Melatonin supplementation decreases prolactin synthesis and release in rat adenohypophysis: correlation with anterior pituitary redox state and circadian clock mechanisms

In the laboratory rat, a number of physiological parameters display seasonal changes even under constant conditions of temperature, lighting, and food availability. Since there is evidence that prolactin (PRL) is, among the endocrine signals, a major mediator of seasonal adaptations, the authors aimed to examine whether melatonin administration in drinking water resembling in length the exposure to a winter photoperiod could affect accordingly the 24-h pattern of PRL synthesis and release and some of their anterior pituitary redox state and circadian clock modulatory mechanisms. Melatonin (3?μg/mL drinking water) or vehicle was given for 1 mo, and rats were euthanized at six time intervals during a 24-h cycle. High concentrations of melatonin (>2000 pg/mL) were detected in melatonin-treated rats from beginning of scotophase (at 21:00?h) to early photophase (at 09:00?h) as compared with a considerably narrower high-melatonin phase observed in controls. By cosinor analysis, melatonin-treated rats had significantly decreased MESOR (24-h time-series average) values of anterior pituitary PRL gene expression and circulating PRL, with acrophases (peak time) located in the middle of the scotophase, as in the control group. Melatonin treatment disrupted the 24-h pattern of anterior pituitary gene expression of nitric oxide synthase (NOS)-1 and -2, heme oxygenase-1 and -2, glutathione peroxidase, glutathione reductase, Cu/Zn- and Mn-superoxide dismutase, and catalase by shifting their acrophases to early/middle scotophase or amplifying the maxima. Only the inhibitory effect of melatonin on pituitary NOS-2 gene expression correlated temporally with inhibition of PRL production. Gene expression of metallothionein-1 and -3 showed maxima at early/middle photophase after melatonin treatment. The 24-h pattern of anterior pituitary lipid peroxidation did not vary after treatment. In vehicle-treated rats, Clock and Bmal1 expression peaked in the anterior pituitary at middle scotophase, whereas that of Per1 and Per2 and of Cry1 and Cry2 peaked at the middle and late photophase, respectively. Treatment with melatonin raised mean expression of anterior pituitary Per2, Cry1, and Cry2. In the case of Per1, decreased MESOR was observed, although the single significant difference found between the experimental groups when analyzed at individual time intervals was increase at early scotophase in the anterior pituitary of melatonin-treated rats. Melatonin significantly phase-delayed expression of Per1, Per2, and Cry1, also phase-delayed the plasma corticosterone circadian rhythm, and increased the amplitude of plasma corticosterone and thyrotropin rhythms. The results indicate that under prolonged duration of a daily melatonin signal, rat anterior pituitary PRL synthesis and release are depressed, together with significant changes in the redox and circadian mechanisms controlling them. (Author correspondence: danielcardinali@uca.edu.ar ; danielcardinali@fibertel.com.ar ).     

Adrenergic response patterns in vas deferens isolated from rats under diurnal rhythms. Influence of stress

The aim of this study was to verify, by means of functional methods, whether the circadian rhythm changes adrenergic response patterns in the epididymal half of the vas deferens isolated from control rats as well as from rats submitted to acute stress. The experiments were performed at 9:00 a.m., 3:00 p.m., 9:00 p.m., and 3:00 a.m. The results showed a light-dark dependent variation of the adrenergic response pattern on organs isolated from control as well as from stressed rats. In the control group, only the phenylephrine sensitivity was changed throughout the circadian rhythm. Under the stress condition, both norepinephrine and phenylephrine response patterns were changed, mainly during darkness. The maximal contractile response to both alpha- and beta-agonist and alpha1-agonist was increased in the dark phase, corresponding to high plasmatic concentrations of endogenous melatonin. The vas deferens isolated from stressed rats during the light phase simultaneously incubated with exogenous melatonin showed the same pattern of response obtained in the dark phase, thus indicating a peripheric action of melatonin on this organ. Therefore, the circadian rhythms are important to the adrenergic response pattern in rat vas deferens from both control and stressed rats. In conclusion, we suggest a melatonin modulation on alpha1-postsynaptic adrenergic response in the rat vas deferens.     

