Female Djungarian Hamsters Research Articles

Ultrasonic vocalisation rate tracks the diurnal pattern of activity in winter phenotype Djungarian hamsters (Phodopus sungorus)

Vocalisations are increasingly being recognised as an important aspect of normal rodent behaviour yet little is known of how they interact with other spontaneous behaviours such as sleep and torpor, particularly in a social setting. We obtained chronic recordings of the vocal behaviour of adult male and female Djungarian hamsters (Phodopus sungorus) housed under short photoperiod (8 h light, 16 h dark, square wave transitions), in different social contexts. The animals were kept in isolation or in same-sex sibling pairs, separated by a grid which allowed non-physical social interaction. On approximately 20% of days hamsters spontaneously entered torpor, a state of metabolic depression that coincides with the rest phase of many small mammal species in response to actual or predicted energy shortages. Animals produced ultrasonic vocalisations (USVs) with a peak frequency of 57 kHz in both social and asocial conditions and there was a high degree of variability in vocalisation rate between subjects. Vocalisation rate was correlated with locomotor activity across the 24-h light cycle, occurring more frequently during the dark period when the hamsters were more active and peaking around light transitions. Solitary-housed animals did not vocalise whilst torpid and animals remained in torpor despite overlapping with vocalisations in social-housing. Besides a minor decrease in peak USV frequency when isolated hamsters were re-paired with their siblings, changing social contexts did not influence vocalisation behaviour or structure. In rare instances, temporally overlapping USVs occurred when animals were socially-housed and were grouped in such a way that could indicate coordination. We did not observe broadband calls (BBCs) contemporaneous with USVs in this paradigm, corroborating their correlation with physical aggression which was absent from our experiment. Overall, we find little evidence to suggest a direct social function of hamster USVs. We conclude that understanding the effects of vocalisations on spontaneous behaviours, such as sleep and torpor, will inform experimental design of future studies, especially where the role of social interactions is investigated.

Social dominance and wheel running in females of Djungarian hamster (Phodopus sungorus)

AbstractWheel running is a behaviour that has a rewarding effect on animals. There are not numerous papers investigating potential relationships between social rank and wheel running in mammals kept in groups, and the majority of published researches were conducted on male house mice (Mus musculus). The aim of our study was to investigate if social dominance and wheel running are related in female Djungarian hamsters (Phodopus sungorus). Hamsters were kept in groups, and social position of every animal was expressed as dominance index calculated on the basis of agonistic behaviour. We found significant positive correlation between dominance index and wheel running (rs = 0.809, n = 18, P < 0.0001), thus dominants used wheel more often than subordinates. Our results are consistent with those published on male mice. In conclusion, we claim that in majority of mammals (independent of their sex) kept in groups with restricted possibility of wheel running, dominants use wheel more often (or in optimal time) than subordinates, what is consistent with the fact that dominants have priority of access to resources.

Enhanced serum oestrogen levels and highly steroidogenic, luteinized atretic follicles in the ovaries of the Djungarian hamster (Phodopus sungorus) kept under a short photoperiod from birth.

In contrast to the elaborate information available on the effects of the photoperiod on the testes of hamsters, little is known about the influence on their ovaries. This study aimed to describe the ovarian follicular development and steroid hormone production in Djungarian hamsters kept from birth under a short daylight regime. Female Djungarian hamsters (Phodopus sungorus) were kept under two different light regimes: (i) 16 h light:8 h darkness (long daylight; LD) and (ii) 4 h light:20 h darkness (short daylight; SD). They were killed at 28, 56 and 80 days after birth; blood and ovaries were collected. Ovaries were either fixed in Bouin's solution or frozen. Fixed material was dehydrated, embedded in paraffin, serially sectioned at 5 microm and stained with haematoxylin and eosin, whereafter all healthy and atretic follicles were classified and counted. 3beta-Hydroxysteroid dehydrogenase (3beta-HSD) was histochemically demonstrated in 10 microm sections of frozen ovaries. Serum oestradiol-17beta and progesterone levels were determined by RIA. The numbers of healthy preantral and antral follicles were higher in LD than in SD hamsters. Antral follicles did not significantly differ in number during development in LD hamsters, but they were completely absent from 80-day-old SD animals. In LD animals the number of apoptotic preantral follicles dramatically increased with age. In SD animals the numbers of apoptotic antral follicles strongly decreased with age, whereas numerous non-apoptotic follicles with luteinized granulosa cells and a degenerated oocyte appeared, and in increasing numbers with age. During development, moderate 3beta-HSD activity was present in interstitial cells, theca cells of healthy follicles, and in both theca and granulosa cells of degenerating follicles. Strong enzyme activity was found in the hypertrophied granulosa cells of luteinized atretic follicles. Mean serum progesterone values varied from 2 to 6 nmol/l and were not different in LD and SD hamsters. Mean serum oestradiol levels varied from 132 to 542 and 325 to 2353 pmol/l in LD and SD hamsters respectively. The highest oestradiol levels were found in SD animals at day 28 of development. Folliculogenesis was dramatically disturbed in Djungarian hamsters raised under a short photoperiod. These animals developed high serum oestradiol levels and numerous luteinized atretic follicles with highly steroidogenic granulosa cells, which appear to be the source of the increased serum oestradiol levels.

