A comparison of flash evoked ERG's and ERG's evoked with sinusoidally modulated light stimuli in a number of rodents

Photic threshold for stimulation of testicular growth and pituitary FSH release in male Djungarian hamsters

While the effects of photoperiod on neuroendocrine-gonadal activity have been extensively studied in a number of species, surprisingly little information concerning the quantitative aspects of light regulating reproductive activity is available. In the present experiment, Djungarian hamsters were exposed to two 10 min pulses of light per day and the light irradiance was systematically varied to determine the threshold for photostimulation by white light. After 10 days testes weight and serum follicle-stimulating hormone (FSH) levels were determined. The data indicate that the irradiance threshold necessary for induction of significant increases of both serum FSH levels and testes weight lies between 0.1 and 0.34 microW/cm2 for 10 min pulses of light. These results demonstrate a strong correlation between the effects of light on serum FSH levels and testes weight and provide the first quantitative assessment of the irradiance threshold for light involved in photoperiodic stimulation of the hypothalamic-pituitary gonadal axis of a mammalian species.

One millisecond of light suffices to suppress nighttime pineal melatonin synthesis in rats

The effect of a single high-intensity light pulse with a duration of 1 ms on nighttime pineal activity of male Sprague-Dawley rats was investigated. 10 minutes after light exposure pineal N-actyltransferase activity and melatonin content were significantly reduced. These results show that the rat pineal is capable of responding to very short light flashes of high intensity.

Dose-response relationship between light irradiance and the suppression of plasma melatonin in human volunteers

This study tested the capacity of different irradiances of monochromatic light to reduce plasma melatonin in normal humans. Six healthy male volunteers, 24-34 years old, were exposed to 0.01, 0.3, 1.6, 5, or 13 microW/cm2 of 509 nm monochromatic light for 1 h during the night on separate occasions. Light irradiance depressed plasma melatonin in a dose-response pattern. The data indicate that the mean threshold irradiance for suppressing melatonin is between 1.6 and 5 microW/cm2. Individual variations in threshold responses to monochromatic light were observed among the volunteers.

Light-induced stimulation of retinal dopamine: a dose-response relationship

Light stimulates dopamine (DA) release in the retina. The purpose of this study was to determine the threshold and dose-response relationship between ocular light exposure and retinal DA synthesis in vivo. Groups of dark-adapted rats were exposed to 0, 1, 3, 5, 10, 25, 50, 100 or 1000 microwatts per square centimeter (microW/cm2) of white light for 15 min. Retinal DA and dihydroxyphenylalanine (DOPA) were subsequently quantified by liquid chromatography with electrochemical detection. Both the DA and DOPA data fit hyperbolic curves significantly (P less than 0.01). Exposure to white light at 25 microW/cm2 or greater appears to elicit the maximum response of these neurons. Threshold irradiation is calculated to be 3-5 microW/cm2. These results indicate that retinal DA synthesis and presumably DA neuron activity have a graded response to increasing irradiances of white light.

Effects of partial receptor cell loss on the electroretinogram of chimaeric mice

A series of chimaeric mice were produced by aggregating morulae from rd/rd and normal (+/+) mice. In the retina of chimaeric mice, produced by aggregating morulae of these two genotypes, loss of rd/rd photoreceptor cells results in a patchy distribution of the surviving normal receptor cells. The number of remaining receptor cells vary between individual chimaeras. The inner retinal layers in the chimaeras, as well as in the two parental genotypes remain intact. Electroretinograms were recorded from 16 chimaeric mice, and various parameters were compared with the amount of visual cells present as estimated by the average thickness of the outer nuclear layer. The amplitudes of the a- and b-wave, showed a linear reduction with decreasing thickness of the outer nuclear layer thickness. However, threshold of the b-wave increased only when the thickness of the outer nuclear layer fell to about 25% of the normal thickness while the time-to-peak of the waves did not change appreciably among the chimaeric individuals. These results suggest that the changes in the electroretinogram of the chimaeric individuals are related to the amount of visual cells present in the retina.

Photoperiodic and light spectral conditions which inhibit circulating concentrations of thyroxine in the male hamster

Adult male Syrian hamsters were exposed daily for 12 weeks to 11 h/day of cool white fluorescent light (350 +/- 50 microW/cm2) followed by an additional 3 h of near ultraviolet (339-317 nm), blue (435-500 nm), green (515-550 nm), yellow (558-636 nm) or red (653-668 nm) light at an irradiance of 0.2 microW/cm2 or to total darkness. Animals exposed to the wavelengths between 558-668 nm (yellow or red half peak bandwidths) or those receiving a total of 13 h of darkness/day had suppressed circulating levels of thyroxine (T4), a depressed free T4 index (FT4I) and a higher T3/T4 ratio compared to animals receiving a total of 14 h of white light (350 +/- 50 microW/cm2). These results suggest that specific wavelengths of light can affect the neuroendocrine-thyroid axis.

Development and degeneration of retina in rds mutant mice: the electroretinogram

In mice, homozygous for the retinal degeneration slow (rds) gene, the photoreceptor cells lack outer segment disc structures, contain low amounts of rhodopsin, and degenerate slowly, while the inner retinal layers remain intact. The electroretinogram (ERG) from 1-month-old mutant mice shows a lower than normal amplitude but the time-to-peak is normal. The ERGs from mutant mice of 2-3 months and 6-7 months of age show a further decline in response amplitude as the receptor cell population is depleted with progress of degeneration while the time-to-peak increases. The ERG is absent in 1-year-old mutant mice which have lost their receptor cells completely. The possible morphological correlations of the ERG and its components in the rds mutant mice are discussed.

The influence of different light spectra on the suppression of pineal melatonin content in the Syrian hamster

The purpose of this study was to test the capacity of different visible wavelengths of light to suppress nocturnal levels of pineal melatonin in hamsters. It was found that the visible wavelengths vary in their ability to perturb pineal melatonin. During the period of peak pineal melatonin production, animals were exposed to fluorescent light sources having half-peak bandwidths of 339-371 nm (near-ultraviolet), 435-500 nm (blue), 510-550 nm (green), 558-636 nm (yellow) and 653-668 nm (red). In each experiment, animals were exposed to equal irradiances of each light source. The different irradiances used were 0.928, 0.200, 0.186, 0.074 and 0.019 microW/cm2. The resultant data demonstrated that blue fluorescent light was the most efficient in suppressing pineal melatonin. Green fluorescent light was found to be the next most efficient light for inhibiting pineal melatonin followed by yellow fluorescent light. Near-ultraviolet and red light were the least capable of suppressing pineal melatonin. These observations suggest that the retinal photopigment responsible for mediating the pineal gland's response to light in the hamster may be either rhodopsin or another blue-sensitive chromophore.