Dopaminergic amacrine neurons of rat retinas with photoreceptor degeneration continue to respond to light

Protecting the melatonin rhythm through circadian healthy light exposure

Currently, in developed countries, nights are excessively illuminated (light at night), whereas daytime is mainly spent indoors, and thus people are exposed to much lower light intensities than under natural conditions. In spite of the positive impact of artificial light, we pay a price for the easy access to light during the night: disorganization of our circadian system or chronodisruption (CD), including perturbations in melatonin rhythm. Epidemiological studies show that CD is associated with an increased incidence of diabetes, obesity, heart disease, cognitive and affective impairment, premature aging and some types of cancer. Knowledge of retinal photoreceptors and the discovery of melanopsin in some ganglion cells demonstrate that light intensity, timing and spectrum must be considered to keep the biological clock properly entrained. Importantly, not all wavelengths of light are equally chronodisrupting. Blue light, which is particularly beneficial during the daytime, seems to be more disruptive at night, and induces the strongest melatonin inhibition. Nocturnal blue light exposure is currently increasing, due to the proliferation of energy-efficient lighting (LEDs) and electronic devices. Thus, the development of lighting systems that preserve the melatonin rhythm could reduce the health risks induced by chronodisruption. This review addresses the state of the art regarding the crosstalk between light and the circadian system.

Circadian organization of the mammalian retina: from gene regulation to physiology and diseases

The retinal circadian system represents a unique structure. It contains a complete circadian system and thus the retina represents an ideal model to study fundamental questions of how neural circadian systems are organized and what signaling pathways are used to maintain synchrony of the different structures in the system. In addition, several studies have shown that multiple sites within the retina are capable of generating circadian oscillations. The strength of circadian clock gene expression and the emphasis of rhythmic expression are divergent across vertebrate retinas, with photoreceptors as the primary locus of rhythm generation in amphibians, while in mammals clock activity is most robust in the inner nuclear layer. Melatonin and dopamine serve as signaling molecules to entrain circadian rhythms in the retina and also in other ocular structures. Recent studies have also suggested GABA as an important component of the system that regulates retinal circadian rhythms. These transmitter-driven influences on clock molecules apparently reinforce the autonomous transcription-translation cycling of clock genes. The molecular organization of the retinal clock is similar to what has been reported for the SCN although inter-neural communication among retinal neurons that form the circadian network is apparently weaker than those present in the SCN, and it is more sensitive to genetic disruption than the central brain clock. The melatonin-dopamine system is the signaling pathway that allows the retinal circadian clock to reconfigure retinal circuits to enhance light-adapted cone-mediated visual function during the day and dark-adapted rod-mediated visual signaling at night. Additionally, the retinal circadian clock also controls circadian rhythms in disk shedding and phagocytosis, and possibly intraocular pressure. Emerging experimental data also indicate that circadian clock is also implicated in the pathogenesis of eye disease and compelling experimental data indicate that dysfunction of the retinal circadian system negatively impacts the retina and possibly the cornea and the lens.

Effects of methimepip and JNJ-5207852 in Wistar rats exposed to an open-field with and without object and in Balb/c mice exposed to a radial-arm maze

The role of the histamine H₃ receptor (H₃R) in anxiety is controversial, due to limitations in drug selectivity and limited validity of behavioral tests used in previous studies. In the present report, we describe two experiments. In the first one, Wistar rats were treated with an H₃R agonist (methimepip), and exposed to an open-field. In the second one, Balb/c mice were treated with H₃R agonist (methimepip) or antagonist (JNJ-5207852), and exposed to an open space 3D maze which is a modified version of the radial-arm maze. C57BL/6J saline treated mice were included for comparisons. When exposed to an empty open field, Wistar rats spent more time in the outer area and made very low number of brief crossings in the central area. However, when an object occupied the central area, rats crossed frequently into and spent a long time in the central area. Administration of a range of different doses of methimepip (selective H₃R agonist) reduced the entries into the central area with a novel object, indicating enhanced avoidance response. In the 3D maze, both Balb/c and C57BL/6J saline-treated mice crossed frequently onto the bridges that radiate from the central platform but only C57BL/6J mice crossed onto the arms which extend the bridges. This suggests that Balb/c mice are more anxious than C57BL/6J mice. Neither methimepip nor JNJ-5207852 (selective H₃R antagonist/inverse agonist) induced entry into the arms of the maze, indicative of lack of anxiolytic effects.

Intrinsically photosensitive retinal ganglion cells

Life on earth is subject to alternating cycles of day and night imposed by the rotation of the earth. Consequently, living things have evolved photodetective systems to synchronize their physiology and behavior with the external light-dark cycle. This form of photodetection is unlike the familiar "image vision," in that the basic information is light or darkness over time, independent of spatial patterns. "Nonimage" vision is probably far more ancient than image vision and is widespread in living species. For mammals, it has long been assumed that the photoreceptors for nonimage vision are also the textbook rods and cones. However, recent years have witnessed the discovery of a small population of retinal ganglion cells in the mammalian eye that express a unique visual pigment called melanopsin. These ganglion cells are intrinsically photosensitive and drive a variety of nonimage visual functions. In addition to being photoreceptors themselves, they also constitute the major conduit for rod and cone signals to the brain for nonimage visual functions such as circadian photoentrainment and the pupillary light reflex. Here we review what is known about these novel mammalian photoreceptors.

