Tsuyoshi Otsuka

Photoperiod regulates corticosterone rhythms by altered adrenal sensitivity via melatonin-independent mechanisms in fischer 344 rats and C57BL/6J mice

Most species living in temperate zones adapt their physiology and behavior to seasonal changes in the environment by using the photoperiod as a primary cue. The mechanisms underlying photoperiodic regulation of stress-related functions are not well understood. In this study, we analyzed the effects of photoperiod on the hypothalamic-pituitary-adrenal axis in photoperiod-sensitive Fischer 344 rats. We first examined how photoperiod affects diurnal variations in plasma concentrations of adrenocorticotropic hormone (ACTH) and corticosterone. ACTH levels did not exhibit diurnal variations under long- and short-day conditions. On the other hand, corticosterone levels exhibited a clear rhythm under short-day condition with a peak during dark phase. This peak was not observed under long-day condition in which a significant rhythm was not detected. To analyze the mechanisms responsible for the photoperiodic regulation of corticosterone rhythms, ACTH was intraperitoneally injected at the onset of the light or dark phase in dexamethasone-treated rats maintained under long- and short-day conditions. ACTH induced higher corticosterone levels in rats examined at dark onset under short-day condition than those maintained under long-day condition. Next, we asked whether melatonin signals are involved in photoperiodic regulation of corticosterone rhythms, and rats were intraperitoneally injected with melatonin at late afternoon under long-day condition for 3 weeks. However, melatonin injections did not affect the corticosterone rhythms. In addition, photoperiodic changes in the amplitude of corticosterone rhythms were also observed in melatonin-deficient C57BL/6J mice, in which expression profiles of several clock genes and steroidgenesis genes in adrenal gland were modified by the photoperiod. Our data suggest that photoperiod regulates corticosterone rhythms by altered adrenal sensitivity through melatonin-independent mechanisms that may involve the adrenal clock.

Photoperiodic responses of depression-like behavior, the brain serotonergic system, and peripheral metabolism in laboratory mice

Seasonal affective disorder (SAD) is characterized by depression during specific seasons, generally winter. The pathophysiological mechanisms underlying SAD remain elusive due to a limited number of animal models with high availability and validity. Here we show that laboratory C57BL/6J mice display photoperiodic changes in depression-like behavior and brain serotonin content. C57BL/6J mice maintained under short-day conditions, as compared to those under long-day conditions, demonstrated prolonged immobility times in the forced swimming test with lower brain levels of serotonin and its precursor l-tryptophan. Furthermore, photoperiod altered multiple parameters reflective of peripheral metabolism, including the ratio of plasma l-tryptophan to the sum of other large neutral amino acids that compete for transport across the blood-brain barrier, responses of circulating glucose and insulin to glucose load, sucrose intake under restricted feeding condition, and sensitivity of the brain serotonergic system to peripherally administered glucose. These data suggest that the mechanisms underlying SAD involve the brain-peripheral tissue network, and C57BL/6J mice can serve as a powerful tool for investigating the link between seasons and mood.

Hypothesis with abnormal amino acid metabolism in depression and stress vulnerability in Wistar Kyoto rats

While abnormalities in monoamine metabolism have been investigated heavily per potential roles in the mechanisms of depression, the contribution of amino acid metabolism in the brain remains not well understood. In additional, roles of the hypothalamus–pituitary–adrenal axis in stress-regulation mechanisms have been of much focus, while the contribution of central amino acid metabolism to these mechanisms has not been well appreciated. Therefore, whether depression-like states affect amino acid metabolism and their potential roles on stress-regulatory mechanisms were investigated by comparing Wistar Kyoto rats, which display depression-like behaviors and stress vulnerability, to control Wistar rats. Brain amino acid metabolism in Wistar Kyoto rats was greatly different from normal Wistar rats, with special reference to lower cystathionine and serine levels. In addition, Wistar Kyoto rats demonstrated abnormality in dopamine metabolism compared with Wistar rats. In the case of stress response, amino acid levels having a sedative and/or hypnotic effect were constant in the brain of Wistar Kyoto rats, though these amino acid levels were reduced in Wistar rats under a stressful condition. These results suggest that the abnormal amino acid metabolism may induce depression-like behaviors and stress vulnerability in Wistar Kyoto rats. Therefore, we hypothesized that abnormalities in amino acid and monoamine metabolism may induce depression, and amino acid metabolism in the brain may be related to stress vulnerability.