General and unspecific damping by malignancy of the circadian amplitude of circulating human melatonin?

The circadian rhythm of serum melatonin of 39 cancer patients is compared with that of 28 healthy subjects matched by gender and age. Each subject provided 6 blood samples at 4-hour intervals for determination of melatonin by RIA. After log10-transformation, data series were analyzed by single and population-mean cosinor and compared between the two groups and among patients subgrouped by cancer site, stage and treatment. A circadian rhythm (P<0.001) is demonstrated for both groups, with a contributing 12-hour harmonic (P<0.001). In the absence of a difference in MESOR, the circadian amplitude of the cancer patients is smaller than that of the healthy subjects (P=0.003). Numerically, nocturnal (00:00 and 04:00) melatonin concentrations are lower and daytime (08:00-20:00) melatonin concentrations are higher in the cancer patients than in the healthy subjects (P=0.032 at 12:00 and P=0.058 at 16:00). In the age ranges examined, no differences are found with age in either group or by gender in health. No differences are found among cancer patients subgrouped either by site, stage (localized vs. metastasized) or treatment. If these results are validated, other Janus-like (two-faced: stimulation or inhibition, depending on chronome stage) effects of malignancy should be taken into consideration for screening and for timing treatment.     

Suppressive effect of delta 9-tetrahydrocannabinol in vitro on phagocytosis by murine macrophages

Incubation of normal mouse peritoneal cells consisting of over 90% phagocytizing macrophages with delta 9-tetrahydrocannabinol (THC) resulted in a inhibition of phagocytic function. The THC in a dose-related manner suppressed the percentage of macrophages per culture which ingested yeast and the average number of yeast particles ingested by the phagocytizing macrophages. The vehicle used to suspend the THC in vitro, i.e., DMSO, had no detectable effect on macrophage function. Suppression of phagocytosis with no effects on viability or cell number occurred with doses of 10 micrograms or less THC per milliliter culture medium. Measurable suppression also occurred after 24- to 48-hr treatment of the macrophages with the THC. This compound had little if any detectable effect on phagocytosis when added directly to the cultures shortly before testing for phagocytosis. Further studies concerning the effects of THC on macrophage function appear warranted.     

The circadian body temperature rhythm in the elderly: effect of single daily melatonin dosing

The present study is part of a more extensive investigation dedicated to the study and treatment of age-dependent changes/disturbances in the circadian system in humans. It was performed in the Tyumen Elderly Veteran House and included 97 subjects of both genders, ranging from 63 to 91 yrs of age. They lived a self-chosen sleep-wake regimen to suit their personal convenience. The experiment lasted 3 wks. After 1 control week, part of the group (n=63) received 1.5 mg melatonin (Melaxen) daily at 22:30 h for 2 wks. The other 34 subjects were given placebo. Axillary temperature was measured using calibrated mercury thermometers at 03:00, 08:00, 11:00, 14:00, 17:00, and 23:00 h each of the first and third week. Specially trained personnel took the measurements, avoiding disturbing the sleep of the subjects. To evaluate age-dependent changes, data obtained under similar conditions on 58 young adults (both genders, 17 to 39 yrs of age) were used. Rhythm characteristics were estimated by means of cosinor analyses, and intra- and inter-individual variability by analysis of variance (ANOVA). In both age groups, the body temperature underwent daily changes. The MESOR (36.38+/-0.19 degrees C vs. 36.17+/-0.21 degrees C) and circadian amplitude (0.33+/-0.01 degrees C vs. 0.26+/-0.01 degrees C) were slightly decreased in the elderly compared to the young adult subjects (p<0.001). The mean circadian acrophase was similar in both age groups (17.19+/-1.66 vs. 16.93+/-3.08 h). However, the inter-individual differences were higher in the older group, with individual values varying between 10:00 and 23:00 h. It was mainly this phase variability that caused a decrease in the inter-daily rhythm stability and lower group amplitude. With melatonin treatment, the MESOR was lower by 0.1 degrees C and the amplitude increased to 0.34+/-0.01 degrees C, a similar value to that found in young adults. This was probably due to the increase of the inter-daily rhythm stability. The mean acrophase did not change (16.93 vs. 16.75 h), although the inter-individual variability decreased considerably. The corresponding standard deviations (SD) of the group acrophases were 3.08 and 1.51 h (p<0.01). A highly significant correlation between the acrophase before treatment and the phase change under melatonin treatment indicates that this is due to a synchronizing effect of melatonin. Apart from the difference in MESOR, the body temperature rhythm in the elderly subjects undergoing melatonin treatment was not significantly different from that of young adults. The data clearly show that age-dependent changes mainly concern rhythm stability and synchronization with the 24 h day. A single daily melatonin dose stabilizes/synchronizes the body temperature rhythm, most probably via hypothermic and sleep-improving effects.     

