George C. Brainard

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.

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 glands response to light in the hamster may be either rhodopsin or another blue-sensitive chromophore.

The effect of different light intensities on pineal melatonin content

In syrian hamsters, elevated night-time pineal melatonin levels are quickly reduced to low daytime levels by exposing the animals to light. The purpose of this study was to determine the lowest light intensity capable of causing a large reduction in night-time levels of pineal melatonin in the male hamster. During the dark phase of the light:dark cycle, groups of hamsters were exposed to one of 8 different intensities of white fluorescent light: 5380, 2798, 151, 20.44, 5.38, 1.08, 0.11 and 0.01 lux. For each light intensity, pineals were collected from 8 hamsters each at 2 min before and at 2, 8 and 32 min after the lights were turned on. Pineal melatonin content was determined by radioimmunoassay. Light intensities of 1.08 lux or greater depressed pineal melatonin content significantly (P less than 0.001). Light intensities of 0.11 or 0.01 lux failed to depress pineal melatonin levels. Thus, the apparent threshold for the action of white fluorescent light on hamster pineal melatonin content lies between l.08 and 0.11 lux.

Pineal melatonin in syrian hamsters: circadian and seasonal rhythms in animals maintained under laboratory and natural conditions.

The object of the following study was to compare pineal melatonin rhythms of hamsters housed in outdoor versus laboratory conditions during five consecutive seasons. For each season, 72 adult male Syrian hamsters were caged under controlled laboratory conditions and 72 were caged in a three-sided shelter outdoors. The light:dark cycle for the animals kept in the laboratory approximated the corresponding day:night lengths of each season. After hamsters were exposed to their respective environments for 3 weeks, pineal glands were collected from 8 animals from each group at 08.00, 12.00, 17.00, 20.00, 22.00, 24.00, 02.00, 04.00 and 06.00 h. Radioimmunoassay was used to determine pineal melatonin content. All groups of animals displayed a circadian rhythm of pineal melatonin with peak nighttime levels of melatonin being 8- to 12-fold greater than daytime levels. Compared to animals kept in the laboratory, hamsters exposed to natural seasonal conditions appear to produce significantly more melatonin during the winter and significantly less melatonin during the summer and fall. A seasonal rhythm of melatonin synthesis was observed in animals kept in the laboratory and outdoors.

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.

The Influence of Various Irradiances of Artificial Light, Twilight, and Moonlight on the Suppression of Pineal Melatonin Content in the Syrian Hamster

The purpose of the present studies using artificial light was to determine how the timing and duration of exposure influence the light‐induced suppression of pineal melatonin levels in hamsters. An 8‐min exposure to 0.186 μW/cm2 of cool white fluorescent light caused a continued depression of pineal melatonin even when animals were returned to darkness. In addition, the pineal gland does not appear to change its sensitivity to light throughout the night. A 20‐min exposure to 0.019 μW/cm2 of cool white fluorescent light did not significantly suppress pineal melatonin during any time of the melatonin peak, whereas a 20‐min exposure to 0.186 μW/cm2was capable of always suppressing melatonin. Furthermore, increasing the duration of 0.019‐μW/cm2 exposure to 30, 60, 120, or 180 min does not increase the capacity of this irradiance to depress melatonin.

Influence of light irradiance on hydroxyindole-O-methyltransferase activity, serotonin-N-acetyltranferase activity, and radioimmunoassayable melatonin levels in the pineal gland of the diurnally active Richardson's ground squirrel

When Richardsons ground squirrels were kept under light:dark cycles of 14:10 h there was no nocturnal rise in pineal hydroxyindole-O-methyltransferase (HIOMT) activity. Conversely, the 10 h dark period was associated with large nocturnal rises in both pineal serotonin-N-acetyltransferase (NAT) activity and radioimmunoassayable melatonin levels. The nighttime rises in pineal NAT and melatonin were not suppressed by the exposure of the animals to a light irradiance of 925 mu W/cm2 during the normal dark period. On the other hand, when the light irradiance was increased to 1850 mu W/cm2 the rise in pineal NAT activity was eliminated while the melatonin rise was greatly reduced. When ground squirrels were acutely exposed to a light irradiance of 1850 mu W/cm2 for 30 min beginning at 5.5 h after lights out, pineal NAT activity and melatonin levels were reduced to daytime values within 30 min. The half-time (t 1/2) for each constituent was less than 10 min. Exposure to a light irradiance of either 5 s or 5 min (beginning at 5.5 h into dark period) was equally as effective as 30 min light exposure in inhibiting pineal NAT activity and melatonin levels. When animals were returned to darkness after a 30 min exposure to a light irradiance of 1850 mu W/cm2 at night, both pineal NAT activity and melatonin levels were restored to high nighttime levels within 2 h of their return to darkness. The results indicate that the pineal gland of the wild-captured, diurnal Richardsons ground squirrel is 9000 X less sensitive to light at night than is the pineal gland of the laboratory raised, nocturnal Syrian hamster.

