Rods progressively escape saturation to drive visual responses in daylight conditions.

Rod and cone photoreceptors support vision across large light intensity ranges. Rods, active under dim illumination, are thought to saturate at higher (photopic) irradiances. The extent of rod saturation is not well defined; some studies report rod activity well into the photopic range. Using electrophysiological recordings from retina and dorsal lateral geniculate nucleus of cone-deficient and visually intact mice, we describe stimulus and physiological factors that influence photopic rod-driven responses. We find that rod contrast sensitivity is initially strongly reduced at high irradiances, but progressively recovers to allow responses to moderate contrast stimuli. Surprisingly, rods recover faster at higher light levels. A model of rod phototransduction suggests that phototransduction gain adjustments and bleaching adaptation underlie rod recovery. Consistently, exogenous chromophore reduces rod responses at bright background. Thus, bleaching adaptation renders mouse rods responsive to modest contrast at any irradiance. Paradoxically, raising irradiance across the photopic range increases the robustness of rod responses.

Melanopsin supports irradiance-driven changes in maintained activity in the superior colliculus of the mouse.

Melanopsin phototransduction allows intrinsically photosensitive retinal ganglion cells (ipRGCs) to maintain firing under sustained illumination and to encode irradiance. ipRGCs project to different parts of the visual system, including the superficial superior colliculus (sSC), but to date there is no description of melanopsin contributions to the activity of that nucleus. We sought to fill that gap using extracellular recordings to describe light response in the sSC. We failed to observe light responses in the sSC of mice lacking rod and cone function, in which melanopsin provides the only photoreception. Nor did the sSC of intact animals track very gradual ramps in irradiance, a stimulus encoded by melanopsin for other brain regions. However, in visually intact mice we did find maintained responses to extended light steps (30 s) and to an irradiance ramp upon which a high frequency (20 Hz) temporal white noise was superimposed. Both of these responses were deficient when the spectral composition of the stimulus was changed to selectively reduce its effective irradiance for melanopsin. Such maintained activity was also impaired in mice lacking melanopsin, and this effect was specific, as responses of this genotype to higher spatiotemporal frequency stimuli were normal. We conclude that ipRGCs contribute to irradiance-dependent modulations in maintained activity in the sSC, but that this effect is less robust than for other brain regions receiving ipRGC input.

Restoration of Vision with Ectopic Expression of Human Rod Opsin.

Many retinal dystrophies result in photoreceptor loss, but the inner retinal neurons can survive, making them potentially amenable to emerging optogenetic therapies. Here, we show that ectopically expressed human rod opsin, driven by either a non-selective or ON-bipolar cell-specific promoter, can function outside native photoreceptors and restore visual function in a mouse model of advanced retinal degeneration. Electrophysiological recordings from retinal explants and the visual thalamus revealed changes in firing (increases and decreases) induced by simple light pulses, luminance increases, and naturalistic movies in treated mice. These responses could be elicited at light intensities within the physiological range and substantially below those required by other optogenetic strategies. Mice with rod opsin expression driven by the ON-bipolar specific promoter displayed behavioral responses to increases in luminance, flicker, coarse spatial patterns, and elements of a natural movie at levels of contrast and illuminance (≈50-100 lux) typical of natural indoor environments. These data reveal that virally mediated ectopic expression of human rod opsin can restore vision under natural viewing conditions and at moderate light intensities. Given the inherent advantages in employing a human protein, the simplicity of this intervention, and the quality of vision restored, we suggest that rod opsin merits consideration as an optogenetic actuator for treating patients with advanced retinal degeneration.

Melanopsin-derived visual responses under light adapted conditions in the mouse dLGN.

A direct projection from melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) reaches the primary visual thalamus (dorsal lateral geniculate nucleus; dLGN). The significance of this melanopsin input to the visual system is only recently being investigated. One unresolved question is the degree to which neurons in the dLGN could use melanopsin to track dynamic changes in light intensity under light adapted conditions. Here we set out to address this question. We were able to present full field steps visible only to melanopsin by switching between rod-isoluminant 'yellow' and 'blue' lights in a mouse lacking cone function (Cnga3-/-). In the retina these stimuli elicited melanopsin-like responses from a subset of ganglion cells. When presented to anaesthetised mice, we found that ~25-30% of visually responsive neurones in the contralateral dLGN responded to these melanopsin-isolating steps with small increases in firing rate. Such responses could be elicited even with fairly modest increases in effective irradiance (32% Michelson contrast for melanopsin). These melanopsin-driven responses were apparent at bright backgrounds (corresponding to twilight-daylight conditions), but their threshold irradiance was strongly dependent upon prior light exposure when stimuli were superimposed on a spectrally neutral ramping background light. While both onset and offset latencies were long for melanopsin-derived responses compared to those evoked by rods, there was great variability in these parameters with some cells responding to melanopsin steps in <1 s. These data indicate that a subset of dLGN units can employ melanopsin signals to detect modest changes in irradiance under photopic conditions.

Increased hippocampal excitability in the 3xTgAD mouse model for Alzheimer's disease in vivo.