Melatonin adjusts the expression pattern of clock genes in the suprachiasmatic nucleus and induces antidepressant-like effect in a mouse model of seasonal affective disorder

Recently, we have shown that C57BL/6J mice exhibit depression-like behavior under short photoperiod and suggested them as an animal model for investigating seasonal affective disorder (SAD). In this study, we tested if manipulations of the circadian clock with melatonin treatment could effectively modify depression-like and anxiety-like behaviors and brain serotonergic system in C57BL/6J mice. Under short photoperiods (8-h light/16-h dark), daily melatonin treatments 2 h before light offset have significantly altered the 24-h patterns of mRNA expression of circadian clock genes (per1, per2, bmal1 and clock) within the suprachiasmatic nuclei (SCN) mostly by increasing amplitude in their expressional rhythms without inducing robust phase shifts in them. Melatonin treatments altered the expression of genes of serotonergic neurotransmission in the dorsal raphe (tph2, sert, vmat2 and 5ht1a) and serotonin contents in the amygdala. Importantly, melatonin treatment reduced the immobility in forced swim test, a depression-like behavior. As a key mechanism of melatonin-induced antidepressant-like effect, the previously proposed phase-advance hypothesis of the circadian clock could not be confirmed under conditions of our experiment. However, our findings of modest adjustments in both the amplitude and phase of the transcriptional oscillators in the SCN as a result of melatonin treatments may be sufficient to associate with the effects seen in the brain serotonergic system and with the improvement in depression-like behavior. Our study confirmed a predictive validity of C57BL/6J mice as a useful model for the molecular analysis of links between the clock and brain serotonergic system, which could greatly accelerate our understanding of the pathogenesis of SAD, as well as the search for new treatments.

Oral administration of D-aspartate, but not L-aspartate, depresses rectal temperature and alters plasma metabolites in chicks

AIMS L-Aspartate (L-Asp) and D-aspartate (D-Asp) are physiologically important amino acids in mammals and birds. However, the functions of these amino acids have not yet been fully understood. In this study, we therefore examined the effects of L-Asp and D-Asp in terms of regulating body temperature, plasma metabolites and catecholamines in chicks. MAIN METHODS Chicks were first orally administered with different doses (0, 3.75, 7.5 and 15 mmol/kg body weight) of L- or D-Asp to monitor the effects of these amino acids on rectal temperature during 120 min of the experimental period. KEY FINDINGS Oral administration of D-Asp, but not of L-Asp, linearly decreased the rectal temperature in chicks. Importantly, orally administered D-Asp led to a significant reduction in body temperature in chicks even under high ambient temperature (HT) conditions. However, centrally administered D-Asp did not significantly influence the body temperature in chicks. As for plasma metabolites and catecholamines, orally administered D-Asp led to decreased triacylglycerol and uric acid concentrations and increased glucose and chlorine concentrations but did not alter plasma catecholamines. SIGNIFICANCE These results suggest that oral administration of D-Asp may play a potent role in reducing body temperature under both normal and HT conditions. The alteration of plasma metabolites further indicates that D-Asp may contribute to the regulation of metabolic activity in chicks.

