Microevolution of neuroendocrine mechanisms regulating reproductive timing in Peromyscus leucopus

A key question in the evolution of life history and in evolutionary physiology asks how reproductive and other life-history traits evolve. Genetic variation in reproductive control systems may exist in many elements of the complex inputs that can affect the hypothalamic-pituitary-gonadal (HPG) or reproductive axis. Such variation could include numbers and other traits of secretory cells, the amount and pattern of chemical message released, transport and clearance mechanisms, and the number and other traits of receptor cells. Selection lines created from a natural population of white-footed mice (Peromyscus leucopus) that contains substantial genetic variation in reproductive inhibition in response to short winter daylength (SD) have been used to examine neuroendocrine variation in reproductive timing. We hypothesized that natural genetic variation would be most likely to occur in the inputs to GnRH neurons and/or in GnRH neurons themselves, but not in elements of the photoperiodic pathway that would have pleiotropic effects on nonreproductive functions as well as on reproductive functions. Significant genetic variation has been found in the GnRH neuronal system. The number of GnRH neurons immunoreactive to an antibody to mature GnRH peptide under conditions maximizing detection of stained neurons was significantly heritable in an unselected control (C) line. Furthermore, a selection line that suppresses reproduction in SD (photoperiod responsive, R) had fewer IR-GnRH neurons than a selection line that maintains reproduction in SD (photoperiod nonresponsive, NR). This supports the hypothesis that genetic variation in characteristics of GnRH neurons themselves may be responsible for the observed phenotypic variation in reproduction in SD. The R and NR lines differ genetically in food intake and iodo-melatonin receptor binding, as well as in other characteristics. The latter findings are consistent with the hypothesis that genetic variation occurs in the nutritional and hormonal inputs to GnRH neurons. Genetic variation also exists in the phenotypic plasticity of responses to two combinations of treatments, (1) food and photoperiod, and (2) photoperiod and age, indicating genetic variation in individual norms of reaction within this population. Overall, the apparent multiple sources of genetic variation within this population suggest that there may be multiple alternative combinations of alleles for both the R and NR phenotypes. If that interpretation is correct, we suggest that this offers some support for the evolutionary "potential" hypothesis and is inconsistent with the evolutionary "constraint" and "symmorphosis" hypotheses for the evolution of complex neuroendocrine pathways.     

Number of immunoreactive GnRH-containing neurons is heritable in a wild-derived population of white-footed mice (Peromyscus leucopus)

The evolution of mammalian brain function depends in part on levels of natural, heritable variation in numbers, location, and function of neurons. However, the nature and amount of natural genetic variation in neural traits and their physiological link to variation in function or evolutionary change are unknown. We estimated the level of within-population heritable variation in the number of gonadotropin-releasing hormone (GnRH) neurons, which play a major role in reproductive regulation, in an unselected outbred population recently derived (<10 generations) from a single natural population of white-footed mice (Peromyscus leucopus, Rafinesque). Young adult male mice exhibited an approximately threefold variation in the number of neurons immunoreactive for GnRH in the brain areas surveyed, as detected using SMI-41 antibody with a single-label avidin-biotin complex method. Consistent with earlier findings of selectable variation in GnRH neurons in this population, the level of genetic variation in this neuronal trait within this single population was high, with broadsense heritability using full-sib analysis estimated at 0.72 (P<0.05). Either weak selection on this trait or environmental variation that results in inconsistent selection on this trait might allow a high level of variation in this population.     

Photoperiod and temperature interact to affect the GnRH neuronal system of male prairie voles (Microtus ochrogaster)