Mouse Alzheimer's disease (AD) models develop age- and region-specific pathology throughout the hippocampal formation. One recently established pathological correlate is an increase in hippocampal excitability in vivo. Hippocampal pathology also produces episodic memory decline in human AD and we have shown a similar episodic deficit in 3xTg AD model mice aged 3-6 months. Here, we tested whether hippocampal synaptic dysfunction accompanies this cognitive deficit by probing dorsal CA1 and DG synaptic responses in anaesthetized, 4-6 month-old 3xTgAD mice. As our previous reports highlighted a decline in episodic performance in aged control mice, we included aged cohorts for comparison. CA1 and DG responses to low-frequency perforant path stimulation were comparable between 3xTgAD and controls at both age ranges. As expected, DG recordings in controls showed paired-pulse depression; however, paired-pulse facilitation was observed in DG and CA1 of young and old 3xTgAD mice. During stimulus trains both short-latency (presumably monosynaptic: 'direct') and long-latency (presumably polysynaptic: 're-entrant') responses were observed. Facilitation of direct responses was modest in 3xTgAD animals. However, re-entrant responses in DG and CA1 of young 3xTgAD mice developed earlier in the stimulus train and with larger amplitude when compared to controls. Old mice showed less DG paired-pulse depression and no evidence for re-entrance. In summary, DG and CA1 responses to low-frequency stimulation in all groups were comparable, suggesting no loss of synaptic connectivity in 3xTgAD mice. However, higher-frequency activation revealed complex change in synaptic excitability in DG and CA1 of 3xTgAD mice. In particular, short-term plasticity in DG and CA1 was facilitated in 3xTgAD mice, most evidently in younger animals. In addition, re-entrance was facilitated in young 3xTgAD mice. Overall, these data suggest that the episodic-like memory deficit in 3xTgAD mice could be due to the development of an abnormal hyper-excitable state in the hippocampal formation.

Episodic-like memory is sensitive to both Alzheimer's-like pathological accumulation and normal ageing processes in mice.

Episodic memory depends on the hippocampus and is sensitive to both Alzheimer's disease (AD) pathology and normal ageing. We showed previously that 3xTgAD mice express a specific, episodic-memory deficit at 6 months of age in the What-Where-Which occasion (WWWhich) task (Davis, Easton, Eacott and Gigg, 2013). This task requires the integration of object-location and contextual cues to form an episodic-like memory. Here, we explore the cumulative effect of AD pathology on WWWhich memory by testing very young and middle-aged mice (3 and 12 months old, respectively). For comparison, we included an alternative episodic-like task (What-Where-When; WWWhen) and an object temporal order (Recency) task to explore claims that WWWhen types of memory are open to non-episodic solutions. We found that in contrast to their performance at 6 months, 3-month-old 3xTgAD mice formed WWWhich episodic-like memories; however, their performance at this age was poorer than in matched controls. In contrast, 3xTgAD and control animals aged 12 months were both impaired on the WWWhich task. Finally, 3xTgAD mice with a WWWhich deficit were unimpaired in both Recency and WWWhen tasks. These results support conclusions that: (1) young 3xTgAD mice express episodic-like memory, albeit depressed relative to controls; (2) age-related changes result in a deficit on the hippocampal-dependent WWWhich episodic memory task; and (3) control and 3xTgAD mice can use recency (trace strength) rather than episodic-like memory for tasks that contain a temporal 'When' component. These results, in combination with our previous findings, support an age-related decline in WWWhich episodic-like memory in mice. Furthermore, this decline is accelerated in the 3xTgAD model.

Episodic-like memory for what-where-which occasion is selectively impaired in the 3xTgAD mouse model of Alzheimer's disease.

Episodic memory loss is a defining feature of early-stage Alzheimer's disease (AD). A test of episodic-like memory for the rat, the What-Where-Which occasion task (WWWhich), requires the association of object, location, and contextual information to form an integrated memory for an event. The WWWhich task cannot be solved by use of non-episodic information such as object familiarity and is dependent on hippocampal integrity. Thus, it provides an ideal tool with which to test capacity for episodic-like memory in the 3xTg murine model for AD. As this model captures much of the human AD phenotype, we hypothesized that these mice would show a deficit in the WWWhich episodic-like memory task. To test the specificity of any episodic-like deficit, we also examined whether mice could perform components of the WWWhich task that do not require episodic-like memory. These included object (Novel Object Recognition), location (Object Location Task, What-Where task), and contextual (What-Which) memory, as well as another three-component task that can be solved without reliance on episodic recall (What-Where-When; WWWhen). The results demonstrate for the first time that control 129sv/c57bl6 mice could form WWWhich episodic-like memories, whereas, 3xTgAD mice at 6 months of age were impaired. Importantly, while 3xTgAD mice showed some deficit on spatial component tasks, they were unimpaired in the more complex WWWhen combination task (which includes a spatial component and is open to non-episodic solutions). These results strongly suggest that AD pathology centered on the hippocampal formation mediates a specific deficit for WWWhich episodic-like memory in the 3xTgAD model.