Effect of photoperiod on LH, FSH, prolactin and melatonin patterns in ovariectomized prepubertal heifers

Angus and Angus crossbred prepubertal heifers were ovariectomized and randomly assigned to either increasing light simulating the photoperiod of the vernal equinox to the summer solstice (I) or decreasing light simulating the photoperiod of the autumnal equinox to the winter solstice (D) for 43 degrees N latitude. Three blood samples were taken each week for 14 weeks, the first at 11:00 h and two others 2 days later, 1 h before lights on (dark), 1 h before lights off (light). At the end of 14 weeks 4 heifers from each treatment group were cannulated and samples were taken for 12 h at 15-min intervals, 6 h in the light and 6 h in the dark. All sera were assayed for LH, FSH and prolactin. In addition, the samples taken at 15-min intervals were assayed for melatonin. In samples taken weekly at 11:00 h circulating concentrations of LH and prolactin were higher among animals in Group I, while FSH concentrations were not different between Groups D and I. In samples collected weekly in the light or the dark, LH and prolactin concentrations were higher in Group I animals. However, prolactin concentrations were higher and LH concentrations tended to be higher in samples taken in the dark. FSH concentrations were not different between either D or I or dark and light. In samples taken at 15-min intervals the prolactin baseline was higher and pulse amplitude tended to be higher for Group I animals. Neither LH nor FSH pulse characteristics differed between I and D; however, LH baseline and LH pulse amplitude were higher in the dark. Melatonin pulse amplitude was higher among animals in Group D and higher in serum collected in the dark. These results suggest that photoperiod alters circulating concentrations of LH and prolactin and alters pulsatile release of LH, prolactin and melatonin in the prepubertal heifer.     

Effects of melatonin feeding on smoltification in masu salmon (Oncorhynchus masou)

Influences of photoperiod on plasma melatonin profiles and effects of melatonin administration on long-day-induced smoltification in masu salmon (Oncorhynchus masou) were investigated in order to reveal the roles of melatonin in the regulation of smoltification in salmonids. Under light-dark (LD) cycles, plasma melatonin levels exhibited daily variation, with higher values during the dark phase than during the light phase. The duration of nocturnal elevation under short photoperiod (LD 8:16) was longer than that under long photoperiod (LD 16:8). Melatonin feeding (0.01, 0.1 and 1 mg/kg body weight) elevated plasma levels of melatonin in a dose-dependent manner for at least 7 h but not for 24 h. When masu salmon reared under short photoperiod were exposed to long photoperiod (LD 16:8) and fed melatonin (1 mg/kg body weight) 7 hours before the onset of darkness, a significantly smaller proportion of smolts appeared in the melatonin-fed group after 32 days than in the control group. However, after 59 days of the treatment, there was no difference in the proportion of smolts between the control and melatonin-treated groups. Thus, melatonin feeding mimicked the effects of short photoperiod, which delays but does not completely suppress smoltification. These results indicate that the day length is transduced into changes in the duration of nocturnal elevation in plasma melatonin levels, and that artificial modification of the plasma melatonin pattern possibly delays the physiological processes of smoltification induced by long-day photoperiodic treatment.     

The effect of exogenous melatonin administration on gonadotropin and prolactin patterns in ovariectomized estradiol-treated heifers exposed to increasing photoperiod

