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Biomedical subjects

D L Foster

Publications and source records attributed to D L Foster.

At least 91 records · Page 5Linked to original sources
PubMed1988

Are ambient short-day cues necessary for puberty in a short-day breeder?

Some exposure to long days (LD) is necessary for female sheep to achieve puberty during short days (SD). In this regard, a 5-wk block of long days in lambs otherwise raised in short days will result in puberty at the normal age. In the present study, Suffolk lambs were raised in various artificial photoperiods to assess the role of short days. An increase in circulating progesterone to luteal phase levels was used as the criterion for puberty. Controls exposed to short days except for 5 wk of long days between 17 and 22 wk (SD-LD-SD) began repetitive reproductive cycles at the expected age (34 +/- 0.7 wk, mean +/- SE). When the final block of short days was eliminated and exposure to long days was continued after 22 wk of age (SD-LD-LD), puberty was delayed (41 +/- 0.8 wk of age). Removal of the first block of short days (LD-LD-SD) did not prevent normal puberty, and the initiation of cycles (29 +/- 0.9 wk) occurred earlier than in controls. Lambs in which the pineal gland was denervated at 22 wk so that it was unable to transduce short-day cues after exposure to long days (SD-LD-X) exhibited puberty at an age (33 +/- 1.2 wk) similar to that of controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗
PubMed1988

Cessation of long day melatonin rhythms time puberty in a short day breeder.

This study tested the hypothesis that in the female sheep, a short day (SD) breeder, puberty can occur normally in the absence of ambient short days. More specifically, the photoperiod cue timing the transition into adulthood is exposure to and then termination of a long day melatonin rhythm. Control lambs born in the spring were exposed to 5 weeks of long days (LD; 16 h of light, 8 h of darkness; 18-23 weeks of age) and were raised in SD (8 h of light, 16 h of darkness) at other times. As expected from previous studies, this alternating photoperiod sequence (SD-LD-SD) induced puberty at the normal age in autumn [33 +/- 2 weeks (mean +/- SEM); n = 6]. The other three groups were exposed only to LD from birth; the superior cervical ganglia were removed bilaterally at different ages to denervate the pineal gland in order to block transduction of subsequent LD cues. Puberty occurred normally (31 +/- 1 weeks; n = 7) after ganglionectomy at 23 weeks of age, indicating that ambient short days are not required to initiate reproductive cycles. LD are necessary, as evidenced by the results for the other two groups ganglionectomized neonatally at 4 weeks of age. With no further treatment, puberty was either delayed (n = 1) or did not occur during the first year of life (n = 5), after which the study ended. This delay was prevented in the other group of ganglionectomized lambs by a 5-week (18-23 weeks of age) exposure to LD melatonin patterns by means of 8-h melatonin infusions nightly; 12 weeks after melatonin replacement therapy, puberty occurred at the normal time (34 +/- 1 weeks; n = 6). The inference is that for puberty to occur in the female lamb the animal must be exposed to relatively limited periods of LD, followed by the blockade or absence of further LD cues (pineal denervation, termination of LD melatonin infusion, or presence of SD). This supports the concept that the LD of summer, followed by their disappearance in autumn, time puberty in the female sheep.

Animals↗
PubMed1988

Control of gonadotrophin secretion during the pubertal and seasonal transitions in the male sheep.

Castration of spring-born, male Suffolk lambs at 2 weeks of age resulted in a rapid increase in gonadotrophin secretion. LH pulse frequency stabilized by 6 weeks of age and did not change throughout the ensuing summer and autumn. In contrast, LH pulse frequency in castrated, adult rams decreased during the summer and increased in the autumn. Testosterone or oestradiol replacement initially suppressed the post-castration increase in gonadotrophin secretion in developing lambs. Responsiveness to steroid inhibition was lost in late spring and summer (8-15 weeks of age). In steroid-treated castrated mature rams, however, an increase in response to the inhibitory effects of testosterone and oestradiol occurred during the summer, as indicated by slower LH pulse frequencies in June, July and August, than at other times of the year. A decrease in response to inhibition by steroids then occurred in the autumn, months after the springtime reactivation of reproductive function (testicular growth, testosterone and gonadotrophin secretion) in testes-intact adult rams. The results reveal fundamental differences between the regulation of gonadotrophin secretion in ram lambs and adults. The pubertal decrease in response to inhibition by steroids, which is not coincident with a steroid-independent increase in gonadotrophin secretion, may play a key role in increasing gonadotrophin secretion to initiate gonadal activity in the spring. The post-pubertal ram also exhibits the first signs of the seasonal resumption of reproductive activity in the spring. However, it is not until the autumn that a decrease in response to inhibitory feedback by steroids occurs, in concert with a steroid-independent increase in gonadotrophin release. Therefore, in the sexually mature ram, rather than stimulating the initial springtime recovery of gonadal function, these two mechanisms may act in concert to achieve gonadotrophin concentrations and/or patterns necessary to optimize reproductive competence in the autumn.