Effect of stress and dexamethasone treatment on circadian rhythms of melatonin and corticosterone in ring dove (Streptopelia risoria)

The possible relationship between the circadian rhythm of blood levels of melatonin and corticosterone in ring dove (Streptopelia risoria) subjected to both immobilization stress and immobilization stress plus dexamethasone treatment were studied. The results show changes in the circadian rhythm of melatonin, with increased day-time levels in situations of stress accompanied by increased corticosterone levels. The highest blood melatonin levels over the 24 h of the study were obtained when the animals were treated with dexamethasone and then subjected to stress. Given the antioxidant role of melatonin, our results support the idea of melatonin-corticosterone coupling with the possibility that melatonin released in situations of stress counteracts the adverse effects of glucocorticoids on the organism.     

Daily Changes of Plasma Corticosterone by an 8-h Daytime Feeding Occur Without Body Weight Loss or Severe Food Restriction in the Rat

Different studies have reported that daytime feeding entrains the circadian rhythm of corticosterone secretion in rats. However, it remained unclear whether calorie restriction or daytime feeding access have an effect. The aim of our study is to evaluate the effect of an 8-h daytime feeding access on the circadian rhythm of plasma corticosterone. Eleven adult male Wistar rats were assigned to two different conditions of access to food: ad lib feeding for one week and daytime feeding for the following two weeks. On the 7th, 14th and 21st day, blood samples were collected every 4 h from 08:00 to 04:00. Food intake and body weight were recorded daily. During daytime feeding, rats ingested 88% of the amount of food ingested over 24 h in the ad lib feeding period. However, body weight increased significantly from the first day to the end of experiment. Peak plasma corticosterone was 12 h shifted during daytime feeding period compared to the ad lib condition. This study showed that an 8-h daytime feeding entrained the circadian rhythm of plasma corticosterone without body weight loss or severe food restriction.     

Daily Changes of Plasma Corticosterone by an 8-h Daytime Feeding Occur Without Body Weight Loss or Severe Food Restriction in the Rat

Different studies have reported that daytime feeding entrains the circadian rhythm of corticosterone secretion in rats. However, it remained unclear whether calorie restriction or daytime feeding access have an effect. The aim of our study is to evaluate the effect of an 8-h daytime feeding access on the circadian rhythm of plasma corticosterone. Eleven adult male Wistar rats were assigned to two different conditions of access to food: ad lib feeding for one week and daytime feeding for the following two weeks. On the 7th, 14th and 21st day, blood samples were collected every 4 h from 08:00 to 04:00. Food intake and body weight were recorded daily. During daytime feeding, rats ingested 88% of the amount of food ingested over 24 h in the ad lib feeding period. However, body weight increased significantly from the first day to the end of experiment. Peak plasma corticosterone was 12 h shifted during daytime feeding period compared to the ad lib condition. This study showed that an 8-h daytime feeding entrained the circadian rhythm of plasma corticosterone without body weight loss or severe food restriction.     