Melatonin Given in the Morning Prevents the Suppressive Action on the Reproductive System of Melatonin Given in Late Afternoon

Melatonin administered to female hamsters kept under light:dark cycles of 14:10 (in hours) normally suppresses reproductive processes only if the indoleamine is administered later than 6.5 h after the beginning of the photoperiod. In the present study, we investigated the influence of morning (11.00 h) injections of melatonin on the reproductive inhibitory effects of afternoon (17.00 h) melatonin injections. Adult female hamsters were exposed to light daily from 06.00 to 20.00 h. The animals were divided into the following experimental groups: group 1, injected with vehicle at both 11.00 and 17.00 h; group 2, injected with vehicle at 11.00 h and 25 microgram melatonin at 17.00 h; group 3, injected with 1 mg melatonin at 11.00 h and 25 microgram melatonin at 17.00 h; group 4, injected with 1 mg melatonin at 11.00 h and vehicle at 17.00 h. Control animals injected with vehicle at both 11.00 and 17.00 h had normal 4-day estrous cycles throughout the 8 weeks of the experiment. 100% of the animals injected with vehicle in the morning and melatonin in the afternoon became acyclic within 7 weeks. However, if the afternoon injections of melatonin were preceded by morning injections of the indoleamine, the animals continued to exhibit normal estrous cycles. Also, hamsters injected with melatonin at 11.00 h and vehicle at 15.00 h had normal estrous cycles throughout the study. At the conclusion of the experiment, the uterine weights and plasma prolactin levels in the animals that received vehicle at 11.00 h and malatonin at 17.00 h were depressed compared to those in the vehicle-vehicle injected controls. Again, the morning injections of melatonin prevented the afternoon injections of melatonin from decreasing either the uterine weights or the plasma prolactin levels. It is concluded that the morning injections of melatonin either down-regulated the melatonin receptors or decreased their number and thereby rendered afternoon injections of melatonin incapable of inhibiting reproductive processes.

Experimentally-induced diabetes reduces nocturnal pineal melatonin content in the Syrian hamster

Male, Syrian hamsters were rendered diabetic by either alloxan (60 mg/kg, i.v.) or streptozotocin (65 mg/kg, i.p.). Diabetic animals had reduced pineal melatonin contents during the night. Basal daytime values were not significantly altered. Diabetes may decrease melatonin synthesis by reducing the availability of glucose for metabolism or by decreasing the transport of tryptophan into pinealocytes for the synthesis of melatonin.

Influence of Morning Melatonin Injections on the Antigonadotrophic Effects of Afternoon Melatonin Administration in Male and Female Hamsters

Male and female hamsters were maintained on a long photoperiod (14L:10D) and were divided into the following groups: group 1, injected with vehicle at both 11.00 and 17.00 h; groups 2-5, injected with various doses of melatonin at 11.00 h (0, 100 micrograms, 500 micrograms, 1 mg) and 25 g of melatonin at 17.00 h. In males, a.m. vehicle p.m. melatonin treatments for 70 days led to atrophy of the testes and accessory sex organs and to a significant depression of both plasma LH and PRL titers. There was a graded inhibition of these actions of exogenous melatonin in animals receiving 100 micrograms, 500 micrograms or 1 mg a.m. injections of melatonin with 1 mg completely preventing the effects of p.m. melatonin. 80% of the female hamsters receiving daily p.m. injections of melatonin for 9 weeks became acyclic with significant decreases in uterine weight and increases in ovarian weight. All doses of melatonin given at 11.00 h suppressed the inhibitory actions of p.m. melatonin with the exception of vaginal cyclicity for which 500 micrograms or more was required to restore normal vaginal cyclicity in all animals. These results demonstrate that morning injections of melatonin can prevent the antigonadotropic effects of afternoon melatonin injections and provide support for the hypothesis that the paradoxical actions of melatonin may be related to the ability of the indole to regulate its own receptors.