Fred Rieke

Nonlinear Signal Transfer from Mouse Rods to Bipolar Cells and Implications for Visual Sensitivity

We investigated the impact of rod-bipolar signal transfer on visual sensitivity. Two observations indicate that rod-rod bipolar signal transfer is nonlinear. First, responses of rods increased linearly with flash strength, while those of rod bipolars increased supralinearly. Second, fluctuations in the responses of rod bipolars were larger than expected from linear summation of the rod inputs. Rod-OFF bipolar signal transfer did not share this strong nonlinearity. Surprisingly, nonlinear rod-rod bipolar signal transfer eliminated many of the rods single-photon responses. The impact on sensitivity, however, was more than compensated for by rejection of noise from rods that did not absorb photons. As a consequence, rod bipolars provide a near-optimal readout of rod signals at light levels near visual threshold.

Essential role of Ca2+-binding protein 4, a Cav1.4 channel regulator, in photoreceptor synaptic function

CaBP1–8 are neuronal Ca2+-binding proteins with similarity to calmodulin (CaM). Here we show that CaBP4 is specifically expressed in photoreceptors, where it is localized to synaptic terminals. The outer plexiform layer, which contains the photoreceptor synapses with secondary neurons, was thinner in the Cabp4−/− mice than in control mice. Cabp4−/− retinas also had ectopic synapses originating from rod bipolar and horizontal cells tha HJt extended into the outer nuclear layer. Responses of Cabp4−/− rod bipolars were reduced in sensitivity about 100-fold. Electroretinograms (ERGs) indicated a reduction in cone and rod synaptic function. The phenotype of Cabp4−/− mice shares similarities with that of incomplete congenital stationary night blindness (CSNB2) patients. CaBP4 directly associated with the C-terminal domain of the Cav1.4 α1-subunit and shifted the activation of Cav1.4 to hyperpolarized voltages in transfected cells. These observations indicate that CaBP4 is important for normal synaptic function, probably through regulation of Ca2+ influx and neurotransmitter release in photoreceptor synaptic terminals.

Controlling the gain of rod-mediated signals in the Mammalian retina.

Effective sensory processing requires matching the gain of neural responses to the range of signals encountered. For rod vision, gain controls operate at light levels at which photons arrive rarely at individual rods, light levels too low to cause adaptation in rod phototransduction. Under these conditions, adaptation within a conserved pathway in mammalian retina maintains sensitivity as light levels change. To relate retinal signals to behavioral work on detection at low light levels, we measured how background light affects the gain and noise of primate ganglion cells. To determine where and how gain is controlled, we tracked rod-mediated signals across the mouse retina. These experiments led to three main conclusions: (1) the primary site of adaptation at low light levels is the synapse between rod bipolar and AII amacrine cells; (2) cellular noise after the gain control is nearly independent of background intensity; and (3) at low backgrounds, noise in the circuitry, rather than rod noise or fluctuations in arriving photons, limits ganglion cell sensitivity. This work provides physiological insights into the rich history of experiments characterizing how rod vision avoids saturation as light levels increase.

Network Variability Limits Stimulus-Evoked Spike Timing Precision in Retinal Ganglion Cells

Visual, auditory, somatosensory, and olfactory stimuli generate temporally precise patterns of action potentials (spikes). It is unclear, however, how the precision of spike generation relates to the pattern and variability of synaptic input elicited by physiological stimuli. We determined how synaptic conductances evoked by light stimuli that activate the rod bipolar pathway control spike generation in three identified types of mouse retinal ganglion cells (RGCs). The relative amplitude, timing, and impact of excitatory and inhibitory input differed dramatically between On and Off RGCs. Spikes evoked by repeated somatic injection of identical light-evoked synaptic conductances were more temporally precise than those evoked by light. However, the precision of spikes evoked by conductances that varied from trial to trial was similar to that of light-evoked spikes. Thus, the rod bipolar pathway modulates different RGCs via unique combinations of synaptic input, and RGC temporal variability reflects variability in the input this circuit provides.

Role of Photoreceptor-specific Retinol Dehydrogenase in the Retinoid Cycle in Vivo

The retinoid cycle is a recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. Photoreceptor-specific retinol dehydrogenase (prRDH) catalyzes reduction of all-trans-retinal to all-trans-retinol and is thought to be a key enzyme in the retinoid cycle. We disrupted mouse prRDH (human gene symbol RDH8) gene expression by targeted recombination and generated a homozygous prRDH knock-out (prRDH–/–) mouse. Histological analysis and electron microscopy of retinas from 6- to 8-week-old prRDH–/– mice revealed no structural differences of the photoreceptors or inner retina. For brief light exposure, absence of prRDH did not affect the rate of 11-cis-retinal regeneration or the decay of Meta II, the activated form of rhodopsin. Absence of prRDH, however, caused significant accumulation of all-trans-retinal following exposure to bright lights and delayed recovery of rod function as measured by electroretinograms and single cell recordings. Retention of all-trans-retinal resulted in slight overproduction of A2E, a condensation product of all-trans-retinal and phosphatidylethanolamine. We conclude that prRDH is an enzyme that catalyzes reduction of all-trans-retinal in the rod outer segment, most noticeably at higher light intensities and prolonged illumination, but is not an essential enzyme of the retinoid cycle.

