PACAP-deficient mice exhibit light parameter-dependent abnormalities on nonvisual photoreception and early activity onset

Feature Detection by Retinal Ganglion Cells

Retinal circuits transform the pixel representation of photoreceptors into the feature representations of ganglion cells, whose axons transmit these representations to the brain. Functional, morphological, and transcriptomic surveys have identified more than 40 retinal ganglion cell (RGC) types in mice. RGCs extract features of varying complexity; some simply signal local differences in brightness (i.e., luminance contrast), whereas others detect specific motion trajectories. To understand the retina, we need to know how retinal circuits give rise to the diverse RGC feature representations. A catalog of the RGC feature set, in turn, is fundamental to understanding visual processing in the brain. Anterograde tracing indicates that RGCs innervate more than 50 areas in the mouse brain. Current maps connecting RGC types to brain areas are rudimentary, as is our understanding of how retinal signals are transformed downstream to guide behavior. In this article, I review the feature selectivities of mouse RGCs, how they arise, and how they are utilized downstream. Not only is knowledge of the behavioral purpose of RGC signals critical for understanding the retinal contributions to vision; it can also guide us to the most relevant areas of visual feature space. Expected final online publication date for the Annual Review of Vision Science, Volume 8 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Effects of light on the sleep-wakefulness cycle of mice mediated by intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are able to synthesize the photosensitive protein melanopsin, which is involved in the regulation of circadian rhythms, the papillary light reflex and other nonimaging visual functions. To investigate whether ipRGCs are involved in mediating the light modulation of sleep-wakefulness in rodents, melanopsin knockout mice (MKO), melanopsin-only mice (MO) and coneless, rodless, melanopsin knockout mice (TKO) were used in this study to record electroencephalogram and electromyography variations in the normal 12:12 h light:dark cycle, and 1 h and 3 h light pulses were administered at 1 h after the light was turned off. In the normal 12:12 h light-dark cycle, the WT, MKO and MO mice had a regular day-night rhythm and no significant difference in wakefulness, rapid eye movement (REM) or nonrapid eye movement (NREM) sleep. However, TKO mice could not be entrained according to the light-dark cycle and exhibited a free-running rhythm. Extending the light pulse durations significantly changed the sleep and wakefulness activities of the WT and MO mice but did not have an effect on the MKO mice. These results indicate that melanopsin significantly affects REM and NREM sleep and that ipRGCs play an important role in light-induced sleep in mice.

Comparative Neurology of Circadian Photoreception: The Retinohypothalamic Tract (RHT) in Sighted and Naturally Blind Mammals

The mammalian eye contains two systems for light perception: an image detecting system constituted primarily of the classical photoreceptors, rods and cones, and a non-image forming system (NIF) constituted of a small group of intrinsically photosensitive retinal ganglion cells driven by melanopsin (mRGCs). The mRGCs receive input from the outer retina and NIF mediates light entrainment of circadian rhythms, masking behavior, light induced inhibition of nocturnal melatonin secretion, pupillary reflex (PLR), and affect the sleep/wake cycle. This review focuses on the mammalian NIF and its anatomy in the eye as well as its neuronal projection to the brain. This pathway is known as the retinohypothalamic tract (RHT). The development and functions of the NIF as well as the knowledge gained from studying gene modified mice is highlighted. Furthermore, the similarities of the NIF between sighted (nocturnal and diurnal rodent species, monkeys, humans) and naturally blind mammals (blind mole rats Spalax ehrenbergi and the Iberian mole, Talpa occidentalis) are discussed in relation to a changing world where increasing exposure to artificial light at night (ALAN) is becoming a challenge for humans and animals in the modern society.