Eight nulliparous Angus and Angus crossbred heifers, which had been ovariectomized and treated with estradiol-17beta (E(2)) S.Q. implants for 6 months, were used to determine the effects of exogenous melatonin on serum gonadotropin and prolactin concentrations. Melatonin (15 mg) or corn oil (vehicle) was administered as a single i.m. injection at 1600 h daily for 12 weeks (March 19 to June 4, 1982). Blood samples taken weekly via jugular venipuncture at approximately 1100 h were assayed for luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin and E(2). At 4-week intervals, animals were fitted with indwelling jugular cannulae at 1100 h and samples were taken for 4 h at 15-min intervals. These samples were used to estimate pulsatile patterns of LH, FSH and prolactin. On the day of the first 15-min sampling, additional blood samples were collected at 30 min intervals from 1500 to 2200 h to determine the acute effect of melatonin injection on concentrations of LH, FSH and prolactin. Melatonin did not affect concentrations of FSH collected at weekly intervals (P=0.03) but tended to inhibit the decrease in concentrations of LH seen in the heifers treated with vehicle (P=0.12). There was a melatonin x time interaction for FSH (P=0.04) and a tendency for this interaction for LH (P=0.11). Circulating concentrations of prolactin were not different between treatment groups (P=0.83) nor was there a melatonin x time interaction (P=0.03). Estradiol was higher in the melatonin treated group (P=0.03) (15.58 +/- 4.17 versus 8.25 +/- 1.25 pg/ml) (X +/- SEM) and the melatonin x time interaction was significant (P=0.001). There was a tendency for a melatonin x time interaction for FSH pulse frequency (P=0.10). Prolactin pulse duration tended to decrease in response to melatonin treatment (P=0.14) (15.92 +/- 9.29 versus 11.04 +/- 4.57 min). These data do not support the hypothesis that melatonin decreases prolactin concentrations in cattle and indicates that other factors must mediate photoperiod regulation of this hormone. However, the interpretation of these data is less clear concerning the hypothesis that melatonin may maintain elevated concentrations of gonadotropins in the presence of increasing photoperiod. Concentrations of FSH appeared to be more affected by melatonin than LH; consistent with previous observations that FSH may be more affected than LH by changes in photoperiod (2). But neither LH or FSH concentrations were clearly shown to be consistantly elevated in the melatonin treatment group.     

Exposure of pregnant dairy heifer to magnetic fields at 60 Hz and 30 microT

Thirty-two pregnant Holstein heifers weighing 499 +/- 45 kg, at 3.1 +/- .7 months of gestation and 21 +/- 2.0 months of age were confined and exposed to 30 microT magnetic fields (MFs) and a 12 h light/12 h dark light cycle. The heifers were divided into two replicates of 16 animals. Each replicate was divided into two groups of eight animals each, one group the non-exposed and the second, the exposed group. The animals were subjected to the different treatments for 4 weeks. After 4 weeks, the animals switched treatment, the exposed group becoming the non-exposed group and vice versa. Then the treatment continued for 4 more weeks. Catheters were inserted into the jugular vein, and blood samples were collected twice a week to estimate the concentration of progesterone (P4), melatonin (MLT), prolactin (PRL), and insulin-like growth factor 1 (IGF-1). Feed consumption was measured daily. The results indicated that exposure of pregnant heifers to MF similar to those encountered underneath a 735 kV high tension electrical power line for 20 h/day during a period of 4 weeks produces slight effects. This is evidenced by statistically significant higher body weight (1.2%), higher weekly body weight gain (30%), and decreases in the concentration of PRL (15%) and IGF-1 (4%) in blood serum. The absence of abnormal clinical signs and the absolute magnitude of the significant changes detected during MF exposure, make it plausible to preclude any major animal health hazard.     

Negative correlation of age and the levels of pineal melatonin, pineal N-acetylserotonin, and serum melatonin in male rats

Pineal concentrations of N-acetylserotonin and melatonin and serum levels of melatonin were studied in 3-wk-old (prepubertal), 8-wk-old (adult), and 17-mo-old (senile) male rats. They were adapted to a photoperiod of 12 h light/12 h darkness for a minimum of 1 wk and killed at mid-light and mid-dark. Melatonin and N-acetylserotonin were determined by radioimmunoassay. The concentrations of pineal N-acetylserotonin and melatonin were high in the dark period and low in the light period. Statistical analysis indicated that pineal N-acetylserotonin and melatonin levels per 100 gm body weight declined with age. Similarly, serum melatonin demonstrated diurnal changes in all the age groups studied. In addition, there was a significant reduction in the levels of serum melatonin with age. The parallel patterns of decrease in pineal and serum melatonin levels with age suggest a decline in pineal secretion of melatonin in the older animals.     

