Reflexive Eye Closure in Response to Cone and Melanopsin Stimulation: A Study of Implicit Measures of Light Sensitivity in Migraine.

BACKGROUND AND OBJECTIVES: To quantify interictal photophobia in migraine with and without aura using reflexive eye closure as an implicit measure of light sensitivity and to assess the contribution of melanopsin and cone signals to these responses. METHODS: Participants were screened to meet criteria for 1 of 3 groups: headache-free (HF) controls, migraine without aura (MO), and migraine with visual aura (MA). MO and MA participants were included if they endorsed ictal and interictal photophobia. Exclusion criteria included impaired vision, inability to collect usable pupillometry, and history of either head trauma or seizure. Participants viewed light pulses that selectively targeted melanopsin, the cones, or their combination during recording of orbicularis oculi EMG (OO-EMG) and blinking activity. RESULTS: We studied 20 participants in each group. MA and MO groups reported increased visual discomfort to light stimuli (discomfort rating, 400% contrast, MA: 4.84 [95% confidence interval 0.33, 9.35]; MO: 5.23 [0.96, 9.50]) as compared to HF controls (2.71 [0, 6.47]). Time course analysis of OO-EMG and blinking activity demonstrated that reflexive eye closure was tightly coupled to the light pulses. The MA group had greater OO-EMG and blinking activity in response to these stimuli (EMG activity, 400% contrast: 42.9%Δ [28.4, 57.4]; blink activity, 400% contrast: 11.2% [8.8, 13.6]) as compared to the MO (EMG activity, 400% contrast: 9.9%Δ [5.8, 14.0]; blink activity, 400% contrast: 4.7% [3.5, 5.9]) and HF control (EMG activity, 400% contrast: 13.2%Δ [7.1, 19.3]; blink activity, 400% contrast: 4.5% [3.1, 5.9]) groups. DISCUSSION: Our findings suggest that the intrinsically photosensitive retinal ganglion cells (ipRGCs), which integrate melanopsin and cone signals, provide the afferent input for light-induced reflexive eye closure in a photophobic state. Moreover, we find a dissociation between implicit and explicit measures of interictal photophobia depending on a history of visual aura in migraine. This implies distinct pathophysiology in forms of migraine, interacting with separate neural pathways by which the amplification of ipRGC signals elicits implicit and explicit signs of visual discomfort.

Light adaptation characteristics of melanopsin.

Following photopigment bleaching, the rhodopsin and cone-opsins show a characteristic exponential regeneration in the dark with a photocycle dependent on the retinal pigment epithelium. Melanopsin pigment regeneration in animal models requires different pathways to rods and cones. To quantify melanopsin-mediated light adaptation in humans, we first estimated its photopigment regeneration kinetics through the photo-bleach recovery of the intrinsic melanopsin pupil light response (PLR). An intense broadband light (~120,000 Td) bleached 43% of melanopsin compared to 86% of the cone-opsins. Recovery from a 43% bleach was 3.4X slower for the melanopsin than cone-opsin. Post-bleach melanopsin regeneration followed an exponential growth with a 2.5 min time-constant (τ) that required 11.2 min for complete recovery; the half-bleaching level (Ip) was ~ 4.47 log melanopic Td (16.10 log melanopsin effective photons.cm-2.s-1; 8.25 log photoisomerisations.photoreceptor-1.s-1). The effect on the cone-directed PLR of the level of the melanopsin excitation during continuous light adaptation was then determined. We observed that cone-directed pupil constriction amplitudes increased by ~ 10% when adapting lights had a higher melanopic excitation but the same mean photometric luminance. Our findings suggest that melanopsin light adaptation enhances cone signalling along the non-visual retina-brain axis. Parameters τ and Ip will allow estimation of the level of melanopsin bleaching in any light units; the data have implications for quantifying the relative contributions of putative melanopsin pathways to regulate the post-bleach photopigment regeneration and adaptation.

Modulations in irradiance directed at melanopsin, but not cone photoreceptors, reliably alter electrophysiological activity in the suprachiasmatic nucleus and circadian behaviour in mice.