The Protective Effects of Endogenous PACAP in Oxygen-Induced Retinopathy

Pituitary adenylate cyclase–activating polypeptide (PACAP) is a neuropeptide having trophic and protective functions in neural tissues, including the retina. Previously, we have shown that intravitreal PACAP administration can maintain retinal structure in the animal model of retinopathy of prematurity (ROP). The purpose of this study is to examine the development of ROP in PACAP-deficient and wild-type mice to reveal the function of endogenous PACAP. Wild-type and PACAP-knockout (KO) mouse pups at postnatal day (PD) 7 were maintained at 75% oxygen for 5 consecutive days then returned to room air on PD12 to develop oxygen-induced retinopathy (OIR). On PD15, animals underwent electroretinography (ERG) to assess visual function. On PD16, eyes were harvested for either immunohistochemistry to determine the percentage of the central avascular retinal area or molecular analysis to assess angiogenesis proteins by array kit and anti-apoptotic protein kinase B (Akt) change by western blot. Retinas of PACAP-deficient OIR mice showed a greater central avascular area than that of the wild types. ERG revealed significantly decreased b-wave amplitude in PACAP KO compared to their controls. Several angiogenic proteins were upregulated due to OIR, and 11 different proteins markedly increased in PACAP-deficient mice, whereas western blot analysis revealed a reduction in Akt phosphorylation, suggesting an advanced cell death in the lack of PACAP. This is the first study to examine the endogenous effect of PACAP in the OIR model. Previously, we have shown the beneficial effect of exogenous local PACAP treatment in the rat OIR model. Together with the present findings, we suggest that PACAP could be a novel retinoprotective agent in ROP.

Behavioral role of PACAP signaling reflects its selective distribution in glutamatergic and GABAergic neuronal subpopulations

The neuropeptide PACAP, acting as a co-transmitter, increases neuronal excitability, which may enhance anxiety and arousal associated with threat conveyed by multiple sensory modalities. The distribution of neurons expressing PACAP and its receptor, PAC1, throughout the mouse nervous system was determined, in register with expression of glutamatergic and GABAergic neuronal markers, to develop a coherent chemoanatomical picture of PACAP role in brain motor responses to sensory input. A circuit role for PACAP was tested by observing Fos activation of brain neurons after olfactory threat cue in wild-type and PACAP knockout mice. Neuronal activation and behavioral response, were blunted in PACAP knock-out mice, accompanied by sharply downregulated vesicular transporter expression in both GABAergic and glutamatergic neurons expressing PACAP and its receptor. This report signals a new perspective on the role of neuropeptide signaling in supporting excitatory and inhibitory neurotransmission in the nervous system within functionally coherent polysynaptic circuits.

Lipocalin-type prostaglandin D synthase regulates light-induced phase advance of the central circadian rhythm in mice

We previously showed that mice lacking pituitary adenylate cyclase-activating polypeptide (PACAP) exhibit attenuated light-induced phase shift. To explore the underlying mechanisms, we performed gene expression analysis of laser capture microdissected suprachiasmatic nuclei (SCNs) and found that lipocalin-type prostaglandin (PG) D synthase (L-PGDS) is involved in the impaired response to light stimulation in the late subjective night in PACAP-deficient mice. L-PGDS-deficient mice also showed impaired light-induced phase advance, but normal phase delay and nonvisual light responses. Then, we examined the receptors involved in the response and observed that mice deficient for type 2 PGD₂ receptor DP2/CRTH2 (chemoattractant receptor homologous molecule expressed on Th2 cells) show impaired light-induced phase advance. Concordant results were observed using the selective DP2/CRTH2 antagonist CAY10471. These results indicate that L-PGDS is involved in a mechanism of light-induced phase advance via DP2/CRTH2 signaling.

Kawaguchi et al. show that mice deficient in lipocalin-type prostaglandin (PG) D synthase (L-PGDS) exhibit impaired light-induced phase advance, but normal phase delay and nonvisual light responses. This study suggests the role of L-PGDS for the light-induced phase advance possibly via a chemoattractant receptor DP2/CRTH2.

Altered light induced EGR1 expression in the SCN of PACAP deficient mice

The brain’s biological clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus and generates circadian rhythms in physiology and behavior. The circadian clock needs daily adjustment by light to stay synchronized (entrained) with the astronomical 24 h light/dark cycle. Light entrainment occurs via melanopsin expressing retinal ganglion cells (mRGCs) and two neurotransmitters of the retinohypothalamic tract (RHT), PACAP and glutamate, which transmit light information to the SCN neurons. In SCN neurons, light signaling involves the immediate-early genes Fos, Egr1 and the clock genes Per1 and Per2. In this study, we used PACAP deficient mice to evaluate PACAP’s role in light induced gene expression of EGR1 in SCN neurons during early (ZT17) and late (ZT23) subjective night at high (300 lux) and low (10 lux) white light exposure. We found significantly lower levels of both EGR1 mRNA and protein in the SCN in PACAP deficient mice compared to wild type mice at early subjective night (ZT17) exposed to low but not high light intensity. No difference was found between the two genotypes at late night (ZT23) at neither light intensities. In conclusion, light mediated EGR1 induction in SCN neurons at early night at low light intensities is dependent of PACAP signaling. A role of PACAP in shaping synaptic plasticity during light stimulation at night is discussed.

