Aggression in adult mice: modification by neonatal injections of gonadal hormones.

Incidence of spontaneous aggression in adult male mice given a single injection of estradiol benzoate (0.4 milligram) when they were 3 days old was less than that of controls injected with oil. Aggressiveness was increased among adult females injected with either estradiol or testosterone propionate (1 milligram) at the same age. The increased aggressiveness noted among females given androgen was further documented during subsequent mating tests, when these females often attacked, wounded, and, in one case, killed naive males.

Release of luteinizing hormone in male mice during exposure to females: habituation of the response.

Male mice release luteinizing hormone when exposed for a short time to a female. In this experiment, multiple blood samples were withdrawn by atrial cannulas from tethered males during either continuous or intermittent exposure to nonreceptive females. After an immediate, transient release of luteinizing hormone, continuous exposure to the same female was accompanied by only random, spontaneous elevations in plasma levels of this hormone. Successive presentations of the same female at 2-hour intervals elicited gradually diminishing luteinizing hormone responses. Exposing such unresponsive males to novel, diestrous females, however, dramatically stimulated their release of the hormone. These results demonstrate habituation of a socially induced, neuroendocrine response involving reproductive hormones.

Social rank in house mice: differentiation revealed by ultraviolet visualization of urinary marking patterns.

Ultraviolet light has been used to examine urine marks deposited by adult male house mice on filter paper on the floors of their cages during overnight tests. Both the urination frequency and the pattern in which urine was deposited on the filter paper depended upon social rank. Dominant males vigorously marked their entire cage floor, whereas subordinate males typically voided urine in only two to four pools in the corners of their cages.

Sexual characteristics of adult female mice are correlated with their blood testosterone levels during prenatal development.

Mice produce litters containing many pups, and the female fetuses that develop between male fetuses have significantly higher concentrations of the male sex steroid testosterone in both their blood and amniotic fluid than do females that develop between other female fetuses. These two types of females differ during later life in many sexually related characteristics. Thus, individual variation in sexual characteristics of adult female mice may be traceable to differential exposure to testosterone during prenatal development because of intrauterine proximity to male fetuses.

Adrenalectomy and coat color in deer mice.

Adrenalectomy in prairie deer mice is followed by a profound darkening of the fur which occurs with in 1 to 3 months. The phenomenon is most noticeable on the normally unpigmented ventral surfaces which turn dark gray or black. A possible mechanism for such hyperpigmentation would involve increased release of inelanocyte-stimulating hormones.

Estrus-inducing pheromone of male mice: transport by movement of air.

The proportion of female SJL/J mice exhibiting estrus whet 2 meters downwind (6 meters per minute) from a group of 15 hybrid mi not significantly less than that of females placed directly under the males posed to their urine. The proportion of mice showing estrus when placed 2 upwind was significantly less than that of mice downwind or of mice beloii but not different from that of females remote from males. These findings sh the pheromone from male mice is volatile and further support the concep, acts through olfactory receptors.

Are humans seasonally photoperiodic?

Humans exhibit seasonal variation in a wide variety of behavioral and physiological processes, and numerous investigators have suggested that this might be because we are sensitive to seasonal variation in day length. The evidence supporting this hypothesis is inconsistent. A new hypothesis is offered here-namely, that some humans indeed are seasonally photoresponsive, but others are not, and that individual variation may be the cause of the inconsistencies that have plagued the study of responsiveness to photoperiod in the past. This hypothesis is examined in relation to seasonal changes in the reproductive activity of humans, and it is developed by reviewing and combining five bodies of knowledge: correlations of human birthrates with photoperiod; seasonal changes in the activity of the neuroendocrine pathway that could link photoperiod to gonadal steroid secretion in humans; what is known about photoperiod, latitude, and reproduction of nonhuman primates; documentation of individual variation in photoresponsiveness in rodents and humans; and what is known about the evolutionary ecology of humans.

Sensitivity of Syrian hamsters (Mesocricetus auratus) to amplitudes and rates of photoperiodic change typical of the tropics.

