The influence of different light intensities on pineal melatonin content in the retinal degenerate C3H mouse and the normal CBA mouse

Methods to Assess Melatonin Receptor-Mediated Phase-Shift and Re-entrainment of Rhythmic Behaviors in Mouse Models

The neurohormone melatonin facilitates entrainment of biological rhythms to environmental light-dark conditions as well as phase-shifts of circadian rhythms in constant conditions via activation of the MT₁ and/or MT₂ receptors expressed within the suprachiasmatic nucleus of the hypothalamus. The efficacy of melatonin and related agonists to modulate biological rhythms can be assessed using two well-validated mouse models of rhythmic behaviors. These models serve as predictive measures of therapeutic efficacy for treatment of circadian phase disorders caused by internal (e.g., clock gene mutations, blindness, depression, seasonal affective disorder) or external (e.g., shift work, travel across time zones) causes in humans. Here we provide background and detailed protocols for quantitative assessment of the magnitude and efficacy of melatonin receptor ligands in mouse circadian phase-shift and re-entrainment paradigms. The utility of these models in the discovery of novel therapeutics acting on melatonin receptors will also be discussed.

Photoperiod integration in C3H rd1 mice

In mammals, the suprachiasmatic nuclei (SCN) constitute the main circadian clock, receiving input from the retina which allows synchronization of endogenous biological rhythms with the daily light/dark cycle. Over the year, the SCN encodes photoperiodic variations through duration of melatonin secretion, with abundant nocturnal levels in winter and lower levels in summer. Thus, light information is critical to regulate seasonal reproduction in many species and is part of the central photoperiodic integration. Since intrinsically photosensitive retinal ganglion cells (ipRGCs) are vital for circadian photoentrainment and other nonvisual functions, we studied the contribution of ipRGCs in photoperiod integration in C3H retinal degeneration 1 (rd1) mice. We assessed locomotor activity and melatonin secretion in mice exposed to short or long photoperiods. Our results showed that rd1 mice are still responsive to photoperiod variations in term of locomotor activity, melatonin secretion, and regulation of the reproductive axis. In addition, retinas of animals exposed to short photoperiod exhibit higher melanopsin labeling intensity compared with the long photoperiod condition, suggesting seasonal-dependent changes within this photoreceptive system. These results show that ipRGCs in rd1 mice can still measure photoperiod and suggest a key role of melanopsin cells in photoperiod integration and the regulation of seasonal physiology.

Limited evidence for affective and diurnal rhythm responses to dim light-at-night in male and female C57Bl/6 mice

Circadian rhythms are recurring patterns in a range of behavioural, physiological and molecular parameters that display periods of near 24 h, and are underpinned by an endogenous biological timekeeping system. Circadian clocks are increasingly recognised as being key for health. Environmental light is the key stimulus that synchronises the internal circadian system with the external time cues. There are emergent health concerns regarding increasing worldwide prevalence of electric lighting, especially man-made light-at-night, and light's impact on the circadian system may be central to these effects. A number of previous studies have demonstrated increased depression-like behaviour in various rodent experimental models exposed to dim light-at-night. In this study we set out to study the impact of dim light-at-night on circadian and affective behaviours in C57Bl/6 mice. We set out specifically to examine the impact of sex on light at night's effects, as well as the impact of housing conditions. We report minimal impact of light-at-night on circadian and affective behaviours, as measured by the tail suspension test, the forced swim test, the sucrose preference test and the elevated plus maze. Light-at-night was also not associated with an increase in body weight, but was associated with a decrease in the cell proliferation marker Ki-67 in the dentate gyrus. In summary, we conclude that experimental contextual factors, such as model species or strain, may be considerable importance in the investigation of the impact of light at night on mood-related parameters.