PAC1- and VPAC2 receptors in light regulated behavior and physiology: Studies in single and double mutant mice

The two sister peptides, pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) and their receptors, the PAC1 -and the VPAC2 receptors, are involved in regulation of the circadian timing system. PACAP as a neurotransmitter in the retinohypothalamic tract (RHT) and VIP as a neurotransmitter, involved in synchronization of SCN neurons. Behavior and physiology in VPAC2 deficient mice are strongly regulated by light most likely as a result of masking. Consequently, we used VPAC2 and PAC1/VPAC2 double mutant mice in comparison with PAC1 receptor deficient mice to further elucidate the role of PACAP in the light mediated regulation of behavior and physiology of the circadian system. We compared circadian rhythms in mice equipped with running wheels or implanted radio-transmitter measuring core body temperature kept in a full photoperiod ((FPP)(12:12 h light dark-cycles (LD)) and skeleton photo periods (SPP) at high and low light intensity. Furthermore, we examined the expression of PAC1- and VPAC2 receptors in the SCN of the different genotypes in combination with visualization of PACAP and VIP and determined whether compensatory changes in peptide and/or receptor expression in the reciprocal knockouts (KO) (PAC1 and VPAC2) had occurred. Our data demonstrate that in although being closely related at both ligand and receptor structure/sequence, PACAP/PAC1 receptor signaling are independent of VIP/VPAC2 receptor signaling and vice versa. Furthermore, lack of either of the receptors does not result in compensatory changes at neither the physiological or anatomical level. PACAP/PAC1 signaling is important for light regulated behavior, VIP/VPAC2signaling for stable clock function and both signaling pathways may play a role in shaping diurnality versus nocturnality.

Double In situ Hybridization for MicroRNAs and mRNAs in Brain Tissues

MicroRNAs (miRNAs) participate in a variety of functions in the brain. Understanding the in vivo localization of miRNAs is an important step for uncovering their roles in brain function. However, the in situ detection of low-abundance miRNAs in brain tissues remains difficult and requires extensive optimization of in situ hybridization (ISH) protocols in individual laboratories. Thus, detailed information regarding experimental conditions would serve as a useful reference for researchers in this field. Here, we investigated and summarized the effects of adjusting a series of critical steps, including tissue fixation, probe accessibility and hybridization stringency, to standardize the currently used miRNA ISH procedures. As a result, we successfully detected several low-abundance miRNAs by ISH using the following experimental conditions: (1) use of fresh brain tissues, (2) digestion of brain samples with proteinase K, (3) LNA-probe hybridization at a temperature 37°C below the melting temperature of the RNA, (4) performance of high-stringency wash steps using 50% formamide in 1 × standard saline citrate (SSC) buffer. RT-PCR of the punched-out tissues using TaqMan^TM primers confirmed the ISH results. Finally, double-fluorescence ISH successfully demonstrated the colocalization of miRNAs and mRNAs. Thus, the detailed information regarding the miRNA ISH procedures used in this study may help to resolve the technical hurdles observed in the in vivo localization of miRNAs, and the elucidation of the specific roles of miRNAs.

A visual circuit uses complementary mechanisms to support transient and sustained pupil constriction

Rapid and stable control of pupil size in response to light is critical for vision, but the neural coding mechanisms remain unclear. Here, we investigated the neural basis of pupil control by monitoring pupil size across time while manipulating each photoreceptor input or neurotransmitter output of intrinsically photosensitive retinal ganglion cells (ipRGCs), a critical relay in the control of pupil size. We show that transient and sustained pupil responses are mediated by distinct photoreceptors and neurotransmitters. Transient responses utilize input from rod photoreceptors and output by the classical neurotransmitter glutamate, but adapt within minutes. In contrast, sustained responses are dominated by non-conventional signaling mechanisms: melanopsin phototransduction in ipRGCs and output by the neuropeptide PACAP, which provide stable pupil maintenance across the day. These results highlight a temporal switch in the coding mechanisms of a neural circuit to support proper behavioral dynamics.