Intrinsically photosensitive retinal ganglion cells convey intrinsic, melanopsin-based, photoreceptive signals alongside those produced by rods and cones to the suprachiasmatic nucleus (SCN) circadian clock. To date, experimental data suggest that melanopsin plays a more significant role in measuring ambient light intensity than cone photoreception. Such studies have overwhelmingly used diffuse light stimuli, whereas light intensity in the world around us varies across space and time. Here, we investigated the extent to which melanopsin or cone signals support circadian irradiance measurements in the presence of naturalistic spatiotemporal variations in light intensity. To address this, we first presented high- and low-contrast movies to anaesthetised mice whilst recording extracellular electrophysiological activity from the SCN. Using a mouse line with altered cone sensitivity (Opn1mwR mice) and multispectral light sources we then selectively varied irradiance of the movies for specific photoreceptor classes. We found that steps in melanopic irradiance largely account for the light induced-changes in SCN activity over a range of starting light intensities and in the presence of spatiotemporal modulation. By contrast, cone-directed changes in irradiance only influenced SCN activity when spatiotemporal contrast was low. Consistent with these findings, under housing conditions where we could independently adjust irradiance for melanopsin versus cones, the period lengthening effects of constant light on circadian rhythms in behaviour were reliably determined by melanopic irradiance, regardless of irradiance for cones. These data add to the growing evidence that modulating effective irradiance for melanopsin is an effective strategy for controlling the circadian impact of light.

Visual and non-visual properties of filters manipulating short-wavelength light.

PURPOSE: Optical filters and tints manipulating short-wavelength light (sometimes called 'blue-blocking' or 'blue-attenuating' filters) are used in the management of a range of ocular, retinal, neurological and psychiatric disorders. In many cases, the only available quantification of the optical effects of a given optical filter is the spectral transmittance, which specifies the amount of light transmitted as a function of wavelength. METHODS: We propose a novel physiologically relevant and retinally referenced framework for quantifying the visual and non-visual effects of these filters, incorporating the attenuation of luminance (luminous transmittance), the attenuation of melanopsin activation (melanopsin transmittance), the colour shift, and the reduction of the colour gamut (gamut reduction). Using these criteria, we examined a novel database of spectral transmittance functions of optical filters (n = 121) which were digitally extracted from a variety of sources. RESULTS: We find a large diversity in the alteration of visual and non-visual properties. The spectral transmittance properties of the examined filters vary widely, in terms of shapes and cut-off wavelengths. All filters show relatively more melanopsin attenuation than luminance attenuation (lower melanopsin transmittance than luminous transmittance). Across the data set, we find that melanopsin transmittance and luminous transmittance are correlated. CONCLUSIONS: We suggest that future studies and examinations of the physiological effects of optical filters quantify the visual and non-visual effects of the filters beyond the spectral transmittance, which will eventually aid in developing a mechanistic understanding of how different filters affect physiology. We strongly discourage comparing the downstream effects of different filters on, e.g. sleep or circadian responses, without considering their effects on the retinal stimulus.

Design of an Ultrafast G Protein Switch Based on a Mouse Melanopsin Variant.

The primary goal of optogenetics is the light-controlled noninvasive and specific manipulation of various cellular processes. Herein, we present a hybrid strategy for targeted protein engineering combining computational techniques with electrophysiological and UV/visible spectroscopic experiments. We validated our concept for channelrhodopsin-2 and applied it to modify the less-well-studied vertebrate opsin melanopsin. Melanopsin is a promising optogenetic tool that functions as a selective molecular light switch for G protein-coupled receptor pathways. Thus, we constructed a model of the melanopsin Gq protein complex and predicted an absorption maximum shift of the Y211F variant. This variant displays a narrow blue-shifted action spectrum and twofold faster deactivation kinetics compared to wild-type melanopsin on G protein-coupled inward rectifying K+ (GIRK) channels in HEK293 cells. Furthermore, we verified the in vivo activity and optogenetic potential for the variant in mice. Thus, we propose that our developed concept will be generally applicable to designing optogenetic tools.

An Investigation of the Role of Macular Pigment in Attenuating Photostress through Comparison between Blue and Green Photostress Recovery Times.