Deletion of PrBP/δ impedes transport of GRK1 and PDE6 catalytic subunits to photoreceptor outer segments

The mouse Pde6d gene encodes a ubiquitous prenyl binding protein, termed PrBP/δ, of largely unknown physiological function. PrBP/δ was originally identified as a putative rod cGMP phosphodiesterase (PDE6) subunit in the retina, where it is relatively abundant. To investigate the consequences of Pde6d deletion in retina, we generated a Pde6d−/− mouse by targeted recombination. Although manifesting reduced body weight, the Pde6d−/− mouse was viable and fertile and its retina developed normally. Immunocytochemistry showed that farnesylated rhodopsin kinase (GRK1) and prenylated rod PDE6 catalytic subunits partially mislocalized in Pde6d−/− rods, whereas rhodopsin was unaffected. In Pde6d−/− rod single-cell recordings, sensitivity to single photons was increased and saturating flash responses were prolonged. Pde6d−/− scotopic paired-flash electroretinograms indicated a delay in recovery of the dark state, likely due to reduced levels of GRK1 in rod outer segments. In Pde6d−/− cone outer segments, GRK1 and cone PDE6α′ were present at very low levels and the photopic b-wave amplitudes were reduced by 70%. Thus the absence of PrBP/δ in retina impairs transport of prenylated proteins, particularly GRK1 and cone PDE, to rod and cone outer segments, resulting in altered photoreceptor physiology and a phenotype of a slowly progressing rod/cone dystrophy.

The Challenges Natural Images Pose for Visual Adaptation

Advances in our understanding of natural image statistics and of gain control within the retinal circuitry are leading to new insights into the classic problem of retinal light adaptation. Here we review what we know about how rapid adaptation occurs during active exploration of the visual scene. Adaptational mechanisms must balance the competing demands of adapting quickly, locally, and reliably, and this balance must be maintained as lighting conditions change. Multiple adaptational mechanisms in different locations within the retina act in concert to accomplish this task, with lighting conditions dictating which mechanisms dominate.

Selective Transmission of Single Photon Responses by Saturation at the Rod-to-Rod Bipolar Synapse

A threshold-like nonlinearity in signal transfer from mouse rod photoreceptors to rod bipolar cells dramatically improves the absolute sensitivity of the rod signals. The work described here reaches three conclusions about the mechanisms generating this nonlinearity. (1) The nonlinearity is caused primarily by saturation of the feedforward rod-to-rod bipolar synapse and not by feedback from horizontal or amacrine cells. This saturation renders the rod bipolar current insensitive to small changes in transmitter release from the rod. (2) Saturation occurs within the G protein cascade that couples receptors to channels in the rod bipolar dendrites, with little or no contribution from presynaptic mechanisms or saturation of the postsynaptic receptors. (3) Between 0.5 and 2 bipolar transduction channels are open in darkness at each synapse, compared to the approximately 30 channels open at the peak of the single photon response.

Timescales of Inference in Visual Adaptation

Adaptation is a hallmark of sensory function. Adapting optimally requires matching the dynamics of adaptation to those of changes in the stimulus distribution. Here we show that the dynamics of adaptation in the responses of mouse retinal ganglion cells depend on stimulus history. We hypothesized that the accumulation of evidence for a change in the stimulus distribution controls the dynamics of adaptation, and developed a model for adaptation as an ongoing inference problem. Guided by predictions of this model, we found that the dynamics of adaptation depend on the discriminability of the change in stimulus distribution and that the retina exploits information contained in properties of the stimulus beyond the mean and variance to adapt more quickly when possible.

Light adaptation in cone vision involves switching between receptor and post-receptor sites

We see over an enormous range of mean light levels, greater than the range of output signals retinal neurons can produce. Even highlights and shadows within a single visual scene can differ ∼10,000-fold in intensity—exceeding the range of distinct neural signals by a factor of ∼100. The effectiveness of daylight vision under these conditions relies on at least two retinal mechanisms that adjust sensitivity in the ∼200 ms intervals between saccades. One mechanism is in the cone photoreceptors (receptor adaptation) and the other is at a previously unknown location within the retinal circuitry that benefits from convergence of signals from multiple cones (post-receptor adaptation). Here we find that post-receptor adaptation occurs as signals are relayed from cone bipolar cells to ganglion cells. Furthermore, we find that the two adaptive mechanisms are essentially mutually exclusive: as light levels increase the main site of adaptation switches from the circuitry to the cones. These findings help explain how human cone vision encodes everyday scenes, and, more generally, how sensory systems handle the challenges posed by a diverse physical environment.