Paternal Care Reduces Maternal Hyperthermia in Djungarian Hamsters (Phodopus campbelli)

WALTON, J. M. AND K. E. WYNNE-EDWARDS Paternal care reduces maternal hyperthermia in Djungarian hamsters(Phodopus campbelli). PHYSIOL BEHAV 63(1) 41–47, 1998.—Throughout lactation, maternal body temperature, nest attendance, activity level and reproductive success of solitary female Djungarian hamsters housed at the recommended ambient temperature of 23°C (Canadian Council on Animal Care guidelines) were compared with those of paired females housed at the same temperature and with solitary females housed at the natural burrow temperature of 18°C. As expected, cooler ambient temperature improved pup survival and weaning weight. Likewise, paternal presence largely compensated for the poor pup growth typical at 23°C. However, the mechanisms were not the same. Females at reduced ambient temperatures were as hyperthermic as females at the higher temperature and spent the same proportion of their day at very high body temperatures. However, the steeper temperature gradient available for passive cooling allowed those females to enhance maternal care by shortening their nest bout absences. In contrast, body temperatures of paired females were tightly regulated compared to the hyperthermia of solitary females and rarely included the highest body temperatures. This alleviation of maternal hyperthermia was not achieved through a reduction in nest attendance. Therefore, maternal hyperthermia in Djungarian hamsters is not essential and may be considered a substantial cost to females when males are not present.

Initial expression of the common alpha-chain in hypophyseal pars tuberalis-specific cells in spontaneous recrudescent hamsters.

When exposed to short-day conditions, hamsters and other long-day breeders undergo gonadal regression. With chronic exposure to short days, however, the animals become photorefractory and gonadal recrudescence occurs. The underlying mechanism for this insensitivity is still unknown. There is growing evidence, however, that specific cells of the pituitary pars tuberalis (PT) mediate these photoperiod/nonphotoperiod-dependent changes as a direct or indirect "Zeitgeber" for the endocrine system. We investigated messenger RNA (mRNA)/protein formation for several hypophyseal hormones (beta-TSH, beta-LH, PRL, common alpha-chain) in the pars distalis (PD) and PT of female Djungarian hamsters in long photoperiod (LP) and after 18, 28, and 38 weeks of short photoperiod (SP). As indicated by gonadal and body weight, the hamsters displayed gonadal regression after 18 and 28 weeks of SP; after 38 weeks of SP, all animals showed recrudescence. At 18 and 28 weeks of SP, only PRL mRNA and protein levels were significantly reduced in the PD and returned to LP values after 38 weeks of SP. The expression of hypothalamic tyrosine hydroxylase in the arcuate nucleus that was determined by immunocytochemistry and by in situ hybridization was also down-regulated in SP18 and SP28 with increasing levels at SP38. Urinary 6-sulfatoxymelatonin levels were elevated in SP with highest levels in the SP18 group. In the PT, beta-TSH mRNA and protein were not detectable in all SP groups compared with the moderate signal intensity in LP. The common alpha-chain mRNA and protein, however, which were also reduced in the animals of the SP18 group, were already elevated after 28 weeks of SP and nearly reached LP-levels after 38 weeks of SP. These results show that, in contrast to LH and TSH, PRL expression in the PD is a sensitive indicator for photoperiod dependent changes of the endocrine system and seems to be tyrosine hydroxylase independent. The increase of common alpha-chain expression in PT-specific cells depending upon duration of SP that precedes the hormonal changes in the PD leads us to speculate that PT-specific cells initiate spontaneous recrudescence via a PT-PD pathway.