Effect of photoperiod on LH, FSH and prolactin patterns in ovariectomized oestradiol-treated heifers

Angus and Angus crossbred heifers were ovariectomized, treated with oestradiol implants and randomly assigned to the natural photoperiod of fall to spring for 43 degrees N latitude or extra light simulating the photoperiod of spring to fall. Weekly blood samples were taken for 6 months (fall to spring equinox). All heifers were cannulated every 4 weeks and blood samples were taken for 4 h at 15-min intervals. Sera were assayed for LH, FSH, prolactin and oestradiol. In samples taken weekly, serum LH and FSH concentrations were higher while serum prolactin was lower in heifers exposed to natural photoperiod. There was a photoperiod X time interaction for both FSH and prolactin with concentrations diverging as photoperiod diverged. Circulating concentrations of oestradiol were not different between groups. In samples taken every 4 weeks at 15-min intervals, baseline concentrations of LH and FSH and LH pulse amplitude were higher while prolactin pulse frequency was lower in heifers exposed to natural photoperiod. There was a photoperiod X time interaction for each of these pulsatile characteristics. The correlation between LH and prolactin concentrations estimated from the 15-min samples differed between the two photoperiod treatment groups. The pooled correlation coefficient (r) was -0.12 under natural photoperiod and +0.50 under extra light. There was also a photoperiod X time interaction with negative correlations occurring when photoperiod was decreasing and positive correlations occurring when photoperiod was increasing. These results support the hypothesis that photoperiod alters serum concentrations of LH, FSH and prolactin in cattle.     

Influence of short days on diurnal patterns of serum gonadotrophins and prolactin concentrations in the male Djungarian hamster, Phodopus sungorus

Exposure to short days for 8 weeks suppressed mean serum concentrations of FSH, LH and prolactin compared to hamsters kept in long days. Hamsters in short days exhibited a small afternoon rise in serum FSH, but serum LH and prolactin did not exhibit 24-h variations. In hamsters under long days, a late afternoon-early evening increase was evident for circulating prolactin but none was detected for the gonadotrophins. A fall in testes weights rapidly occurred by 14-28 days after transfer to short days. This was accompanied or preceded by a decrease in serum gonadotrophins and prolactin. Reductions in serum FSH and LH occurred in short days in blood samples taken at 09:00 h or 15:00 h. However, the nadir in serum prolactin was first achieved (at 09:00 h), at least 7 days before that at 15:00 h (i.e. Day 14 versus Day 21 of short photoperiod, respectively). The ability to secrete gonadotrophins was further tested in hamsters that had undergone gonadal regression. Castration of hamsters exposed to short days or injected with melatonin in the afternoon, a treatment known to mimic short day effects, induced a 3- to 5-fold increase in serum gonadotrophins. However, this rise in FSH and LH was significantly attenuated compared to the 10-fold response in controls in long days. The results indicate that gonadal involution induced by short days may be mediated by the decline in mean gonadotrophin secretion which, in turn, is regulated by responsiveness to steroids, as well as a mechanism independent of the negative feedback action of gonadal steroids.     

Advancement of reproductive activity, seasonal reduction in prolactin secretion and seasonal pelage changes in pubertal red deer hinds (Cervus elaphus) subjected to artificially shortened daily photoperiod or daily melatonin treatments

Prepubertal red deer hinds were subjected to shortened daily photoperiod (8 h light per day, N = 3) or a daily (afternoon) melatonin injection (N = 4) for 83 days starting on 8 January, 2 weeks after the summer solstice. Compared with control hinds (N = 3) these treatments caused premature moulting of summer pelage, reduced serum prolactin concentrations to barely detectable levels about 34 days earlier than usual and advanced the date of mating. Calves were born earlier (P less than 0.005) in the hinds exposed to a shortened photoperiod (12 November +/- 1.7 days) and melatonin treatment (11 November +/- 3.2 days) than in control hinds (13 December +/- 7.9 days). Serum progesterone levels recorded before the first detected oestrus indicated that silent ovulations had occurred in many of the hinds (6 of 10) in this experiment. This study demonstrated the role of shortened daily photoperiod in red deer and indicated that the effects of reduced photoperiod observed were mediated by melatonin.     

