Larry J. Petterborg

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.

Variation in pineal melatonin content during the estrous cycle of the rat

Abstract Pineal levels of melatonin exhibit a circadian rhythm in the rat. To determine if this rhythm varies during the estrous cycle, adult rats were sacrificed every 2 hr from 2000 hr (prior to lights off) until 0800 hr (2 hr after lights on) throughout each day of the 4-day estrous cycle. Pineal glands were assayed for melatonin by radioimmunoassay. A significant circadian rhythm in the pineal content of melatonin was evident each day of the cycle with peak levels exhibited during the dark phase. There were significant differences in titers of melatonin present at 0200, 0400, and 0600 hr between various stages of the cycle. In general, highest nighttime levels occurred during the evenings of metestrus and diestrus, with levels at a minimum on the evening of estrus. These data indicate that the circadian pineal melatonin rhythm is a function of estrous cycle stage in the rat, and, thus, important differences exist in the pineal biosynthetic dynamics of this species relative to those of seasonal breeders such as the hamster and sheep. Additionally, the results suggest that hydroxyindole-O-methyltransferase, and not serotonin N-acetyltransferase, is the regulatory enzyme responsible for these estrous stage differences in pineal melatonin content.

Pineal biosynthetic activity and neuroendocrine physiology in the aging hamster and gerbil

Abstract The effects of advancing age on the ability of the pineal gland to produce melatonin and on the neuroendocrine physiology of Syrian hamsters and Mongolian gerbils were investigated. In 2-month-old male and female hamsters the nocturnal rise in pineal melatonin levels is on the order of 8-fold. As hamsters age the nighttime increases become progressively more depressed until in 18-month-old animals the nighttime rise in pineal melatonin is only twice the levels measured during the daylight hours. In male gerbils, the normal nocturnal increase in pineal melatonin levels observed in 2-month-old animals has completely disappeared by the time gerbils reach 19 months of age. The results suggest a marked reduction in pineal biosynthetic activity as animals age. In hamsters, neither hypothalamic norepinephrine nor dopamine levels were altered in 18-month-old animals compared to those in hamsters killed 8 weeks after birth. The reproductive system of both male and female hamsters and male gerbils seem to function relatively normally until the animals are 1 1 2 years old. Thus, the majority of the 18-month-old female hamsters exhibit normal vaginal cyclicity. Likewise, the patterns of gonadotrophin and prolactin secretion are quite similar to those in the young females. At the light microscope level, most of the ovaries of the old hamsters, like those of the young animals, were found to contain pre-antral and antral follicles and corpora lutea. The uteri of the old hamsters exhibited the most pronounced changes. There was an obvious paucity of endometrial glands and the submucosa appeared fibrous. The testes of neither the old (18 month) hamsters nor the old (19 month) gerbils appeared greatly different from those of the young animals. Likewise, in advancing age there were no marked effects on circulating levels of gonadotrophins and prolactin in male hamsters. Finally, old male gerbils had plasma levels of testosterone similar to those observed in much younger animals.

Influence of propranolol, phenoxybenzamine or phentolamine on the in vivo nocturnal rise of pineal melatonin levels in the syrian hamster

Abstract In the Syrian hamster, pineal melatonin levels exhibit a 15-fold rise during the dark phase of the light: dark cycle. This rise is believed to be mediated by the release of norepinephrine from the postganglionic sympathetic fibers which terminate within the pineal. In order to determine the nature of the adrenergic receptor involved in the norepinephrine mediated nocturnal increase in melatonin, male hamsters were treated with either α- or β-adrenergic blockers just prior to lights out. Subsequently, radioimmunoassayable levels of melatonin were measured at 7, 8 and 9 hours (0300, 0400 and 0500 h, respectively) into the dark period. Propranolol (20 mg/kg) completely suppressed the nocturnal rise of melatonin while phentolamine (10 mg/kg) had no effect upon the increase. The minimum amount of propranolol necessary to block the nighttime rise of melatonin was determined to lie between 1 mg/kg and 10 mg/kg. Phenoxybenzamine (20 mg/kg) exhibited a slight, although statistically significant, blockade of the nocturnal melatonin rise.

Effect of long or short photoperiod on pineal melatonin content in the white-footed mouse, Peromyscus leucopus.

Abstract Adult white-footed mice were maintained under either a long photoperiod (LP, LD 16:8, lights out at 2100) or a short photoperiod (SP, LD 8:16, lights out at 1700) for six weeks. Subgroups from each lighting regime were killed at specific times over a 24 hour period. Pineal radioimmunoassayable melatonin levels were significantly elevated at night compared to daytime values. Pineal melatonin content appears to be elevated for a longer period of time in the SP mice than in the LP animals. The apparent increased melatonin production observed in white-footed mice maintained under short and reproductively repressive daylengths may help to explain the ability of chronically available exogenous melatonin to cause gonadal atrophy in this species.

