Biomedical subjects

F H Bronson

Publications and source records attributed to F H Bronson.

At least 19 recordsLinked to original sources

PubMed1992

Lack of reproductive photoresponsiveness and correlative failure to respond to melatonin in a tropical rodent, the cane mouse.

Cane mice (Zygodontomys brevicauda) are year-round breeders in Venezuela. As shown previously, these animals are not reproductively responsive to variation in photoperiod. In the present experiments, male cane mice were maintained on long or short day lengths (16L:8D or 8L:16D, respectively) and challenged with each of three experimental treatments known to "unmask" reproductive photoresponsiveness in laboratory rats: olfactory bulbectomy, prolonged food restriction, and exposure as neonates to a single injection of testosterone. Variation in photoperiod had no inhibitory effect on the responses of cane mice to any of these three treatments, as assessed by the weight of their testes and seminal vesicles. A fourth experiment demonstrated that cane mice are insensitive to 10 wk of continuous exposure to pharmacological levels of melatonin, again as assessed by reproductive organ weight. Likewise, a fifth experiment documented a lack of response to 10 wk of late-afternoon injections of massive amounts of melatonin. The cane mouse apparently is unique among the animals challenged so far in these ways in that it seems to have no vestige of reproductive photoresponsiveness.

Animals↗

PubMed1992

Life history and bioeconomy of the house mouse.

1. More is known about the western European house mouse, Mus (musculus) domesticus than any other non-human mammal. If laboratory and field information is combined, an extremely valuable understanding of the species' bioeconomy could be obtained. 2. The seven stages of mouse life-history are surveyed (up to birth, nest life, sex life, social structure, population statics and stability, senescence, and death), and the interactions between the changing phenotype and the environment are described. 3. These interactions can be used to build up a model of the opportunities and compromises which result in the fitness of individual mice. It is not yet possible to quantify such a model, but this should in principle be achievable.

Animals↗

PubMed1992

A pseudoseasonal reproductive strategy in a tropical rodent, Peromyscus nudipes.

A population of cloud forest mice (Peromyscus nudipes) at latitude 10 degrees N near Monteverde, Costa Rica, was sampled four times by live-trapping twice during the 7-8 month wet season and twice during the 4-5 month dry season in 1989 and 1990. Body weights were lower during the early part of the dry season in males and throughout the dry season in females than at other times. Testes and seminal vesicles were somewhat lighter early in the dry season, but epididymal spermatozoa were abundant in most males throughout the year. Adult females ovulated, mated and became pregnant in the wet and dry seasons, but young were produced only during the wet season. Most embryos failed to implant during the dry season, and the few that did complete implantation were reabsorbed before midpregnancy. Apparently, every year, the females in this population spend several months actively engaged in a behavioural and metabolically costly process that is doomed to be unsuccessful. This reproductive strategy is termed pseudoseasonal, because reproductive success is highly seasonal, but attempts to reproduce are nonseasonal. Implantation failures similar to those seen in the wild were induced in the laboratory using mild restriction of food or water. Field evidence points to food restriction as the more important cause of pregnancy losses in the wild. Exposure to the gradually changing daylengths typical of Costa Rica had no effect on the production of young by adults, and maintenance on light cycles of 8 h light: 16 h dark, 11 h light: 13 h dark, 13 h light: 11 h dark and 16 h light: 8 h dark had no effect on the reproductive development of young animals of either sex.

Animal Nutritional Physiological Phenomena↗

PubMed1992

Seasonal reproduction of a tropical bat, Anoura geoffroyi, in relation to photoperiod.

Anoura geoffroyi (Chiroptera, Phyllostomidae, Glossophaginae), Geoffroy's hairy-legged long-tongued bat, were collected from September 1984 to August 1985, and these bats were found to breed seasonally in the wild on Trinidad, West Indies, at 10 degrees N latitude. Histological examination of these samples indicated that females became pregnant in July or August, and young were born in late November or early December. The testes and epididymides were small from September to mid-April, increased threefold in weight between mid-April and late May, reached a peak weight in July, and decreased in weight in August. Spermatogenesis occurred throughout the testes of males captured from May to August. In 1990, the timing of parturition in females that gave birth in the laboratory to young conceived in the wild was similar to the timing in the field in 1984-1985. Groups of 10-13 males were subjected in the laboratory to (i) a gradually changing, civil twilight photoperiod that mimicked the natural cycle of annual change at 10 degrees N latitude, (ii) the same gradually changing cycle of photoperiod accelerated to a six-month period, or (iii) a constant photoperiod (light 12:54 h: dark 11:06 h). These treatments began in mid-December, four months before the initiation of testicular recrudescence in the wild. In all three groups, testicular volume remained low until April, and then increased two- to threefold between late April and late June, rising to a peak in July, as occurred in the wild.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗

PubMed1991

Lability of fat stores in peripubertal wild house mice.