A Role for the Transcription Factor Nk2 Homeobox 1 in Schizophrenia: Convergent Evidence from Animal and Human Studies

Schizophrenia is a highly heritable disorder with diverse mental and somatic symptoms. The molecular mechanisms leading from genes to disease pathology in schizophrenia remain largely unknown. Genome-wide association studies (GWASs) have shown that common single-nucleotide polymorphisms associated with specific diseases are enriched in the recognition sequences of transcription factors that regulate physiological processes relevant to the disease. We have used a “bottom-up” approach and tracked a developmental trajectory from embryology to physiological processes and behavior and recognized that the transcription factor NK2 homeobox 1 (NKX2-1) possesses properties of particular interest for schizophrenia. NKX2-1 is selectively expressed from prenatal development to adulthood in the brain, thyroid gland, parathyroid gland, lungs, skin, and enteric ganglia, and has key functions at the interface of the brain, the endocrine-, and the immune system. In the developing brain, NKX2-1-expressing progenitor cells differentiate into distinct subclasses of forebrain GABAergic and cholinergic neurons, astrocytes, and oligodendrocytes. The transcription factor is highly expressed in mature limbic circuits related to context-dependent goal-directed patterns of behavior, social interaction and reproduction, fear responses, responses to light, and other homeostatic processes. It is essential for development and mature function of the thyroid gland and the respiratory system, and is involved in calcium metabolism and immune responses. NKX2-1 interacts with a number of genes identified as susceptibility genes for schizophrenia. We suggest that NKX2-1 may lie at the core of several dose dependent pathways that are dysregulated in schizophrenia. We correlate the symptoms seen in schizophrenia with the temporal and spatial activities of NKX2-1 in order to highlight promising future research areas.

Light-sensitive brain pathways and aging

Notwithstanding its effects on the classical visual system allowing image formation, light acts upon several non-image-forming (NIF) functions including body temperature, hormonal secretions, sleep-wake cycle, alertness, and cognitive performance. Studies have shown that NIF functions are maximally sensitive to blue wavelengths (460–480 nm), in comparison to longer light wavelengths. Higher blue light sensitivity has been reported for melatonin suppression, pupillary constriction, vigilance, and performance improvement but also for modulation of cognitive brain functions. Studies investigating acute stimulating effects of light on brain activity during the execution of cognitive tasks have suggested that brain activations progress from subcortical regions involved in alertness, such as the thalamus, the hypothalamus, and the brainstem, before reaching cortical regions associated with the ongoing task. In the course of aging, lower blue light sensitivity of some NIF functions has been reported. Here, we first describe neural pathways underlying effects of light on NIF functions and we discuss eye and cerebral mechanisms associated with aging which may affect NIF light sensitivity. Thereafter, we report results of investigations on pupillary constriction and cognitive brain sensitivity to light in the course of aging. Whereas the impact of light on cognitive brain responses appears to decrease substantially, pupillary constriction seems to remain more intact over the lifespan. Altogether, these results demonstrate that aging research should take into account the diversity of the pathways underlying the effects of light on specific NIF functions which may explain their differences in light sensitivity.

Constructing the suprachiasmatic nucleus: a watchmaker's perspective on the central clockworks

The circadian system constrains an organism's palette of behaviors to portions of the solar day appropriate to its ecological niche. The central light-entrained clock in the suprachiasmatic nucleus (SCN) of the mammalian circadian system has evolved a complex network of interdependent signaling mechanisms linking multiple distinct oscillators to serve this crucial function. However, studies of the mechanisms controlling SCN development have greatly lagged behind our understanding of its physiological functions. We review advances in the understanding of adult SCN function, what has been described about SCN development to date, and the potential of both current and future studies of SCN development to yield important insights into master clock function, dysfunction, and evolution.

Rhythmic control of activity and sleep by class B1 GPCRs

Members of the class B1 family of G-protein coupled receptors (GPCRs) whose ligands are neuropeptides have been implicated in regulation of circadian rhythms and sleep in diverse metazoan clades. This review discusses the cellular and molecular mechanisms by which class B1 GPCRs, especially the mammalian VPAC2 receptor and its functional homologue PDFR in Drosophila and C. elegans, regulate arousal and daily rhythms of sleep and wake. There are remarkable parallels in the cellular and molecular roles played by class B1 intercellular signaling pathways in coordinating arousal and circadian timekeeping across multiple cells and tissues in these very different genetic model organisms.

