Season- and age-related variation in neural cGnRH-I and cGnRH-II immunoreactivity in house sparrows (Passer domesticus)

Green light inhibits GnRH-I expression by stimulating the melatonin-GnIH pathway in the chick brain

To study the mechanism by which monochromatic light affects gonadotrophin-releasing hormone (GnRH) expression in chicken hypothalamus, a total of 192 newly-hatched chicks were divided into intact, sham-operated and pinealectomy groups and exposed to white (WL), red (RL), green (GL) and blue (BL) lights using a light-emitting diode system for 2 weeks. In the GL intact group, the mRNA and protein levels of GnRH-I in the hypothalamus, the mean cell area and mean cell optical density (OD) of GnRH-I-immunoreactive (-ir) cells of the nucleus commissurae pallii were decreased by 13.2%-34.5%, 5.7%-39.1% and 9.9%-17.3% compared to those in the chicks exposed to the WL, RL and BL, respectively. GL decreased these factors related to GnRH-I expression and the effect of GL was not observed in pinealectomised birds. However, the mRNA and protein levels of hypothalamic gonadotrophin-inhibitory hormone (GnIH) and GnIH receptor (GnIHR), the mean cell area and mean cell OD of the GnIH-ir cells of the paraventricularis magnocellularis, and the plasma melatonin concentration in the chicks exposed to GL were increased by 18.6%-49.2%, 21.1%-60.0% and 8.6%-30.6% compared to the WL, RL and BL intact groups, respectively. The plasma melatonin concentration showed a negative correlation with GnRH-I protein and a positive correlation with GnIH and GnIHR proteins. Protein expression of both GnRH-I and GnIHR showed a negative correlation in the hypothalamus. After pinealectomy, GnRH-I expression increased, whereas plasma melatonin concentration, GnIH and GnIHR expression decreased, and there were no significant differences among the WL, RL, GL and BL groups. Double-labelled immunofluorescence showed that GnIH axon terminals were near GnRH-I neurones, some GnRH-I neurones coexpressed with GnIHR and GnIH neurones coexpressed with melatonin receptor subtype quinone reductase 2. These results demonstrate that green light inhibits GnRH-I expression by increasing melatonin secretion and stimulating melatonin receptor-GnIH-GnIH receptor pathway in the chick brain.

Concurrent hypothalamic gene expression under acute and chronic long days: Implications for initiation and maintenance of photoperiodic response in migratory songbirds

Hypothalamic expression of the thyroid hormone (TH) responsive gonadostimulatory (eya3, cga, tshβ, dio2, dio3, gnrh, gnih) and neurosteroid pathway genes (androgen receptor [ar], aromatase [cyp19], estrogen receptor [er] α and β) was examined in photosensitive redheaded buntings exposed to 2 (acute, experiment 1) or 12 (chronic, experiment 2) long days (16L:8D). Experiment 2 also included a photorefractory group. Acute long days caused a significant increase in eya3, cga, tshβ, dio2 and gnrh and decrease in dio3 mRNA levels. eya3, cga and tshβ expressions were unchanged after the chronic long days. We also found increased cyp19, erα and erβ mRNA levels after acute, and increased cyp19 and decreased erβ levels after the chronic long-day exposure. Photorefractory buntings showed expression patterns similar to that in the photosensitive state, except for high gnrh and gnih and low dio3 mRNA levels. Consistent with gene expression patterns, there were changes in fat deposition, body mass, testis size, and plasma levels of testosterone, tri-iodothyronine and thyroxine. These results show concurrent photostimulation of the TH-signalling and neurosteroid pathways, and extend the idea, based on differences in gene expression, that transitions in seasonal photoperiodic states are accomplished at the transcriptional levels in absolute photorefractory species.

Food availability, energetic constraints and reproductive development in a wild seasonally breeding songbird

In many organisms, food availability is a proximate cue that synchronizes seasonal development of the reproductive system with optimal environmental conditions. Growth of the gonads and secondary sexual characteristics is orchestrated by the hypothalamic–pituitary–gonadal (HPG) axis. However, our understanding of the physiological mechanisms by which food availability modulates activity of the HPG axis is limited.

