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At least 865 records · Page 48Linked to original sources
PubMed2006

The effect of a competitive season and environmental factors on pulmonary function and aerobic power in varsity hockey players.

This study examined the effect of a competitive season and environmental factors on pulmonary function and aerobic power in varsity hockey players. Fourteen male subjects completed testing before and after a 7-month varsity hockey season within ice arena conditions. All subjects completed an aerobic power (.)VO(2) (max) test on a cycle ergometer. Pulmonary function tests were performed at rest and 1, 10, 15, and 25 min after the (.)VO(2) (max) test. The arena environment was monitored during testing and throughout the season for temperature, relative humidity, gaseous chemicals, moulds, and fungi. There was no change in (.)VO(2) (max) during the season. The percent change in forced expiratory flow in 1 s (FEV1) post-exercise compared to resting FEV1 and forced vital capacity (FVC) after the (.)VO(2) (max) test were significantly lower after the season. The arena temperature and relative humidity ranged between 13 and 16 degrees C and between 30% and 45% over the course of the season. Sulfur dioxide (0.7-4.5 ppm) was found in the arena and no airborne moulds unique to the dressing room environment were found to exceed Health Canada's guideline of 50 CFU/m(3) for indoor air quality. It was concluded that some hockey players experience limitations to pulmonary function over the course of a competitive season.

Adult↗
PubMed1989

Control of torpor and body weight patterns by a seasonal timer in Siberian hamsters.

Two experiments were designed to assess whether the short-day-induced patterns of shallow daily torpor, body weight, and other seasonal responses (food intake and pelage pigmentation) exhibited by Siberian hamsters (Phodopus sungorus sungorus) are under the control of a "seasonal timekeeping mechanism" that is independent of reproductive status [testosterone, (T)]. We examined whether the patterning and expression of these seasonal responses were altered by decreases in serum T that accompany gonadal regression during the first 8 wk of short-day exposure (i.e., the "preparatory phase" of the torpor season) or by experimental increases in serum T after this phase. Short-day-housed, castrated hamsters bearing T implants had long-day levels of the hormone and did not exhibit torpor. Appropriate seasonal patterns and levels of torpor, body weight, pelage color stage, and food intake were exhibited after T implant removal although serum T was clamped to long-day levels during the preparatory phase. In animals that were gonad intact during the preparatory phase and were subsequently castrated and given T implants, torpor did not occur as long as the implants were in place. However, the patterns and levels of daily torpor, food intake, and body weight rapidly returned to appropriate seasonal values compared with the castrated, blank-implanted controls on T implant removal; these effects occurred whether the T implants were removed when torpor frequency was increasing, at its peak, or decreasing across the torpor season. T did not affect pelage color stage under any condition.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Seasonal variation in the human circadian rhythm: dissociation between sleep and temperature rhythm.

The circadian rhythms of sleep-wakefulness, rectal temperature, and plasma melatonin were measured in 10 healthy male subjects for five consecutive seasons. To minimize direct effects of seasonally changing environmental factors, the subjects stayed in a living facility for 4 days in each season, where ambient temperature, humidity, and social contacts were controlled, while the light intensity of the living room was substantially influenced by natural daylight. Seasonal variations were found in the timing of sleep, the mean body temperature, the phases of circadian temperature and melatonin rhythms, and the phase relation between sleep and the rectal temperature rhythm. The subjects went to bed earliest in summer, intermediate in spring and autumn, and latest in winter. A similar but more pronounced seasonality was observed in the wake-up time, which was earlier in summer than in winter. The acrophases of the rectal temperature and plasma melatonin rhythms, which were calculated by fitting a cosine curve, were located in an earlier time of day in summer than in winter. The phase-angle difference of the rectal temperature rhythm to sleep varied seasonally and was more positive in summer than in winter. These findings indicate that not only the external (to the local time) but also the internal (between circadian rhythms) phase relations of the human circadian rhythms depend on season.

Adult↗
PubMed2005

Season, sun, sex, and cervical cancer.