The daily melatonin pattern in Djungarian hamsters depends on the circadian phenotype

Djungarian hamsters (Phodopus sungorus) bred at the Institute of Halle reveal three different circadian phenotypes. The wild type (WT) shows normal locomotor activity patterns, whereas in hamsters of the DAO (delayed activity onset) type, the activity onset is continuously delayed. Since the activity offset in those hamsters remains coupled to "light-on," the activity time becomes compressed. Hamsters of the AR (arrhythmic) type are episodically active throughout the 24 h. Previous studies showed that a disturbed interaction of the circadian system with the light-dark (LD) cycle contributes to the phenomenon observed in DAO hamsters. To gain better insight into the underlying mechanisms, the authors investigated the daily melatonin rhythm, as it is a reliable marker of the circadian clock. Hamsters were kept individually under standardized laboratory conditions (LD 14:10, T=22°C±2°C, food and water ad libitum). WT, DAO (with exactly 5 h delay of activity onset), and AR hamsters were used for pineal melatonin and urinary 6-sulfatoxymelatonin (aMT6s) measurement. Pineal melatonin content was determined at 3 time points: 4 h after "light-off" [D+4], 1 h before "light-on" [L-1], and 1h after "light-on" [L+1]). The 24-h profile of melatonin secretion was investigated by transferring the animals to metabolic cages for 27?h to collect urine at 3-h intervals for aMT6s analysis. WT hamsters showed high pineal melatonin content during the dark time (D+4, L-1), which significantly decreased at the beginning of the light period (L+1). In contrast, DAO hamsters displayed low melatonin levels during the part of the dark period when animals were still resting (D+4). At the end of the dark period (L-1), melatonin content increased significantly and declined again when light was switched on (L+1). AR hamsters showed low melatonin levels, comparable to daytime values, at all 3 time points. The results were confirmed by aMT6s data. WT hamsters showed a marked circadian pattern of aMT6s excretion. The concentration started to increase 3?h after "light-off" and reached daytime values 5 h after "light-on." In DAO hamsters, in contrast, aMT6s excretion started about 6?h later and reached significantly lower levels compared to WT hamsters. In AR animals, aMT6s excretion was low at all times. The results clearly indicate the rhythm of melatonin secretion in DAO hamsters is delayed in accord with their delayed activity onset, whereas AR hamsters display no melatonin rhythm at all. Since the regulatory pathways for the rhythms of locomotor activity and melatonin synthesis (which are downstream from the suprachiasmatic nucleus [SCN]) are different but obviously convey the same signal, we conclude that the origin of the phenomenon observed in DAO hamsters must be located upstream of the SCN, or in the SCN itself.

Loss of circadian rhythm and light-induced suppression of pineal melatonin levels in Cry1 and Cry2 double-deficient mice

Cryptochrome 1 and 2 (Cry1 and Cry2) are considered essential for generating circadian rhythms in mammals. The role of Cry1 and Cry2 in circadian rhythm expression and acute light-induced suppression of pineal melatonin was assessed using Cry1 and Cry2 double-deficient mice (Cry1(-/-) /Cry2(-/-) ) developed from the C3H strain that synthesizes melatonin. We examined the circadian variation of pineal melatonin under a 12:12-h light-dark (LD) cycle and constant darkness (DD). Light suppression of pineal melatonin concentration was analyzed by subjecting a 30-min light pulse at the peak phase of melatonin concentration. Wild-type mice showed significant rhythmicity in pineal melatonin concentration with the highest level at Zeitgeber time 22 (ZT22, where time of light on was defined as ZT0) under LD or ZT18 on the first day of DD. In contrast, Cry1(-/-) /Cry2(-/-) mice did not show significant circadian rhythmicity, with only a small peak observed at ZT22 in LD. Nevertheless, a significant daily variation could be observed under DD, with a small increase at ZT6 and ZT18 h. Melatonin concentration was significantly suppressed by acute light pulse at ZT22 in wild-type mice but not in Cry1(-/-) /Cry2(-/-) mice. The present results suggest that Cry genes are required for regulating pineal melatonin synthesis via circadian and photic signals from the suprachiasmatic nucleus of the hypothalamus (SCN).