No short‐term effects of high‐frequency electromagnetic fields on the mammalian pineal gland

There is ample experimental evidence that changes of earth-strength static magnetic fields, pulsed magnetic fields, or alternating electric fields (60 Hz) depress the nocturnally enhanced melatonin synthesis of the pineal gland of certain mammals. No data on the effects of high-frequency electromagnetic fields on melatonin synthesis is available. In the present study, exposure to 900 MHz electromagnetic fields [0.1 to 0.6 mW/cm2, approximately 0.06 to 0.36 W/kg specific absorption rate (SAR) in rats and 0.04 W/kg in Djungarian hamsters; both continuous and/or pulsed at 217 Hz, for 15 min to 6 h] at day or night had no notable short-term effect on pineal melatonin synthesis in male and female Sprague-Dawley rats and Djungarian hamsters. Pineal synaptic ribbon profile numbers (studied in rats only) were likewise not affected. The 900 MHz electromagnetic fields, unpulsed or pulsed at 217 Hz, as applied in the present study, have no short-term effect on the mammalian pineal gland.

Afternoon injections of melatonin in the Djungarian hamster Phodopus sungorus: long lasting sex-specific effects and influence of acute treatment on the endogenous pineal melatonin rhythm.

The effects of exogenous melatonin on gonadal function and on the endogenous melatonin profiles of male and female Djungarian hamsters were investigated. Daily afternoon subcutaneous injections of melatonin (50 micrograms) in hamsters kept in long photoperiods (LD 16:8) led to gonadal inhibition in all the treated females (n = 13) but in only 2 of 13 males within the treatment period of 8 weeks (uteri: 244 +/- 11 mg in controls vs. 79 +/- 4 mg in treated hamsters, P < 0.001; ovaries: 13.6 +/- 0.6 mg in controls vs. 7.9 +/- 0.7 mg in treated hamsters, P < 0.001; testes: 1,021 +/- 54 mg in controls vs. 732 +/- 100 mg in treated hamsters, P < 0.05; and accessory glands: 641 +/- 38 mg in controls vs. 548 +/- 70 mg in treated hamsters, P > 0.05). These results indicate that there are some circumstances under which the gonadal responses of the reproductive organs of male and female Djungarian hamsters differ. Interestingly, pineal concentrations of melatonin were found to be significantly higher in males (P < 0.01), possibly indicating a more robust endogenous supply with the hormone, whereas serum melatonin levels were not significantly different between males and females. However, the subcutaneous injection of melatonin (5, 25, and 125 micrograms) exerted no acute effect on the endogenous, circadian melatonin-profile, independent of dosage and sex.

Diurnal changes in the content of indoleamines, catecholamines, and methoxyindoles in the pineal gland of the Djungarian hamster (Phodopus sungorus): effect of photoperiod.