Animals↗
PubMed1988

Photoperiod requirements for puberty differ from those for the onset of the adult breeding season in female sheep.

Reproductive responses to photoperiod were directly compared in mature ewes and in their spring-born twin female lambs. All females were ovariectomized and treated with oestradiol implants before transfer into artificial photoperiod; serum LH concentrations and pulsatile LH patterns provided an index of neuroendocrine reproductive activity. Mothers were transferred from natural photoperiod to artificial long days (16 h light:8 h dark) at the summer solstice so that no decrease in photoperiod would be experienced. These ewes began reproductive activity synchronously at the expected time in the autumn. One of each pair of twin lambs was treated exactly as the mothers; to determine the normal timing of puberty the remaining twin was maintained in a photoperiod simulating the natural decrease in daylength. In all 6 control lambs experiencing the simulated natural photoperiod, reproductive activity occurred synchronously at 28 +/- 1 weeks of age (2 October +/- 7 days). However, in their twin sisters which did not experience a decrease in photoperiod, only 2 of 6 lambs had begun reproductive activity by the end of the experiment at 52 weeks of age (March), and these were both delayed relative to their twin control lambs exposed to decreasing daylength. Therefore, a decrease in photoperiod is necessary for the normal timing of puberty in the spring-born, female sheep, whereas seasonally anoestrous, mature sheep can enter the breeding season at a normal time in the absence of decreasing photoperiod. We suggest that the requirement for a decreasing photoperiod by the spring-born lamb reflects its limited photoperiodic history as compared to the adult.

Animals↗
PubMed1988

Neuroendocrine responsiveness to light during the neonatal period in the sheep.

Circulating prolactin concentrations were monitored during the early postnatal period in sheep to evaluate their response to photoperiod. In the first experiment, male and female lambs were exposed from 1 week of age, with their mothers, to either long days (16 h light: 8 h darkness; n = 15) or short days (8 h light: 16 h darkness; n = 16) to test whether they could discriminate different day lengths. In both sexes, serum prolactin concentrations were higher on long than on short days during the first 7 weeks after birth. In the second experiment, female lambs (n = 21) were raised on long days from 2 weeks of age. The superior cervical ganglia were removed bilaterally at 4 weeks of age from 14 lambs to lesion the sympathetic innervation to the pineal gland, and thus ablate the nocturnal increase in pineal melatonin secretion. After surgery, serum prolactin concentrations on long days were significantly lower in ganglionectomized lambs than in the intact controls. In the third experiment, the amplitude of the night-time melatonin rise was artificially increased in female lambs (n = 8) between 2 and 7 weeks of age to adult levels. Unrestrained lambs were infused during the 8-h dark phase of each day with melatonin by means of a self-contained, computerized syringe-pump. Concentrations of circulating prolactin did not differ from those in uninfused lambs (n = 8) with lower endogenous nocturnal melatonin. These results reveal that the sheep can discriminate photoperiod cues during the early postnatal period, and suggest that the low-amplitude melatonin rhythm in the neonatal lamb is sufficient to mediate this response.

Animals↗
PubMed1987

Absence of an increase in gonad-independent drive to pulsatile luteinizing hormone secretion during photoperiod-induced puberty.