Empirical data suggest that reproductive photoresponsiveness occurs in some populations of mammals above 13 degrees of latitude, but may be absent in populations from 0 degrees to 10 degrees of latitude. The present experiments examined the degree to which the low amplitude of change in photoperiod in the tropics constrains mammals from using daylength as a seasonal cue. The Syrian hamster, a temperate-zone species, was studied because of its well-documented ability to respond to small changes in photoperiod, and because of the absence of an alternative robustly responding species from the tropics. We subjected adult male hamsters to photoperiods that mimicked the amplitude and rate of photoperiod change of 30 degrees, 20 degrees, 10 degrees, and 5 degrees of latitude, but centered around an estimate of their critical daylength. For comparison, a fifth group was subjected to an abrupt change in daylength of a magnitude equal to the total annual variation occurring at 30 degrees. The two groups experiencing the gradually changing daylengths of 30 degrees and 20 degrees showed less within-group synchrony during testicular regression; in other dimensions of the annual testis cycle, including the degree of synchrony exhibited during recrudescence, they reacted similarly to the hamsters given the abrupt change in daylength. Some of the hamsters exposed to the gradually changing daylengths of 10 degrees responded to this challenge, as did a few in the 5 degrees treatment--in both cases, with poor within-group synchrony and a submaximal decrease in testis size. In an abbreviated second experiment, hamsters given abrupt decreases in daylength of magnitudes equal to those of the 10 degrees and 5 degrees groups responded slightly more frequently, and with maximal decreases in testis size. This suggests that mammals may not be constrained absolutely by an inability to respond to changes in photoperiod at 5 degrees to 10 degrees latitude. Seasonally breeding populations of mammals in the deep tropics that do not use photoperiod to regulate reproduction may use nonphotoperiodic cues because they offer a higher signal-to-noise ratio than do tropical changes in photoperiod.

Food-restricted, prepubertal, female rats: rapid recovery of luteinizing hormone pulsing with excess food, and full recovery of pubertal development with gonadotropin-releasing hormone.

Prepubertal female rats were maintained continuously at 45% of their expected 50-day body weight by restricting their food intake. Uteri and ovaries declined in weight under these conditions. No evidence of pulsatile LH release was seen when these animals were examined at 50 days of age. Allowing unlimited access to food at this time caused rapid pubertal development. LH pulsing began in some females within 12 h; strong LH pulsing was seen in most females within 24 h, and all ovulated after only 2 1/2 or 3 1/2 days of ad libitum feeding. These were fertile ovulations, accompanied by mating and resulting in pregnancy. Administering GnRH in a pulsatile manner to 50-day-old, food-restricted animals also yielded full pubertal development. Uteri and ovaries gradually increased in weight, and ovulation occurred in 3 1/2 to 5 1/2 days. These findings support a contention that the major reproductive deficit resulting from food restriction relates to the control of GnRH secretion. In toto they also suggest a close metabolic coupling between some dimension of nutrient and/or energy processing and the GnRH pulse generator in the normally growing female.

Male-induced precocial puberty in female mice: confirmation of the role of estrogen.

Previous work has shown that exposure of prepubertal female mice to an adult male results in an immediate, sequential release of LH and estrogen. An ovulating release of LH follows in a predictable 3-day period, providing that male-exposure occurs at a particular body weight. Using this three day system, the present objective was to determine how effectively exogenous estrogen would substitute for the male's presence. Control experiments established that neither social isolation nor exposure to a castrated male would yield ovulatory puberty within the 3-day experimental period. Immature females were then subjected to a variety of sequences of three daily treatments including male-exposure and/or steroid injection. Two days of male exposure and no treatment on the third day was as effective in eliciting ovulation on the third night as 3 full days of cohabitation. Furthermore, either or both of the first two days of male-exposure could be mimicked by single injections of estrogen if followed by male-exposure for the remainder of the experimental period. A single injection of estrogen on the first day was ineffective in eliciting ovulation unless followed by male-exposure for 2 days, by a second injection of estrogen on day 2, or by an injection of progesterone on the third day. It was concluded that, as far as the induction of puberty is concerned, the important action of the male's presence is to elicit estrogen secretion (via a subovulating release of LH), and that this process must continue for 2 days, even in immediately prepubertal females, if the pubertal ovulation is to occur during the third night.