Effect of staged ovariectomy on measures of mammary growth and development in prepubertal dairy heifers

Previous studies in prepubertal heifers suggest that the magnitude of reduction in mammary parenchymal growth in response to ovariectomy varies with the age at which surgery is performed. We hypothesized that ovarian secretions are essential for initiating mammary development but not required to maintain allometric mammary growth in prepubertal dairy heifers. The objectives of this study were to determine the effect of staged ovariectomy during the prepubertal period on mammary growth and tissue composition and the expression of selected genes. Prepubertal Holstein heifers at 2, 3 or 4 months of age were randomly assigned to one of two treatments, ovariectomized (OVX; n = 12) or sham operated (INT; n = 12). Mammary parenchyma (PAR) and fat pad (MFP) were harvested 30 days after surgery. Proximate composition of PAR and MFP (DNA, protein and lipid) as well as expression of the selected estrogen-responsive genes stanniocalcin1 (STC1), tissue factor pathway inhibitor precursor (TFPI) and proliferating cell nuclear antigen (PCNA) were determined in PAR and MFP by quantitative real-time PCR. The relative amount of epithelium and proportion of epithelia cell nuclei expressing the proliferation marker Ki67 were determined by histological and immunohistochemical analyses, respectively. MFP mass was not impacted by treatment but was decreased with age as was lipid content and concentration (P ⩽ 0.01). The mass of mammary PAR was reduced in OVX and increased with age (P ⩽ 0.01). Parenchymal tissue tended to have less total DNA, protein and lipid in OVX heifers. Parenchymal tissue concentrations of protein and DNA were increased with age and there was an age × treatment interaction. Treatment had no effect on either the Ki67 labeling index or percent epithelial area. The relative abundances of STC1, TFPI and PCNA mRNA in PAR were reduced in OVX. We did not find a significant impact of ovariectomy on mRNA expression when surgery was performed at 2 months compared with surgery at 3 or 4 months of age. However, having nearly undetectable PAR in two heifers ovariectomized at the earliest period (2 months of age) suggests that early ovariectomy is especially detrimental to subsequent parenchymal development.     

Response of prepubertal Bos taurus and Bos indicus x Bos taurus heifers to melengestrol acetate with or without gonadotropin-releasing hormone

The effectiveness of treatments to induce estrus in prepubertal beef heifers was evaluated. Angus x Hereford (n = 148) and Brahman x Hereford (n = 148) heifers were sorted after weaning by body weight into light and heavy weight blocks. Heifers were assigned to diets, calculated to reach a target weight of 55% or 65% of their projected mature weight by the start of breeding. Cyclicity was determined after a 160-d observation period and from concentrations of progesterone in serum determined 10 d before and on the day that treatments began to induce puberty. The remaining nonpubertal heifers, with concentrations of progesterone in serum of less than 1 ng/ml (0 or 10 d before treatment), were assigned randomly within breed and nutrition group to either a melengestrol acetate + saline (MGA+S) or MGA + gonadotropin-releasing hormone (MGA+GnRH) treatment. Prepubertal Angus x Hereford heifers (n = 11) and Brahman x Hereford heifers (n = 49) were fed 0.5 mg MGA for 7 d. Forty-eight hours after MGA, heifers were injected with 500 ug s.c. GnRH or 5 ml of saline. Blood samples were collected from all prepubertal heifers every 3 d after GnRH or saline for 30 d. There was no difference between treatments in the proportion of heifers that exhibited estrus by Day 7 after treatment. However, a larger (P<0.05) proportion of MGA+S-treated heifers exhibited estrus within 14 d after treatment than MGA+GnRH-treated heifers (87 vs 63%). Among heifers that exhibited estrus during that time period, the proportion with increased progesterone was higher (P<0.10) for the MGA+GnRH group than for the MGA+S group (71 vs 41%, Day 7; 79 vs 54%, Day 14). There was no difference in conception rate at first service between treatment groups. Thirty-seven and 53%, respectively, of the MGA+S and MGA+GnRH-treated heifers had short estrous cycles after treatment, and 44 and 50%, respectively, of those short cycles were repeated. Pregnancy rates at the end of 45 d were numerically higher for MGA+S heifers than for MGA+GnRH treated counterparts (63 vs 53%).     