Previous studies in Syrian hamster have shown that the correlations between the daily fluctuations in the contents of pineal indoleamines and methoxyindoles are influenced by the photoperiod, and that dopamine may play a role in the regulation of pineal function. The present study investigated the 24 hour changes in the content of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), N-acetylserotonin (NAS), melatonin, 5-methoxytryptophol (5-MT), noradrenaline (NA), dopamine (DA), and 3,4-dihydroxyphenylacetic acid (DOPAC) in the pineal gland of female Djungarian hamsters exposed to long (LP; LD 16:8) or short (SP; LD 10:14) photoperiods for 10 weeks. Pronounced nocturnal increases of N-acetylserotonin and melatonin content were observed irrespective of the photoperiod regime. The content of 5-HT was markedly decreased during the first hour of the night in LP, which contrasted with the lack of changes in NAS and melatonin content at this time. In SP, an increased 5-HTP content and a less obvious decrease in 5-HT content was observed during the night, although melatonin and NAS content were even higher than in LP. Similar daily patterns as for 5-HT were observed in the 5-HT oxidative metabolites (5-HIAA and 5-MT). When considering values throughout the day, a poor correlation for 5-HT vs.. NAS and melatonin content was observed, which was particularly evident during the nighttime. These data indicate that the daily variation of pineal indoles may not only be dependent on changes in the N-acetyltransferase activity but also in other mechanisms regulating pinealocyte 5-HT availability. As previously reported in other species of hamster, pineal NA content did not show daily variations in LP conditions, although a nocturnal increase was detected in SP. In contrast, pronounced nocturnal increases were observed in the content of DA and its acid metabolite, DOPAC, irrespective of the photoperiod. These data indicate the existence of an increased dopaminergic turnover during the night and further support a role for DA in the regulation of melatonin synthesis and in the synchronization of the pineal functions.

Daily Melatonin Treatments Regulate the Circadian Melatonin Rhythm in the Adult Djungarian Hamster

The present study tested the hypothesis that daily melatonin treatments influence the biological clock mechanism controlling the circadian melatonin rhythm. Adult male and female Djungarian hamsters in light:dark = 16L:8D (lights on 0300-1900 h) were administered melatonin subcutaneously (s.c.) each day (5 micrograms/0.2 ml saline) in the morning at 1000 h (AM) or late afternoon at 1700 h (PM); controls received a vehicle injection (CON). After 14 days, pineal and serum melatonin concentrations were determined at various times on the last day of treatment and the next day in constant darkness (no treatment). The rhythm in pineal gland melatonin content was similar in each of the three groups on the last day of treatment (about 6 h duration). On the next day in constant dark, the rising phase was advanced and duration extended by 2 h or more in melatonin-treated hamsters compared to that in CONs (ANOVA). In circulation, the melatonin rhythm in AM and PM groups was phase advanced (onset and peak) on both days of the study. Thus duration was extended by up to 4.5 h compared to that in saline-treated controls. Moreover, amplitude of the nighttime serum melatonin rise was elevated up to fivefold relative to that in the CON group (ANOVA and Accumulated Sums analysis). The effects of repeated melatonin treatments on amplitude and phase of the serum melatonin rhythm raise the possibility that the circadian clock that controls pineal gland production of melatonin may also regulate melatonin secretion. From this and another study, the apparent half-life of melatonin in circulation was estimated to be 7.5 min; the melatonin injection initially produced pharmacological concentrations that were followed by low serum melatonin levels within 2 h. Thus, in both melatonin treatment groups, the data suggest that two distinct periods of elevated serum melatonin were present each day. The cellular mechanism for melatonin action must take into consideration how a brief interruption in elevated melatonin in circulation (about 1 h in the PM group) is recognized as a continuous duration (short daylength), whereas a more extended baseline period is transduced as an abbreviated or long daylength (about 7 h in the AM group). These data further suggest that the biological clock mechanism that generates the circadian melatonin rhythm is responsive to the influence of daily melatonin treatments and presumably to the feedback action of endogenous melatonin on its own rhythm in the Djungarian hamster in long days.

Daily patterns of pituitary prolactin secretion and their role in regulating maternal serum progesterone concentrations across pregnancy in the Djungarian hamster (Phodopus campbelli).

Profiles of serum prolactin (PRL) and progesterone (P4) were determined in repeatedly (every 2 h) sampled female Djungarian hamsters (Phodopus campbelli) over a 24-h period on Days 3, 6, 9, 12, and 15 of the 18-day pregnancy. The first half of pregnancy was characterized by significant surges of PRL within a 2-h period around dawn (0500 h) and dusk (1900 h), with some females also showing a weak midday surge at 1300 h. By Day 9, dusk and midday surges were absent, but the dawn surge remained at its initial amplitude. On Day 12, no PRL surges were seen. Resumption of both the dusk and dawn PRL surges occurred on Day 15 of gestation. Considerable interindividual variability in the amplitude of PRL surges, the timing of PRL surges, and the number of surges per day was detected and would complicate any assessment of PRL levels based on single samples per female. Serum P4 concentrations were 8-10 ng.ml-1 before doubling on Day 15. A 3-day treatment of 50 and 300 micrograms bromocryptine (CB 154; on Days 13-15) effectively suppressed PRL during late pregnancy (Day 15) but did not alter serum P4 concentrations or interfere with parturition. Therefore, surges of PRL are not an essential luteotropic stimulus during late gestation. Compared to oil-injected controls, CB 154-treated females had a higher incidence of infanticide postpartum. Growth rates of the pups, mammary gland development, and successful delivery of milk to pups, however, did not differ between groups. Further studies will be required to determine the function of late-gestation PRL surges.