Indirect blue light does not suppress nocturnal salivary melatonin in humans in an automobile setting

In 2007, the International Agency for Research on Cancer (IARC) classified shift work that involves circadian disruption as being probably carcinogenic to humans (Group 2A). In this context, light exposure during the night plays a key role because it can suppress nocturnal melatonin levels when exposures exceed a certain threshold. Blue light around 464 nm is most effective in suppressing melatonin because of the spectral sensitivity of melanopsin, a recently discovered photopigment in retinal ganglion cells; the axons of these cells project to the suprachiasmatic nucleus, a circadian master clock in the brain. Due to advances in light technologies, normal tungsten light bulbs are being replaced by light-emitting diodes which produce quasi-monochromatic or white light. The objective of this study was to assess whether the light-melanopsin-melatonin axis might be affected in automobiles at night which employ the new generation diodes. To this end, we have tested in an experimental automobile setting whether indirect blue light (lambda(max) = 465 nm) at an intensity of 0.22 or 1.25 lx can suppress salivary melatonin levels in 12 male volunteers (age range 17-27 years) who served as their own controls. Daytime levels were low (2.7 +/- 0.5 pg/mL), and night-time levels without light exposure were high (14.5 +/- 1.1 pg/mL), as expected. Low-intensity light exposures had no significant effect on melatonin levels (0.22 lx: 17.2 +/- 2.8 pg/mL; P > 0.05; 1.25 lx: 12.6 +/- 2.0 pg/mL; P > 0.05). It is concluded that indirect blue light exposures in automobiles up to 1.25 lx do not cause unintentional chronodisruption via melatonin suppression.

Determination of endogenous melatonin in the individual pineal glands of inbred mice using precolumn oxidation reversed-phase micro-high-performance liquid chromatography

The amount of endogenous melatonin in the individual pineal glands of inbred mice has been determined using reversed-phase micro-high-performance liquid chromatography after precolumn oxidation of melatonin to a compound having strong fluorescence. The fluorescent compound was identified as N-[(6-methoxy-4-oxo-1,4-dihydroquinolin-3-yl)methyl]acetamide. The excitation and emission wavelengths of this compound are 245 and 380 nm, respectively, and the fluorescence intensity is 6.8 times greater than that of melatonin. Molar absorptivity and fluorescence quantum yield of this compound are 46,300[L mol(-1)cm(-1)] and 0.31 (245 nm), respectively. The lower quantification limit of melatonin in biological samples using this precolumn oxidation method is 200 amol, and the calibration curve of spiked melatonin is linear from 200 amol to 50 fmol (r>0.999). The sensitivity of the present method is almost 10 times higher than that of the previous method. The values of endogenous melatonin obtained for ICR, C57BL, BALB/c, and AKR mice are 4.7, 6.1, 7.4, and 18.8 fmol/pineal gland, respectively. The amounts of endogenous pineal melatonin of these strains had not been clearly reported due to the poor enzymatic activities for melatonin biosynthesis; this is the first report that clearly demonstrates the existence of endogenous melatonin in these inbred mice.

Melatonin and activity rhythm responses to light pulses in mice with the Clock mutation

Melatonin and wheel-running rhythmicity and the effects of acute and chronic light pulses on these rhythms were studied in Clock(Delta19) mutant mice selectively bred to synthesize melatonin. Homozygous melatonin-proficient Clock(Delta19) mutant mice (Clock(Delta19/Delta19)-MEL) produced melatonin rhythmically, with peak production 2 h later than the wild-type controls (i.e., just before lights on). By contrast, the time of onset of wheel-running activity occurred within a 20-min period around lights off, irrespective of the genotype. Melatonin production in the mutants spontaneously decreased within 1 h of the expected time of lights on. On placement of the mice in continuous darkness, the melatonin rhythm persisted, and the peak occurred 2 h later in each cycle over the first two cycles, consistent with the endogenous period of the mutant. This contrasted with the onset of wheel-running activity, which did not shift for several days in constant darkness. A light pulse around the time of expected lights on followed by constant darkness reduced the expected 2-h delay of the melatonin peak of the mutants to approximately 1 h and advanced the time of the melatonin peak in the wild-type mice. When the Clock(Delta19/Delta19)-MEL mice were maintained in a skeleton photoperiod of daily 15-min light pulses, a higher proportion entrained to the schedule (57%) than melatonin-deficient mutants (9%). These results provide compelling evidence that mice with the Clock(Delta19) mutation express essentially normal rhythmicity, albeit with an underlying endogenous period of 26-27 h, and they can be entrained by brief exposure to light. They also raise important questions about the role of Clock in rhythmicity and the usefulness of monitoring behavioral rhythms compared with hormonal rhythms.