This study was conducted to determine if photoperiod can influence the pattern of luteinizing hormone (LH) secretion in the absence of the ovaries in the developing female sheep. Lambs were raised in a photoperiod sequence (short, long, short days) known to induce puberty between 30 and 35 wk of age, or in a photoperiod (only short days) that prevents puberty during the first year. Their ovaries were removed at 10 wk of age, and the detailed pattern of LH was assessed (samples at 12-min intervals for 4 h) each 3- to 5-wk period between 9 and 45 wk of age. Rapid LH pulses (40- to 50-min interpulse interval) were evident within a few weeks after ovariectomy in both groups of females. Those exposed to the artificial photoperiod sequence that induces normal sexual maturity did not increase their pulse frequency further during the pubertal period. Moreover, their LH pulse frequencies were not greater than those in agonadal females exposed to the photoperiod that delays puberty. These findings indicate that photoperiodic induction of puberty in the sheep does not require steroid-independent modulation of pulsatile LH secretion.

Animals↗
PubMed1986

Control of gonadotropin secretion in the male during puberty: a decrease in response to steroid inhibitory feedback in the absence of an increase in steroid-independent drive in the sheep.

The gonadostat hypothesis, i.e. that a decrease in response to the inhibitory feedback action of gonadal steroids occurs during puberty, was tested in the male lamb. Also investigated was whether a simultaneous steroid-independent rise in gonadotropin secretion could be the underlying mechanism for the reduction in steroid feedback sensitivity during puberty. Sexual maturation in intact Suffolk lambs was characterized by increases in all of the following parameters: Serum FSH and LH and LH pulse frequency (during 4-7 weeks of age), testicular size and testosterone (T) concentrations (during 7-28 weeks of age). Estradiol (E2) levels were elevated at 32 weeks. Motile spermatozoa were produced by 16-18 weeks of age. Castration at 5 weeks of age resulted in a prompt increase in gonadotropin concentrations. LH pulse frequency reached a plateau of approximately 5 pulses/4 h (n = 5) at 7 weeks of age and did not change thereafter. T or E2 replacement suppressed pulsatile LH secretion for several weeks. However, despite maintenance of constant serum T (approximately 1.0 ng/ml) or E2 (3-5 pg/ml) concentrations, LH pulse frequency began to increase after 9 weeks of age, and by 13 weeks, pulsatile secretion was apparent in all steroid-replaced castrated lambs. This was well after LH pulse frequency had ceased to increase in untreated castrated animals. These data support the gonadostat hypothesis for puberty in the male lamb. Furthermore, the temporal dissociation of increasing LH secretion in untreated castrated lambs and steroid-replaced castrated lambs suggests that a steroid-independent increase in gonadotropin secretion is not the mechanism underlying the decrease in responsiveness to steroid negative feedback.

Aging↗
PubMed1986

Melatonin rhythms time photoperiod-induced puberty in the female lamb.

Photoperiod is an important environmental cue timing puberty in the domestic female sheep, a seasonal breeder. Because the effects of photoperiod on reproductive function in the adult sheep are mediated by the pineal melatonin rhythm, the present study determined whether the pineal through its secretion of melatonin is involved in the pubertal process. Neonatal denervation of the pineal by removal of the superior cervical ganglia (SGX) abolished the nocturnal melatonin rhythm and delayed the onset of reproductive cycles beyond 1 yr of age in female lambs reared under natural conditions. Puberty was also delayed in SGX lambs reared under an artificial photoperiod sequence of long days (experienced at 17-22 weeks of age) followed by short days. By comparison, intact lambs under this same photoperiod treatment began cycles at the normal age. Another group of SGX lambs was treated each night with melatonin; the infusion replicated the duration of the nighttime rise in serum melatonin occurring in intact lambs exposed to 5 weeks of long days followed by short days (9-h or 15-h infusions, respectively). Melatonin treatment of these SGX lambs restored puberty to the normal age. These findings suggest that the pineal melatonin rhythm is an essential component of the photoperiodic mechanism timing puberty in the female sheep.

Age Factors↗
PubMed1986

Delayed puberty in lambs chronically treated with oestradiol.