PURPOSE: Photostress recovery time (PSRT) is the time required for the macula to return to its normal functioning after the bleaching of cone photopigments due to light exposure, usually white. This work investigates the role of macular pigment (MP) as an optical filter that attenuates photostress by analyses of PSRT at different wavelengths. METHODS: Thirty-nine subjects (19-28 years) were exposed to blue/green photostress varying in irradiance. During photostress, pupil constriction (Cp) was measured. Twenty-seven subjects (20-27 years) were exposed to white photostress. After 25 s of photostress, the time (PSRT) required to read correctly a 0.2 logMAR letter was measured. Correlation was studied between PSRT, CP, and irradiance. Statistical significance of differences between PSRTs was evaluated at Log(irradiance(quanta s-1 cm-2)) = 14 by Student's t statistics. RESULTS: Cp and PSRT were found linearly correlated to Log(irradiance) for blue, green, and white. At Log(irradiance(quanta s-1 cm-2)) = 14, blue and green mean PSRTs resulted different (p < 0.001) with 3.8 ± 0.8 s and 6.7 ± 1.7 s, respectively. After correcting irradiance for the optical absorption of MP, mean blue PSRT became 6.6 ± 0.8 s, at the logarithm of MP-corrected irradiance in quanta s-1 cm-2 equal to 14 (p = 0.571 compared to green PSRT). For white light, at the logarithm of MP-corrected irradiance in quanta s-1 cm-2 equal to 14, mean PSRT was 7.5 ± 2.2 s, not significantly different from blue and green PSRT (p > 0.05). CONCLUSIONS: MP plays the role of an optical filter attenuating photostress. PSRT was substantially proportional to the number of incident photons corrected for the MP optical absorption, regardless of their wavelength.

Hydrogen/Deuterium Exchange Mass Spectrometry of Human Green Opsin Reveals a Conserved Pro-Pro Motif in Extracellular Loop 2 of Monostable Visual G Protein-Coupled Receptors.

Opsins comprise the protein component of light sensitive G protein-coupled receptors (GPCRs) in the retina of the eye that are responsible for the transduction of light into a biochemical signal. Here, we used hydrogen/deuterium (H/D) exchange coupled with mass spectrometry to map conformational changes in green cone opsin upon light activation. We then compared these findings with those reported for rhodopsin. The extent of H/D exchange in green cone opsin was greater than in rhodopsin in the dark and bleached states, suggesting a higher structural heterogeneity for green cone opsin. Further analysis revealed that green cone opsin exists as a dimer in both dark (inactive) and bleached (active) states, and that the predicted glycosylation sites at N32 and N34 are indeed glycosylated. Comparison of deuterium uptake between inactive and active states of green cone opsin also disclosed a reduced solvent accessibility of the extracellular N-terminal region and an increased accessibility of the chromophore binding site. Increased H/D exchange at the extracellular side of transmembrane helix four (TM4) combined with an analysis of sequence alignments revealed a conserved Pro-Pro motif in extracellular loop 2 (EL2) of monostable visual GPCRs. These data present new insights into the locus of chromophore release at the extracellular side of TM4 and TM5 and provide a foundation for future functional evaluation.

Retinal Attachment Instability Is Diversified among Mammalian Melanopsins.

Melanopsins play a key role in non-visual photoreception in mammals. Their close phylogenetic relationship to the photopigments in invertebrate visual cells suggests they have evolved to acquire molecular characteristics that are more suited for their non-visual functions. Here we set out to identify such characteristics by comparing the molecular properties of mammalian melanopsin to those of invertebrate melanopsin and visual pigment. Our data show that the Schiff base linking the chromophore retinal to the protein is more susceptive to spontaneous cleavage in mammalian melanopsins. We also find this stability is highly diversified between mammalian species, being particularly unstable for human melanopsin. Through mutagenesis analyses, we find that this diversified stability is mainly due to parallel amino acid substitutions in extracellular regions. We propose that the different stability of the retinal attachment in melanopsins may contribute to functional tuning of non-visual photoreception in mammals.

Using Flickering Light to Enhance Nonimage-Forming Visual Stimulation in Humans.

PURPOSE: Intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate nonimage-forming visual functions such as pupillary constriction and circadian photoentrainment. Optimizing daytime nonimage-forming photostimulation has health benefits. We aimed to enhance ipRGC excitation using flickering instead of steady light. METHODS: Human subjects were tested with a three-dimensional matrix of flickering 463-nm stimuli: three photon counts (13.7, 14.7 and 15.7 log photons cm(-2)), three duty cycles (12%, 47%, and 93%) and seven flicker frequencies (0.1, 0.25, 0.5, 1, 2, 4, and 7 Hz). Steady-state pupil constrictions were measured. RESULTS: Among stimuli containing 13.7 log photons cm-2, the one flickering at 2 Hz with a 12% duty cycle evoked the greatest pupil constriction of 48% ± 4%, 71% greater than that evoked by an equal-intensity (12.3 log photons cm(-2) s(-1)) continuous light. This frequency and duty cycle were also best for 14.7 log photons cm-2 stimuli, inducing a 58% ± 4% constriction which was 38% more than that caused by an equal-intensity (13.3 log photons cm(-2) s(-1)) constant light. For 15.7 log photons cm-2 stimuli, the 1-Hz, 47% duty cycle flicker was optimal although it evoked the same constriction as the best 14.7 log photons cm(-2) flicker. CONCLUSIONS: Pupillary constriction depends on flicker frequency and duty cycle besides intensity. Among the stimuli tested, the one with the lowest photon count inducing a maximal response is 13.3 log photons cm(-2) s(-1) flickering at 2 Hz with 12% duty cycle. Our data could guide the design of healthier architectural lighting and better phototherapy devices for treating seasonal affective disorder and jet lag.