Increased oxidation of pineal serotonin as a possible explanation for reduced melatonin synthesis in the aging Djungarian hamster ( Phodopus sungorus)

Groups of young adult (1.7 months) and old adult (9.7 months) female Djungarian hamsters were exposed to short-day photoperiods (SD; 8L: 16D) or long-day photoperiods (LD; 16L: 8D) for 6 weeks. Pineal and serum melatonin rhythms were dampened in old SD animals when compared with young SD animals. The melatonin precursor serotonin showed diurnal variations with opposite phases to melatonin synthesis. However, pineal serotonin levels were even lower in old SD animals than in young SD animals, contrary to the expected accumulation of the precursor. The synthesis of a metabolite of serotonin, 5-hydroxyindoleacetic acid (5-HIAA), was significantly lower in younger hamsters, under both photoperiods. These results indicate that the age-associated decline in the production of melatonin is a consequence of an increased oxidation of its precursors.

Prolactin, Follicle-Stimulating Hormone, and Luteinizing Hormone Levels during Preimplantation in the Djungarian Hamster (Phodopus Campbelli)1

The reproductive physiology of female Djungarian hamsters (Phodopus campbelli) varies in several ways from that of more conventional laboratory models. In this study we investigated serum and pituitary levels of prolactin (PRL), LH, and FSH during preimplantation pregnancy. Data were collected from independent females every 2 h from 2200 h on proestrus through 0600 h on Day 5. PRL levels in serum showed a distinct, biphasic, nocturnal surge pattern. Levels during the light phase were low, with variable PRL during Day 2, when both estrogen and progesterone are at low levels in serum. FSH and LH levels in serum were determined twice daily. Elevated FSH and LH levels were found on Day 0 (proestrus). FSH then remained at a basal level while LH began to increase, with evidence for a daily rhythm in serum LH levels. Both FSH and LH accumulated in the pituitary over the first 4 days of pregnancy, although patterns were difficult to interpret. Data were consistent with a hypothesis that ovarian follicular development and preparation for a possible ovulation occur during preimplantation.

Substance P-like immunoreactivity in the hypothalamic suprachiasmatic nucleus of Phodopus sungorus — relation to daytime, photoperiod, sex and age

The immunohistochemical distribution of substance P (SP) in the hypothalamic suprachiasmatic nucleus (SCN) was studied in adult male and female Djungarian hamsters ( Phodopus sungorus) held under either long or short photoperiods. Intact animals were killed by perfusion with a fixative at the middle of the light or dark periods, respectively. The tissue was processed by routine immunohistochemical methods. Perikarya exhibiting SP-like immunoreactivity (LI) were found in the SCN of animals of all groups. These cell bodies predominantly were restricted to a distinct portion of the nucleus extending less than 150 μm rostrocaudally and were often concentrated in its lateral aspect. SP-LI fibers were rarely observed in the SCN, however, other hypothalamic parts, e.g. anterior and paraventricular regions, exhibit strong SP-LI innervation patterns. Sex-related differences were not observed. Long-term exposure to short days decreased the number ofneurons exhibiting SP-LI by approximately 60% when compared to long-day animals at both day- and nighttime. At night, SP-LI neurons were augmented in number by 34% (long-day group) and 56% (short-day group). Further, the numbers of SP-LI perikarya in the SCN of aged hamsters at day- and nighttime were augmented 3- to 4-fold when compared to adult animals. These results suggest that substance P in the SCN is involved in the regulation of circadian and seasonal mechanisms in this highly photoperiodic rodent species.