Intact female lambs were chronically treated with low levels of oestradiol by Silastic implant from 20 weeks of age. Reproductive cycles were initiated in only 33% of these lambs (3 of 9) compared to 80% of untreated females (11 of 14) by 45 weeks when the study was terminated. Moreover, in the 3 oestradiol-treated lambs which began cycles, the age at first oestrus was delayed 3 weeks (37 +/- 1 weeks of age vs 34 +/- 1 weeks of age for untreated controls). Retardation of the pubertal process was not due to absence of the pubertal rise in circulating LH. At about 32 weeks of age, chronic oestradiol treatment was no longer able to suppress tonic LH secretion and serum LH increased in intact, oestradiol-treated lambs. These results indicate that a maturational decrease in responsiveness to oestradiol inhibition of tonic LH secretion can be demonstrated in the intact female, as in the ovariectomized female. However, chronic oestradiol suppression of prepubertal LH secretion also delays onset of reproductive cycles. This finding raises the possibility that low tonic LH secretion, presumably in the form of slow pulses, is necessary for development or maintenance of ovarian function before puberty. In the absence of LH during the last part of sexual maturation, the ability of the ovary to respond to the high frequency LH pulses during the pubertal gonadotrophin rise may be delayed.

Animals↗
PubMed1985

Purification and characterization of the aspartate chemoreceptor.

The chemoreceptor for aspartate in Salmonella typhimurium was purified from an Escherichia coli strain containing a plasmid bearing the receptor's structural gene (tar). The receptor was solubilized from salt-washed membranes with the nonionic detergent octyl-beta-D-glucopyranoside and purified by a combination of ion exchange, molecular sieve and hydroxyapatite-agarose chromatography. The inclusion of glycerol and 1,10-phenanthroline in all buffers used prior to ion exchange chromatography prevented scission of the receptor by an endogenous proteolytic activity. The solubilized receptor was estimated to have a molecular weight of 248,000 from its behavior on Sephacryl S-300, suggesting that the receptor may be organized as a multimer containing 4 +/- 1 identical subunits. Circular dichroic measurements of the purified protein indicate that 78% of its residues are arranged in helical secondary structures.

Bacterial Proteins↗
PubMed1985

Proteolytic fragments identified with domains of the aspartate chemoreceptor.

Two proteolytic fragments generated during the preparation of the aspartate receptor from Salmonella typhimurium have been purified. These fragments are the products of a single cleavage by an endogenous protease after amino acid 259 in the sequence of the intact receptor. Proteolytic fragment 1 (PF1) represents amino acids 1-259 (Mr = 29,000); this unit retains the aspartate-binding function of the intact receptor. The second fragment (PF2) includes residues 260-552 (Mr = 31,000) and has the normal sites of reversible methylation for the receptor. Like the purified intact receptor, this fragment can be methylated in vitro, although at a much slower rate. Circular dichroic measurements suggest that both proteolytic fragments contain substantial alpha-helical structure, approximately 95 and 53% for PF1 and PF2, respectively. No beta-structure could be detected in either fragment. Molecular sieve chromatography in the presence of detergent suggests that PF1 occurs as a stable multimer of an order equivalent to that observed for the detergent-solubilized aspartate receptor, i.e. a tetramer (+/- 1). PF2 is found to have a multimeric form which is sensitive to the removal of detergent. It is proposed that these fragments represent structural and functional domains of the aspartate receptor.

Amino Acid Sequence↗
PubMed1985

Effect of restricted nutrition on puberty in the lamb: patterns of tonic luteinizing hormone (LH) secretion and competency of the LH surge system.

The effects of undernutrition on the timing of puberty and regulation of LH secretion by the inhibitory and stimulatory feedback action of estradiol were examined in female sheep. The first experiment determined that maintenance of low BW (ca., 20 kg, four lambs) between 10 and 45 weeks of age prevented initiation of ovulation at the usual time (30 weeks of age). Ad libitum feeding of such growth-retarded lambs (n = 7) resulted in rapid catch-up growth and onset of reproductive cycles. The second experiment determined the effects of level of nutrition on tonic LH secretion in the presence and absence of inhibitory steroid feedback. Detailed LH patterns were obtained from agonadal lambs (ovariectomy at 20 weeks) that either remained undernourished (ca., 20 kg, 10-39 weeks of age) or were initially undernourished (10-27 weeks) and subsequently fed ad libitum (28-39 weeks). In undernourished lambs (n = 7), LH pulse frequency was slow, and only an occasional LH pulse was detected in the absence of steroid negative feedback; chronic treatment with low levels of estradiol (Silastic capsule), beginning at ovariectomy, prevented pulsatile LH secretion (six lambs). Ad libitum feeding (six lambs) produced a progressive severalfold increase in LH pulse frequency. Estradiol during ad libitum feeding (six lambs) markedly reduced amplitude of LH pulses and retarded the increase in LH pulse frequency. The third experiment, conducted at 40 weeks of age, determined the response to the stimulatory feedback action of estradiol. LH surges were readily induced in undernourished (20 kg) ovariectomized lambs (peak height 51 +/- 16 ng/ml, n = 6 of seven); chronic pretreatment with estradiol markedly reduced the magnitude of the LH surge (peak height 7 +/- 3 ng/ml, four of six). These findings raise the possibility that severe undernutrition prevents ovulation in the lamb by impairing the system governing GnRH secretion and its production of high-frequency LH pulses for follicular development to the preovulatory stage and its establishment of sufficient pituitary LH reserves for release by estradiol stimulatory feedback.