Traditionally, the adaptive value of mammalian white fat stores is considered in relation to long-term needs such as providing protection against the vagaries of winter or signalling the reproductive system when energy reserves are sufficient to risk pregnancy. As shown here, the fat stores of young house mice could not serve such needs. Despite prolonged acclimation and excess nesting material, food deprivation at 10 degrees C significantly lowered the fat stores of peripubertal female house mice in only 12 h, and would exhaust them in 30 h. Even close to thermoneutrality (24 degrees C) the calculated time to exhaustion was only 70 h. The fat stores of a young house mouse are obviously too meager to offer any meaningful protection over a winter of several months duration, or even over a 5-6-week cycle of pregnancy and lactation. Furthermore, in a wild habitat where food availability and ambient temperature can vary rapidly and greatly, such fat stores would be too labile to effectively coordinate puberty with somatic development.

Adaptation, Physiological↗

PubMed1991

Characteristics of a genetic polymorphism for reproductive photoresponsiveness in the white-footed mouse (Peromyscus leucopus).

Wild populations of Peromyscus are often composed of individuals that vary greatly in their reproductive response to photoperiod. A population of white-footed mice (P. leucopus) from Michigan (43 degrees N) was subjected to mass selection in the laboratory both for and against reproductive photoresponsiveness for four generations. The first generation of selection yielded one line of mice in which about 80% of the individuals were classified as reproductively photoresponsive (i.e., with undeveloped reproductive tracts when reared in short days, 8L: 16D) and another in which only about 20% were reproductively photoresponsive. Some and perhaps most of this difference was accounted for by changes in degree of responsiveness to photoperiod rather than by alterations in the proportion of discrete responsive vs. unresponsive phenotypes. Alteration of critical day length was not a factor. Three more generations of selection failed to change the proportions noted above significantly. Although the genetic control of reproductive photoresponsiveness is undoubtedly complex, a single variable locus may be responsible for much of the heritable variation present in this population. These results also suggest that natural populations contain genetically determined phenotypes that are intermediate between absolutely photoresponsive and absolutely unresponsive. The factors that might promote maintenance of heterogeneity of reproductive photoresponsiveness in a wild population of rodents are considered.

Animals↗

PubMed1991

The energetic regulation of ovulation: a realistic role for body fat.

This review weighs the evidence for and against the hypothesis that ovulation is regulated by a critical amount of body fat. The evidence supporting this hypothesis is correlative, and most of it stems from observations made in humans. On balance, the evidence from human studies does not support the hypothesis, however, and the results of animal studies argue strongly against it. In the latter regard, a variety of experimental approaches have been tried in both adult and peripubertal females of several species, and the results almost uniformly show little relationship between fatness and ovulation. There is no doubt that ovulation can be regulated somehow in relation to whole-body energy balance and that fat stores are an important component of energy balance, but there is no reason to accord body fat a direct causal role in regulating ovulation.

Adipose Tissue↗

PubMed1991

Suppression of puberty in rats by exercise: effects on hormone levels and reversal with GnRH infusion.

In an effort to better understand the effects of prolonged exercise on the hypothalamic-pituitary-ovarian axis, we made a variety of comparisons between young female rats in two treatment groups: 1) prolonged exercise, in which growth and reproductive development were arrested at a peripubertal stage by requiring rats to run for long periods of time in order to obtain food; and 2) voluntary exercise, in which same-aged control rats were fed ad libitum and given free access to a running wheel. The pulsatile secretion of luteinizing hormone (LH) and growth hormone were completely suppressed by the prolonged exercise treatment. Mean levels of follicle-stimulating hormone, prolactin, and thyroid-stimulating hormone were not affected. Prolonged exercise elevated corticosterone titers, and the secretory pattern of this steroid was changed out of phase with running activity. Tissue levels of gonadotropin-releasing hormone (GnRH) in the hypothalamus and LH in the pituitary were enhanced, not suppressed, by prolonged exercise. Most importantly, pulsatile infusions of GnRH reinstated normal pubertal development and ovulation in rats still growth restricted by the prolonged exercise treatment. The results of this study indicate that the suppressive effects of prolonged exercise somehow affect the production of the hypothalamic GnRH-pulse generator signal.

Animals↗

PubMed1990

Photoperiod, melatonin secretion, and sexual maturation in a tropical rodent.