Individuals of numerous species limit energy expenditure during winter by inhibiting reproduction and other nonessential functions. To time these adaptations appropriately with the annual cycle, animals rely on environmental cues that predict, well in advance, the onset of winter. The most commonly studied environmental factor that animals use to time reproduction is photoperiod. Rodents housed in short photoperiods in the laboratory or in naturally declining day lengths exhibit pronounced alterations in reproductive function concomitant with alterations in the hypothalamic gonadotropin-releasing hormone neuronal system. Because animals in their natural environment use factors in addition to photoperiod to time reproduction, the present study sought to determine the independent effects of photoperiod and temperature, as well as the interaction between these factors, on reproductive parameters and the GnRH neuronal system. Male prairie voles were housed in either long (LD 16:8) or short (LD 8:16) day lengths for 10 weeks. Animals in each photoperiod were further subdivided into groups housed in either mild (i.e., 20 degrees C) or low (i.e., 8 degrees C) temperatures. As shown with immunohistochemistry, voles that underwent gonadal regression in response to short photoperiods and long-day voles housed in low temperatures (and maintained large gonads) exhibit higher GnRH-immunoreactive (GnRH-ir) neuron numbers in the preoptic area/anterior hypothalamus (POA/AH) relative to all other groups. In addition, voles that underwent gonadal regression in response to both short days and low temperatures did not exhibit an increase in GnRH-ir neuron numbers compared to long-day, mild-temperature controls. These data suggest that photoperiod and temperature interact to influence reproductive function potentially by alterations of the GnRH neuronal system.     

Phenotypic differences in the GnRH neuronal system of deer mice Peromyscus maniculatus under a natural short photoperiod

The neural mechanisms by which short photoperiod induces gonadal regression among seasonally breeding mammals are not well understood. One hypothesis suggests that the proximate cause of seasonal gonadal regression is a photoperiod-induced modification in GnRH secretion. This hypothesis is indirectly supported by our recent findings using immunocytochemistry which identified specific photoperiod-induced adjustments in the number and morphology of GnRH containing neurones between reproductively competent and reproductively regressed laboratory housed male deer mice. Herein, we report that the GnRH neuronal system is similarly affected in reproductively responsive and nonresponsive wild male deer mice Peromyscus maniculatus exposed to a natural short photoperiod. The distribution of immunoreactive (IR)-GnRH neurones was nearly identical in field caught animals and those housed under artificial photoperiod in the laboratory. Compared with reproductively nonresponsive males, reproductively responsive mice from the field population possessed a greater total number of IR-GnRH neurones, a greater number of IR-GnRH neurones within the lateral hypothalamus, and a greater proportion of bipolar IR-GnRH neurones. Each of these distributional and morphological characters was consistent with our findings in laboratory housed male deer mice exposed to an artificial short photoperiod. Taken together, these data underscore the validity of using an artificial photoperiod to evaluate seasonal adjustments in reproductive function in the laboratory.     

Short photoperiod-induced gonadal regression: effects on the gonadotropin-releasing hormone (GnRH) neuronal system of the white-footed mouse, Peromyscus leucopus

The peroxidase-antiperoxidase method was used to determine quantitatively the effect of short photoperiod-induced gonadal regression on the immunoreactive gonadotropin-releasing hormone (GnRH) neuronal system of female Peromyscus leucopus. In mice exposed to either long (16L:8D) or short (8L:16D) photoperiod, immunoreactive cell bodies were loosely organized into six groups: olfactory peduncle, diagonal band of Broca, septum, preoptic area (POA), anterior hypothalamus (AH), and basal hypothalamus. The POA and AH contain the largest number of cell bodies, which supply the major GnRH innervation to the median eminence (ME) and several extrahypothalamic brain sites. Exposure to short photoperiod increased the number of immunoreactive cell bodies within the anterior hypothalamus and preoptic area (AHPOA) and also increased the optical density for staining of immunoreactive cell bodies in the AHPOA and olfactory peduncle. The ME of mice exposed to short photoperiod had a higher density of GnRH fibers relative to that of mice exposed to long photoperiod, and the content of GnRH fibers in the rostral ME was correlated with the optical content for immunostaining of cell bodies in the AHPOA. These results are evidence that gonadal regression induced by short photoperiod (mediated by the pineal gland) involves alterations of GnRH neuronal activity. Notably, data from this study are consistent with the hypothesis that suppressed release of GnRH from neurovascular terminals in the ME, rather than lack of availability of the decapeptide, promotes gonadal regression.     