Alteration of the bed nucleus of the stria terminalis (BNST) in young female Djungarian hamsters ( Phodopus campbelli) exposed to adult males

Central components of the vomeronasal system appear to mediate extrahypothalamic control of gonadotropin release during male-induced acceleration of puberty onset in female rodents. In order to describe the bed nucleus of the stria terminalis (BNST) and to determine whether this component of the vomeronasal system is altered following early puberty onset, the anterior BNST of female Djungarian hamsters (Phodopus campbelli) was subdivided into three regions: medial, lateral, and ventral. The cross-sectional area, soma size, and neuronal density of each subdivision was compared among female hamsters in three groups: (1) young females showing accelerated reproductive development following a period of housing with an adult male, (2) young females matched by age with the accelerated females but housed alone during the same period, and (3) older females at the age of spontaneous puberty. Females housed with an adult male and undergoing accelerated puberty onset had significantly smaller lateral anterior BNST subdivisions than females housed alone for the same period (10 days following weaning) or housed alone until the age of spontaneous puberty onset (25 days following weaning). The size of the ventral and medial subdivisions was not different in the three groups. Furthermore, although soma size and neuronal density differed markedly among the anterior BNST subdivisions, these subdivision characteristics were similar in the accelerated, prepubertal, and older pubertal females. Our finding demonstrates that a central component of the vomeronasal system undergoes neuroanatomical alteration in response to environmental stimuli and recommends further examination of the BNST during this dynamic ontogenetic period.

Influence of short photoperiods on reproductive organs and estrous cycles of normal and pinealectomized female Djungarian hamsters, Phodopus sungorus.

This study was designed to analyze morphological changes in the reproductive organs and endocrine changes in the serum and pituitaries of female Djungarian hamsters during regression, the period of involution and early spontaneous recrudescence in response to short photoperiods. In addition, the involvement of the pineal gland on the inhibitory response to short photoperiods was investigated. In the first study, 63 adult female hamsters were divided into five groups, which were either pinealectomized at the time of transfer or 11 wk after transfer into short photoperiods, or exposed to different light regimens. The estrous cycles of these hamsters were recorded by vaginal smears for a period of 34 wk. In a second study, 156 adult female hamsters were divided into 14 groups and maintained under the influence of long or short photoperiods for 0-33 wk. The animals were killed by decapitation at 3-wk intervals and analyzed for changes in body weight and morphology of the vagina, ovary, and uterus. A loss of cyclic activity and complete involution of uterus and vagina were observed between Week 9 and Week 27 after transfer into short photoperiods. During this time period, no corpora lutea were observed in the ovaries. Under the influence of short photoperiods, the numbers of medium and antral follicles declined continuously until Week 15, and afterwards gradually recovered to maximal numbers in Week 27. This biphasic response to short photoperiods was not observed in the number of small follicles. These morphological changes were accompanied by changes in serum and pituitary FSH concentrations. Pinealectomy prevented the loss of estrous cyclicity after transfer into short photoperiods.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of photoperiod on pineal melatonin synthesis, fur color, body weight, and reproductive function in the female Djungarian hamster, Phodopus sungorus.

In order to investigate female Djungarian hamsters' reactions to changes of the photoperiod, the following two experiments were performed. Experiment I: Age-matched female hamsters were exposed to either short (8L:16D) or long days (16L:8D) for 38 weeks. Initially, the short-day group showed a decline in body weight, associated with changes in gonadal function and fur color. This was not maintained by the short-day group which returned, on the most part, to long-day levels, thus becoming insensitive to this regressive lighting regimen. The time courses of these events compare well with those observed in males, which suggests a common mechanism. Experiment II: Two groups of female hamsters were exposed for 8 weeks to either long days or short days. At the end of the test period, the diurnal variations in pineal content of melatonin, serotonin, hydroxyindole acetic acid, and serum melatonin were estimated, revealing marked differences between the two groups. Not only was there a prolongation of melatonin synthesis observed in the short-day animals, but there was also a significant elevation of the melatonin levels when compared to the long-day animals. Together with recent findings in males, these findings lend support to the hypothesis that, in the Djungarian hamster, the elevation of nocturnal melatonin levels may be of additional significance, with respect to the physiological changes induced by short-day photoperiods.