Descendants of a sample of cane mice (Zygodontomys brevicauda) trapped at 8 degrees latitude in Venezuela were tested for reproductive photoresponsiveness. This species breeds continuously, year around, despite living in a seasonally harsh habitat. At 50 days of age there were no differences in the weights of the testes or seminal vesicles or in sperm counts of males born and reared on 16L:8D, 13L:11D, 11L:13D, or 8L:16D photoperiods, although there were small differences in body weight. Females born and reared on 16L:8D vs. 8L:16D cycles became pregnant at the same rates and ages when paired with males at 21 or 31 days of age. The daily duration of melatonin secretion depended on the length of the dark phase of the cycle in both sexes. Circulating levels of melatonin were elevated for 8 h on a 16L:8D cycle and for between 9 and 16 h on an 8L:16D cycle. In this tropical species, the neuroendocrine pathway that links photoperiod to reproduction apparently is disconnected somewhere between melatonin and gonadotropin secretion, causing cane mice to be reproductively unresponsive to variation in photoperiod.

Animals↗

PubMed1990

Short-term hormonal responses to food intake in peripubertal female rats.

These studies focused on the phenomenon of "catch-up" pubertal development. Circulating levels of several hormones were characterized in 8-wk-old female rats whose growth and reproductive development had been blocked before puberty by restricting their food intake. Some of these females were fed ad libitum for 24 h to initiate rapid pubertal development. Blood levels of luteinizing hormone (LH) and growth hormone (GH) were suppressed by food restriction and then partially restored to adult diestrus levels by 24 h of ad libitum feeding. Prolactin titers were also suppressed by food restriction but not significantly elevated by 24 h of ad libitum feeding. Circulating levels of follicle-stimulating hormone (FSH) and thyroid-stimulating hormone (TSH) were unaffected by either treatment. It is concluded that GH could play an active supplementary role to LH in eliciting catch-up pubertal development but that FSH and TSH could play only passive roles at best. The role of prolactin remains uncertain. On a finer time scale, when food-restricted females were examined in relation to the time of day at which they were fed, most showed high-amplitude LH pulses 2-4 h after eating but rarely at any other time. Thus under some conditions LH secretion can be modulated by food intake on an almost hour-by-hour basis. Overall, blood levels of corticosterone were generally but not always inversely correlated with the frequency of LH pulsing in these experiments. Finally, the present results argue against the concept that puberty is dependent on a critical whole body characteristic.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenal Cortex Hormones↗

PubMed1989

Epinephrine release in response to sexual activity in male versus female rats.

Sequential blood samples were collected from adult male and female rats before, during and after they interacted with either a sexually-active or a gonadectomized adult of the opposite sex. As assessed by plasma levels of epinephrine, both sexes showed a much greater response to a sexual encounter than they did to a nonsexual encounter. The release of epinephrine in response to sexual activity differed in males and females in at least two ways, however. First, a sexual encounter caused an absolutely greater but a proportionally smaller response in females. Second, plasma levels of epinephrine in males but not in females reflected the alternating periods of copulation and noncopulation that characterize sexual activity in this species. Blood levels of epinephrine were low and stable in males between copulatory bouts and relatively high during these bouts. In contrast, circulating levels of epinephrine in females were almost as high in the interval between copulatory bouts as they were during copulation.

Animals↗

PubMed1989

Effects of prolonged exercise on puberty and luteinizing hormone secretion in female rats.

Immature female rats were required to run for prolonged periods of time to obtain food. The amount of food they earned was adequate for full pubertal development and moderate growth under nonworking conditions, but both processes were blocked by the exercise requirement. Prolonged exercise also blocked the pulsatile release of luteinizing hormone (LH); only two LH pulses were seen in seven exercising females during a total of 24 h of monitoring at 8 wk of age. By comparison, almost 1 pulse/h was seen in postpubertal, normally growing females of this same age during metestrus. When the exercising females' running requirement was relaxed at 8 wk of age they experienced rapid catch-up growth and reproductive development. Both basal secretion and LH pulse frequency increased markedly within 48 h, and most of these females ovulated during the third dark period after relaxation. Altogether, the experimental paradigm and techniques employed here yield highly predictable results, and they should prove useful for exploring other neuroendocrine pathways through which excessive exercise antagonizes reproduction.

Animals↗

PubMed1988

Patterns of sexual receptivity in the female musk shrew (Suncus murinus).

Female musk shrews (Suncus murinus) were tested daily to examine patterns of sexual receptivity. When only mounting was used as a criterion (to avoid pregnancy), nonpregnant females remained sexually receptive to males every day for 14 consecutive days. When insemination was allowed, most females continued to copulate for the first 5 days of pregnancy. Receptivity declined markedly around Day 10 of gestation, but a few females were receptive even into late pregnancy. Lactating females copulated with males 5 and 10 days after parturition. In general, unlike most mammals studied in the laboratory, the nonpregnant female musk shrew has no behavioral estrous cycle. Musk shrews are ready to mate anytime except in mild to late pregnancy, and even then occasionally mating is found.