INTRODUCTION: Sunlight's UV B component, a known cellular immunosupressant, carcinogen, and activator of viral infections, is generally seasonally available. Venereal human papillomavirus (HPV) transmission, at least in part, causes cervical cancer. We have previously inspected the monthly rates of venereal HPV infection and sunlight fluency in Southern Holland over 16 consecutive years. Both peak in August with at least 2-fold seasonality. The amount of available sunlight and the rate of Papanicolaou (Pap) smear screen-detected HPV are positively correlated. We now investigate whether premalignant and malignant cervical epithelial changes are also seasonal and related to seasonal sunlight fluency. METHODS: We have studied >900,000 consecutive, serially independent, interpretable screening Pap smears obtained by a single cervical cancer screening laboratory in Leiden, Holland, during a continuous 16-year span from 1983 through 1998. The average monthly rates of premalignant and malignant epithelial change were inspected and the annual patterns contrasted to the annual pattern of sunlight fluency at this global location and to monthly average HPV infection rate. Because HPV is venereally transmitted, Dutch seasonal sexual behavior was evaluated by assessment of the annual pattern of Dutch conception frequency as a competing cause for cervical cancer seasonality. RESULTS: (a) Twice as many premalignant and malignant epithelial changes were found among Pap smears obtained in the summer months, with an August peak concurrent with histopathologic evidence of HPV infection and sunlight fluency in Southern Holland. (b) Monthly sunlight fluency is correlated positively with both the monthly rates of Pap smear-detected cervical epithelial dysplasia and carcinomatous histopathology, as well as HPV. (c) Conception frequency, in this location, peaks in Spring not summer, and has a 4.8% annual amplitude. CONCLUSIONS: (a) Cervical epithelial HPV infection and HPV-induced cervical epithelial dysplasia and carcinomatous change may each be novel sun exposure risks and thereby behaviorably avoidable. (b) Because screening Pap smears uncover many abnormalities that resolve spontaneously (false positives), these data may argue for screening and follow-up Pap smear examinations in seasons other than summer in the Northern Hemisphere, to diminish the false-positive smear rate. Global data are available to confirm and further test each of these conclusions.

Adult↗
PubMedDate not supplied

Seasonal changes in blood pressure and frequency of related complications in elderly Israeli patients with essential hypertension.

BACKGROUND: Earlier studies on hypertension demonstrated seasonal variations in different age groups. However, slightly greater fluctuations were found in the hypertensive elderly. OBJECTIVE: We conducted a prospective 5-year study from January 1997 to December 2001 to evaluate the seasonal variation in blood pressure and the variables of age, gender, body mass index and related complications in elderly Israeli patients with essential hypertension. METHODS: Blood pressure was measured in four seasons in 182 patients (98 men and 84 women; age range 65-91 years) treated for hypertension in our outpatient clinic. RESULTS: Both systolic and diastolic mean blood pressures were higher during winter compared to summer (165 +/- 11.6 and 90 +/- 13.7 and 134 +/- 47.3 and 74 +/- 8.5 mm Hg, respectively; p < 0.001). There were no significant seasonal differences between spring and autumn or any correlation between the seasonal winter-summer difference in blood pressure and other studied parameters. Patients aged 65-75 years were unexpectedly more sensitive to winter-summer changes than older patients. There was a correlation between a large winter-summer difference in systolic blood pressure and a body mass index between 20 and 30, but there was none in lower or higher ranges. Supplementary antihypertension treatment was required during winter in 38% of these selected patients. Complications such as myocardial infarctions and strokes occurred twice as frequently in winter than in any other season (p < 0.0001). CONCLUSIONS: Both systolic and diastolic blood pressures were highest during winter. Hypertension complications were more frequent in winter. Our results refute those of earlier studies that failed to find significant seasonal variations in blood pressure among the elderly.

Age Factors↗
PubMed2005

Seasonal patterns of incidence and case fatality of stroke in Malmo, Sweden: the STROMA study.