[Intrinsically Photosensitive Retinal Ganglion Cells]. / Vnitrne fotosenzitivní gangliové bunky sítnice.

Recently discovered intrinsically photosensitive melanopsin-containing retinal ganglion cells contribute to circadian photoentrainment and pupillary constriction; recent works have also brought new evidence for their accessory role in the visual system in humans. Pupil light reaction driven by individual photoreceptors can be isolated by means of the so called chromatic pupillography. The use of chromatic stimuli to elicit different pupillary responses may become an objective clinical pupil test in the detection of retinal diseases and in assessing new therapeutic approaches particularly in hereditary retinal degenerations like retinitis pigmentosa. In advanced stages of disease, the pupil light reaction is even more sensitive than standard electroretinography for detecting residual levels of photoreceptor activity. This review summarizes current knowledge on intrinsically photosensitive retinal cells and highlights its possible implications for clinical practice.

[Circadian clock and non-visual functions: the role of light in humans]. / Horloge circadienne et fonctions non visuelles : rôle de la lumière chez l'Homme.

Hormonal secretion, cognitive performance, motor activity, metabolic processes, the sleep wake cycle and, most recently shown, cell division and ADN repair show a 24 h rhythmicity that is driven by the circadian timing system (the biological clock). Their appropriate activity over the 24 h requires appropriate entrainment of the circadian clock, which is achieved through the synchronizing effects of ocular light exposure. The activation of melanopsin-expressing ganglion cells in the retina depends on timing, quality, intensity, and history of light exposure. Inappropriate lighting leads to inappropriate synchronization of the clock, and activation of non-visual functions (mood, wakefulness, cognition, etc.). In turn, a deficit of circadian entrainment to the 24 h is responsible for alterations of a large number of functions, and leads to altered sleep, wake, mood, neurobehavioral processes and cell division, but also to pathologies. The crucial role of the circadian clock and the nature of the non-visual functions activated by light give rise to the concept that light is a biological need fundamental to health. Without an appropriate light hygiene, the clock receives an odd tempo, and it is cacophony!

Optical control demonstrates switch-like PIP3 dynamics underlying the initiation of immune cell migration.

There is a dearth of approaches to experimentally direct cell migration by continuously varying signal input to a single cell, evoking all possible migratory responses and quantitatively monitoring the cellular and molecular response dynamics. Here we used a visual blue opsin to recruit the endogenous G-protein network that mediates immune cell migration. Specific optical inputs to this optical trigger of signaling helped steer migration in all possible directions with precision. Spectrally selective imaging was used to monitor cell-wide phosphatidylinositol (3,4,5)-triphosphate (PIP3), cytoskeletal, and cellular dynamics. A switch-like PIP3 increase at the cell front and a decrease at the back were identified, underlying the decisive migratory response. Migration was initiated at the rapidly increasing switch stage of PIP3 dynamics. This result explains how a migratory cell filters background fluctuations in the intensity of an extracellular signal but responds by initiating directionally sensitive migration to a persistent signal gradient across the cell. A two-compartment computational model incorporating a localized activator that is antagonistic to a diffusible inhibitor was able to simulate the switch-like PIP3 response. It was also able simulate the slow dissipation of PIP3 on signal termination. The ability to independently apply similar signaling inputs to single cells detected two cell populations with distinct thresholds for migration initiation. Overall the optical approach here can be applied to understand G-protein-coupled receptor network control of other cell behaviors.

Non-circadian direct effects of light on sleep and alertness: lessons from transgenic mouse models.