Animals↗

PubMed1988

Variation in litter size encountered in utero influences the bioenergetic characteristics of adult female mice.

This experiment establishes the importance of in utero litter size in shaping the bioenergetic characteristics of the adult female mouse. The potential number of fetuses carried by a pregnant female was reduced to half by blocking one oviduct prior to mating. The resulting offspring were reared in normal sized litters after birth. As adults these female offspring were heavier and they consumed less food than control females who had shared their mother's uterus with a normal complement of fetuses. As assessed with running wheels, females from reduced litters were less than half as active during adulthood as control females. Variation in prenatal litter size apparently has far reaching effects on the acquisition and use of energy by mice.

Animals↗

PubMed1988

Mammalian reproductive strategies: genes, photoperiod and latitude.

This paper considers how and why natural selection might promote or block the photoperiodic regulation of a mammal's reproduction. The factors most important in making this decision would seem to be the following: life expectancy, length of the female's cycle, feeding strategy, the presence or absence of survival mechanisms like hibernation, and the nature of the seasonal challenges offered by the mammal's habitat. A speculative scheme is offered for the potential utility of this type of regulation dependent upon life expectancy and latitude of residence.

Animals↗

PubMed1988

Experiential and endocrine dependence of gonadotropin responses in male mice to conspecific urine.

Previous research has shown that a urinary pheromone of female mice acts via the vomeronasal organ of the accessory olfactory system to elicit rapid release of luteinizing hormone (LH) in conspecific males. Several experiments were conducted to examine the importance of sexual experience for gonadotropin responses in male mice to female urine, male urine, saline, or mixtures of these stimuli. Both sexually naive and sexually experienced male mice had significantly higher plasma LH levels after presentations of female urine than after presentations of male urine. However, sexual experience appeared to increase the reliability of the short-latency gonadotropin response to female urine relative to a sexually neutral component of urine such as sodium chloride, and male urine appeared to suppress spontaneous LH secretion episodes in both naive and sexually experienced males. Subsequent experiments with sexually experienced subjects demonstrated that male mouse urine is a powerful suppressant of LH release in other males. Specifically, female mouse urine mixed with male urine failed to elicit LH responses in male subjects, whereas female urine mixed with saline was highly effective. Urine obtained from castrated male donors was as potent as urine from intact males in suppressing the gonadotropin response to female urine. The suppressive activity in male mouse urine thus does not appear to be critically dependent on gonadal hormones. The existence of a potent stimulatory pheromone in female urine and a potent suppressive pheromone in male urine makes male mice an excellent model system for studying the neural regulation of LH secretion.

Animals↗

PubMed1988

Chemical properties of a female mouse pheromone that stimulates gonadotropin secretion in males.

Female mouse urine contains a pheromone that acts via the vomeronasal organ of conspecific males to stimulate a rapid increase in circulating levels of luteinizing hormone. A bioassay based on this male response was used to test biochemical preparations of female urine. Retention of significant biological activity by the urine after dialysis indicated that the activity is associated with urinary protein. Complete loss of activity from the urine after adsorption chromatography on a neutral polystyrene column suggested that the protein functions as a pheromone carrier. Assay of gel permeation chromatography fractions, before and after degradation of the urinary proteins with proteolytic enzymes, demonstrated that the protein is not necessary for the male response in the bioassay. Its resistance to vigorous proteolytic enzyme treatment further indicates that the pheromone is not a peptide. High biological activity, indistinguishable from that of the unfractionated urine, was isolated in a protein-depleted, presumably low molecular weight fraction containing compounds that are retarded by adsorption on Sephadex. The chemical properties of this female mouse pheromone are markedly different from those of a recently purified female hamster pheromone that also acts via the vomeronasal organ.

Animals↗

PubMed1988

A novel postcopulatory block of reproduction in white-footed mice.

Female white-footed mice (Peromyscus leucopus) fail to produce offspring when paired with a male from weaning until 150 days of age if an adult female or her odor is also present. The present study delineates more clearly which stage of the young female's reproductive cycle is inhibited by the chemosignal of the older female. Age at vaginal opening and first estrus are delayed by the presence of the older female, but only for about 10 days. The presence of a male counteracts this delay. Thus, the basis for the block is not a delay in puberty. The older female's presence does not influence the number of estrous cycles experienced during the 30 days following first estrus, nor does it influence the number of corpora lutea found at autopsy. The presence of sperm in the vaginae of young females indicates that they were copulating. Likewise, examination of embryos 2 and 3 days after copulation reveals normal developmental progress. However, implantation does not occur in young females that have been exposed to an adult female. Thus, the block occurs either during the final stages of embryo transport or in relation to the implantation process itself.

Animals↗