Suppression of kisspeptin expression and gonadotropic axis sensitivity following exposure to inhibitory day lengths in female Siberian hamsters

To avoid breeding during unsuitable environmental or physiological circumstances, the reproductive axis adjusts its output in response to fluctuating internal and external conditions. The ability of the reproductive system to alter its activity appropriately in response to these cues has been well established. However, the means by which reproductively relevant cues are interpreted, integrated and relayed to the reproductive axis remain less well specified. The neuropeptide kisspeptin has been shown to be a potent positive stimulator of the hypothalamo-pituitary-gonadal (HPG) axis, suggesting a possible neural locus for the interpretation/integration of these cues. Because a failure to inhibit reproduction during winter would be maladaptive for short-lived female rodents, female Siberian hamsters (Phodopus sungorus) housed in long and short days were examined. In long "summer" photoperiods, kisspeptin is highly expressed in the anteroventral periventricular nucleus (AVPV), with low expression in the arcuate nucleus (Arc). A striking reversal in this pattern is observed in animals held in short, "winter" photoperiods, with negligible kisspeptin expression in the AVPV and marked staining in the Arc. Although all studies to date suggest that both populations act to stimulate the reproductive axis, these contrasting expression patterns of AVPV and Arc kisspeptin point to disparate roles for these two cell populations. Additionally, we found that the stimulatory actions of exogenous kisspeptin are blocked by acyline, a gonadotropin-releasing hormone (GnRH) receptor antagonist, suggesting an action of kisspeptin on the GnRH system rather than pituitary gonadotropes. Finally, females held in short day lengths exhibit a reduced response to exogenous kisspeptin treatment relative to long-day animals. Together, these findings indicate a role for kisspeptin in the AVPV and Arc as an upstream integration center for reproductively relevant stimuli and point to a dual mechanism of reproductive inhibition in which kisspeptin expression is reduced concomitant with reduced sensitivity of the HPG axis to this peptide.     

Inhibition of reproductive maturation and somatic growth of Fischer 344 rats by photoperiods shorter than L14:D10 and by gradually decreasing photoperiod

Photoperiod is the major regulator of reproduction in temperate-zone mammals. Laboratory rats are generally considered to be nonphotoresponsive, but young male Fischer 344 (F344) rats have a uniquely robust response to short photoperiods of 8 h of light. Rats transferred at weaning from a photoperiod of 16 h to photoperiods of < 14 h of light slowed in both reproductive development and somatic growth rate. Those in photoperiods < 13 h of light underwent the strongest responses. The critical photoperiod of F344 rats can be defined as 13.5 h of light, but photoperiods of 相似文献   

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.     

Neurons of the lateral preoptic area/rostral anterior hypothalamic area are required for photoperiodic inhibition of estrous cyclicity in sheep

Photoperiod determines the timing of reproductive activity in many species, yet the neural pathways whereby day length is transduced to a signal influencing gonadotropin-releasing hormone (GnRH) release are not fully understood. Physical lesions of the lateral preoptic area (lPOA)/rostral anterior hypothalamic area (rAHA) in female sheep extend the period of estrous cyclicity during inhibitory photoperiods. In the present study we sought to determine whether destroying only neurons and not fibers of passage in this area would lead to similar resistance to photosuppression. Additionally, neural tract-tracing was used to map connectivity between the lPOA/rAHA and other hypothalamic areas implicated in photoperiodic regulation of reproduction. Progesterone secretion was monitored in six sheep to determine estrous cycles for 90 days during a short-day (permissive) photoperiod. Three sheep then received bilateral injections of the excitotoxic glutamate analog, n-methyl-aspartic acid, directed toward the lPOA/rAHA, whereas three others served as controls. All were then exposed to a long-day (suppressive) photoperiod for 120 days. Control sheep ceased cycling at 40 ± 10 days (mean ± SEM), whereas lesioned sheep continued cycling through the end of the study. The results of the tract-tracing study revealed both afferent and efferent projections to the medial POA, retrochiasmatic area, arcuate nucleus, and premammillary region. Furthermore, close proximal associations with GnRH neurons from efferent projections were observed. We conclude that neurons located within the lPOA/rAHA are important for timing cessation of estrous cycles during photosuppression and that this area communicates directly with GnRH neurons and other hypothalamic areas involved in the photoperiodic regulation of reproduction.     