Puberty in female mice is not associated with increases in either body fat or leptin.

It has been hypothesized that puberty is triggered when body fat and hence circulating levels of leptin exceed critical thresholds. Four kinds of experiments tested that hypothesis in female mice. When age was the independent variable, body fat and circulating levels of leptin decreased rather than increased before the onset of puberty. When stage of reproductive development was the independent variable, neither body fat nor circulating levels of leptin correlated with the onset of puberty. In sharp contrast, reproductive development was well correlated with body weight. A significant nocturnal peak in circulating levels of leptin was seen before and at all stages of reproductive development, but the highest levels were seen after rather than before the first estrous cycle was initiated. Neither acceleration nor deceleration of puberty by varying the female's social environment had any effect on either body fat or leptin. There is no support in any of these experiments for the hypothesis that an increase in body fat and thus an increase in circulating levels of leptin triggers puberty in female mice.

The regulation of luteinizing hormone secretion by estrogen: relationships among negative feedback, surge potential, and male stimulation in juvenile, peripubertal, and adult female mice.

These experiments queried the potential for developmental change in the positive and negative feedback sensitivities of LH secretion to estradiol. Serum LH levels were compared in juvenile, peripubertal, and adult female mice after ovariectomy and challenge with a variety of 4-day steroid treatments. Since male stimuli accelerate the attainment of puberty in this species, comparisons also were made between age- and weight-matched groups of pre- vs. postpubertal females, obtained by housing them in either the presence or absence of males. The following conclusions were derived from eight experiments. 1) Progressively more estradiol is required to suppress the postovariectomy rise in serum LH as the female mouse develops the juvenile stage to full adulthood. 2) This slow, progressive change is due in part to a steadily increasing capacity to secrete LH when unhindered by any negative feedback regulation by the ovary; it is not due to developmental changes in estradiol metabolism. 3) The LH surging mechanism is functional and already coupled to its circadian oscillator in the juvenile mouse, but surges of adult magnitude could not be induced with exogenous steroid until the peripubertal period. 4) The estradiol requirements for preovulatory LH surging change markedly after the peripubertal stage of development; a wide range of temporal changes in blood estradiol levels will elicit LH surging in the adult mouse, but the immature female requires a rigid pattern of change involving only a narrow range of blood levels of this steroid. 5) Exposure of peripubertal females to male stimuli alters neither their negative feedback sensitivity to estradiol nor their estrogen requirements for LH surging. These conclusions are integrated into two working models that focus first on the estrogen requirements for LH surging in the adult mouse, and second on the maturation of this process and the attainment of pubertal ovulation. (Endocrinology 108: 506, 1981)

Suppression of serum follicle-stimulating hormone by follicular fluid in the maximally estrogenized, ovariectomized mouse.

Serum levels of FSH in ovariectomized female mice are only partially suppressed by estradiol regardless of the dosage administered. Conversely, within the limits of detection, serum FSH in intact females is totally suppressed by estradiol. Thus, the present experiments used the maximally estrogenized, ovariectomized female as a test system for evaluating other nonestrogenic factors which could depress serum FSH. It was found that a transplanted ovary lowered serum FSH dramatically under such conditions. Of the large number of steroid and pituitary hormones tested, however, only large doses of testosterone propionate yielded any further suppression of serum FSH (24%). Charcoal-extracted follicular fluid (porcine) markedly lowered serum FSH concentrations (50% suppression), while a charcoal-extracted saline homogenate of PMS-treated mouse ovaries also was active in this regard (26% suppression). Finally, charcoal-extracted porcine follicular fluid was administered to nonestrogenized, ovariectomized females, where it both depressed serum FSH and markedly elevated serum LH concentrations. These studies, when viewed in toto, support a contention that follicular fluid contains a nonsteroidal factor which is capable of acting additively with estradiol to regulate FSH secretion. The question of whether this factor also regulates LH secretion in a reciprocal fashion in the mouse requires further work.