Light exerts a strong non-visual influence on human physiology and behavior. Additionally light is known to affect sleep indirectly through the phase shifting of circadian rhythms, and directly, promoting alertness in humans and sleep in nocturnal species. Little attention has been paid to the direct non-image-forming influence of light until recently with the discovery and emerging knowledge on melanopsin, a photopigment which is maximally sensitive to the blue spectrum of light and expressed in a subset of intrinsically photosensitive retinal ganglion cells. Indeed, the development of transgenic mouse models targeting different phototransduction pathways has allowed researchers to decipher the mechanisms by which mammals adapt sleep to their light environment. This review summarizes the novel concepts and discrepancies from recent publications relating to the non-circadian effects of light on sleep and waking. Specifically, we discuss whether darkness, in addition to light, affects their quality. Furthermore, we seek to understand whether longer sustained periods of light exposure can influence sleep, if the direct photic regulation depends on time of day, and whether this affects the homeostatic sleep process. Moreover, the neural pathways by which light exerts a direct influence on sleep will be discussed including the respective role of rods/cones and melanopsin. Finally, we suggest that light weighs on the components of the flip-flop switch model to induce respectively sleep or waking, in nocturnal and diurnal animals. Taking these data into account we therefore propose a novel model of sleep regulation based on three processes; the direct photic regulation interacting with the circadian and homeostatic drives to determine the timing and quality of sleep and waking. An outlook of promising clinical and non-clinical applications of these findings will be considered as well as directions for future animal and human research.

Ultraviolet visual sensitivity in three avian lineages: paleognaths, parrots, and passerines.

Ultraviolet (UV) light-transmitted signals play a major role in avian foraging and communication, subserving functional roles in feeding, mate choice, egg recognition, and nestling discrimination. Sequencing functionally relevant regions of the short wavelength sensitive type 1 (SWS1) opsin gene that is responsible for modulating the extent of SWS1 UV sensitivity in birds allows predictions to be made about the visual system's UV sensitivity in species where direct physiological or behavioral measures would be impractical or unethical. Here, we present SWS1 segment sequence data from representative species of three avian lineages for which visually based cues for foraging and communication have been investigated to varying extents. We also present a preliminary phylogenetic analysis and ancestral character state reconstructions of key spectral tuning sites along the SWS1 opsin based on our sequence data. The results suggest ubiquitous ultraviolet SWS1 sensitivity (UVS) in both paleognaths, including extinct moa (Emeidae), and parrots, including the nocturnal and flightless kakapo (Strigops habroptilus), and in most, but not all, songbird (oscine) lineages, and confirmed violet sensitivity (VS) in two suboscine families. Passerine hosts of avian brood parasites were included both UVS and VS taxa, but sensitivity did not co-vary with egg rejection behaviors. The results should stimulate future research into the functional parallels between the roles of visual signals and the genetic basis of visual sensitivity in birds and other taxa.

Light-dependent phosphorylation of the carboxy tail of mouse melanopsin.

Melanopsin-based phototransduction is involved in non-image forming light responses including circadian entrainment, pupil constriction, suppression of pineal melatonin synthesis, and direct photic regulation of sleep in vertebrates. Given that the functions of melanopsin involve the measurement and summation of total environmental luminance, there would appear to be no need for the rapid deactivation typical of other G-protein coupled receptors. In this study, however, we demonstrate that heterologously expressed mouse melanopsin is phosphorylated in a light-dependent manner, and that this phosphorylation is involved in regulating the rate of G-protein activation and the lifetime of melanopsin's active state. Furthermore, we provide evidence for light-dependent phosphorylation of melanopsin in the mouse retina using an in situ proximity ligation assay. Finally, we demonstrate that melanopsin preferentially interacts with the GRK2/3 family of G-protein coupled receptor kinases through co-immunoprecipitation assays. Based on the complement of G-protein receptor kinases present in the melanopsin-expressing retinal ganglion cells, GRK2 emerges as the best candidate for melanopsin's cognate GRK.

Vertebrate ancient-long opsin has molecular properties intermediate between those of vertebrate and invertebrate visual pigments.

VA/VAL opsin is one of the four kinds of nonvisual opsins that are closely related to vertebrate visual pigments in the phylogenetic tree of opsins. Previous studies indicated that among these opsins, parapinopsin and pinopsin exhibit molecular properties similar to those of invertebrate bistable visual pigments and vertebrate visual pigments, respectively. Here we show that VA/VAL opsin exhibits molecular properties intermediate between those of parapinopsin and pinopsin. VAL opsin from Xenopus tropicalis was expressed in cultured cells, and the pigment with an absorption maximum at 501 nm was reconstituted by incubation with 11-cis-retinal. Light irradiation of this pigment caused cis-to-trans isomerization of the chromophore to form a state having an absorption maximum in the visible region. This state has the ability to activate Gi and Gt types of G proteins. Therefore, the active state of VAL opsin is a visible light-absorbing intermediate, which probably has a protonated retinylidene Schiff base as its chromophore, like the active state of parapinopsin. However, this state was apparently photoinsensitive and did not show reverse reaction to the original pigment, unlike the active state of parapinopsin, and instead similar to that of pinopsin. Furthermore, the Gi activation efficiency of VAL opsin was between those of pinopsin and parapinopsin. Thus, the molecular properties of VA/VAL opsin give insights into the mechanism of conversion of the molecular properties from invertebrate to vertebrate visual pigments.