Mice deficient of glutamatergic signaling from intrinsically photosensitive retinal ganglion cells exhibit abnormal circadian photoentrainment

Several aspects of behavior and physiology, such as sleep and wakefulness, blood pressure, body temperature, and hormone secretion exhibit daily oscillations known as circadian rhythms. These circadian rhythms are orchestrated by an intrinsic biological clock in the suprachiasmatic nuclei (SCN) of the hypothalamus which is adjusted to the daily environmental cycles of day and night by the process of photoentrainment. In mammals, the neuronal signal for photoentrainment arises from a small subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) that send a direct projection to the SCN. ipRGCs also mediate other non-image-forming (NIF) visual responses such as negative masking of locomotor activity by light, and the pupillary light reflex (PLR) via co-release of neurotransmitters glutamate and pituitary adenylate cyclase-activating peptide (PACAP) from their synaptic terminals. The relative contribution of each neurotransmitter system for the circadian photoentrainment and other NIF visual responses is still unresolved. We investigated the role of glutamatergic neurotransmission for circadian photoentrainment and NIF behaviors by selective ablation of ipRGC glutamatergic synaptic transmission in mice. Mutant mice displayed delayed re-entrainment to a 6 h phase shift (advance or delay) in the light cycle and incomplete photoentrainment in a symmetrical skeleton photoperiod regimen (1 h light pulses between 11 h dark periods). Circadian rhythmicity in constant darkness also was reduced in some mutant mice. Other NIF responses such as the PLR and negative masking responses to light were also partially attenuated. Overall, these results suggest that glutamate from ipRGCs drives circadian photoentrainment and negative masking responses to light.

PACAP is essential for the adaptive thermogenic response of brown adipose tissue to cold exposure

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a widely distributed neuropeptide that acts as a neurotransmitter, neuromodulator, neurotropic factor, neuroprotectant, secretagogue, and neurohormone. Owing to its pleiotropic biological actions, knockout of Pacap (Adcyap1) has been shown to induce several abnormalities in mice such as impaired thermoregulation. However, the underlying physiological and molecular mechanisms remain unclear. A previous report has shown that cold-exposed Pacap null mice cannot supply appropriate levels of norepinephrine (NE) to brown adipocytes. Therefore, we hypothesized that exogenous NE would rescue the impaired thermogenic response of Pacap null mice during cold exposure. We compared the adaptive thermogenic capacity of Pacap(-/-) to Pacap(+/+) mice in response to NE when housed at room temperature (24 °C) and after a 3.5-week cold exposure (4 °C). Biochemical parameters, expression of thermogenic genes, and morphological properties of brown adipose tissue (BAT) and white adipose tissue (WAT) were also characterized. Results showed that there was a significant effect of temperature, but no effect of genotype, on the resting metabolic rate in conscious, unrestrained mice. However, the normal cold-induced increase in the basal metabolic rate and NE-induced increase in thermogenesis were severely blunted in cold-exposed Pacap(-/-) mice. These changes were associated with altered substrate utilization, reduced β3-adrenergic receptor (β3-Ar (Adrb3)) and hormone-sensitive lipase (Hsl (Lipe)) gene expression, and increased fibroblast growth factor 2 (Fgf2) gene expression in BAT. Interestingly, Pacap(-/-) mice had depleted WAT depots, associated with upregulated uncoupling protein 1 expression in inguinal WATs. These results suggest that the impairment of adaptive thermogenesis in Pacap null mice cannot be rescued by exogenous NE perhaps in part due to decreased β3-Ar-mediated BAT activation.