Tau differences between short-day responsive and short-day nonresponsive white-footed mice (Peromyscus leucopus) do not affect reproductive photoresponsiveness

In laboratory-bred rodent populations, intraspecific variation in circadian system organization is a known cause of individual variation in reproductive photoresponsiveness. The authors sought to determine whether circadian system variation accounted for individual variation in reproductive photoresponsiveness in a single, highly genetically variable population of Peromyscus leucopus recently derived from the wild. Running-wheel activity patterns of male and female mice, aged 70 to 90 days, from artificially selected lines of reproductively photoresponsive (R) and nonresponsive (NR) lines were monitored under short-day photoperiod (8 h light, 16 h dark), long-day photoperiod (16 h light, 8 h dark), and constant darkness (DD). NR mice displayed a significantly longer mean free-running period (24.08 h) in DD compared with R mice (23.75 h), due in large part to a difference between NR and R females (24.25 h vs. 23.74 h, respectively). All other entrainment characteristics (alpha, phase angle of activity) under short days, long days, and DD were similar between R and NR mice. Variation in free-running period and entrainment characteristics has been shown to affect photoresponsiveness in other rodent species by altering the manner in which the circadian system interprets short days. To determine whether variation in photoresponsiveness in P. leucopus is due to differences in free-running period instead of variation downstream from the central circadian clock in the pathway controlling photoresponsiveness, the authors exposed young R and NR mice to DD and measured the effect on reproductive organ development. If variation in free-running period affected how the circadian system of mice interpreted short days, then both R and NR mice exposed to DD should have exhibited a delay in gonadal development. Only R mice exhibited pubertal delay in DD. NR mice exhibited large paired testes, paired seminal vesicles, paired ovaries, and uterine weight typical of mice nonresponsive to short days, whereas R mice exhibited reproductive organ weight typical of mice responsive to short days. These data suggest that despite significant differences in free-running period between R and NR mice, individual variation in photoresponsiveness is not due to differences in how the circadian systems of R and NR mice interpret the LD cycle.     

Sexual dimorphism of gonadotropin-releasing hormone type-III (GnRH3) neurons and hormonal sex reversal of male reproductive behavior in Mozambique tilapia

In tilapia, hormone treatment during the period of sexual differentiation can alter the phenotype of the gonads, indicating that endocrine factors can cause gonadal sex reversal. However, the endocrine mechanism underlying sex reversal of reproductive behaviors remains unsolved. In the present study, we detected sexual dimorphism of gonadotropin-releasing hormone type III (GnRH3) neurons in Mozambique tilapia Oreochromis mossambicus. Our immunohistochemical observations showed sex differences in the number of GnRH3 immunoreactive neurons in mature tilapia; males had a greater number of GnRH3 neurons in the terminal ganglion than females. Treatment with androgen (11-ketotestosterone (11-KT) or methyltestosterone), but not that with 17β-estradiol, increased the number of GnRH3 neurons in females to a level similar to that in males. Furthermore, male-specific nest-building behavior was induced in 70% of females treated with 11-KT within two weeks after the onset of the treatment. These results indicate androgen-dependent regulation of GnRH3 neurons and nest-building behavior, suggesting that GnRH3 is importantly involved in sex reversal of male-specific reproductive behavior.     

Photoperiodic polyphenisms in rodents: neuroendocrine mechanisms, costs, and functions