It is thought that many factors, including energetic status, modulate seasonal reproductive activation. We tested the hypothesis that food availability modulates the activity of the HPG axis in a songbird. Specifically, we food‐restricted captive adult male Abert's Towhees Melozone aberti for 2 or 4 weeks during photoinduced reproductive development. A third group (control) received ad libitum food throughout. We measured multiple aspects of the reproductive system including endocrine activity of all three levels of the HPG axis [i.e. hypothalamic gonadotropin‐releasing hormone‐I (GnRH‐I), plasma luteinizing hormone (LH) and testosterone (T)], and gonad morphology. Furthermore, because gonadotropin‐inhibitory hormone (GnIH) and neuropeptide Y (NPY; a potent orexigenic peptide) potentially integrate information on food availability into seasonal reproductive development, we also measured the brain levels of these peptides.

At the hypothalamic level, we detected no effect of food restriction on immunoreactive (ir) GnRH‐I, but the duration of food restriction was inversely related to the size of ir‐GnIH perikarya. Furthermore, the number of ir‐NPY cells was higher in food‐restricted than control birds. Food restriction did not influence photoinduced testicular growth, but decreased plasma LH and T, and width of the cloacal protuberance, an androgen‐sensitive secondary sexual characteristic. Returning birds to ad libitum food availability had no effect on plasma LH or T, but caused the cloacal protuberance to rapidly increase in size to that of ad libitum‐fed birds.

Our results support the tenet that food availability modulates photoinduced reproductive activation. However, they also suggest that this modulation is complex and depends upon the level of the HPG axis considered. At the hypothalamic level, our results are consistent with a role for the GnIH and NPY systems in integrating information on energetic status. There also appears to be a role for endocrine function at the anterior pituitary gland and testicular levels in modulating reproductive development in the light of energetic status and independently of testicular growth.

Seasonal plasticity in the peptide neuronal systems: potential roles of gonadotrophin-releasing hormone, gonadotrophin-inhibiting hormone, neuropeptide Y and vasoactive intestinal peptide in the regulation of the reproductive axis in subtropical Indian weaver birds

Two experiments examined the expression of gonadotrophin-releasing and inhibiting hormones (GnRH-I, GnRH-II and GnIH), neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) in subtropical Indian weaver birds, which demonstrate relative photorefractoriness. Experiment 1 measured peptide expression levels in the form of immunoreactive (-IR) cells, percentage cell area and cell optical density in the preoptic area (GnRH-I), midbrain (GnRH-II), paraventricular nucleus (GnIH), mediobasal hypothalamus [dorsomedial hypothalamus (DMH), infundibular complex (INc), NPY and VIP] and lateral septal organ (VIP) during the progressive, breeding, regressive and nonbreeding phases of the annual reproductive cycle. GnRH-I was decreased in the nonbreeding and VIP was increased in INc in the breeding and regressive states. GnRH-II and NPY levels did not differ between the testicular phases. Double-labelled immunohistochemistry (IHC) revealed a close association between the GnRH/GnIH, GnRH/NPY, GnRH/VIP and GnIH/NPY peptide systems, implicating them interacting and playing roles in the reproductive regulation in weaver birds. Experiment 2 further measured these peptide levels in the middle of day and night in weaver birds that were maintained under short days (8 : 16 h light /dark cycle; photosensitive), exposed to ten long days (16 : 8 h light /dark cycle; photostimulated) or maintained for approximately 2 years on a 16 : 8 h light /dark cycle (photorefractory). Reproductively immature testes in these groups precluded the possible effect of an enhanced gonadal feedback on the hypothalamic peptide expression. There were group differences in the GnRH-I (not GnRH-II), GnIH, NPY and VIP immunoreactivity, albeit with variations in immunoreactivity measures in the present study. These results, which are consistent with those reported in birds with relative photorefractoriness, show the distribution and possibly a complex interaction of key neuropeptides in the regulation of the annual reproductive cycle in Indian weaver birds.