Light regulation of retinal dopamine that is independent of melanopsin phototransduction

Light-dependent release of dopamine (DA) in the retina is an important component of light-adaptation mechanisms. Melanopsin-containing inner retinal photoreceptors have been shown to make physical contacts with DA amacrine cells, and have been implicated in the regulation of the local retinal environment in both physiological and anatomical studies. Here we determined whether they contribute to photic regulation of DA in the retina as assayed by the ratio of DA with its primary metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), and by c-fos induction in tyrosine hydroxylase (TH)-labelled DA amacrine cells. Light treatment (approximately 0.7 log W/m(2) for 90 min) resulted in a substantial increase in DA release (as revealed by an increase in the DOPAC : DA ratio), as well as widespread induction of nuclear c-fos in DA amacrine cells in wild-type mice and in mice lacking melanopsin (Opn4(-/-)). Light-induced DA release was also retained in mice lacking rod phototransduction (Gnat1(-/-)), although the magnitude of this response was substantially reduced compared with wild-types, as was the incidence of light-dependent nuclear c-fos in DAergic amacrines. By contrast, the DAergic system of mice lacking both rods and cones (rd/rd cl) showed no detectable light response. Our data suggest that light regulation of DA, a pivotal retinal neuromodulator, originates primarily with rods and cones, and that melanopsin is neither necessary nor sufficient for this photoresponse.

Intraretinal signaling by ganglion cell photoreceptors to dopaminergic amacrine neurons

Retinal dopaminergic amacrine neurons (DA neurons) play a central role in reconfiguring retinal function according to prevailing illumination conditions, yet the mechanisms by which light regulates their activity are poorly understood. We investigated the means by which sustained light responses are evoked in DA neurons. Sustained light responses were driven by cationic currents and persisted in vitro and in vivo in the presence of L-AP4, a blocker of retinal ON-bipolar cells. Several characteristics of these L-AP4-resistant light responses suggested that they were driven by melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs), including long latencies, marked poststimulus persistence, and a peak spectral sensitivity of 478 nm. Furthermore, sustained DA neuron light responses, but not transient DA neuron responses, persisted in rod/cone degenerate retinas, in which ipRGCs account for virtually all remaining retinal phototransduction. Thus, ganglion-cell photoreceptors provide excitatory drive to DA neurons, most likely by way of the coramification of their dendrites and the processes of DA neurons in the inner plexiform layer. This unprecedented centrifugal outflow of ganglion-cell signals within the retina provides a novel basis for the restructuring of retinal circuits by light.

Static and extremely low frequency electromagnetic field exposure: reported effects on the circadian production of melatonin

The circadian rhythm of melatonin production (high melatonin levels at night and low during the day) in the mammalian pineal gland is modified by visible portions of the electromagnetic spectrum, i.e., light, and reportedly by extremely low frequency (ELF) electromagnetic fields as well as by static magnetic field exposure. Both light and non-visible electromagnetic field exposure at night depress the conversion of serotonin (5HT) to melatonin within the pineal gland. Several reports over the last decade showed that the chronic exposure of rats to a 60 Hz electric field, over a range of field strengths, severely attenuated the nighttime rise in pineal melatonin production; however, more recent studies have not confirmed this initial observation. Sinusoidal magnetic field exposure also has been shown to interfere with the nocturnal melatonin forming ability of the pineal gland although the number of studies using these field exposures is small. On the other hand, static magnetic fields have been repeatedly shown to perturb the circadian melatonin rhythm. The field strengths in these studies were almost always in the geomagnetic range (0.2 to 0.7 Gauss or 20 to 70 mu tesla) and most often the experimental animals were subjected either to a partial rotation or to a total inversion of the horizontal component of the geomagnetic field. These experiments showed that several parameters in the indole cascade in the pineal gland are modified by these field exposures; thus, pineal cyclic AMP levels, N-acetyltransferase (NAT) activity (the rate limiting enzyme in pineal melatonin production), hydroxyindole-O-methyltransferase (HIOMT) activity (the melatonin forming enzyme), and pineal and blood melatonin concentrations were depressed in various studies. Likewise, increases in pineal levels of 5HT and 5-hydroxyindole acetic acid (5HIAA) were also seen in these glands; these increases are consistent with a depressed melatonin synthesis. The mechanisms whereby non-visible electromagnetic fields influence the melatonin forming ability of the pineal gland remain unknown; however, the retinas in particular have been theorized to serve as magnetoreceptors with the altered melatonin cycle being a consequence of a disturbance in the neural biological clock, i.e., the suprachiasmatic nuclei (SCN) of the hypothalamus, which generates the circadian melatonin rhythm. The disturbances in pineal melatonin production induced by either light exposure or non-visible electromagnetic field exposure at night appear to be the same but whether the underlying mechanisms are similar remains unknown.