Annual changes in daylength figure prominently in the generation of seasonal rhythms in reproduction, and a wide variety of mammals use ambient photoperiod as a proximate cue to time critical reproductive events. Nevertheless, within many reproductively photoperiodic mammalian species, there exist individuals--termed "photoperiod nonresponders"--that fail to adopt a seasonal breeding strategy and instead exhibit reproductive competence at a time of year when their conspecifics are reproductively quiescent. Photoperiod nonresponsiveness has been principally characterized by laboratory observations--over half of the species known to be reproductively photoperiodic contain a proportion of nonresponsive individuals. The study of nonresponders has generated basic insights regarding photic regulation of reproduction in mammals. The neuroendocrine mechanisms by which the short-day photoperiodic signal is degraded or lost in nonresponders varies between species: differences in features of the circadian pacemaker, which provides photoperiodic input to the reproductive neuroendocrine system, have been identified in hamsters; changes in the responsiveness of hypothalamic gonadotrophs to melatonin and as-yet-unspecified inhibitory signals have been implicated in voles and mice. Individuals that continue to breed when their conspecifics refrain might enjoy higher fitness under certain circumstances. Statements regarding the adaptive function of reproductive nonresponsiveness to photoperiod require additional information on the costs (metabolic and fitness) of sustaining reproductive function during the winter months and how these costs vary as a function of environmental conditions. Reproductive nonresponders thus continue to represent a challenge to theories that extol the adaptive function of seasonality. Several nonexclusive hypotheses are proposed to account for the maintenance of nonresponsive individuals in wild rodent populations.     

Adaptive strategies of overwintering adults: Reproductive diapause and mating behavior in a grasshopper,Stenocatantops splendens (Orthoptera: Catantopidae)

Abstract To understand the adaptive strategies of the overwintering adults of Stenocatantops splendens, the mechanism of maintenance and termination of the reproductive diapause, the variation in mortality between overwintering females and males, and the mating strategy of the males were investigated. The results indicated that the adult reproductive diapause in natural conditions was mainly regulated by photoperiod in the fall – long photoperiods promoted reproductive development and short photoperiods maintained reproductive diapause, and the sensitivity of the overwintering adults to photoperiod was over before the end of the winter. When transferred from natural conditions to controlled laboratory conditions on dates from September through February, pre‐oviposition became increasingly shorter with increasingly deferred transfer dates regardless of photoperiod conditions. The adults treated with low temperature for 30 days in September through November had significantly shorter pre‐oviposition, suggesting that low temperatures in winter had an important role in the termination of reproductive diapause. The female had a significantly lower supercooling point than the male, which was related to their lower mortality after winter. In addition, observations of wild populations of the species indicated that mating behavior prior to winter and the duration of pre‐mating period were not affected by photoperiod; mating and sperm transfer were mostly completed by November. Compared with females only mating before winter, females mating in the spring had shorter life span, longer pre‐oviposition, lower hatching rate and laid fewer egg pods while showing no significant difference with regard to ovipositional interval, per pod number of eggs and nymph dry weight.     

The olfactory organ modulates gonadotropin-releasing hormone types and nest-building behavior in the tilapia Oreochromis niloticus

Direct olfactory inputs to any of the known gonadotropin-releasing hormone (GnRH) containing neurons have not been demonstrated. Therefore, the rationale of this study was to examine whether olfactory inputs might in some way interact with the GnRH system(s) to synchronize reproductive behaviors. In order to establish this, we used anosmic mature male tilapia to investigate changes in reproductive behaviors, gonadal morphology, and GnRH1, GnRH2, and GnRH3 cellular morphology and change in GnRH mRNA levels by real-time polymerase chain reaction. Bilateral removal of the olfactory rosettes followed by occlusion of the nasal cavity (ORX) inhibited nest-building behavior, but had no effect on aggressive and sexual behaviors or gonadal morphology. ORX failed to alter the morphological features of GnRH1, GnRH2, and GnRH3 (cell number, size, GnRH optical density), but significantly decreased copies of GnRH1 and GnRH2 mRNAs. GnRH immunoreactive fibers were not evident in the olfactory nerve and rosettes. DiI application to the olfactory nerve labeled inputs primarily to the glomerular layer of the olfactory bulbs and extrabulbar inputs to the forebrain but not to GnRH neurons. These results provide evidence that the olfactory rosette is crucial for modulating nest-building behavior through second-order olfactory pathways interacting with GnRH1 and GnRH2 neuronal systems.     