Annual gonadal cycles in birds: modeling the effects of photoperiod on seasonal changes in GnRH-1 secretion

This paper reviews current knowledge of photoperiod control of GnRH-1 secretion and proposes a model in which two processes act together to regulate GnRH1 secretion. Photo-induction controls GnRH1 secretion and is directly related to prevailing photoperiod. Photo-inhibition, a longer term process, acts through GnRH1 synthesis. It progresses each day during daylight hours, but reverses during darkness. Thus, photo-inhibition gradually increases when photoperiods exceed 12h, and reverses under shorter photoperiods. GnRH1 secretion on any particular day is the net result of these two processes acting in tandem. The only difference between species is their sensitivity to photo-inhibition. This can potentially explain differences in timing and duration of breeding seasons between species, why some species become absolutely photorefractory and others relatively photorefractory, why breeding seasons end at the same time at different latitudes within species, and why experimental protocols sometimes produce results that appear counter to what happens naturally.

Gonadotropin releasing hormone and brooding behavior in the native Thai hen

Changes in the number of hypothalamic gonadotropin releasing hormone-I (GnRH-I) neurons within the Nucleus commissurae pallii (nCPa) were associated with the reproductive cycle of native Thai chickens. In order to further understand the association of GnRH-I in the regulation of brooding behavior in this bird, the native Thai chickens were divided into two groups; chick-rearing (R) and non-chick-rearing (NR) hens. Numbers of visible of GnRH-I-immunoreactive (GnRH-I-ir) neurons in the hypothalamus of R and NR hens were compared utilizing immunohistochemistry. Numbers of visible GnRH-I-ir neurons within the Nucleus anterior medialis hypothalami, Nucleus suprachaiasmaticus, pars medialis, Nucleus septalis lateralis, Nucleus paraventricularis magnocellularis, and Regio lateralis hypothalami areas were observed in both groups, but no differences were seen between R and NR hens. The number of visible GnRH-I neurons in the nCPa was higher (P<0.05) in the NR than in R hens, and increased in NR hens by day 14 after chick removal. These findings suggest, for the first time, an association of the GnRH system with brooding behavior in continuously breeding birds. Furthermore, the expression of brooding behavior of native Thai chickens might be regulated, in part, by GnRH-I neurons in the nCPa.

Distribution and variation in gonadotropin releasing hormone-I (GnRH-I) immunoreactive neurons in the brain of the native Thai chicken during the reproductive cycle

Gonadotropin releasing hormone-I (GnRH-I) is known to regulate the avian reproductive system. We investigated the roles of GnRH-I in the regulation of the reproductive system of the native Thai chicken. The distribution of GnRH-I neurons and changes in GnRH-I-immunoreactive (-ir) neurons throughout the reproductive stages and between incubating and nest-deprived hens were analyzed utilizing immunohistochemical techniques. The results revealed that GnRH-I-ir neurons were distributed in a discrete region lying close to the third ventricle from the level of preoptic area through the anterior hypothalamus, with the greatest abundance found within the nucleus commissurae pallii (nCPa). The number of GnRH-I-ir neurons in the nCPa was highest in laying hens when compared with that in the other reproductive stages. Nest deprivation caused an increase in the number of GnRH-I-ir neurons in the nCPa of nest-deprived hens when compared with incubating hens. These results indicate that GnRH-I expression is correlated with the reproductive state in the native Thai chicken and may be, in part, regulated by it. This study also confirms a pivotal role of GnRH-I in controlling avian reproduction of this non-seasonal breeding, equatorial species.

Gonadotropin-releasing hormone plasticity: a comparative perspective

Gonadotropin-releasing hormone 1 (GnRH1) is a key regulator of the reproductive neuroendocrine system in vertebrates. Recent developments have suggested that GnRH1 neurons exhibit far greater plasticity at the cellular and molecular levels than previously thought. Furthermore, there is growing evidence that sub-populations of GnRH1 neurons in the preoptic area are highly responsive to specific environmental and hormonal conditions. In this paper we discuss findings that reveal large variation in GnRH1 mRNA and protein expression that are regulated by social cues, photoperiod, and hormonal feedback. We draw upon studies using histochemistry and immediate early genes (e.g., c-FOS/ZENK) to illustrate that specific groups of GnRH1 neurons are topographically organized. Based on data from diverse vertebrate species, we suggest that GnRH1 expression within individuals is temporally dynamic and this plasticity may be evolutionarily conserved. We suggest that the plasticity observed in other neuropeptide systems (i.e. kisspeptin) may have evolved in a similar manner.