BACKGROUND: Studies on the temporal variation in stroke incidence have reported inconsistent results. Few have studied the temporal variations in case fatality. No study on incidence and case fatality of stroke by season in Sweden has been found. This study explores the weekly, monthly and seasonal variations in incidence and 28-day case fatality of stroke. METHODS: A total of 7,129 patients with first-ever stroke during the period 1989-1999 were retrieved from the Stroke Register of Malmo, Sweden. chi(2) test was performed to test the seasonal differences and Poisson regression analysis was used to calculate the case fatality ratios adjusted for sex and age. RESULTS: The stroke cases were on the whole randomly distributed over the study period of 4,017 days. Incidence of all types of stroke, intracerebral hemorrhage and subarachnoid hemorrhage showed no variation by season, month or weekday, but incidence of cerebral infarction for the total population was higher in autumn and winter. Twenty-eight-day case fatality (930 fatal events, 13%) clustered significantly over the study period. Winter emerged as the peak season among men (12.5%), women (17.2%) and total population (15.1%). No consistent variation of incidence and case fatality of stroke by month or weekday was found. CONCLUSION: Case fatality after stroke demonstrates a seasonal variation with a peak in winter. Incidence of stroke showed no consistent association with season, month or weekday.

Aged↗
PubMed2004

Seasonal variation in hospitalization and case fatality of subarachnoid hemorrhage - a nationwide danish study on 9,367 patients.

Seasonal variation in the occurrence of cerebrovascular disease has been reported, but data about subarachnoid hemorrhage (SAH) are few and inconclusive. We conducted a nationwide population-based study in Denmark to examine any seasonal pattern of hospitalization and case fatality of SAH. We identified 9,367 patients with SAH and found a modest indication of overall seasonal variation for the risk of hospitalization with this diagnosis. The seasonal pattern, with the highest incidence in January and the nadir in July, was mostly apparent for subjects aged >65 years (peak-to-trough ratio = 1.18; 95% CI 1.04-1.32). There was little difference by sex. The overall 30-day case fatality rate was 38% and showed less seasonal variation. We found evidence of weak seasonal variation in hospitalization for SAH and almost no seasonal variation in a 30-day case fatality rate after SAH.

Age Factors↗
PubMed2006

Seasonal variation in stroke and stroke-associated mortality in patients with a hospital diagnosis of nonvalvular atrial fibrillation or flutter. A population-based study in Denmark.

AIM: There are few data on seasonal variation in stroke and seasonal variation in mortality after stroke in patients with atrial fibrillation. We examined the seasonal pattern in stroke occurrence and the effect of the season on mortality after stroke in patients with a history of nonvalvular atrial fibrillation. METHODS: We identified all individuals, aged 40-89 years, with an incident diagnosis of stroke of any nature (ischemic or hemorrhagic) in the 1980-2002 period and no history of heart valve disease and a previous or concomitant diagnosis of atrial fibrillation or flutter in the Danish National Registry of Patients. Subjects were followed in the Danish Civil Registration System for emigration and vital status. We used periodic regression models to estimate the peak-trough ratio stratified by sex, age and comorbid medical conditions. Seasonal effect on mortality after stroke was analyzed in a Cox proportional hazards model. RESULTS: The relative incidence of stroke estimated as the ratio of the incidence in the month of the peak (January) to the incidence in the month of the trough (July) was 1.11 (95% confidence interval: 1.07-1.15). The relative incidence of stroke was similar for men and women, did not differ by age (stratified by age 75 years) and was essentially similar for comorbid conditions considered. There was no seasonal effect on mortality after stroke. CONCLUSIONS: The occurrence of stroke in patients with atrial fibrillation is modestly higher during the winter. Stroke-associated mortality does not vary by season.

Adult↗
PubMed2007

Seasonal variation of serum lipid levels in stable renal transplant recipients.