Melatonin in mice: rhythms, response to light, adrenergic stimulation, and metabolism

There has been relatively little research conducted on pineal melatonin production in laboratory mice, in part, due to the lack of appropriate assays. We studied the pineal and plasma rhythm, response to light, adrenergic stimulation, and metabolism of melatonin in CBA mice. With the use of a sensitive and specific melatonin RIA, melatonin was detected in the pineal glands at all times of the day >21 fmol/gland in CBA mice but not in C57Bl mice. Both plasma and pineal melatonin levels peaked 2 h before dawn in a 12:12-h light-dark photoperiod (162 +/- 31 pM and 1,804 +/- 514 fmol/gland, respectively). A brief light pulse (200 lx/15 min), 2 h before lights on, suppressed both plasma and pineal melatonin to near basal levels within 30 min. Exposure to light pulses 4 h after lights off or 2 h before lights on resulted in delays and advances, respectively, in the early morning decline of plasma and pineal melatonin on the next cycle. Administration of the beta-adrenergic agonist isoproterenol (20 mg/kg) 2 and 4 h after lights on in the morning resulted in a fivefold increase in plasma and pineal melatonin 2.5 to 3 h after the first injection. In the mouse, unlike the rat, melatonin was shown to be metabolized almost exclusively to 6-glucuronylmelatonin rather than 6-sulphatoxymelatonin. These studies have shown that the appropriate methodological tools are now available for studying melatonin rhythms in mice.

Non-visual ocular photoreception

At least six light-regulated phenomena are preserved in the eyes of retinally degenerate mice, including the entrainment of circadian rhythms, the gating of ocular immune response, and pupillary reactivity. Some of these phenomena have also been observed in blind human patients. These findings have prompted the search for a non-visual ocular phototransduction mechanism. Molecular genetic studies have identified several candidate genes for these effects. These include genes encoding novel ocular opsins, such as melanopsin, as well as potential flavin-based photopigments. Data linking these potential photoreceptors to these phenomena are discussed, and the clinical implications of these findings are explored.

Neural basis and biological function of masking by light in mammals: suppression of melatonin and locomotor activity

Light influences mammalian circadian rhythms in two different ways: (1) It entrains endogenous oscillators (clocks), which regulate physiology and behavior; and (2) it affects directly and often immediately physiology and behavior (these effects are also referred to as masking). Masking effects of light on pineal melatonin, locomotor activity, and the sleep-wake cycle in mammals and man are reviewed. They seem to represent a universal response in this group. The review reveals that the mechanism of photic inhibition of melatonin is fairly well understood, whereas only little is known about the influence of light on other circadian rhythm outputs, such as locomotor activity.

Standardizing tests of mouse behavior: reasons, recommendations, and reality

As more investigators with widely varying backgrounds enter the field of mouse behavioral genetics, there is a growing need to standardize some of the more popular tests because differences between laboratories in the details of behavioral testing and the pretesting environment can contribute to failures to replicate results of genetic experiments. It is argued here that we have sufficient knowledge to warrant a wise choice of a short list of standard strains and even details of apparatus and protocols for several kinds of behavioral tests. Equating the laboratory environment does not appear to be feasible. Instead, we need to learn what kinds of behavioral tests yield the most stable results in different labs and what kinds are most sensitive to the ubiquitous variations among test sites. Methods for making an informed choice of sample size for evaluating interactions between the laboratory environment and genotype are available and should be utilized in standardization trials. New resources for convenient sharing of data will greatly aid in collaborative and comparative studies involving several sites. Like the sequencing of an entire genome, test standardization is something that needs to be done only once if it is done properly, and the work will then benefit the field of behavioral and neural genetics for many years.