Experimental tests of endocrine function in breeding and nonbreeding raptors

Many long-lived avian species defer reproduction for several years, often displaying a "floating" behavior characterized by the lack of mates and exclusive territories. Understanding the proximate mechanisms regulating floating behavior is a relevant topic of research for physiologists, behavioral ecologists, and population biologists because a prolonged period of nonbreeding can negatively affect lifetime fitness and change population dynamics. Here we tested two hypotheses linking endocrine function to floating status: (a) floaters undergo a period of sexual immaturity characterized by lower gonadal function (hypothesis of sexual immaturity), and (b) floating status is socially imposed by dominant conspecifics and revealed by the adrenocortical response to stress (hypothesis of social subordination). The two hypotheses were tested in a population of free-living black kites Milvus migrans in Doñana National Park (southwest Spain), where breeders coexist with young floaters that defer reproduction for 3-7 yr. Hypophysial-gonadal function, estimated as androgen production in response to experimental challenge with gonadotropin-releasing hormone (c-GnRH-I), was similar in magnitude and timing between floating and breeding males. The same treatment was, however, unable to elicit any response in terms of increasing estradiol or total androgen levels in females regardless of their breeding status. Following experimental capture and restraint, the adrenocortical response to stress (estimated as circulating corticosterone levels) was higher in floating than in breeding males, while females showed the opposite pattern (i.e., lower response to stress in young floaters compared with breeders). Contrary to the hypothesis of sexual immaturity, our results suggest that floating males are physiologically capable of reproducing. The reported differences in adrenocortical function support the idea that floaters are socially subordinate to breeders, and corticosterone responses reflect the sex-specific roles during competition in socially monogamous species.

Newborn GnRH neurons in the adult forebrain of the ring dove

The preoptic area of the hypothalamus is a key area that produces gonadotrophin-releasing hormone (GnRH). In birds, the chicken GnRH-I-form neurons are responsible for the hypothalamus-pituitary-gonadal system, which controls reproduction. In the ring dove, electrolytic lesion in the adult hypothalamus induces neurogenesis. In this study, we determined whether adult neurogenesis is involved in repairing GnRH neurons, specifically by generating newborn cells exhibiting GnRH-I immunoreactive properties. We selectively applied electrolytic lesions to three different regions of the diencephalon, including the preoptic area, which contains GnRH-I neurons, and identified new cells (BrdU-positive cells) that co-labeled with GnRH-I-immunoreactive cells. The BrdU(+)/GnRH(+) double labeled cells were then confirmed with confocal laser analysis. In brains of both male and female ring doves we found new neurons at the lesion site of the preoptic region that were GnRH-I immunoreactive. However, the total number of GnRH neurons in the lesioned brains was less than that of sham-lesioned brains. When two other regions of the diencephalon that contain GnRH-I neurons were damaged, no recruitment of new GnRH-I neurons was detected. The rate of neurogenesis depends on the bird's reproductive phase when the lesion was applied. We found BrdU(+)/GnRH(+) double-labeled cells almost exclusively during the pre-laying phase when birds are engaged in active courtship that leads to egg laying. Our observations suggest that recruitment of GnRH immunoreactive new neurons is restricted to the hypothalamic region and is sensitive to the reproductive stage of the birds.

Seasonal changes in hypothalamic gonadotropin-releasing hormone-I immunoreactivity in relation with testicular volume in adult male free-living European starlings (Sturnus vulgaris)

Birds from the temperate and cold zones show annual sexual activity accompanied by gonadal changes and fluctuation in their brain gonadotropin-releasing hormone (GnRH) levels. However, most of the studies were done on captive birds where the constant environment can profoundly modify periodical changes. Therefore our aim was to reveal annual variations of hypothalamic and gonadal changes in male, free-living European starlings (Sturnus vulgaris) captured directly from their natural environment. We analyzed hypothalamic GnRH-I immunoreactivity and testes volume. Four key time points of the active reproductive cycle and the photorefractory phase were studied. GnRH-I immunoreactivity was analyzed in the preoptic area (POA) and the median eminence (ME). Photorefractory birds (August) with regressed gonads had the lowest level of GnRH-I immunoreactivity compared to other birds from the active reproductive phases. These results suggest that parallel with the gonadal volume GnRH-I undergoes seasonal changes in adult male free-living European starlings.