Differential effects of male stimuli on follicle-stimulating hormone, luteinizing hormone, and prolactin secretion in prepubertal female mice.

Puberty may be induced rapidly in young female mice by exposing them to adult males. The relevant male stimuli include a urinary pheromone and tactile cues, the latter acting in a potentiating capacity. The specific action of the urinary pheromone on pituitary gonadotropins was the subject of the present research. Immature females of a standard size were either paired with adult males, isolated and their bedding sprayed with male urine, or maintained as isolated controls. Exposure to male urine resulted in a rapid and significant release of LH (30 min) which was maintained at an average of 50% higher than levels in isolated controls throughout the 48 h experiment; no immediate changes were observed in either serum FSH or prolactin, but FSH was depressed and prolactin rose progressively during later sampling periods. Cohabitation with an adult male yielded parallel but decidedly stronger reponses in the concentrations of all three hormones. Exogenous estrogen, administered in a second experiment at a dosage previously found to mimic the puberty-inducing action of a male, suppressed serum FSH and LH while elevating serum prolactin. Thus, the present results suggest a) that the male's urinary stimulus exerts its action on immature females via LH release with no immediate and/or direct effects on the other two tropic hormones, and b) that the delayed alterations in serum FSH and prolactin concentrations accompanying male- and/or urine-exposure are secondary consequences of an LH-induced release of estradiol. Attempts to induce the entire pubertal cycle in intact females with subovulating doses of LH, nevertheless, were unsuccessful.

Reproductive failure in aged CBF1 male mice: interrelationships between pituitary gonadotropic hormones, testicular function, and mating success.

Circulating pituitary gonadotropins and testicular function were examined in aged CBF1 male mice using two experimental designs: a) a longitudinal analysis of 2 to 30-month-old males, and b) a direct comparison of weight-matched, sexually-active vs. sexually-inactive 24-month-old males, all of whom were relatively robust. Measurements included serum concentrations of testosterone, follicle stimulating hormone, and luteinizing hormone, gonadotropic responsiveness to castration, success in a 4-day mating test, testicular sperm content, and reproductive organ weights. The longitudinal analysis showed progressive losses in mating success and sperm production between 18 and 30 months, changes which were correlated with decreased levels of serum LH and testosterone but not with FSH. The direct comparison of robust, sexually-active vs.-inactive males provided a better design for identifying reproductive-specific effects of aging, as opposed to debilitative changes that are general to several supporting systems. Such comparisons in 24-month-old CBF1 males suggest the existence of a subpopulation of mice in which reproductive failure is specifically correlated with a loss in the episodic release of LH.

Gonadotropic responses of male mice to female urine.

Five experiments consistently demonstrated the existence of a factor(s) in the urine of female mice which stimulates gonadotropin secretion in male mice. The factor, presumably a priming pheromone, is not found in the urine of intact or castrated males nor is it in the urine of female hamsters. Furthermore, the potency of female mouse urine is apparently independent of ovarian state. In the response of the male, the release of LH is more dramatic than that of Fsh. as little as 10 min of exposure to female urine produced significantly elevated LH levels at 30 min. Finally, the male's responsiveness is largely, and possibly entirely, independent of his sexual experience. These results are discussed within the framework of a two-way pheromonal system by way of which urinary factors of both sexes modulate gonadotropin and gonadal hormone secretion in the opposite sex.

Endocrine responses to sexual arousal in male mice.