Aging↗
PubMed1985

Alternate photoperiods time puberty in the female lamb.

In the spring-born lamb, puberty, the onset of repetitive ovarian cycles, occurs between 25 and 35 weeks of age during the decreasing day length of autumn. This study was conducted to determine the photoperiod conditions required for puberty. Several artificial photoperiod treatments were used, beginning from birth or soon after. Continuous exposure to long days or to short days delayed the first normal luteal cycle beyond 1 yr of age. Similarly, a single block of 4 or 10 weeks of long days at early ages, from either 3-7 or 3-13 weeks of age (afterwards under short days), produced only a few consecutive cycles within the first year after birth. However, exposure to blocks of long days at later ages resulted in the onset of cycles during the normal pubertal period. Lambs that experienced a 10-week block of long days (12-22 weeks of age; otherwise under short days) began reproductive cycles at 34 weeks of age. Reduction of the single block of long days to 5 weeks (17-22 weeks of age) or 1 week (week 22 of age) also resulted in the onset of normal luteal cycles within the normal age range for puberty. The results indicate that a sequence of long days followed by short days is required to initiate and sustain ovulatory cycles at the normal age in the lamb. Moreover, the age when long days are experienced is important, with later exposure to long days being more effective in producing consecutive cycles than early exposure to long days. These findings raise the possibility that the spring-born lamb uses a portion of the long days of summer to time the onset of puberty during the short day lengths of autumn.

Animals↗
PubMed1985

Photoperiodic time measurement is maintained in undernourished lambs with delayed puberty.

Beginning at 42 weeks of age undernourished females that had been maintained outdoors were exposed to long days (15L:9D) or short days (9L:15D). After 6 weeks, both groups were placed on short days, and ad-libitum feeding was begun. Rapid 'catch-up' growth occurred similarly in both groups. However, the response to oestradiol negative feedback regulation of LH secretion differed greatly. Short-day lambs remained hyperresponsive to oestradiol inhibition, and circulating LH remained low, a condition that typifies immaturity of the system governing LH secretion. In the females exposed to the long-day-short-day sequence, circulating LH began to increase 10 weeks after the end of long days; this change is characteristic of the neuroendocrine alteration that occurs during puberty. These findings indicate that the growth-retarded lamb can differentiate long days from short days, and can therefore continue to accumulate photoperiod information during prolonged periods of undernutrition.

Animal Nutritional Physiological Phenomena↗
PubMed1985

Internal and external determinants of the timing of puberty in the female.

A working hypothesis is proposed to account for the timing of puberty in female sheep. In the immature female, the frequency of LH pulses is low, and ovarian follicles do not develop to an advanced stage. During the pubertal transition, the frequency of LH pulses increases to drive follicular development and the production of oestradiol which evokes the gonadotrophin surge and ovulation. Central to the hypothesis is the hypothalamic pulse generator for GnRH that directs the pattern and level of LH secretion. Growth-related cues are monitored to regulate the activity of the GnRH pulse generator. When a sufficient body size is attained, the frequency of LH pulses increases both because the sensitivity to oestradiol inhibitory feedback decreases and because the GnRH pulse generator can be accelerated by the steroid. This increase in LH pulse frequency occurs provided the female has experienced the requisite exposure to photoperiod, i.e. the long days of summer followed by the short days of autumn. These photoperiodic cues are transduced by the pineal gland into a humoral signal which is an increased nocturnal production of melatonin. Failure to grow to the appropriate body size or to experience the appropriate exposure to photoperiod leads to a maintenance of the prepubertal anovulatory condition because the GnRH pulse generator operates at low frequency.

Animal Nutritional Physiological Phenomena↗