Endogenous circannual cycles of ovarian activity and changes in prolactin and melatonin secretion in wild and domestic female sheep maintained under a long-day photoperiod

The present study examines the ovulatory activity of wild and domesticated ewes subjected to either a constant photoperiod of long days (16L:8D) or natural changes in daily photoperiod for 16 mo. The aim was to determine whether an endogenous reproductive rhythm controls seasonal reproductive activity in these sheep, and how the photoperiod might affect this. The effects of long-day photoperiods on long-term changes in prolactin and melatonin secretion were also evaluated. The two species showed changes in reproductive activity under the constant photoperiod conditions, suggesting the existence of an endogenous rhythm of reproduction. This rhythm was differently expressed in the two types of ewe (P < 0.05), with the domestic animals exhibiting much greater sensitivity to the effects of long days. A circannual rhythm of plasma prolactin concentration was also seen in both species and under both photoperiod conditions, although in both species the amplitude was always lower in the long-day animals (P < 0.01). The duration of the nocturnal melatonin plasma concentrations reflected the duration of darkness in both species and treatments. The peak melatonin concentration did not differ between seasons either under natural or long-day photoperiods.     

Time of parturition in rats after melatonin administration or change of photoperiod

Two experiments were carried out with rats isolated at mating (Day 1 of gestation) and kept in a standard light regimen of 14 h light (14L:10D). All treatments started on Day 8 of gestation; periodicities of 23:45 h, 24:00 h and 24:15 h were applied to the light phase (14L) in Exp. I and to daily treatment with melatonin (0.3 mg/rat) or its vehicle in Exp. II. In Exp. II, the animals were placed in a continuous dim light regimen and injections were given at a time corresponding to lights off. In all groups, rats delivered on the afternoon of Day 22 and on the morning of Day 23 after a cessation of parturitions. The rates of births during these two times depended on the periodicities of the light phase or those of melatonin administration. With a periodicity of 24:15 h, 85.7% of rats in Exp. I and 85.7% of rats in Exp. II delivered on Day 22. With a periodicity of 23:45 h, 83.0% of births occurred on Day 23 in Exp. I and 57.7% in Exp. II with melatonin instead of 25.9% in the corresponding vehicle controls. These results suggest that melatonin secretion may be a mechanism whereby photoperiod regulates the time of parturition in the rat.     

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.     

Effects of 60 Hz magnetic field exposure on the pineal and hypothalamic-pituitary-gonadal axis in the Siberian hamster (Phodopus sungorus)

Experiments using the dwarf Siberian hamster Phodopus sungorus were carried out to determine possible neuroendocrine consequences of one-time and repeated exposures to 60 Hz magnetic fields (MF). Animals were maintained in either a short-light (SL, 8 h light:16 h dark) or long-light (LL, 16 h light:8 h dark) photoperiod. Acute (one-time, 15 min) exposure of male SL animals to a linearly polarized, horizontally oriented, 60 Hz MF (0.1 mT) gave rise to a statistically significant (P < .005) reduction in pineal melatonin content as determined 3 and 5 h after onset of darkness. In LL animals, acute exposure to 0.10 mT resulted in a significant decrease in pineal melatonin as measured 4 h after onset of darkness, whereas acute exposure to 50 microT showed no effect compared with sham exposure. In SL animals, an increase in norepinephrine was observed in the medial basal hypothalamus (including the suprachiasmatic nucleus) after acute exposure (P < .01). Daily MF exposure of SL animals to a combination of steady-state and on/off 60 Hz magnetic fields (intermittent exposure) at 0.1 mT for 1 h per day for 16 days was associated with a reduction in melatonin concentrations at 4 h after onset of darkness and an increase in blood prolactin concentrations (P < .05). Exposure of SL animals to a steady state 60 Hz MF for 3 h/day for 42 days resulted in a statistically significant reduction in body weight (ANOVA: P > .05), compared with sham-exposed SL animals. At 42 days, however, no significant changes in overnight melatonin or prolactin levels were detected. In both repeated exposure experiments, gonadal weights were lowest in the MF-exposed groups. This difference was statistically significant (P < .05) after 42 days of exposure. These data indicate that both one-time and repeated exposure to a 0.1 mT, 60 Hz MF can give rise to neuroendocrine responses in Phodopus.     

Role of photoperiod in reproductive maturation and peripubertal hormone concentrations in male collared lemmings (Dicrostonyx groenlandicus).