Guinea pigs reared in a monochromatic environment exhibit changes in cone density and opsin expression.

This study aimed to determine if a monochromatic environment will affect the development of cones in a guinea pig model. Thirty 3-day-old guinea pigs were randomized into three groups and exposed to green, violet, and white light (control) for 8 weeks. The animals were sacrificed and the density of middle-wavelength cones (M cones) and short-wavelength sensitive (S cones) and expression of M-opsin and S-opsin were determined. The density of M cones was increased in the green light group as compared to the control group, and decreased in the violet light group as compared to the control group (both, p < 0.05). There was no significant difference in the density of the S cones among the groups (all, p > 0.05). The density of coexpressing cones in the middle retina was significantly increased in the green light group in comparison to the violet light group (p < 0.01). In addition, there was a significant increase in the level of M-opsin as determined by Western blotting and M-opsin mRNA expression as determined by PCR analysis in the green light group as compared to the control group and a significant decrease in violet light group as compared to the control group (all, p < 0.05). No significant difference in S-opsin level or S-opsin mRNA expression was noted among the groups. We concluded that monochromatic lighting affected the density of cones and expression of opsins in a guinea pig model, and this indicates that the retinal color visual system of the guinea pig possess developmental plasticity.

Perinatal development of melanopsin expression in the mouse retina.

The melanopsin-expressing retinal ganglion cells are specialized in measuring irradiance for several functions, including daily photoentrainment and regulation of pupil size. In the present study, these cells were analyzed in mice during their perinatal period, from embryonic day (E) 15 to postnatal day (P) 1. Melanopsin expression was detected at E15 in cells that did not co-express the transcription factor Brn3a. Under light/dark (LD) cycles, the number of melanopsin-expressing cells did not change between E16 and E19, while a very significant increase was observed during the short interval around birth, between E19 (the day before birth) and P0 (the day of birth). As these samples were collected after lights on, to determine whether such increase in melanopsin expression was driven by light, we also analyzed samples collected 0-4 hours after birth (during the night period), which revealed that the cell number increase was already present and, therefore, was not induced by the early post-birth light exposure. To clarify the role of ambient light conditions during this period, P1 retinas from pups under constant light or darkness conditions were also analyzed and compared to those of mice under LD cycles. No variation in the number of immunostained cells was detected among the groups studied, indicating that ambient conditions did not provoke the increase in melanopsin expression detected. Rather, it might have been induced by either a maternal or a developmental signal and is likely related to the first connections between the retina and the suprachiasmatic nucleus reported by other authors.

Neuronal regulation of photo-induced pineal photoreceptor proteins in carp Catla catla.

In the present in vitro study on the pineal in carp Catla catla, specific agonist and antagonists of receptors for different neuronal signals and regulators of intra-cellular Ca(++) and cAMP were used to gather basic information on the neuronal signal transduction cascade mechanisms in the photo-induced expression of rod-like opsin and alpha-transducin-like proteins in any fish pineal. Western-blot analysis followed by quantitative analysis of respective immunoblot data for both the proteins revealed that photo-induced expression of each protein was stimulated by cholinergic (both nicotinic and muscarinic) agonists and a dopaminergic antagonist, inhibited by both cholinergic antagonists and a dopaminergic agonist, but not affected by any agonists or antagonists of adrenergic (alpha(1), alpha(2) and beta(1)) receptors. Moreover, expression of each protein was stimulated by voltage gated L type calcium channel blocker, adenylate cyclase inhibitor and phosphodiesterase activator; but suppressed by the activators of both calcium channel and adenylate cyclase, and by phosphodiesterase inhibitor. Collectively, we report for the first time that both cholinergic and dopaminergic signals play an important, though antagonistic, role in the photo-induced expression of photoreceptor proteins in the fish pineal through activation of a signal transduction pathway in which both calcium and cAMP may act as the intracellular messengers.