Gonadotrophin-releasing hormone neurons in a photoperiodic songbird express fos and egr-1 protein after a single long day

Birds use a variety of environmental cues, such as day length, temperature and social interactions, to time reproductive efforts. For most seasonally breeding birds, day length is the most important cue and takes precedence over all others. Experimental manipulation of day length has shown that, in a number of galliformes and passeriformes, exposure to a single long day induces a rise in plasma luteinising hormone (LH). The mechanisms underlying this response are only beginning to be understood. In Japanese quail and Zonotrichia sparrows, one long day causes striking up-regulation of the protein products of immediate early genes (IEGs) in the mediobasal hypothalamus, near gonadotrophin-releasing hormone (GnRH) axons and terminals. Photoperiodic induction of the same proteins in the GnRH somata themselves, however, has not been described in these species. In the present study, we used immunohistochemistry to assay the induction of two IEGs, Fos and Egr-1, in the GnRH somata of male and female white-throated sparrows (Zonotrichia albicollis) exposed to a single long day. We found that immunoreactivity for both proteins increased in a subset of the GnRH neurones of the septo-preoptic area by the morning after the long day. Photo-induced expression of Egr-1 or Fos protein in GnRH neurones was limited to a population of cells in the medial preoptic area. Males showed significantly greater induction of both proteins in this population of GnRH neurones than did females, which is consistent with the hypothesis that males may be more sensitive to photic cues. Overall, the results obtained suggest that photostimulation stimulates new protein synthesis in GnRH neurones on a relatively rapid time scale. Further research is required to determine whether the GnRH somata are themselves integrating photic cues, or whether they are responding rapidly to an increased demand for GnRH synthesis.

Anatomical localization of the effects of reproductive state, castration, and social milieu on cells immunoreactive for gonadotropin-releasing hormone-I in male European starlings (Sturnus vulgaris)

Gonadotropin-releasing hormone-I (GnRH-I) cells are localized primarily to the septopreoptic area (POA) and are responsible for regulating gonadotropin release from the anterior pituitary. Some songbird species exhibit dramatic seasonal variation in the number of detectable GnRH-I immunoreactive cells, with higher numbers being observed during the breeding season. Here we investigated the anatomical distribution of GnRH-I-immunoreactive cells in male starlings that varied in response to manipulations of reproductive state, social context, and gonadal condition. We housed photostimulated, intact and castrated male starlings with a female or alone. Additionally, a fifth treatment group consisted of photorefractory males (i.e., in a nonreproductive state) housed alone. All photostimulated males had significantly greater numbers of GnRH-I cells compared with photorefractory male starlings. There was a significant main effect of castration and social context. Castrated males had significantly greater numbers of GnRH-I cells compared with intact males, and males housed in male-female dyads also had greater numbers of GnRH-I cells. Furthermore, the significant main effects of castration and social context were the result of an increase in GnRH-I cell numbers specifically in the rostral and intermediate regions of the POA. These findings indicate that social context and hormonal milieu have profound effects on GnRH-I immunoreactivity in addition to the previously described effects of reproductive state. These data provide novel insight into the environmental regulation of the hypothalamopituitary axis and suggest that gonadal hormones and female presence independently regulate GnRH-I cells in specific regions of the POA in male starlings.

Photoperiodic condition is associated with region-specific expression of GNRH1 mRNA in the preoptic area of the male starling (Sturnus vulgaris)