Influence of near-ultraviolet radiation on reproductive and immunological development in juvenile male Siberian hamsters

The aim of this study was to characterize the lenticular ultraviolet transmission of the Siberian hamster (Phodopus sungorus) and to probe the range of near-ultraviolet (UV-A, 315–400nm) and visible wavelengths (400–760nm) for modulating the photoperiodic regulation of its reproductive and immune systems. Ocular lenses from adult hamsters were found to transmit UV-A wavelengths at similar levels to visible wavelengths, with a short-wavelength cut-off of 300nm. Five separate studies compared the responses of juvenile male hamsters to long photoperiods (16h:8h L:D), short photoperiods (10h:14h L:D) and short photoperiods interrupted by an equal photon pulse of monochromatic light of 320, 340, 360, 500 or 725nm during the night. The results show that UV-A wavelengths at 320, 340 and 360nm can regulate both reproductive and immune short-photoperiod responses as effectively as visible monochromatic light at 500nm. In contrast, long-wavelength visible light at 725nm did not block the short-photoperiod responses. These results suggest that both wavelengths in the visible spectrum, together with UV-A wavelengths, contribute to hamster photoperiodism in natural habitats.

Characterization of an ocular photopigment capable of driving pupillary constriction in mice

This work demonstrates that transgenic mice lacking both rod and cone photoreceptors (rd/rd cl) retain a pupillary light reflex (PLR) that does not rely on local iris photoreceptors. These data, combined with previous reports that rodless and coneless mice show circadian and pineal responses to light, suggest that multiple non-image-forming light responses use non-rod, non-cone ocular photoreceptors in mice. An action spectrum for the PLR in rd/rd cl mice demonstrates that over the range 420-625 nm, this response is driven by a single opsin/vitamin A-based photopigment with peak sensitivity around 479 nm (opsin photopigment/OP479). These data represent the first functional characterization of a non-rod, non-cone photoreceptive system in the mammalian CNS.

The influence of long-term exposure of mice to randomly varied power frequency magnetic fields on their nocturnal melatonin secretion patterns

Disruption of the normal melatonin rhythm has many implications in health and disease. Exposure to magnetic fields is alleged to suppress nocturnal melatonin production, which could implicate magnetic fields in the development of, for example, breast cancer. Magnetic fields of overhead powerlines allegedly pose a risk in the development of childhood leukemia, and the question arises whether changed pineal function could play a role here. In this study two strains of mice were exposed to a rms 50-Hz magnetic field which varied randomly between 0.5 and 77 microT with an average of 2.75 microT and compared to sham-exposed groups. The male mice were exposed for 24 h per day from conception until adult age. Nighttime plasma melatonin values were determined using radioimmunoassay (n=9 for each time point). Statistical comparison was done by nonparametric 95% confidence intervals for median differences to determine nocturnal elevated melatonin values. Although a shortcoming of the study was the small sample size, no statistically significant difference in the nocturnal median elevated melatonin values between exposed and sham-exposed groups could be demonstrated. Long-term and continuous exposure to simulated powerline magnetic fields did not result in a decreased nocturnal melatonin secretion in mice.

Responses to light after retinal degeneration

Transgenic rodless mice were given 1-h pulses of light of varying brightness at times of the night when they were normally active. The rodless mice showed decreases in locomotor activity during light pulses brighter than 2 lux; these decreases were significantly greater than those in wildtypes (ANOVA, P < 0.01). However, with very dim light, rodless mice showed no changes in activity, whereas wildtype mice actually increased their activity. It is suggested that irradiance detection could be enhanced by absence of image-forming vision. Enhanced inhibition of activity around twilight may be adaptive for mice in some circumstances and so help maintain genes for retinal degeneration in natural populations.