BACKGROUND/AIMS: Seasonal variation in lipid levels is well described in the general population, but has not been examined in renal transplant recipients (RTR). We sought to determine whether seasonal differences exist in RTR, a group at high risk for hyperlipidemia. METHODS: We reviewed our population of 920 adults, identifying primary allograft recipients with survival > or =1 year, stable function, and > or =1 pair of post-6 months 'winter' (December 21 to March 20) plus 'summer' (June 21 to September 22) fasting lipid measurements within the same year. Correlations between factors affecting lipids and lipid level change were followed by multiple linear regression analysis. RESULTS: 243 patients contributed 344 pairs. When most recent seasonal pair (n = 243) and all pairs (n = 344) were separately analyzed, no seasonal total cholesterol difference (winter vs. summer) was seen (5.08 vs. 5.05 mmol/l, p = 0.80; 5.11 vs. 5.09 mmol/l, p = 0.81 respectively). Opposing variation was seen between hyperlipidemic and nonhyperlipidemic patients (0.08 vs. -0.18 mmol/l for winter minus summer, p = 0.02). In multivariate analysis, seasonal cholesterol variation was predicted by level (p < 0.0001) and hemoglobin change (p = 0.01), while triglyceride variation was predicted only by level (p = 0.01). CONCLUSION: RTR do not exhibit seasonal variation in lipids, unlike the general population. Factors unique to RTR such as immunosuppressive therapies may act to suppress any seasonal effects.

Adult↗
PubMed1987

Seasonal cholesterol cycles: the Lipid Research Clinics Coronary Primary Prevention Trial placebo group.

Seasonal variation of plasma cholesterol levels was studied in 1446 hypercholesterolemic 35- to 59-year-old male participants in the Lipid Research Clinics Coronary Primary Prevention Trial placebo group. Each man's serial cholesterol data, obtained at bimonthly intervals for 2.0 to 6.5 years, were analyzed as a separate periodic time series, and distributions of cycle zeniths and amplitudes were calculated. A highly significant (chi 2= 706, 2 degrees of freedom) seasonal effect, 7.4 mg/dl higher on December 30 than on June 30, was found. This effect was similar among the 12 LRC centers, including such disparate climates as those of Minneapolis and San Diego, and tended to be larger in the southern centers. Its magnitude was independent of baseline levels of plasma cholesterol and other baseline characteristics. Observed seasonal differences in weight and diet explained less than one-third of the seasonal variation in plasma cholesterol levels. Plasma low- and high-density lipoprotein cholesterol levels, analyzed similarly, also showed significant synchronous seasonal cycles. Plasma triglyceride levels showed a weaker irregular seasonal pattern, highest in midsummer and late autumn and lowest in spring. The etiologies and mechanisms for these seasonal patterns remain largely unknown.

Adult↗
PubMed1996

Seasonal variations in house dust mite influence the circadian peak expiratory flow amplitude.

The aim of the study was to investigate whether seasonal differences in house dust mite (HDM) allergen exposure influence the circadian peak expiratory flow (PEF) amplitude in asthmatic children. Asthmatic children (n = 25) with a solitary allergy to HDM were studied in spring and in autumn. All used inhaled corticosteroids (ICS) regularly. Six days after withdrawal of ICS, PEF amplitude (every 4 h during 24 h, highest-lowest/percentage of mean value) was assessed. HDM allergen (HDMA) in living rooms, bedrooms, and mattresses was collected. HDMA levels were not always highest in autumn. PEF amplitudes in spring and autumn did not correlate with HDMA levels in the same season. However, the seasonal difference in PEF amplitude (autumn value - spring value) correlated positively and significantly with the seasonal difference in HDMA exposure levels from the mattresses (rho = 0.34, p < 0.05). Multivariate analysis showed that the seasonal difference in HDMA exposure in the mattress was the single parameter explaining seasonal difference in PEF amplitudes by 21.0% (p = 0.02). Our cross-sectional study showed a higher PEF amplitude not to be significantly associated with higher HDMA exposure in mattresses in a group of HDM-allergic asthmatic children. However, the change in HDMA exposure over seasons contributed significantly to the change in PEF amplitude after withdrawal of ICS in HDM-allergic asthmatic children.

Allergens↗
PubMed2001

Season of birth among patients with schizophrenia and their siblings: evidence for the procreational habits hypothesis.

