Seasonal changes in plasma levels of thyroid hormones and the effects of the hormones on cellular ATP content in common bottlenose dolphin.

In general, thyroid hormones (THs) stimulate cellular metabolism by inducing ATP utilization that collaterally leads to thermogenesis. However, in cetaceans, TH functions and the contribution of THs to cold adaptation are not fully understood. To investigate the role of THs in metabolism of cetaceans, seasonal changes in circulating levels in THs were investigated in the common bottlenose dolphin Tursiops truncatus that were monitored under two different conditions for two years, with routine measurements of body temperature (BT), water temperature (WT) and air temperature (AT). The effects of THs on ATP synthesis were determined using cultured cells. Blood samples were collected from the species kept in different conditions at the Taiji Whale Museum located in the temperate zone and at Okinawa Expo Park in the subtropical zone. Circulating levels in total thyroxine (T4) for the dolphins at both aquaria and total 3,5,3'-tri-iodothyronine (T3) levels in dolphins at Taiji were measured by enzyme-linked immunoassay methods, respectively, and average concentrations were compared among seasons. To confirm the effects of THs on ATP synthesis, T3 or T4 was administrated to cultured kidney cells from the same species and cellular ATP contents were quantified at 0, 24, 48, 96 and 192 h after administration. BT of common bottlenose dolphins in each aquarium was measured for health check by chance in Taiji and every morning in Okinawa. WT in pools and AT were also measured every morning. Circulating T4 levels in autumn and winter were lower than those in spring and summer in dolphins in Taiji where WT and AT varied greatly from season to season. T4 levels showed a small difference between spring and autumn in dolphins in Okinawa with warmer WT and AT in smaller amplitude ranges than in Taiji. Total T3 level in Taiji was highest in spring and lowest in autumn as T4 levels, but not significant. The BT of dolphins in Taiji was also lower in autumn and winter compared with those in spring and summer, whereas the BT of dolphins in Okinawa fell in autumn but rose in summer, albeit to a lesser extent than in Taiji. Cellular ATP was increased by administration of both T3 and T4 compared to control. Collectively, these results suggest that the cellular metabolic activities regulated by THs may be enhanced in dolphins exposed to increasing surrounding temperature for lipolysis and reduced in dolphins exposed to colder conditions for fat accumulation.

Effect of short-term decrease in water temperature on body temperature and involvement of testosterone in steelhead and rainbow trout, Oncorhynchus mykiss.

The Pacific salmonid species Oncorhynchus mykiss is separated into a migratory form (steelhead trout) and a non-migratory form (rainbow trout). A decrease in water temperature is likely a cue triggering downstream behavior in the migratory form, and testosterone inhibits onset of this behavior. To elucidate differences in sensitivity to water temperature decreases between the migratory and non-migratory forms and effect of testosterone on the sensitivity, we examined two experiments. In experiment 1, we compared changes in body temperature during a short-term decrease in water temperature between both live and dead steelhead and rainbow trout. In experiment 2, we investigated effects of testosterone on body temperature decrease in steelhead trout. Water temperature was decreased by 3°C in 30min. The body temperature of the steelhead decreased faster than that of the rainbow trout. In contrast, there was no significant difference in the decrease in body temperature between dead steelhead and rainbow trout specimens. The body temperature of the testosterone-treated steelhead trout decreased more slowly than that of control fish. Our results suggest that the migratory form is more sensitive to decreases in water temperature than the non-migratory form. Moreover, testosterone might play an inhibitory role in sensitivity to such decreases.

Endocrine control of sexual behavior in teleost fish.

Sexual behavior is one of the most profound events during the life cycle of animals that reproduce sexually. After completion of gonadal development that is mediated by various hormones, oviparous teleosts perform a suite of behaviors, often termed as spawning behavior. This is particularly important for teleosts that have their gametes fertilized externally as the behavior patterns ensures the close proximity of both sexes for gamete release, fusion and ultimately the production of offspring. As in other vertebrates, sexual behavior of fish is also under the control of hormones. Testicular androgen is a requirement for male sexual behavior to occur in most fish species that have been studied. Unlike tetrapods, however, ovarian estrogen does not appear to be essential for the occurrence of female sexual behavior for fish that have their gametes fertilized externally. Prostaglandins produced in the ovary after ovulation act as a trigger in some teleosts to induce female sexual behavior. Potentiating effects of gonadotropin-releasing hormone in the brain on sexual behavior are reported in some species. Under endocrine regulation, male and female fish exhibit gender-typical behavior during spawning, but in some fish species there is also some plasticity in their sexual behavior. Sex changing fish can perform both male-typical and female-typical sexual behaviors during their lifetime and this sexual plasticity can also be observed in non-sex changing fish when undergoing hormonal treatment. Although the neuroanatomical basis is not clear in fish, results of field and laboratory observations suggest that some teleosts possess a sexually bipotential brain which can regulate two types of behaviors unlike most other vertebrates which have a discrete sex differentiation of their brain and can only perform gender-typical sexual behavior.

Effects of growth hormone and cortisol on the downstream migratory behavior in masu salmon, Oncorhynchus masou.

The effects of ovine growth hormone (oGH) and cortisol on downstream migratory behavior in yearling (1(+)) smolts and underyearling (0(+)) parr of masu salmon, Oncorhynchus masou, were examined during the downstream migratory period in spring using artificial raceways. In May, each of 22 1(+) smolts and 0(+) parr were implanted with cholesterol pellets containing 250 microg of oGH and/or 2 mg of cortisol. Their downstream migratory behavior was subsequently observed in artificial raceways, along with control groups 4-23 days after implantation. In 1(+) smolts, the frequency of downstream migratory behavior was 23%, 18%, 72%, and 82% in the control, oGH, cortisol, and oGH+cortisol-treated groups, respectively. The frequency was significantly higher in the cortisol and oGH+cortisol-treated groups than in the control and oGH-treated groups. In 0(+) parr, the frequency of downstream migratory behavior in the cortisol (82%) and cortisol+oGH-treated (90%) groups was significantly higher than in the control (18%) and oGH-treated (0%) groups. These results indicate that cortisol is an important endocrine factor inducing downstream migratory behavior in both 1(+) smolt and 0(+) parr of masu salmon.