Photoperiodic control of seasonality in birds

This review examines how birds use the annual cycle in photoperiod to ensure that seasonal events--breeding, molt, and song production--happen at the appropriate time of year. Differences in breeding strategies between birds and mammals reflect basic differences in biology. Avian breeding seasons tend to be of shorter duration and more asymmetric with respect to changes in photoperiod. Breeding seasons can occur at the same time each year (predictable) or at different times (opportunistic), depending on the food resource. In all cases, there is evidence for involvement of photoperiodic control, nonphotoperiodic control, and endogenous circannual rhythmicity. In predictable breeders (most nontropical species), photoperiod is the predominant proximate factor. Increasing photoperiods of spring stimulate secretion of gonadotropin-releasing hormone (GnRH) and consequent gonadal maturation. However, breeding ends before the return of short photoperiods. This is the consequence of a second effect of long photoperiods--the induction of photorefractoriness. This dual role of long photoperiods is required to impart the asymmetry in breeding seasons. Typically, gonadal regression through photorefractoriness is associated with a massive decrease in hypothalamic GnRH, essentially a reversal to a pre-pubertal condition. Although breeding seasons are primarily determined by photoperiodic control of GnRH neurons, prolactin may be important in determining the exact timing of gonadal regression. In tropical and opportunistic breeders, endogenous circannual rhythmicity may be more important. In such species, the reproductive system remains in a state of "readiness to breed" for a large part of the year, with nonphotic cues acting as proximate cues to time breeding. Circannual rhythmicity may result from a temporal sequence of different physiological states rather than a molecular or cellular mechanism as in circadian rhythmicity. Avian homologues of mammalian clock genes Per2, Per3, Clock, bmal1, and MOP4 have been cloned. At the molecular level, avian circadian clocks appear to function in a similar manner to those of mammals. Photoperiodic time measurement involves interaction between a circadian rhythm of photoinducibility and, unlike mammals, deep brain photoreceptors. The exact location of these remains unclear. Although the eyes and pineal generate a daily cycle in melatonin, this photoperiodic signal is not used to time seasonal breeding. Instead, photoperiodic responses appear to involve direct interaction between photoreceptors and GnRH neurons. Thyroid hormones are required in some way for this system to function. In addition to gonadal function, song production is also affected by photoperiod. Several of the nuclei involved in the song system show seasonal changes in volume, greater in spring than in the fall. The increase in volume is, in part, due to an increase in cell number as a result of neurogenesis. There is no seasonal change in the birth of neurons but rather in their survival. Testosterone and melatonin appear to work antagonistically in regulating volume.     

Variability of GnRH secretion in two goby species with socially controlled alternative male mating tactics

Male reproductive phenotypic plasticity related to environmental-social conditions is common among teleost fish. In several species, males adopt different mating tactics depending on their size, monopolizing mates when larger, while parasitizing dominant male spawns when smaller. Males performing alternative mating tactics are often characterized by a strong dimorphism in both primary and secondary reproductive traits. According to studies on sex-changing species and on species where only one male morph is reproductively active, male alternative phenotypes are expected to vary also in gonadotropin-releasing hormone (GnRH) neurons in forebrain preoptic area (POA). Here, we compared the intra- and inter-sexual variations in number and size of GnRH neurons, along with gonads and male accessory structure investment, in two goby species, the grass goby, Zosterisessor ophiocephalus, and the black goby, Gobius niger, characterized by male alternative mating phenotypes. In both species, older and larger males defend nests, court and perform parental care, while younger and smaller ones try to sneak territorial male spawning. We found that grass goby and black goby have different patterns of GnRH expression. Grass goby presents a clear intra-sexual dimorphism in GnRH expression, related to the occurrence of alternative mating tactics, while in the black goby, only inter-sexual differences are observed. The inter- and intra-specific variability in the GnRH neurons in these two goby species is discussed in light of the differences in migratory behavior, nest type, and mating system.     