OBJECTIVE: The birth rate of patients with schizophrenia during the winter and spring months is 5%-8% higher worldwide than the birth rate of the general population in the winter and spring months. Seasonal variation of births among the unaffected siblings of patients with schizophrenia has not been studied with adequate sample sizes. The authors investigated the seasonal variation of births among siblings of patients with schizophrenia in a large, nationwide, representative patient and sibling population. METHOD: Finnish patients with schizophrenia born from 1950 to 1969 (N=15,389) were identified from three nationwide health care registers. Unaffected siblings of these patients born in the same time period (N=37,819) were identified from the Finnish National Population REGISTER: The seasonal variation of births among patients and siblings were examined by using a log-linear model. Explanatory variables were sex, year of birth categorized into four 5-year groups, and seasonal variation, which was analyzed by fitting a short Fourier series to the monthly birth data. RESULTS: The odds for having been born during the winter-spring months were slightly higher among both siblings and patients in all birth-year groups. However, patients born from 1955 to 1959 showed prominent seasonal variation of births, but the magnitude of this variation remained unchanged among siblings. CONCLUSIONS: Seasonal variation of births among patients with schizophrenia may consist of two factors: 1) parental procreational habits causing a slight excess of births of both patients and unaffected siblings during the winter-spring months and 2) irregular environmental factors that considerably increase the magnitude of the seasonal variation of births among patients but not their siblings.

Adult↗
PubMed2002

Seasonal changes in adiposity: the roles of the photoperiod, melatonin and other hormones, and sympathetic nervous system.

It appears advantageous for many non-human animals to store energy body fat extensively and efficiently because their food supply is more labile and less abundant than in their human counterparts. The level of adiposity in many of these species often shows predictable increases and decreases with changes in the season. These cyclic changes in seasonal adiposity in some species are triggered by changes in the photoperiod that are faithfully transduced into a biochemical signal through the nightly secretion of melatonin (MEL) via the pineal gland. Here, we focus primarily on the findings from the most commonly studied species showing seasonal changes in adiposity-Siberian and Syrian hamsters. The data to date are not compelling for a direct effect of MEL on white adipose tissue (WAT) and brown adipose tissue (BAT) despite some recent data to the contrary. Thus far, none of the possible hormonal intermediaries for the effects of MEL on seasonal adiposity appear likely as a mechanism by which MEL affects the photoperiodic control of body fat levels indirectly. We also provide evidence pointing toward the sympathetic nervous system as a likely mediator of the effects of MEL on short day-induced body fat decreases in Siberian hamsters through increases in sympathetic drive on WAT and BAT. We speculate that decreases in the SNS drive to these tissues may underlie the photoperiod-induced seasonal increases in body fat of species such as Syrian hamsters. Clearly, we need to deepen our understanding of seasonal adiposity, although, to our knowledge, this is the only form of environmentally induced changes in body fat where the key elements of its external trigger have been identified and can be traced to and through their transduction into a physiological stimulus that ultimately affects identified responses of white adipocyte physiology and cellularity. Finally, the comparative physiological approach to the study of seasonal adiposity seems likely to continue to yield significant insights into the mechanisms underlying this phenomenon and for understanding obesity and its reversal in general.

Adipose Tissue↗
PubMed2004

Seasonal ovulatory activity exists in tropical Creole female goats and Black Belly ewes subjected to a temperate photoperiod.

BACKGROUND: Seasonality of ovulatory activity is observed in European sheep and goat breeds, whereas tropical breeds show almost continuous ovulatory activity. It is not known if these tropical breeds are sensitive or not to temperate photoperiod. This study was therefore designed to determine whether tropical Creole goats and Black-Belly ewes are sensitive to temperate photoperiod. Two groups of adult females in each species, either progeny or directly born from imported embryos, were used and maintained in light-proof rooms under simulated temperate (8 to 16 h of light per day) or tropical (11 - 13 h) photoperiods. Ovulatory activity was determined by blood progesterone assays for more than two years. The experiment lasted 33 months in goats and 25 months in ewes. RESULTS: Marked seasonality of ovulatory activity appeared in the temperate group of Creole female goats. The percentage of female goats experiencing at least one ovulation per month dramatically decreased from May to September for the three years (0%, 27% and 0%, respectively). Tropical female goats demonstrated much less seasonality, as the percentage of goats experiencing at least one ovulation per month never went below 56%. These differences were significant. Both groups of temperate and tropical Black-Belly ewes experienced a marked seasonality in their ovulatory activity, with only a slightly significant difference between groups. The percentage of ewes experiencing at least one ovulation per month dropped dramatically in April and rose again in August (tropical ewes) or September (temperate ewes). The percentage of ewes experiencing at least one ovulation per month never went below 8% and 17% (for tropical and temperate ewes respectively) during the spring and summer months. CONCLUSIONS: An important seasonality in ovulatory activity of tropical Creole goats was observed when females were exposed to a simulated temperate photoperiod. An unexpected finding was that Black-Belly ewes and, to a lesser extent, Creole goats exposed to a simulated tropical photoperiod also showed seasonality in their ovulatory activity. Such results indicate that both species are capable of showing seasonality under the photoperiodic changes of the temperate zone even though they do not originate from these regions.