The visual input stage of the mammalian circadian pacemaking system: I. Is there a clock in the mammalian eye?

Threads of evidence from recent experimentation in retinal morphology, neurochemistry, electrophysiology, and visual perception point toward rhythmic ocular processes that may be integral components of circadian entrainment in mammals. Components of retinal cell biology (rod outer-segment disk shedding, inner-segment degradation, melatonin and dopamine synthesis, electrophysiological responses) show self-sustaining circadian oscillations whose phase can be controlled by light-dark cycles. A complete phase response curve in visual sensitivity can be generated from light-pulse-induced phase shifting. Following lesions of the suprachiasmatic nuclei, circadian rhythms of visual detectability and rod outer-segment disk shedding persist, even though behavioral activity becomes arrhythmic. We discuss the converging evidence for an ocular circadian timing system in terms of interactions between rhythmic retinal processes and the central suprachiasmatic pacemaker, and propose that retinal phase shifts to light provide a critical input signal.

Responses of the mammalian retina to experimental alteration of the ambient magnetic field

The detection of earth strength magnetic fields by rodents has been demonstrated previously by numerous physiological and behavioral techniques. This phenomenon appears to require input from the eyes. In an effort to better understand this phenomenon retinal melatonin synthesis and catecholamine contents were assayed in rats exposed at night to an alteration of the ambient magnetic field. In normal animals both dopamine and norepinephrine levels in the retina were reduced by this stimulus, while retinal melatonin synthesis was unaffected. Animals that had lost their intact photoreceptors as a result of 8 weeks of previous constant light exposure did not show a catecholamine response to the magnetic stimulus. These results support the view that the mammalian retina participates in the relaying of magnetic information into the central nervous system.

Geomagnetic field detection in rodents

In addition to behavioral evidence for the detection of "earth-strength" magnetic fields (MF) by rodents, recent investigations have revealed that electrophysiological and biochemical responses to MF occur in the pineal organ and retina of rodents. In addition, ferrimagnetic deposits have been identified in the ethmoidal regions of the rodent skull. These findings point to a new sensory phenomenon, which interfaces with many fields of biology, including neuroscience, psychophysics, behavioral ecology, chronobiology and sensory physiology.

Extended exposure to continuous low intensity light abolishes the photosensitivity of retinal dopamine neurons

Male Sprague-Dawley rats were divided into 2 groups. One group (experimental) was housed for 6 months in continuous low intensity light while the other (control) was exposed to standard 14 hr light: 10 hr dark cyclic lighting conditions for the entire time. For both the control and experimental groups the light intensity was 350-700 Lux. After 6 months, the experimental rats were returned to cyclic lighting. At one week and again at 2 months the light aversion behavior of all rats was tested in a light/dark test box. The experimental rats chose the dark side of the box only 58% of the time while control animals preferred the dark 79% of the time. Since rats normally are nocturnal and avoid light, these results suggest that the experimental rats may have permanently lost a functionally significant portion of the ability to detect light. After the second behavioral test all rats were dark adapted and 15 hr later the effect of short term (30 or 60 min) exposure to light on DA turnover in one retina from each rat was assessed. The other retina from each rat was fixed and examined histologically. Light significantly enhanced the alpha methyl-p-tyrosine induced decline of DA in the retinas of the control rats but exerted no similar effect in the experimental animals. The retinal DA contents of the experimental rats were substantially depleted. Histological examination suggested that the outer nuclear layers of the experimental retinas were more severely damaged than those from rats exposed to continuous light for 4 months but still contained a few pycnotic photoreceptor nuclei and nearly normal looking inner neural layers. These results indicate that extended exposure to light eventually abolishes light aversion behavior and at this time there is also a loss of the photosensitivity of the dopaminergic amacrine neurons.

Transsynaptic regulation of olfactory bulb catecholamines in mice and rats

Norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylalanine (DOPA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured simultaneously by high performance liquid chromatography with electrochemical detection in extracts of olfactory bulbs at various intervals after chemical or surgical deafferentation. Chemical deafferentation of mice by intranasal irrigation with Triton X-100 or of rats by olfactory axotomy resulted in a rapid progressive decline of DA and DOPAC and an associated rise in NE in the olfactory bulb. However, after several weeks, these values returned to prelesion levels concomitant with reinnervation of the bulb by the afferent neurons. In contrast, deafferentation by procedures known to prevent reinnervation of the bulb by the afferent chemoreceptor neurons (i.e., a ZnSo4 solution in mice or a surgical procedure in rats) completely blocked the return to pre-lesion values of DA, DOPAC, and NE. The specificity of these effects was demonstrated by the inability of intranasal administration of the neurotoxin 6-hydroxydopamine to alter DA levels, resulting instead in a significant decline in olfactory bulb NE content. These data demonstrate that the DA content of the olfactory bulb can be influenced by either chemical or surgical modulation of the afferent pathway in two different species. This offers additional support for our hypothesis of transsynaptic regulation of intrinsic DA neurons of the bulb by the afferent olfactory chemoreceptor neurons.