Onset of sexual maturation was determined in weanling male collared lemmings exposed to one of three experimental regimens of different photoperiods before and after weaning. Animals gestated in photoperiods of either 16 h light:8 h dark or 8 h light:16 h dark. Those from 16 h light:8 h dark were transferred at 19 days of age to either 20 h light:4 h dark or 8 h light:16 h dark; those gestated under 8 h light: 16 h dark remained in that photoperiod throughout the experiment. After exposure for 15, 20, 25 or 30 days to the postweaning photoperiod, animals were killed and the following parameters assessed: body weight, testes weight, seminal vesicle weight, the presence or absence of epididymal spermatozoa and serum concentrations of prolactin, testosterone and corticosterone. All parameters except serum testosterone were significantly influenced by photoperiod. Animals housed under 8 h light:16 h dark had significantly greater body weights than those housed under 20 h light:4 h dark, a response that differs from that reported for other arvicoline rodents. The group gestated on 16 h light:8 h dark and transferred on day 19 to 8 h light:16 h dark had lower testes and seminal vesicle weights than the other two groups, and mature spermatozoa in the epididymides appeared 5 days later than in the 20 h light:4 h dark group. Serum prolactin was largely undetectable in animals from both 8 h light:16 h dark groups, but all males housed in 20 h light:4 h dark had 2.0-15.0 ng prolactin ml-1. Concentration of serum corticosterone was higher in animals weaned into long photoperiod, and decreased with age. These data indicate that weanling male D. groenlandicus are reproductively photoresponsive, but use a decrease in photoperiod, not static short-photoperiod exposure, to alter the rate of development. Prolactin was largely undetectable in animals exposed to short photoperiod, indicating that high concentrations of this hormone are not important for maturation. Low prolactin concentrations in animals in short photoperiods may mediate the annual moult to white pelage. The short-photoperiod-mediated decrease in corticosterone may play a role in seasonal changes in body weight and composition.     

Daily changes in concentrations of prolactin in serum of prepubertal bulls exposed to short- or long-day photoperiods

Early temporal changes in concentrations of prolactin (PRL) in serum after a sudden change in photoperiod and daily responsiveness to PRL-releasing and inhibiting factors were investigated in prepubertal Holstein bull calves exposed to different photoperiods. In calves switched from 8-hr light: 16-hr dark to 16-hr light:8-hr dark, there was no observable change in the daily pattern of serum concentrations of PRL after 1, 2, or 4 days. On the other hand, in animals switched from 16-hr light:8-hr dark to 8-hr light:16-hr dark, there was a consistent increase in serum PRL from 33.4 ng/ml on Day 0 to maximum values of 57.3, 62.7, and 78.9 ng/ml between 14 and 18 hr after onset of light on Days 1, 2, and 4, respectively. Thus, absence of light allowed expression of a daily rhythm in serum concentrations of PRL that persisted for at least 4 days after the photoperiod switch. There were no differences in L-dopa inhibition of PRL release in animals exposed to 16-hr light:8-hr dark at 3 or 15 hr after onset of light. However, thyrotropin-releasing hormone-induced release of PRL was greater 3 hr after onset of light (11 hr after onset of dark) compared with release at 9, 15, and 21 hr after onset of light in animals exposed to 16-hr light:8-hr dark, but not in bulls exposed to 8-hr light:16-hr dark. The results provide evidence that the cue for the putative photosensitive period of PRL secretion in cattle may be more closely associated with onset of dark, not onset of light.     

The effect of continuous light on prolactin secretion in prepubertal Holstein bulls

Effects of 16 (16 light:8 dark) and 8 (8L:16D) h of daily light were compared with continuous light (24L:0D) exposure on prolactin (PRL) concentrations in serum of prepubertal bulls. Concentrations of PRL in serum were 2 to 3 fold greater in bulls exposed to 24L:0D or 16L:8D as compared with 8L:16D. However, PRL concentrations attained a maximum approximately 3 weeks later in calves exposed to 24L:0D than in calves given 16L:8D. Continuous low intensity (11 to 16 lux) lighting supplemented with 16 or 8 h of high intensity (449 to 618 lux) light per day increased PRL concentrations in serum of prepubertal bulls 1.5 to 2.5 fold relative to 8L:16D (470 lux). We found that relative to 8L:16D, 1) photoperiods of 16 or 24 h of light per day increased serum concentrations of PRL in prepubertal bulls; however, the time required to achieve maximum PRL concentrations was longer in animals exposed to 24L:0D, 2) continuous low intensity lighting supplemented with 16 or 8 h of high intensity daily light also increased concentrations of PRL in serum.     