Animals↗
PubMed2003

Seasonal plasticity within the gonadotropin-releasing hormone (GnRH) system of the ewe: changes in identified GnRH inputs and glial association.

The annual reproductive cycle in sheep may reflect a functional remodeling within the GnRH system. Specifically, changes in total synaptic input and association with the polysialylated form of neural cell adhesion molecule have been observed. Whether seasonal changes in a specific subset(s) of GnRH inputs occur or whether glial cells specifically play a role in this remodeling is not clear. We therefore examined GnRH neurons of breeding season (BS) and nonbreeding season (anestrus) ewes and tested the hypotheses that specific (i.e. gamma-aminobutyric acid, catecholamine, neuropeptide Y, or beta-endorphin) inputs to GnRH neurons change seasonally, and concomitant with any changes in neural inputs is a change in glial apposition. Using triple-label immunofluorescent visualization of GnRH, glial acidic fibrillary protein and neuromodulator/neural terminal markers combined with confocal microscopy and optical sectioning techniques, we confirmed that total numbers of neural inputs to GnRH neurons vary with season and demonstrated that specific inputs contribute to these overall changes. Specifically, neuropeptide Y and gamma-aminobutyric acid inputs to GnRH neurons increased during BS and beta-endorphin inputs were greater during either anestrus (GnRH somas) or BS (GnRH dendrites). Associated with the changes in GnRH inputs were seasonal changes in glial apposition, glial acidic fibrillary protein density, and the thickness of glial fibrils. These findings are interpreted to suggest an increase in net stimulatory inputs to GnRH neurons during the BS contributes to the seasonal changes in GnRH neurosecretion and that this increased innervation is perhaps stabilized by glial processes.

Animals↗
PubMed2006

Morphological plasticity in the neural circuitry responsible for seasonal breeding in the ewe.

An increase in the response of GnRH neurons to estrogen negative feedback is responsible for seasonal anestrus in the ewe, but the underlying neural mechanisms remain largely unknown. Neural plasticity may play an important role because the density of synaptic input to GnRH neurons changes with seasons. Moreover, the transition from breeding to anestrous season requires thyroid hormones, which are also required for neuronal development. In the first experiment, we examined whether the decrease in synapses on GnRH neurons is critical for the transition to anestrus by comparing synaptic input in thyroidectomized and thyroid-intact controls, using electron microscopic analysis. Thyroidectomized ewes remained in the breeding season, but the number of synaptic contacts on their GnRH cells was not different from those in thyroid-intact ewes that were anestrus. The next experiment tested whether there was a seasonal change in morphology of the A15 dopaminergic neurons that mediate estrogen negative feedback during anestrus by analyzing synapsin-positive close contacts onto A15 neurons with confocal microscopy. There was a 2-fold increase in these close contacts onto dendrites of A15 neurons in anestrus and a corresponding increase in the length of A15 dendrites at this time of year. The increase in dendritic length was blocked by thyroidectomy, but this procedure did not significantly affect synaptic input to A15 neurons. These results provide initial evidence that the seasonal change in synapses on GnRH neurons is not sufficient for the transition into anestrus but that plasticity of the A15 dopaminergic neurons mediating estrogen negative feedback may contribute to this seasonal alteration.

Anestrus↗
PubMed1988

Changes in brain aromatase and 5 alpha-reductase activities correlate significantly with seasonal reproductive cycles in goldfish (Carassius auratus).

