Pupillary reflex and behavioral masking responses to light as functional measures of retinal degeneration in mice

Neuroplastin 65 deficiency leads to the impairment of visual function through affecting ribbon synapse in retina of mice

Neuroplastin 65 (NP65) is a synapse-enriched glycoprotein in the central nervous system and is implicated in synaptic plasticity. In the present study, we found that NP65 knockout (NP65 KO) mice exhibit impaired visual function, including reductions in the amplitude of b-wave in scotopic flash electroretinogram (fERG), the amplitude of N1 and P1 waves in flash visual evoked potentials (fVEP), and the constriction rate in pupillary light reflexes (PLR). In wild-type (WT) mice, NP65 is specifically enriched in the synaptic ribbon (SR) of ribbon synapses labeled by Ribeye in the retina. We found that NP65 KO mice display nearly normal architecture of the retina. However, NP65 KO mice show a significant decrease in the immunoreactivity of presynaptic postsynaptic density protein 95 (PSD95), synaptophysin (SYN) and Ribeye in the outer plexiform layer (OPL). Moreover, the electron microscopy displays a decrease in synaptic ribbons and defects in postsynaptic structures in the ribbon synapses of the OPL in NP65 KO mice. In addition, we found that the apposition of presynaptic photoreceptor axonal terminals and postsynaptic bipolar cell dendrites in the OPL is misplaced in NP65 KO mice. Finally, we show that intravitreous injection of AAV-NP65 reverses the visual dysfunction, increases Ribeye expression and restores the normal arrangement in the OPL of NP65 KO mice. Together, our findings reveal that NP65 deficiency leads to visual function impairment by affecting ribbon synapses in the OPL of mice, suggesting that NP65 is critical for visual function in mammals and a potential target for degenerative retinopathy.

Deprivation of visual input alters specific subset of inhibitory neurons and affect thalamic afferent terminals in V1 of rd1 mouse

Retinitis Pigmentosa (RP) is a heterogenous group of inherited disorder, and its progression not only affects the retina but also the primary visual cortex. This manifests imbalances in the excitatory and inhibitory neurotransmission. Here, we investigated if changes in cortical functioning is linked to alterations in GABAergic population of neurons and its two important subsets, somatostatin (SST) and parvalbumin (PV) neuron in rd1 model of retinal degeneration (RD). We demonstrate marked decrease in the proportion of SST neurons in different layers of cortex whereas PV neurons were less affected. Moreover, we found reduced expression of glutamatergic thalamic afferents (VGLUT2) due to lack of visual activity. These results suggest PV neurons are likely recruited by the cortical circuitry to increase the inhibitory drive and compensate the disrupted inhibition-excitation balance. However, reduced SST expression perhaps results in weakening of stimulus selectivity. Delineating their functional role during RD will provide insights for acquisition of high-resolution vision thereby improving current state of vision restoration.

Regulation of mouse exploratory behaviour by irradiance and cone-opponent signals

BACKGROUND

Animal survival depends on the ability to adjust behaviour according to environmental conditions. The circadian system plays a key role in this capability, with diel changes in the quantity (irradiance) and spectral content ('colour') of ambient illumination providing signals of time-of-day that regulate the timing of rest and activity. Light also exerts much more immediate effects on behaviour, however, that are equally important in shaping daily activity patterns. Hence, nocturnal mammals will actively avoid light and dramatically reduce their activity when light cannot be avoided. The sensory mechanisms underlying these acute effects of light are incompletely understood, particularly the importance of colour.

RESULTS

To define sensory mechanisms controlling mouse behaviour, we used photoreceptor-isolating stimuli and mice with altered cone spectral sensitivity (Opn1mwR), lacking melanopsin (Opn1mwR; Opn4-/-) or cone phototransduction (Cnga3-/-) in assays of light-avoidance and activity suppression. In addition to roles for melanopsin-dependent irradiance signals, we find a major influence of spectral content in both cases. Hence, remarkably, selective increases in S-cone irradiance (producing a blue-shift in spectrum replicating twilight) drive light-seeking behaviour and promote activity. These effects are opposed by signals from longer-wavelength sensitive cones, indicating a true spectrally-opponent mechanism. Using c-Fos-mapping and multielectrode electrophysiology, we further show these effects are associated with a selective cone-opponent modulation of neural activity in the key brain site implicated in acute effects of light on behaviour, the subparaventricular zone.

CONCLUSIONS

Collectively, these data reveal a mechanism whereby blue-shifts in the spectrum of environmental illumination, such as during twilight, promote mouse exploratory behaviour.

Effects of Cage Position and Light Transmission on Home Cage Activity and Circadian Entrainment in Mice

Light is known to exert powerful effects on behavior and physiology, including upon the amount and distribution of activity across the day/night cycle. Here we use home cage activity monitoring to measure the effect of differences in home cage light spectrum and intensity on key circadian activity parameters in mice. Due to the relative positioning of any individually ventilated cage (IVC) with regard to the animal facility lighting, notable differences in light intensity occur across the IVC rack. Although all mice were found to be entrained, significant differences in the timing of activity onset and differences in activity levels were found between mice housed in standard versus red filtering cages. Furthermore, by calculating the effective irradiance based upon the known mouse photopigments, a significant relationship between light intensity and key circadian parameters are shown. Perhaps unsurprisingly given the important role of the circadian photopigment melanopsin in circadian entrainment, melanopic illuminance is shown to correlate more strongly with key circadian activity parameters than photopic lux. Collectively, our results suggest that differences in light intensity may reflect an uncharacterized source of variation in laboratory rodent research, with potential consequences for reproducibility. Room design and layout vary within and between facilities, and caging design and lighting location relative to cage position can be highly variable. We suggest that cage position should be factored into experimental design, and wherever possible, experimental lighting conditions should be characterized as a way of accounting for this source of variation.

Melatonin Adjusts the Phase of Mouse Circadian Clocks in the Cornea Both Ex Vivo and In Vivo

The presence of an endogenous circadian clock within most mammalian cells is associated with the amazing observation that within a given tissue, these clocks are largely in synchrony with each other. Different tissues use a variety of systemic or environmental cues to precisely coordinate the phase of these clocks. The cornea is a unique tissue in that it is largely isolated from the direct blood supply that most tissues experience, it is transparent to visible light, and it is exposed directly to environmental light and temperature. Melatonin is a hormone that has been implicated in regulation of the cornea's circadian clocks. Here, we analyze the ability of rhythmic melatonin to entrain corneas ex vivo, and analyze the phase of corneal circadian clocks in vivo both in light: dark cycles and in constant darkness. We find that the presence of a retina from a melatonin-proficient mouse strain, C3Sn, can photoentrain the circadian clocks of a co-cultured mouse cornea, but a retina from a melatonin-deficient strain, C57Bl/6, cannot. Furthermore, pharmacologic blockade of melatonin or use of a retina with advanced retinal degeneration, Pde6b^rd1, blocks the photoentraining effect. Corneal circadian clocks in vivo adopt an advanced phase in C3Sn mice compared with C57Bl/6, but the circadian clocks in the liver are unaffected. This observation is not attributable to a shorter endogenous period of the cornea or behavior between the strains. Some transcripts of circadian genes in the corneas of C3Sn mice also show an advanced phase of expression in a light: dark cycle, while the transcript of Per2 exhibits a light-dependent transient induction at the onset of darkness. We conclude that melatonin acts as a phase modifying factor in a rhythmic manner for the circadian clocks of the cornea.