Estrus synchronization in beef heifers with progestin-based protocols. I. Differences in response based on pubertal status at the initiation of treatment

Two progestin-based protocols for estrus synchronization in replacement beef heifers were compared on the basis of estrous response, interval to and synchrony of estrus, and pregnancy rate. The objective was to determine, whether addition of GnRH to a melengestrol acetate (MGA)-prostaglandin F2alpha (PGF2alpha) estrus synchronization protocol would improve synchrony of estrus without compromising fertility in yearling beef heifers. Heifers at two locations (Location 1, n = 60 and Location 2, n = 64) were assigned randomly to one of two treatments by breed and pubertal status. Heifers were defined as, pubertal when concentrations of progesterone in serum were elevated (> or = 1 ng/mL) in either one of two samples obtained 10 and 1 day prior to treatment initiation. Prior to MGA administration, 18/60 (30%) and 36/64 (56%) of the heifers at Locations 1 and 2, respectively, were pubertal. Heifers in both treatments were fed MGA (0.5 mg/head/day in 1.8 kg/head/day supplement) for 14 days followed by 25 mg of PGF2alpha i.m. (MGA-PGF2alpha) 19 days after MGA withdrawal (Day 33 of treatment). One-half of the heifers at each location received 100 microg of GnRH i.m. 12 days after MGA withdrawal (Day 26 of treatment; MGA Select). The control group received only MGA-PGF2alpha. Heifers were observed for signs of behavioral estrus continuously during daylight hours for 7 days beginning on the day PGF2alpha was administered. Heifers were inseminated 12 h after observed estrus. There was a treatment by location by pubertal status interaction (P < 0.05) for interval to estrus. Compared to the respective control treatment at each location, prepubertal heifers assigned to the MGA Select protocol at Location 1 had longer intervals to estrus, whereas at Location 2, prepubertal heifers assigned to the MGA-PGF2alpha protocol had longer intervals to estrus. The higher number of pubertal heifers at Location 2 was associated with a reduced variance in the interval to estrus among MGA Select treated heifers. Total estrous response and synchronized conception rates were similar between treatments at both locations. These data suggest that addition of GnRH to the MGA-PGF2alpha protocol may improve synchrony of estrus, however, the degree of synchrony may be influenced by pubertal status of heifers at the time treatments are imposed. Further studies are needed to define production systems in which the MGA Select protocol is warranted for use in beef heifers.     

Leptin gene expression, circulating leptin, and luteinizing hormone pulsatility are acutely responsive to short-term fasting in prepubertal heifers: relationships to circulating insulin and insulin-like growth factor I(1)

In the present study, we tested the hypothesis that short-term fasting would reduce leptin gene expression, circulating leptin, and LH pulsatility in prepubertal heifers in association with a decrease in circulating concentrations of insulin and insulin-like growth factor I (IGF-I). Twelve prepubertal crossbred heifers (mean +/- SD = 315 +/- 5 kg body weight) were assigned randomly to one of two treatments in two replicates: 1) control; normal feed consumption (n = 6) and 2) fasted; 48 h of total feed restriction (n = 6). Blood samples were collected at 15-min intervals for 8 h on Days 0 and 2 of the experiment and twice on Day 1. Subcutaneous fat samples were collected before treatment onset (Day -1) and at the end of the intensive blood sampling on Day 2. Acute feed restriction markedly reduced leptin mRNA in adipose tissue (P < 0.01) and circulating concentrations of leptin (P < 0.05), IGF-I (P < 0.01), and insulin (P = 0.05) as compared with controls on Day 2. Moreover, the treatment x day interaction (P < 0.076) and within-day contrasts (expressed as a percentage of Day 0 values) revealed that the mean frequency of LH pulses in the fasted group was lower (P < 0.06) than in controls on Day 2. Neither mean concentrations of growth hormone (GH) nor GH secretory dynamics were affected by acute feed restriction. Fasting-mediated decreases in leptin gene expression and circulating leptin, in association with reductions in secretion of IGF-I, insulin, and LH, provide a basis for investigating leptin as a hormone signaling energy status to the central reproductive axis in cattle.