Refractoriness to the antigonadotrophic effects of melatonin in male hamsters and its interruption by exposure of the animals to long daily photoperiods.

Abstract Exposure of adult male Syrian hamsters to artificially shortened photoperiods in the laboratory was followed by gonadal involution. Return of these animals to long days (light:ddark cycles of 14:10) resulted in regrowth of the reproductive organs, following which the animals were refractory to the normally antigonadotrophic effects of daily melatonin injections given late in the afternoon. When hamsters were kept under naturally shortening photoperiods beginning on October 19, their reproductive organs involuted and remained infantile until the following spring when they again recrudesced to adult size. These animals too were refractory to the inhibitory effects of daily afternoon melatonin injections. The refractory period to melatonin was interrupted by exposure of the animals to long days for 22 weeks.

Effects of Injections and/or Chronic Implants of Melatonin and 5-Methoxytryptamine on Plasma Thyroid Hormones in Male and Female Syrian Hamsters

Plasma thyroid hormone concentrations in male and female Syrian hamsters were evaluated in five experiments after two pineal indoles, melatonin (MEL) and 5-methoxytryptamine (5-MT), were administered as chronic s.c. implants and/or daily afternoon injections. Circulating concentrations of thyroxine (T4), ordinarily maintained by the long photoperiod (LD 14:10), were inhibited by daily afternoon injections of 25 micrograms MEL but not 5-MT in male (experiment 1) and female (experiment 2) Syrian hamsters. The suppressive effect of MEL injections on T4 concentration in the long photoperiod was prevented in both sexes by a s.c. beeswax pellet containing either 1 mg MEL or 5-MT. In the third experiment, various doses of MEL or 5-MT were injected each afternoon for 12 weeks to compare the effectiveness of these two indoles in reducing T4 concentrations in hamsters maintained in the long photoperiod. Only the highest dose (200 micrograms) of 5-MT effectively suppressed T4 concentrations whereas all doses of MEL (greater than or equal to 5 micrograms) significantly reduced plasma T4. MEL or 5-MT (1-1,000 micrograms) were implanted in beeswax pellets in male hamsters exposed to short photoperiod (LD 10:14) to determine if either indole could prevent the short photoperiod-induced suppression of T4 (experiments 4 and 5). Regression and covariance analyses showed a significant log dose-related elevation of T4 in these hamsters, indicating equal potency of MEL and 5-MT in preventing the short photoperiod-induced suppression of T4. 5-MT given by either injection or implantation raised plasma triiodothyronine (T3) in female (experiment 2) but not in male hamsters.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of testicular and thyroid function by chronic exposure to short photoperiod: a comparison in four rodent species

Exposure of male Syrian hamsters (Mesocricetus auratus) for 10 weeks to short photoperiod (SP) providing 10 hr light: 14 hr darkness (10:14 LD) produced a significant reduction in the weights of the reproductive organs, plasma thyroxine (T4) levels and free T4 index (FT4I) compared to the values of animals exposed to long photoperiod (LP, 14:10 LD). C57bl male house mice (Mus musculus) kept in SP (10:14 LD) had reproductive organ weights equivalent to those of mice kept in long days (14:10 LD) and lower T3 uptake (T3U) values. Male gerbils (Meriones unguiculatus) exposed to 13 weeks of SP (10:14 LD) had lower body weights, testes and seminal vesicle weights and higher T3U values compared to LP (14:10 LD) controls. However, no effect was seen on plasma T4 and triiodothyronine (T3) values nor the FT4I and free T3 index (FT3I). White-footed male mice (Peromyscus leucopus) exposed to SP (8:16 LD) had significantly lower testes and seminal vesicle weights while plasma T4 and T3 levels were unaffected. Snell strain house mice (Mus musculus) exposed to SP (8:16 LD) had normal reproductive organ weights compared to the values of LP-exposed (16:8 LD) control animals. However, there was a significant depression in T3 and in the FT3I in the SP animals.

Ultrastructure of pinealocytes of the cotton rat, Sigmodon hispidus.

SummaryFine structural features of pinealocytes of cotton rats (Sigmodon hispidus) were examined. Golgi complexes, mitochondria, endoplasmic reticulum and polysomes are usual organelles seen in the perikaryonal cytoplasm of pinealocytes. Many non-granulated vesicles (40 to 80 nm in diameter) and a few granulated vesicles (about 100 nm in diameter) are associated with the Golgi cisternae. Occasionally, the cisternae contain granular materials. The perikaryonal cytoplasm of pinealocytes is characterized by the presence of inclusion bodies. These bodies are usually round in shape, not bounded by a limiting membrane and composed of fine granular or filamentous materials of high electron-opacity, which are similar in appearance to the substance seen in the nucleolonema. Pinealocyte processes, filled with abundant non-granulated vesicles and some granulated vesicles, are mainly found within the parenchyma and occasionally in perivascular spaces.