Investigation of PACAP Fragments and Related Peptides in Chronic Retinal Hypoperfusion

Pituitary adenylate cyclase activating polypeptide (PACAP) has neuroprotective effects in different neuronal and retinal injuries. Retinal ischemia can be effectively modelled by permanent bilateral common carotid artery occlusion (BCCAO), which causes chronic hypoperfusion-induced degeneration in the entire rat retina. The retinoprotective effect of PACAP 1-38 and VIP is well-established in ischemic retinopathy. However, little is known about the effects of related peptides and PACAP fragments in ischemic retinopathy. The aim of the present study was to investigate the potential retinoprotective effects of different PACAP fragments (PACAP 4-13, 4-22, 6-10, 6-15, 11-15, and 20-31) and related peptides (secretin, glucagon) in BCCAO-induced ischemic retinopathy. Wistar rats (3-4 months old) were used in the experiment. After performing BCCAO, the right eyes of the animals were treated with PACAP fragments or related peptides intravitreal (100 pM), while the left eyes were injected with saline serving as control eyes. Sham-operated (without BCCAO) rats received the same treatment. Routine histology was performed 2 weeks after the surgery; cells were counted and the thickness of retinal layers was compared. Our results revealed significant neuroprotection by PACAP 1-38 but did not reveal retinoprotective effect of the PACAP fragments or related peptides. These results suggest that PACAP 1-38 has the greatest efficacy in ischemic retinopathy.

Central CRTH2, a second prostaglandin D2 receptor, mediates emotional impairment in the lipopolysaccharide and tumor-induced sickness behavior model

Chemoattractant receptor-homologous molecule expressed on T helper type 2 cells (CRTH2) is a second prostaglandin D2 receptor involved in mediating the allergic response; however, its central function is not yet known. Here, we demonstrate that central CRTH2 mediates emotional impairment. Lipopolysaccharide (LPS)-induced decreases in social interaction and novel exploratory behavior were observed in wild-type (CRTH2(+/+)) mice but not CRTH2-deficient (CRTH2(-/-)) mice, but both genotypes showed hypolocomotion and anorexia following LPS injection. Tumor (colon 26) inoculation, a more pathologically relevant model, induced decreases in social interaction and novel exploratory behavior in CRTH2(+/+), but not CRTH2(-/-) mice. In addition, the CRTH2 antagonists including clinically available ramatroban reversed impaired social interaction and novel exploratory behavior after either LPS or tumor inoculation in CRTH2(+/+) mice. Finally, LPS-induced c-Fos expression in the hypothalamic paraventricular nucleus (PVN) and central amygdala (CeA) was selectively abolished in CRTH2(-/-) mice. These results show that CRTH2 participates in LPS-induced emotional changes and activation in the PVN and CeA. Our study provides the first evidence that central CRTH2 regulates specific emotional behaviors, and that CRTH2 antagonism has potential as a therapeutic target for behavioral symptoms associated with tumors and infectious diseases.

Aging of non-visual spectral sensitivity to light in humans: compensatory mechanisms?

The deterioration of sleep in the older population is a prevalent feature that contributes to a decrease in quality of life. Inappropriate entrainment of the circadian clock by light is considered to contribute to the alteration of sleep structure and circadian rhythms in the elderly. The present study investigates the effects of aging on non-visual spectral sensitivity to light and tests the hypothesis that circadian disturbances are related to a decreased light transmittance. In a within-subject design, eight aged and five young subjects were exposed at night to 60 minute monochromatic light stimulations at 9 different wavelengths (420-620 nm). Individual sensitivity spectra were derived from measures of melatonin suppression. Lens density was assessed using a validated psychophysical technique. Although lens transmittance was decreased for short wavelength light in the older participants, melatonin suppression was not reduced. Peak of non-visual sensitivity was, however, shifted to longer wavelengths in the aged participants (494 nm) compared to young (484 nm). Our results indicate that increased lens filtering does not necessarily lead to a decreased non-visual sensitivity to light. The lack of age-related decrease in non-visual sensitivity to light may involve as yet undefined adaptive mechanisms.

Glutamatergic neurotransmission from melanopsin retinal ganglion cells is required for neonatal photoaversion but not adult pupillary light reflex

Melanopsin-expressing retinal ganglion cells (mRGCs) in the eye play an important role in many light-activated non-image-forming functions including neonatal photoaversion and the adult pupillary light reflex (PLR). MRGCs rely on glutamate and possibly PACAP (pituitary adenylate cyclase-activating polypeptide) to relay visual signals to the brain. However, the role of these neurotransmitters for individual non-image-forming responses remains poorly understood. To clarify the role of glutamatergic signaling from mRGCs in neonatal aversion to light and in adult PLR, we conditionally deleted vesicular glutamate transporter (VGLUT2) selectively from mRGCs in mice. We found that deletion of VGLUT2 in mRGCs abolished negative phototaxis and light-induced distress vocalizations in neonatal mice, underscoring a necessary role for glutamatergic signaling. In adult mice, loss of VGLUT2 in mRGCs resulted in a slow and an incomplete PLR. We conclude that glutamatergic neurotransmission from mRGCs is required for neonatal photoaversion but is complemented by another non-glutamatergic signaling mechanism for the pupillary light reflex in adult mice. We speculate that this complementary signaling might be due to PACAP neurotransmission from mRGCs.