Top-down approaches to the study of natural variation in complex physiological pathways using the white-footed mouse (Peromyscus leucopus) as a model

Variation in complex physiological pathways has important effects on human function and medical treatment. Complex pathways involve cells at multiple locations, which serve different functions regulated by many genes and include complex neuroendocrine pathways that regulate physiological function. One of two competing hypotheses regarding the effects of selection on complex pathways predicts that variability should be common within complex pathways. If this hypothesis is correct, then we should expect wide variation in neuroendocrine function to be typical within natural populations. To test this hypothesis, a complex neuroendocrine pathway that regulates photoperiod-dependent changes in fertility in a natural population of white-footed mice (Peromyscus leucopus) was used to test for natural genetic variability in multiple components of the pathway. After testing only six elements in the photoperiod pathway in P. leucopus, genetic variation in the following four of these elements was evident: the circadian clock, melatonin receptor abundance or affinity, sensitivity of the reproductive axis to steroid negative feedback, and gonadotropin-releasing hormone neuronal activity. If this result can be extended to humans, the prediction would be that significant variation at multiple loci in complex neuroendocrine pathways is common among humans, and that variation would exist even in human populations from a common genetic background. This finding could only be drawn from an "exotic" animal model derived from a natural source population, confirming the continuing importance of nontraditional models alongside the standard laboratory species.     

Reproductive status and torpor of the marsupial Sminthopsis crassicaudata: Effect of photoperiod

1. 1. We investigated the effects of photoperiod on the reproductive state and the occurrence and pattern of torpor in male Sminthopsis crassicaudata.

2. 2. Testes regressed when animals were exposed to a short photoperiod (L:D 8:16) and recrudesced under a long photoperiod (L:D 16:8).

3. 3. Animals entered torpor under both photoperiods and no significant differences were observed in the frequency or physiological variables of torpor of S. crassicaudata between the short and long photoperiods.

4. 4. The differences in the response to photoperiod in thermal physiology and reproduction suggest that, unlike in many rodent species, torpor and reproduction in S. crassicaudata are controlled by separate environmental cues and mechanisms.

     

The effects of running activity on the reproductive axes of rodents

Access to a running wheel causes gonadal recrudescence in Syrian hamsters whose reproductive axes have been suppressed by housing them under short day lengths (Borer et al. 1983). The first experiment tested the generality of this phenomenon in a population of rodents that is genetically heterogeneous for reproductive photoresponsiveness. Male meadow voles (Microtus pennsylvanicus) of the two extreme phenotypes — reproductively photoresponsive and non-responsive — were either provided with a running wheel or housed without one. After 4 weeks with a wheel, the responsive voles had recovered full reproductive function, while the reproductive axes of responsive voles housed without wheels remained suppressed. Three experiments queried whether the use of a wheel would have reproductively stimulative effects in other rodents. First, intact male mice given access to wheels showed no increase in testis size when compared to mice housed without wheels. Likewise, locomotor activity had no effect on male rats whose testes were partially regressed in response to testosterone implants or on female mice whose estrous cycles were pheromonally suppressed by housing them in groups. Thus the neuroendocrine pathway used by locomotor activity to enhance the secretion of gonadotropin is specifically allied with the pathway used by photoperiod to control GnRH secretion.Abbreviations GnRH gonadotropin-releasing hormone - LH luteinizing hormone     

Effects of age and photoperiod on reproduction and the spleen in the marsh rice rat (Oryzomys palustris)

To examine the interactions between age and photoperiod on reproduction and spleen weights, we exposed adult male and female rice rats of various ages to photoperiods of 16:8-h light-dark photoperiods (16L:8D) or 12L:12D. After 10 wk, animals were killed and the following data were recorded: weights of testes, seminal vesicles, uterus, ovaries, body, and spleen and, in addition, vaginal patency. Young adult males displayed a greater degree of testicular and seminal vesicle regression in short photoperiods than did older males; the testes of most older males did not regress in response to short photoperiods. Spleen weight was unresponsive to short photoperiods in all males, but was affected by age. Females, however, exhibited reproductive organ regression and decreased vaginal patency in response to short photoperiods at all ages examined. Body weights were affected by photoperiod in young females, and, as in males, photoperiod had no effect on spleen weights. These data suggest that the reproductive response to photoperiod in adult male rice rats declines with age, whereas in adult females it does not.