Social influences on reproductive development and fertility in female Djungarian hamsters ( Phodopus campbelli)

Social influences on the sexual maturation of female Djungarian hamsters were investigated in two experiments. In the first experiment females were housed from weaning with an adult male, by themselves, or with a weanling sister. Maturation was accelerated in females housed with males as indicated by younger age at first ovulation, increased rates of ovarian and uterine growth, and lower LH levels at some ages. Maturation was delayed in females housed with sisters compared to those housed alone as measured by time of first ovulation and by lower estradiol levels at some ages. The most marked differences between groups occurred 8 to 12 days after weaning, suggesting that events during this period are particularly important in the social mediation of sexual maturation. In the second experiment the effects of reproductive suppression (caused by living with a sister) on the subsequent fertility of females housed with males were examined. If male-female pairs were housed in clean cages, no effects were observed; however, pairs housed in cages previously soiled by the female and her sister had fewer young surviving until 1 week of age despite no differences in the age of pregnancy onset or in the initial litter size. Thus, even cues present in unrenewed soiled bedding may have subtle but long lasting effects on reproductive function.

Photoperiod effects on bombesin- and cholecystokinin-like immunoreactivity in the suprachiasmatic nuclei of the Djungarian hamster ( Phodopus sungorus)

The immunocytochemical distribution of the putative satiety peptides bombesin (BBS) and cholecystokinin (CCK) were studied in the hypothalamic suprachiasmatic nuclei (SCN) of male and female Djungarian hamsters ( Phodopus sungorus) held under either long (light/dark, LD 16:8 h) or short (LD 8:16) photoperiod. The animals were killed by perfusion with a fixative at the middle of the light period and the tissue was processed by routine immunohistochemical methods. Perikarya exhibiting BBS- or CCK-like immunoreactivity (LI) were found in the SCN of animals of all groups. Sex-related differences were not observed. In contrast, long-term exposure to short days decreased the number of neurons exhibiting CCK-LI dramatically when compared to long-day animals, while BBS-LI was not affected. The results suggest that CCK in the SCN is involved in the regulation of differential food intake in this highly photoperiodic rodent species.

Inhibition of 5′-Deiodination of Thyroxine Suppresses the Cold-Induced Increase in Brown Adipose Tissue Messenger Ribonucleic Acid for Mitochondrial Uncoupling Protein without Influencing Lipoprotein Lipase Activity*

Young adult male and female Djungarian hamsters were exposed to ambient temperatures of 23 or 0 C for 12 h; half of the animals in each group were treated with iopanoic acid to suppress the peripheral conversion of T4 to the thermotropically active thyroid hormone T3 by the enzyme 5'-deiodinase (5'D). Brown adipose tissue (BAT) mRNA for uncoupling protein (UCP), BAT lipoprotein lipase (LPL) activity, and 5'D activity were measured at the conclusion of the study. A temperature of 0 C produced large rises in 5'D and LPL activities and a similar large increase in UCP mRNA within the 12-h exposure period. When 5'D activity was inhibited with iopanoic acid, mRNA for UCP was reduced, while LPL activity was unaffected. The results show that the optimal production of mRNA for BAT UCP depends on the availability of T3; however, T3 is not required for the cold-induced activation of LPL activity in BAT.

Sustained reproductive responses in Djungarian hamsters (Phodopus sungorus) exposed to a single long day

Male and female Djungarian hamsters maintained from birth in a short photoperiod (8 h light per day; 8L:16D) showed substantial testicular and uterine growth in response to a single long photoperiod or a 15-min light pulse that interrupted the 16-h dark period at 18 days of age. These light regimens resulted in heavier testes and uteri at 30 and 35 days of age when compared with those of control animals. Similar results were obtained in hamsters maintained from birth to Day 18 in a long photoperiod (16L:8D), given a single longer day (20L:4D) or constant light on Day 18 and then transferred to a short photoperiod (8L:16D) on Day 19. At 35 days of age animals that received extended light treatment on Day 18 had significantly more developed reproductive structures than did control hamsters. The marked effects of brief light treatment in producing long-term changes in the reproductive axis provide a convenient mammalian model system in which to study neuroendocrine events that underlie photoperiodism.