Relationship of atypical melatonin rhythm with two circadian clock outputs in B6D2F(1) mice

Circadian rhythms in body temperature, locomotor activity, and the circadian changes of plasma and pineal melatonin content were investigated in B6D2F(1) mice synchronized by 12 h of light and 12 h of darkness. During 8 wk continuous recording, activity and temperature displayed a marked stable and reproducible circadian rhythm, with both peaks occurring near the middle of darkness. Both 24- and 12-h rhythmic components were also significantly detected. Mean plasma melatonin concentration rose steadily during the light span and reached a maximum (30.6 +/- 10.0 pg/ml) at 11 h after light onset (HALO), then gradually decreased after the onset of darkness to a nadir (4.7 +/- 0.4 pg/ml) at 20 HALO. Mean pineal content followed a pattern parallel to that of plasma concentration (peak at 11 HALO: 17.7 +/- 1.0 pg/gland; trough at 17 HALO: 4.7 +/- 1.0 pg/gland). In addition, a second sharp peak was observed at 21 HALO (20.2 +/- 3.5 pg/gland). Plasma and pineal contents displayed large and statistically significant circadian changes, with a composite rhythm of period (24 + 12 h). This mouse model has predominant production and secretion of melatonin during the day. This possibly contributes to a similar coupling between chronopharmacology mechanisms and the rest-activity cycle in these mice and in human subjects.

Determination of pineal melatonin by precolumn derivatization reversed-phase high-performance liquid chromatography and its application to the study of circadian rhythm in rats and mice

Determination of minute amounts of endogenous melatonin in rat and mouse pineal gland was performed using an RP-HPLC system. Melatonin was separated following precolumn derivatization and determined with a fluorescence detector at the emission wavelength of 380 nm with the excitation at 245 nm. The calibration curve of melatonin constructed by adding known amounts of melatonin to the homogenates of mouse pineal gland was linear over the range of 1-500 fmol (injection amount/20 microl). The detection limit of added melatonin was 1 fmol (S/N = 5). Repeatability and day-to-day precision for the melatonin spiked sample of mouse pineal gland was 4.0 and 3.8% (RSD), respectively. Using the present method, circadian changes of melatonin content in rat (Wistar) and mouse (C3H) pineal gland were determined. In addition, a minute amount of melatonin in ddY mouse pineal gland was determined, because pineal melatonin of many inbred mouse strains has been reported to be lower than the detection limit.

Neither functional rod photoreceptors nor rod or cone outer segments are required for the photic inhibition of pineal melatonin

Pineal melatonin production is rapidly suppressed by light. In mammals, the photoreceptors mediating this response are ocular; however, definitive information regarding their nature and precise location is absent. In an attempt to define these photoreceptors, we examined the sensitivity of pineal melatonin production to inhibition by controlled irradiance monochromatic green light (lambda max 509 nm) in C3H mice bearing either of two mutations affecting the retina: retinal degeneration (rd), a disruption of rod phototransduction, and retinal degeneration slow (rds), an ablation of photoreceptor outer segments. Diurnal profiles of pineal melatonin content were similar in both mutant genotypes and in wild-type mice; melatonin peaked between 3-5 h before lights on. All three genotypes exhibited irradiance dependent inhibition of pineal melatonin content; 2.6 x 10(-2) microwatts/cm2 509 nm light induced complete suppression in all three genotypes, whereas lower irradiances were ineffective in all cases. Bilateral enucleation abolished responses even to 6 microwatts/cm2 509 nm light. These results demonstrate that the process of irradiance detection for pineal melatonin inhibition is buffered against considerable loss of photoreceptive capacity and that neither rod photoreceptors nor rod or cone outer segments are required for mediating this response in mice.

Spatial responses to light in mice with severe retinal degeneration

It is known that mice homozygous for the retinal degeneration (rd) mutation are able to synchronize their circadian rhythms to light-dark cycles. In the present experiments mice were given a choice of a dark or an illuminated living and nesting area. C3H, CBA and C57 rd/rd mice spent more time in the dark than in the illuminated area. Also, they spent as much time in the dark area as did wildtype controls. This shows that, despite advanced retinal degeneration, light can be used to control behaviour in space as well as in time. This was true of mutant mice over a year old, when retinal degeneration is very severe, and also of a transgenic strain of mice whose rods are destroyed as they begin to develop in the first few weeks after birth.