Aromatization and 5 alpha-reduction are known to be required for the full expression of testosterone actions in neuroendocrine tissues. Although aromatase and 5 alpha-reductase activities in brain and pituitary can be experimentally manipulated by castration and steroid replacement, naturally occurring variations during seasonal reproductive cycles have not been examined in any species. Goldfish (Carassius auratus) were selected for study because they exhibit exceptionally high levels of aromatase in both brain and pituitary, although 5 alpha-reductase levels resemble the vertebrate norm. Four animals of each sex were tested monthly through three breeding seasons (2.5 yr). Using previously validated techniques, the enzymes were assayed by product formation from [3H]androstenedione in homogenates of anterior hypothalamus-preoptic area (AHPOA), remaining telencephalon (TEL), whole pituitary, ovary, and testis. Seasonal variations in aromatase were most dramatic in the AHPOA of female fish, exhibiting a peak in April and May that was 6-fold higher than the nadir in July. As judged by changes in the appearance and weight of the gonads, maximal aromatase coincided with the spawning season, whereas low enzyme levels corresponded to reproductive inactivity. Seasonal variations were similar but of a lesser magnitude in the TEL of females and in the AHPOA and TEL of males (2- to 3-fold, peak to nadir). Both ovarian and testicular aromatase showed cyclic changes; however, activity was much lower than that in brain at all times of the year (4.5, 1.2, and 47.0 pmol/mg protein, maximal values in ovaries, testes, and AHPOA, respectively). Pituitary aromatase varied from 5-22 pmol/mg protein, but was not consistently correlated with season. Cyclic changes in 5 alpha-reductase were distinctly different from those in aromatase, with maximal values in both brain and pituitary occurring when fish were reproductively inactive. In general, circulating sex steroids were high when aromatase was high and low when reductase was maximal; however, there was no apparent causal relationship suggested by temporal changes in a given steroid. Variations in testosterone metabolism, by regulating the quantity and quality of active hormone in close proximity to receptor sites, may be responsible for the changes in feedback sensitivity and behavioral responsiveness that are known to occur in seasonal breeders.

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗
PubMed1997

A critical period for thyroid hormone action on seasonal changes in reproductive neuroendocrine function in the ewe.

Thyroid hormones are obligatory for the annually recurring termination of reproductive activity in a spectrum of seasonal breeders, including sheep. Previous studies involving thyroidectomy and T4 replacement have led to the hypothesis that, in the ewe, thyroid hormones are necessary only during a limited interval late in the breeding season for the neuroendocrine processes that cause the transition to anestrus. The present series of experiments tested this hypothesis by assessing the influence of thyroidectomy, with or without T4 replacement for specific durations and at different times of the year, on the transition to anestrus. Seasonal alterations in reproductive neuroendocrine activity were monitored by changes in serum LH concentration in ovariectomized ewes bearing s.c. SILASTIC brand silicon tubing implants containing estradiol. Thyroidectomy in mid-December, just before the putative period of thyroid hormone action, prevented the development of the neuroendocrine anestrous season (fall in LH in this animal model). T4 replacement for 90 days beginning in late December (i.e., during the postulated period of thyroid hormone action) overcame the blockade of anestrus, causing LH to fall in ewes thyroidectomized several months previously. The minimal effective duration of exposure to thyroid hormones required for the transition to anestrus was estimated to be 60-90 days. Further, exposure to T4 for 60-90 days beginning in late December was found to be the only time of the year that thyroid hormones were required to maintain seasonal changes in reproductive neuroendocrine activity. Finally, replacement of T4 for 90 days at a different time of year (beginning in August) failed to provoke development of neuroendocrine anestrus in thyroidectomized ewes. These results support the hypothesis that thyroid hormones are necessary only during a limited interval late in the breeding season to promote seasonal reproductive suppression in the ewe. Further, the reproductive neuroendocrine axis is not equally responsive to thyroid hormone at all times of the year. This suggests there is a critical period of responsiveness during which thyroid hormones must be present for anestrus to develop.

Anestrus↗