Acute stress disrupts temporal patterns of behavioral and biochemical parameters of rats

This study aimed to answer these questions: (i) Can the temporal patterns of locomotor and exploratory activities and anxiety-like behavior be disrupted by a single session of stress? (ii) Can the temporal patterns of corticosterone, melatonin, and glucose be disrupted by a single session of stress and how long does the effect of stress upon their circadian rhythm? Adult male Wistar rats were exposed to restrain stress at four Zeitgeber Times and subdivided according to evaluation times of behavioral (open field test and elevated plus-maze test) and biochemistry (corticosterone, melatonin, and glucose levels in serum) parameters. There were temporal patterns within a course of 24 h in anxiety-like behaviors and glucose levels. Circadian rhythm to corticosterone and melatonin levels was confirmed and a single session of stress can disrupt these temporal patterns. We showed an immediate disruptive effect of stress in biomarkers levels and a delayed appearance of behavioral and locomotion pattern disruption.     

Effect of orally administered l-tryptophan on serotonin,melatonin, and the innate immune response in the rat

To assess the effects of external administration of L-tryptophan on the synthesis of serotonin and melatonin as well as on the immune function of Wistar rats, 300 mg of the amino acid were administered through an oral cannula either during daylight (08:00) or at night (20:00) for 5 days. Brain, plasma, and peritoneal macrophage samples were collected 4 h after the administration. The accumulation of 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition was used to measure the rate of tryptophan hydroxylation in vivo. Circulating melatonin levels were determined by radioimmunoassay, and the phagocytic activity of macrophages was measured by counting, under oil-immersion phase-contrast microscopy, the number of particles ingested. The results showed a diurnal increase (p < 0.05) in the brain 5-HTP, serotonin (5-hydroxytryptamine, 5-HT), and 5-hydroxyindolacetic acid (5-HIAA) of the animals which had received tryptophan at 08:00 and were killed 4 h later. In the animals which received tryptophan during the dark period, the 5-HT declined but the 5-HT/5-HIAA ratio remained unchanged. There was also a significant increase (p < 0.05) in nocturnal circulating melatonin levels and in the innate immune response of the peritoneal macrophages in the animals which had received tryptophan at 20:00. The results indicated that the synthesis of serotonin and melatonin, as well as the innate immune response, can be modulated by oral ingestion of tryptophan.     

Diurnal Variation of Plasmatic Melatonin,Corticosterone and Variation of General Activity in Pigeons under Light-Dark Cycle and Constant Light

This work analyzed the diurnal variation of general activity and plasmatic levels of melatonin and corticosterone in pigeons submitted to a 12:00:12:00 h light-dark cycle (lights on at 6:00 a.m.) or to constant light. In both conditions pigeons were observed in 5-min sessions at times 03:00, 06:00, 09:00, 12:00, 15:00, 18:00, 21:00 and 24:00 h during two successive days. Behavior was video taped in the home cages for posterior categorization and quantification. Radioimmunoassays were used to evaluate plasmatic levels of melatonin and corticosterone. Blood samples were obtained at the times of behavioral observation. In the light-dark condition the results showed day-night variation of general activity (p &lt; 0.001) and a robust diurnal rhythm of plasmatic melatonin (p &lt; 0.001). Both of these variations as well as the oscillatory secretion of corticosterone disappeared under constant light condition. The parallel changes in general activity and blunting of melatonin rhythm secretion in constant light condition agree with previous evidences that melatonin may regulate behavioral oscillations in the pigeon. The present data are related to the proposition that the timing system in pigeons may involve neuroendocrine relations characterized by interactions between blood born signalization by melatonin and corticosterone.     