Central injection of L- and D-aspartate attenuates isolation-induced stress behavior in chicks possibly through different mechanisms

Intracerebroventricular (i.c.v.) injection of L- and D-aspartate (L- and D-Asp) has been shown to have a sedative effect with and without a hypnotic effect, respectively, in neonatal chicks experiencing isolation stress. However, the mechanisms of the different stress-attenuating functions of L- and D-Asp have not yet been fully clarified. In the present study, we investigated the involvement of the N-methyl-D-aspartate (NMDA) receptor in order to reveal the receptor-mediated function of L- and D-Asp. To reveal whether L-and D-Asp act through the NMDA receptor, (+)-MK-801, which is an antagonist of NMDA receptors, was used in the current study. In experiment 1, the chicks were injected i.c.v. with either saline, (+)-MK-801, L-Asp or L-Asp plus (+)-MK-801. The sedative and hypnotic effects induced by L-Asp were blocked by co-administration with (+)-MK-801. In experiment 2, the chicks were injected i.c.v. with either saline, (+)-MK-801, D-Asp or D-Asp plus (+)-MK-801. Importantly, the sedative effects induced by D-Asp were shifted to hypnotic effects by co-administration with (+)-MK-801. Taken together, L-Asp could induce sedative and hypnotic effects for stress behaviors through the NMDA receptor, but the attenuation of stress behaviors by D-Asp might be via simultaneous involvement of other receptors besides the NMDA receptor in this process. These differences may explain the different functional mechanisms of L- and D-Asp in the central nervous system.

Serotonin levels in the dorsal raphe nuclei of both chipmunks and mice are enhanced by long photoperiod, but brain dopamine level response to photoperiod is species-specific.

Seasonal affective disorder (SAD) is a subtype of major depressive or bipolar disorders associated with the shortened photoperiod in winter. This depressive disorder is integrally tied to the seasonal regulation of the brains serotonergic system. Recently, we found that C57BL/6J mice subjected to a forced-swim test exhibited immobility, a photoperiod-dependent depression-associated behavior, and suppression of brain serotonin levels. However, mice are nocturnal animals, and it is unclear whether the brain serotonergic system responds similarly to photoperiod in nocturnal and diurnal species. This study compared the responses of brain serotonergic and dopaminergic systems to photoperiod in diurnal chipmunks and nocturnal C57BL/6J mice. In both species, serotonin levels in the dorsal raphe nuclei were higher under long-day conditions than short-day conditions, suggesting a similarity in the photoperiod responses of the serotonergic systems. However, photoperiod affected dopamine levels in various brain regions differently in the two species. Some chipmunk brain regions exhibited stronger photoperiod-induced changes in dopamine levels than those of C57BL/6J mice, and the direction of the changes in the hypothalamus was opposite. In conclusion, photoperiod may regulate the brain serotonergic system through similar mechanisms, regardless of whether the animals are diurnal or nocturnal, but photoperiod-dependent regulation of brain dopamine is species-specific.

Photoperiod regulates dietary preferences and energy metabolism in young developing Fischer 344 rats but not in same-age Wistar rats

The effects of photoperiod on dietary preference were examined using young growing Fischer 344 and Wistar rats, which are seasonal and nonseasonal breeders, respectively. Rats were provided a low-fat, high-carbohydrate diet (LFD: 66/10/24% energy as carbohydrate/fat/protein) and high-fat, low-carbohydrate diet (HFD: 21/55/24% energy as carbohydrate/fat/protein) simultaneously under long- (LD: 16 h light/day) and short-day (SD: 8 h light/day) conditions for 3 wk. Fischer 344 rats preferred the LFD to the HFD under the LD condition, whereas preference for both diets was equivalent under the SD condition. Consequently, their body weight and total energy intake exhibited 11-15 and 10-13% increases, respectively, under the LD condition. Calculation of energy intake from macronutrients revealed that rats under the LD condition consumed 20-24 and 9-13% higher energy of carbohydrates and proteins, respectively, than those under the SD condition. In contrast, Wistar rats preferred the LFD to the HFD irrespective of photoperiod and exhibited no photoperiodic changes in any parameters examined. Next, Fischer 344 rats were provided either the LFD or HFD for 3 wk under LD or SD conditions. Calorie intake was 10% higher in the rats fed the LFD than those fed the HFD under SD condition. However, rats under LD condition exhibited 5-10, 14, and 64% increases in body weight, epididymal fat mass, and plasma leptin levels, respectively, compared with those under the SD condition irrespective of dietary composition. In conclusion, photoperiod regulates feeding and energy metabolism in young growing Fischer 344 rats via the interactions with dietary macronutrient composition.