The endocrine correlates of sexual arousal were explored in male mice. Using indwelling atrial cannulae, temporal changes in the blood levels of six hormones were monitored in sexually rested vs. sexually sated males before and during their exposure to receptive females. The vaginae of some test females were artificially occluded to assess separately the impact of intromission on hormone levels. Sexually rested males responded to receptive females with dramatic increases in their blood levels of corticosterone, epinephrine, and norepinephrine. These changes occurred independently of intromission. Blood titers of these three hormones did not change in sexually sated males when they encountered a receptive female. Both sexually sated and sexually rested males discharged LH in a reflex-like manner immediately upon perceiving a test female. PRL was released only in response to ejaculation, and FSH titers were unaffected by the presence of a female. In general, a receptive female elicits a variety of hormonal responses in a male mouse. Some hormones seem to be discharged in response to sexual arousal per se, whereas others are released with varying degrees of independence of this process. Viewed from another perspective, the present results allow the presentation of a relatively detailed scheme that temporally relates sensory input to hormonal response during the sexual interactions of male mice.

Episodic release of luteinizing hormone in male mice: antagonism by a neural refractory period.

A refractory period of LH secretion follows both an internally generated and a female-induced, episodic release of LH in male mice. These experiments examined the duration of the refractory period and investigated its physiological basis. Plasma LH was measured in sequential blood samples that were withdrawn from chronically cannulated male mice. The males were exposed to two successive stimuli that were presented at 10-, 25-, or 45-min intervals; the two stimuli were a female both times, LHRH both times, or LHRH first and a female second. Males that were exposed to two successive females released LH in response to the second female only if 45 min had elapsed between exposures. All males in the LHRH-LHRH treatment group released LH after both injections of LHRH at all three stimulus intervals. The female stimulus evoked LH secretion in the LHRH-female treatment group at all three times after LHRH administration. In a second set of experiments, testosterone capsules that maintained the seminal vesicle weights of castrated males at well above normal levels did not block their LH responses to a female stimulus. When taken together, these results indicate that a refractory period lasting between 25 and 45 min follows episodes of LH release that are socially generated, that this refractory period has its basis in altered neural rather than pituitary activity and, finally, that this neurally based refractory period is not a result of inhibitory hormonal feedback.

Control of the preovulatory release of luteinizing hormone by steroids in the mouse.

The regulation of the preovulatory release of LH by steroids was examined in the mouse, a species in which ovulation is strongly influenced by priming pheromones. Ovariectomized mice were implanted with estradiol in Silastic capsules to involve negative feedback. Preovulatory-like LH surges then were induced by injections of either estradiol benzoate (EB) or progesterone. LH surges were not observed in the absence of steroid injection. LH surges always occurred at lights out on a 14-h light, 10-h dark cycle on the day after EB injection but occurred on the same day as progesterone injection. The amount of EB or progesterone injected seemed unimportant but, in either case, had to be given within a limited diurnal period of sensitivity. LH surges comparable to those of intact proestrous females were produced either by injecting both EB and progesterone or by manipulating the background dose of encapsulated estradiol. In the latter regard, when ovariectomized females were implanted with a wide range of doses of estradiol (0.1--1000 microgram/capsule), a decided window phenomenon became apparent. That is, LH surging could be induced by steroid injections only within a limited range of background doses of encapsulated estradiol. The relationship of the above findings to the pheromonal control of LH secretion and ovulation in mice is as yet unclear.

Behavioral and physiological responses of female house mice to foraging variation.

Peripubertal female house mice were required to work for their food at either 23 degrees C or 9 degrees C. We used a special caging system in which animals had to emerge from a thermally-buffered burrow and run a programmable number of running wheel revolutions to obtain a pellet of food. Of concern here were the behavioral and physiological adjustments necessary to accommodate growth and reproductive development when faced with the need to forage for different lengths of time at different temperatures. When female house mice are confronted with poor foraging at cool temperatures they allot their highest priorities to maintaining energy balance; body growth is next, and reproductive development and nonforaging activity have the lowest priorities. Our results also demonstrate that the time spent foraging while exposed to low ambient temperature is critical for this species. This relationship probably determines whether or not house mice will breed continuously or seasonally in a particular habitat.