The effect of melatonin on cleavage rate of C57BL/6 and CBA/Ca preimplantation embryos cultured in vitro

There is growing interest in using melatonin as a therapeutic agent for the treatment of a variety of medical conditions, including cancer, heart disease, glaucoma, stress, jet lag, and sleep disorders. In addition, melatonin is being evaluated in a clinical trial to test its efficacy as an oral contraceptive. In order to test any possible adverse effects of melatonin on preimplantation embryos, we used the mouse as a model system. Two strains of mice, a Ped fast, melatonin-deficient strain, C57BL/6, and a Ped slow strain previously found to have detectable melatonin levels at nighttime, CBA/Ca, were studied. Two cell embryos were incubated with melatonin concentrations from 10(-5) M to 10(-13) M for 48 or 72 hours and the number of cells per embryo assessed quantitatively at the end of the incubation period. We used sufficiently high levels of melatonin to mimic the pharmacological concentration used in the oral contraceptive. It was found that there was no effect of melatonin on embryos from either mouse strain at any of the concentrations tested. Our results suggest that if conception occurs while melatonin is being administered to treat a range of conditions, it would not adversely affect the embryo.

Increased melatonin levels after hemorrhagic shock in male and female C3H/HeN mice

Although hemorrhagic shock leads to significant alterations of several hormones, e.g. ACTH, corticosterone and beta-endorphin, it is not known whether plasma melatonin levels are affected under this condition and if so, whether the effects are comparable in males and females. Using a radioimmunoassay, it was found that plasma melatonin levels were significantly increased in male and proestrus female C3H/HeN mice immediately after hemorrhagic shock. However, in male mice, by two hours after hemorrhage and resuscitation, plasma melatonin returned to levels comparable to those seen in control and sham-operated animals. Proestrus female mice, on the other hand, showed significantly increased plasma melatonin levels at two hours after surgery when compared to unoperated control animals. Although the significance and biological role of the transient increased plasma melatonin levels after hemorrhagic shock remain to be determined, it appears that the pineal gland and/or an extrapineal source of melatonin, of both male and proestrus female mice responds to severe hypotension by increased release of melatonin.

Differences in circadian photosensitivity between retinally degenerate CBA/J mice (rd/rd) and normal CBA/N mice (+/+)

Using the magnitude of phase shift of circadian locomotor rhythms induced by a single pulse of white fluorescent light, we compared the sensitivity of the circadian system to light in retinally degenerate mice and in normal mice. In the first experiment, phase response curves (PRCs) for 10-lux white light were generated in CBA/J mice with retinal degeneration (rd/rd) and CBA/N mice with normal retinas (+/+). Although large phase delays early in the subjective night and small phase advances in the late subjective night were observed in CBA/N mice, CBA/J mice showed only small phase delays early in the subjective night. In the second experiment, we found that the magnitude of phase shifts at circadian time (CT) 16 for delays and CT 24 for advances in CBA/J mice became larger with increasing light intensity, and that CBA/J mice could show the same amount of phase shift as CBA/N mice when higher intensities were used. These findings indicate that the differences in the shapes of PRCs are not due to differences in the nature of the oscillating system, but to differences in circadian photosensitivity between these strains. Because the genetic background for the rd loci was not completely identical in the CBA/N and CBA/J mice, it was possible that genes other than the rd gene might have caused different photosensitivity in these mice. Therefore, in the last experiment, we studied the circadian photosensitivity in F1 hybrids between CBA/N and CBA/J mice and in the backcross progeny with different genotypes (+/rd and rd/rd) obtained from the crossing between F1 hybrids between CBA/N and CBA/J mice and in the backcross progeny with different genotypes (+/rd and rd/rd) obtained from the crossing between F1 and CBA/J mice. In these mice with heterogeneous genetic backgrounds as well, mice with retinal degeneration were always less sensitive to light, suggesting that reduced circadian photosensitivity is caused by retinal degeneration. These results are discussed in relation to recent findings in retinally degenerate C57BL mice, which have been found to have normal circadian sensitivity to light.