Effects of time of l-ornithine administration on the diurnal rhythms of plasma growth hormone, melatonin, and corticosterone levels in mice

The synthesis and secretion of many hormones such as growth hormone (GH), melatonin, and corticosterone, exhibit temporal variations over each day and night. Oral administration of several nutritional factors, including l-ornithine, modulates these hormonal secretions and induces an acute increase in plasma GH levels. However, the impact of l-ornithine on the diurnal rhythms of hormone secretion remains unclear. In this study, we evaluated whether the diurnal rhythms of plasma GH, melatonin, and corticosterone secretion were altered by the daily administration of l-ornithine as well as the timing of the administration, in CBA/N mice. Our results showed that the plasma GH levels that peaked at light phase were amplified by l-ornithine (500 mg/kg) administered at Zeitgeber time (ZT) 22, but not at ZT10. Additionally, l-ornithine (1000 mg/kg) administered at ZT22 advanced the onset of the nocturnal rise of melatonin, which resulted in the elongation of the melatonin peak. On the other hand, l-ornithine (500 and 1000 mg/kg) administered at ZT10, but not at ZT22, suppressed the diurnal rhythm peaks of plasma corticosterone. The effects of l-ornithine on plasma GH rhythms lasted for at least 2 days after cessation of the daily administration. Running wheel activity during the active phase was slightly elevated by l-ornithine administration at ZT22, but the overall patterns were only slightly affected. l-Ornithine levels in the plasma and hypophysis after a single administration of l-ornithine at ZT22 were lower than those after administration at ZT10, suggesting that the metabolic rate of l-ornithine differs between day and night. In conclusion, our data suggest that a daily administration of l-ornithine regulates the diurnal rhythms of GH, melatonin, and corticosterone in a manner dependent on administration time, which might be related to the diurnal rhythms of l-ornithine metabolism.

Antidepressant-like effect of bright light is potentiated by L-serine administration in a mouse model of seasonal affective disorder.

Bright light therapy is used as the primary treatment for seasonal affective disorder; however, the mechanisms underlying its antidepressant effect are not fully understood. Previously, we found that C57BL/6J mice exhibit increased depression-like behavior during a short-day condition (SD) and have lowered brain serotonin (5-HT) content. This study analyzed the effect of bright light on depression-like behaviors and the brain serotonergic system using the C57BL/6J mice. In the mice maintained under SD, bright light treatment (1000 lx, daily 1 h exposure) for 1 week reduced immobility time in the forced swimming test and increased intake of saccharin solution in a saccharin intake test. However, the light treatment did not modify 5-HT content and selective 5-HT uptake in the amygdala, or temporal patterns of core body temperature and wheel-running activity throughout a day. In the next experiment, we attempted to enhance the effect of bright light by using L-serine, a precursor of D-serine that acts as an N-methyl-D-aspartic acid receptor coagonist. Daily subcutaneous injection of L-serine for 2 weeks prior to the bright light strongly reduced the immobility time in the forced swimming test, suggesting a synergistic effect of light and L-serine. Furthermore, bright light increased the total number of 5-HT-immunoreactive cells and cells that had colocalized 5-HT and c-Fos immunosignals in several subregions of the raphe nuclei. These effects were potentiated by prior injection of L-serine. These data suggest that the bright light may elicit an antidepressant-like effect via enhanced 5-HT signals in the brain and L-serine can enhance these effects.