicate that this regularly timed, multivesicular release promotes transmission of single-photon responses to post-synaptic rod-bipolar cells. Furthermore, the mechanism is efficient, requiring lower overall release rates than uniquantal release governed by Poisson statistics.

Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear

Calcium influx through voltage-gated Ca (Ca_V) channels is the first step in synaptic transmission. This review concerns Ca_V channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the Ca_V channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of Ca_V channels at presynaptic, ribbon-type active zones, because the spatial relationship between Ca_V channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, Ca_V1.3, and Ca_V1.4 channels or their protein modulatory elements.

Elevated cAMP improves signal-to-noise ratio in amphibian rod photoreceptors

Vertebrate photoreceptors need to distinguish light signals from background noise to convey visual information to downstream bipolar cells. By affecting both signal and noise, Astakhova et al. find that increases in intracellular cAMP can improve the signal-to-noise ratio by twofold.

The absolute sensitivity of vertebrate retinas is set by a background noise, called dark noise, which originates from several different cell types and is generated by different molecular mechanisms. The major share of dark noise is produced by photoreceptors and consists of two components, discrete and continuous. Discrete noise is generated by spontaneous thermal activations of visual pigment. These events are undistinguishable from real single-photon responses (SPRs) and might be considered an equivalent of the signal. Continuous noise is produced by spontaneous fluctuations of the catalytic activity of the cGMP phosphodiesterase. This masks both SPR and spontaneous SPR-like responses. Circadian rhythms affect photoreceptors, among other systems by periodically increasing intracellular cAMP levels ([cAMP]_in), which increases the size and changes the shape of SPRs. Here, we show that forskolin, a tool that increases [cAMP]_in, affects the magnitude and frequency spectrum of the continuous and discrete components of dark noise in photoreceptors. By changing both components of rod signaling, the signal and the noise, cAMP is able to increase the photoreceptor signal-to-noise ratio by twofold. We propose that this results in a substantial improvement of signal detection, without compromising noise rejection, at the rod bipolar cell synapse.

Visual threshold is set by linear and nonlinear mechanisms in the retina that mitigate noise: how neural circuits in the retina improve the signal-to-noise ratio of the single-photon response

In sensory biology, a major outstanding question is how sensory receptor cells minimize noise while maximizing signal to set the detection threshold. This optimization could be problematic because the origin of both the signals and the limiting noise in most sensory systems is believed to lie in stimulus transduction. Signal processing in receptor cells can improve the signal-to-noise ratio. However, neural circuits can further optimize the detection threshold by pooling signals from sensory receptor cells and processing them using a combination of linear and nonlinear filtering mechanisms. In the visual system, noise limiting light detection has been assumed to arise from stimulus transduction in rod photoreceptors. In this context, the evolutionary optimization of the signal-to-noise ratio in the retina has proven critical in allowing visual sensitivity to approach the limits set by the quantal nature of light. Here, we discuss how noise in the mammalian retina is mitigated to allow for highly sensitive night vision.