Daily rhythms of pituitary-ovarian function in the immature hamster are independent of adrenal and pineal influence

In female hamsters, the daily rhythm of LH appeared on the 15th or 16th day after birth with a peak occurring at about 16:00 h (14L:10D, lights on 06:00 h). Progesterone concentrations increased and became rhythmic a few days later. In serum samples collected at 14, 16, 18, 20, 25, 30, 40 and 60-62 days of age between 13:00 and 23:00 h, significant rhythms of serum cortisol and corticosterone concentrations were not detected before 25 days of age; furthermore, the phase of the rhythms did not stabilize to the adult pattern until about 40 days of age. As in the adult, significant rhythms were present in both sexes and the levels of cortisol were greater than those of corticosterone. Injection of pig ACTH (50 i.u./kg body wt, i.p.) significantly increased serum cortisol by 10 days of age, but corticosterone did not respond until 25 days of age. Thus, for cortisol at least, the appearance of 24-h rhythms in the serum is probably not dependent on the ability of the adrenal to respond to ACTH. Ovariectomy had no effect on the late afternoon surge of serum cortisol; similarly, adrenalectomy of immature females did not abolish the surge of LH. Ovariectomy did not alter the daily rhythm of pineal melatonin content and pinealectomy had no effect on the daily afternoon surge of LH. These results demonstrate functional independence of circadian rhythms in the pituitary-gonadal axis and the pituitary-adrenal axis of the immature hamster and also independence of daily rhythms of pineal melatonin and pituitary release of LH.     

Stimulation of the phagocytic function in guinea pig peritoneal macrophages by physical activity stress.

A study was made of all the different stages of the phagocytic function in peritoneal macrophages from male guinea pigs [3 (SD 1) months old] before, immediately after, and 24 h after being subjected to stress from physical activity (swimming until exhaustion). The early (10 min) and late (40 min) adherence to tissue substrates, chemotaxis, attachment and phagocytosis of Candida albicans, ingestion of inert particles (latex beads), and basal oxidative metabolism [measured by nitroblue tetrazolium (NBT) reduction] were significantly stimulated by the physical activity. After 24 h, late adherence, attachment capacities, and basal oxidative metabolism returned to basal values, whereas early adherence, chemotaxis, phagocytosis of cells and inert particles, and microbicidal capacity (production of superoxide anion measured by NBT reduction in presence of ingested material) remained significantly increased. The stress produced by physical activity, reflected in increased serum corticosterone values, led to a global stimulation of the phagocytic function.     

Comparative study of the heterophil phagocytic function in young and old ring doves (<Emphasis Type="Italic">Streptopelia risoria</Emphasis>) and its relationship with melatonin levels

A functional connection between the pineal gland (via the hormone melatonin) and the immune system has been suggested. In our previous results in the ring dove, we observed diurnal oscillations in the levels of this neurohormone in young animals and a decline in its plasma levels with advancing age (which is accompanied by the absence of diurnal rhythm). We also noted enhanced phagocytic activity of heterophils from old animals after in vitro incubation with both physiological and pharmacological doses of melatonin. Here, we evaluate the functional capacity of ring dove (Streptopelia risoria) heterophils in young (2 years of age) and old (8 years and more) animals at different times of day (0:00, 10:00 and 16:00, the times when the maximum, minimum, and mean values, respectively, of melatonin levels are observed in young animals). The phagocytic capacities for the ingestion of latex beads and Candida albicans were evaluated, as well as the oxidative metabolism which accompanies phagocytosis. At all three times of day studied, the heterophil phagocytic function with both latex and C. albicans was significantly greater in the young than in the old animals, and in the young animal cells it was significantly higher at 0:00. In addition, in the presence of latex beads, there was a significant decline at 10:00 and 0:00 of superoxide anion levels in the young animals relative to the old. In the young animals, there was a decline at 0:00 in comparison with both 10:00 and 16:00, and in the old animals there was a decline at both 0:00 and 16:00 compared with 10:00. These results could be due, at least in part, to the absence of a diurnal rhythm of melatonin in old animals, and to an enhancing effect of that hormone on young animals heterophil phagocytic function, which would also neutralize the oxidative stress deriving from this immune function.Abbreviations PBS phosphate-buffered saline - MIF macrophage migration-inhibition factor - PI phagocytosis index - PP phagocytosis percentage - PE phagocytosis efficiencyCommunicated by G. Heldmaier     