Differential effects of retinal degeneration on sleep and wakefulness responses to short light-dark cycles in albino mice

This study characterizes the different response patterns of sleep and wakefulness (W) to short light-dark (LD) cycles in albino mice and examines whether retinal degeneration resulting from prolonged bright light treatment and/or rd/rd mutation alters such response patterns. Eight young male Institute for Cancer Research (ICR) mice with normal eyes, seven young male rd/rd Friend Virus B type (FVB) mice, six young ICR and five young rd/rd FVB mice receiving 48-h bright light treatment, and five older rd/rd FVB mice were implanted with skull and muscle electrodes to record sleep and W. All the mice were maintained in 12-h-12-h LD cycles at baseline and received 2 days of short LD cycle treatment, which included 5-min-5-min LD cycles for a total of 24 cycles presented 4h after lights-on and again 4h after lights-off. All the five mouse groups maintained photo-entrainment of sleep and W rhythms at the baseline and showed a preference for paradoxical sleep (PS) occurrence in the 5-min dark period and non-rapid eye movement sleep (NREMS) in the 5-min light period and a brief alerting effect of light onset on experimental days. Retinal degeneration rising from bright light treatment and/or genetic mutation failed to eliminate or reduce the response of PS and NREMS to short LD cycles, although it enhanced the LD contrast of W, i.e., bright light treatment prolonged the alerting effect of light and the rd mutation increased the suppressing effect of the dark on W. These results suggest that sleep responses to short LD cycles and the brief alerting effect of light were independent of the photoreceptors in the outer retina. Furthermore, the residual photoreceptors in the outer retina and/or the photosensitive cells in the inner retina may actively modulate the effect of light and dark signals on W.

Mice deficient in pituitary adenylate cyclase activating polypeptide (PACAP) are more susceptible to retinal ischemic injury in vivo

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuroprotective peptide exerting protective effects in neuronal injuries. We have provided evidence that PACAP is neuroprotective in several models of retinal degeneration in vivo. Our previous studies showed that PACAP treatment ameliorated the damaging effects of chronic hypoperfusion modeled by permanent bilateral carotid artery occlusion. We have also demonstrated in earlier studies that treatment with PACAP antagonists further aggravates retinal lesions. It has been shown that PACAP deficient mice have larger infarct size in cerebral ischemia. The aim of this study was to compare the degree of retinal damage in wild type and PACAP deficient mice in ischemic retinal insult. Mice underwent 10 min of bilateral carotid artery occlusion followed by 2-week reperfusion period. Retinas were then processed for histological analysis. It was found that PACAP deficient mice had significantly greater retinal damage, as shown by the thickness of the whole retina, the morphometric analysis of the individual retinal layers, and the cell numbers in the inner nuclear and ganglion cell layers. Exogenous PACAP administration could partially protect against retinal degeneration in PACAP deficient mice. These results clearly show that endogenous PACAP reacts as a stress-response peptide that is necessary for endogenous protection against different retinal insults.

The emerging roles of melanopsin in behavioral adaptation to light

The adaptation of behavior and physiology to changes in the ambient light level is of crucial importance to life. These adaptations include the light modulation of neuroendocrine function and temporal alignment of physiology and behavior to the day:night cycle by the circadian clock. These non-image-forming (NIF) responses can function independent of rod and cone photoreceptors but depend on ocular light reception, suggesting the participation of novel photoreceptors in the eye. The discovery of melanopsin in intrinsically photosensitive retinal ganglion cells (ipRGCs) and genetic proof for its important role in major NIF responses have offered an exciting entry point to comprehend how mammals adapt to the light environment. Here, we review the recent advances in our understanding of the emerging roles of melanopsin and ipRGCs. These findings now offer new avenues to understand the role of ambient light in sleep, alertness, dependent physiologies and potential pharmacological intervention as well as lifestyle modifications to improve the quality of life.