The Circadian Body Temperature Rhythm in the Elderly: Effect of Single Daily Melatonin Dosing

The present study is part of a more extensive investigation dedicated to the study and treatment of age‐dependent changes/disturbances in the circadian system in humans. It was performed in the Tyumen Elderly Veteran House and included 97 subjects of both genders, ranging from 63 to 91 yrs of age. They lived a self‐chosen sleep‐wake regimen to suit their personal convenience. The experiment lasted 3 wks. After 1 control week, part of the group (n=63) received 1.5 mg melatonin (Melaxen?) daily at 22:30 h for 2 wks. The other 34 subjects were given placebo. Axillary temperature was measured using calibrated mercury thermometers at 03:00, 08:00, 11:00, 14:00, 17:00, and 23:00 h each of the first and third week. Specially trained personnel took the measurements, avoiding disturbing the sleep of the subjects. To evaluate age‐dependent changes, data obtained under similar conditions on 58 young adults (both genders, 17 to 39 yrs of age) were used. Rhythm characteristics were estimated by means of cosinor analyses, and intra‐ and inter‐individual variability by analysis of variance (ANOVA). In both age groups, the body temperature underwent daily changes. The MESOR (36.38±0.19°C vs. 36.17±0.21°C) and circadian amplitude (0.33±0.01°C vs. 0.26±0.01°C) were slightly decreased in the elderly compared to the young adult subjects (p<0.001). The mean circadian acrophase was similar in both age groups (17.19±1.66 vs. 16.93±3.08 h). However, the inter‐individual differences were higher in the older group, with individual values varying between 10:00 and 23:00 h. It was mainly this phase variability that caused a decrease in the inter‐daily rhythm stability and lower group amplitude. With melatonin treatment, the MESOR was lower by 0.1°C and the amplitude increased to 0.34±0.01°C, a similar value to that found in young adults. This was probably due to the increase of the inter‐daily rhythm stability. The mean acrophase did not change (16.93 vs. 16.75 h), although the inter‐individual variability decreased considerably. The corresponding standard deviations (SD) of the group acrophases were 3.08 and 1.51 h (p<0.01). A highly significant correlation between the acrophase before treatment and the phase change under melatonin treatment indicates that this is due to a synchronizing effect of melatonin. Apart from the difference in MESOR, the body temperature rhythm in the elderly subjects undergoing melatonin treatment was not significantly different from that of young adults. The data clearly show that age‐dependent changes mainly concern rhythm stability and synchronization with the 24 h day. A single daily melatonin dose stabilizes/synchronizes the body temperature rhythm, most probably via hypothermic and sleep‐improving effects.     

Influence of melatonin on rat thyroid, adrenals and testis secretion during the day.

The effect of pinealectomy and exogenous melatonin on circadian rhythm of triiodothyronine (T3), thyroxin (T4), corticosterone and testosterone in sham-operated and pinealectomized rats were investigated. The hormones concentration were RIA-measured and the circadian rhythm secretion were analysed by cosinor method. The findings suggest that pinealectomy abolishes the rhythmical character of corticosterone secretion and disturbs the circadian rhythm of T3, T4 and testosterone. Exogenous melatonin has the suppressive effect of diurnal secretion of T3, T4 and testosterone in pinealectomized rats but stimulates the rhythmical corticosterone secretion.     

Twenty-four-hour rhythm patterns of plasma melatonin in short-day and long-day breeders maintained under natural environmental conditions

ABSTRACT

Photoperiodic treatments have been of practical interest in controlling seasonal reproduction in sheep, goats and horses. Melatonin is the principal mediator of the environmental photoperiodic message. To investigate the intra- and inter-subject variability of melatonin 24 h rhythm, ten female Italian Saddle horses (8–10 yrs old, mean body weight 525 ± 30 kg), ten female Sarda breed sheep (2–3 yrs old, mean body weight 40.5 ± 2.8 kg) and ten female Sarda breed goats (3–4 yrs old, mean body weight 38.9 ± 4.1 kg), housed individually in a 4 × 4 m soundproof box equipped with 50 × 100 cm opening windows, were subjected to a natural photoperiod of the vernal equinox (sunrise 06:00 h; sunset 18:00 h). Blood samples were collected from each animal, every 3 h over a 48 h period starting at 00:00 h of day 1 and ending at 00:00 h of day 3. Plasma melatonin concentrations were determined by direct radioimmunoassay (MelatoninDirect RIA, Labor Diagnostika Nord GmbH, Nordhorn, Germany). The application of single cosinor method substantiated a circadian rhythm of melatonin with a nocturnal peak in all studied species. The application of two-way ANOVA on the rhythmic parameters indicated statistically significant differences between the three species in all of the cosinor analysis-derived parameters of MESOR, amplitude, acrophase and robustness of rhythm. Analyses of intra- and inter-subject variability indicate that organization of the melatonin 24 h rhythm is characterized by great accuracy of control within and between the individuals of a breed. In conclusion, features of the 24 h rhythm of melatonin among species; however, the 24 h rhythmicity of melatonin each species showed high stability within the various subjects and within the same subject. These findings must be taken into consideration when applying photoperiod and melatonin treatments for breeding purposes.     

Are There Circadian Variations of Polymorphonuclear Phagocytosis in Man?

Eleven male subjects were investigated to detect a possible circadian rhythm of the polymorphonuclear phagocytosis. Both cell activity and serum opsonins were studied for numerical detection of granulocytes having ingested at least one particle and for the mean number of ingested particles per cell. No significant temporal differences (ANOVA and cosinor) were found.     

Reproductive phase dependent circadian variations of plasma melatonin, testosterone, thyroxine and corticosterone in Indian palm squirrel, Funambulus pennanti

To explain the complex mechanism of environmental influence along with internal hormonal (factors) milieu on daily variations in the circulating levels of melatonin, testosterone, thyroxine and corticosterone were analyzed with the help of inferential statistics (Cosinor rhythmometry) in a seasonally breeding tropical rodent, F. pennanti during the reproductively active (RAP) and inactive phases (RIP). Plasma melatonin, thyroxine and corticosterone levels exhibited a significant circadian oscillation during both the active and inactive phases of the annual reproductive cycle. Melatonin showed higher amplitude during RIP in the circulating plasma. Testosterone presented a peak level during evening hours (16:00 - 18:00 h) during RAP only. The phase of thyroxine was noted ∼09:76 h and ∼10.35 h during active and inactive phases, respectively. Corticosterone showed a peak level at ∼12.00 h during both phases of the reproductive cycle. Further, in this tropical rodent, the minimum difference in photoperiod (∼3 - 4 hours) and maximum variation in temperature (max. 18°C - min. 10°C during RIP and max. 45°C - min. 32°C during RAP) and humidity (85% during RIP and 35% during RAP) regulated the diurnal rhythm of circulating melatonin circadian rhythm by ∼1 hour phase advance during RIP. In conclusion, the studied hormonal rhythms may be part of an integrative system to coordinate reproduction and physiological processes successfully with environmental factors.     

Ultrastructure and phagocytic activity of rat peritoneal macrophages exposed to low temperatures in vitro

Hypothermia affects various components of the immune system, leading to impaired immune resistance. To examine the in vitro effect of low temperature on the ultrastructure and phagocytic function of rat peritoneal macrophages, cells were incubated at 4, 10, 24, and 37 degrees C for 60 min. Subsequently, their ultrastructure and capacity to engulf latex particles and generate superoxide anions were evaluated. The results showed a close inverse relationship between incubation temperature and ultrastructural changes, i.e., the lower the temperature, the higher the number of altered cells. In addition, at lower temperatures the number of cells capable of phagocytosis was reduced; the cells engulfed fewer particles per cell and generated less superoxide anions. These findings may be relevant for explaining the increased susceptibility to bacterial infections under hypothermic conditions.