Many seasonally breeding avian species exhibit marked changes in hypothalamic content of gonadotropin-releasing vhormone 1 (GNRH1) protein that are reflective of breeding condition. We recently cloned the GNRH1 cDNA in European starlings and demonstrated that changes in GNRH1 mRNA levels occur with a time course similar to what has been observed with GNRH1 protein. However, we did not previously resolve whether these differences were attributable to changes in the number of cells expressing the gene. Herein, we investigated photoperiod-induced changes in the number and distribution of GNRH1 mRNA-expressing cells in the preoptic area of male starlings. GNRH1 mRNA-expressing cell number was significantly greater in breeding birds than in nonbreeding birds. Starlings maintained in short nonstimulatory day length (i.e., prebreeding) showed intermediate cell numbers. Detailed analysis of the rostrocaudal and mediolateral distribution revealed that breeding birds had greater numbers of cells expressing GNRH1 mRNA in the medial intermediate, mediocaudal, and lateral intermediate preoptic area compared with prebreeding and nonbreeding birds. These data demonstrate that photoperiodic changes in reproductive state in starlings are associated with region-specific alterations in the number of cells expressing the GNRH1 gene. It remains to be determined whether these changes reflect quantitative differences in gene expression among an otherwise stable population of cells or a phenotypic switch in which cells gain or lose the ability to make GNRH1 mRNA in response to environmental cues.

Cloning of gonadotropin-releasing hormone I complementary DNAs in songbirds facilitates dissection of mechanisms mediating seasonal changes in reproduction

Temperate zone animals exhibit seasonal variation in reproductive physiology. In most cases, seasonal changes in reproductive states are regulated by changes in GnRH1 secretion, rather than synthesis, from the preoptic area (POA)/anterior hypothalamus. An important exception occurs in some songbirds that become photorefractory to the stimulatory effects of long days and show profound decreases in brain GnRH1 protein content. Whether this decline reflects changes in gene expression is unknown because of past failures to measure GNRH1 mRNA levels, due in large part to the absence of available GNRH1 gene sequence in this taxon. Here, we report the first cloning of GNRH1 cDNAs in two songbirds: European starlings and zebra finches. Consistent with the size of the prepro-hormone in other avian and non-avian species, the open-reading frames predict proteins of 91 and 92 amino acids, respectively. Whereas the decapeptide in both species is perfectly conserved with chicken GnRH1, the amino acid identity in the signal peptide and GNRH associated peptide subdomains are significantly less well conserved. At the nucleotide level, the starling and zebra finch coding sequences are approximately 88% identical to each other but only approximately 70% identical to chicken GNRH1. In situ hybridization using radiolabeled cRNA probes demonstrated GNRH1 mRNA expression primarily in the POA, consistent with previous studies on the distribution of the GnRH1-immunoreactive cell bodies. Furthermore, we provide evidence for photoperiod-dependent regulation of GNRH1 mRNA in male starlings. Declines in GNRH1 mRNA levels occur in parallel with testicular involution. Thus, photorefractoriness is associated with decreases in GNRH1 gene expression in the medial POA.

Control of the annual cycle in birds: endocrine constraints and plasticity in response to ecological variability

This paper reviews information from ecological and physiological studies to assess how extrinsic factors can modulate intrinsic physiological processes. The annual cycle of birds is made up of a sequence of life-history stages: breeding, moult and migration. Each stage has evolved to occur at the optimum time and to last for the whole duration of time available. Some species have predictable breeding seasons, others are more flexible and some breed opportunistically in response to unpredictable food availability. Photoperiod is the principal environmental cue used to time each stage, allowing birds to adapt their physiology in advance of predictable environmental changes. Physiological (neuroendocrine and endocrine) plasticity allows non-photoperiodic cues to modulate timing to enable individuals to cope with, and benefit from, short-term environmental variability. Although the timing and duration of the period of full gonadal maturation is principally controlled by photoperiod, non-photoperiodic cues, such as temperature, rainfall or food availability, could potentially modulate the exact time of breeding either by fine-tuning the time of egg-laying within the period of full gonadal maturity or, more fundamentally, by modulating gonadal maturation and/or regression. The timing of gonadal regression affects the time of the start of moult, which in turn may affect the duration of the moult. There are many areas of uncertainty. Future integrated studies are required to assess the scope for flexibility in life-history strategies as this will have a critical bearing on whether birds can adapt sufficiently rapidly to anthropogenic environmental changes, in particular climate change.

Gonadotropin-releasing hormone II: a multi-purpose neuropeptide

Close to 30 forms of gonadotropin releasing hormone (GnRH) and at least five GnRH receptors have been identified in a wide variety of vertebrates and some invertebrates. One form, now called GnRH II, has the broadest distribution and the most ancient and conserved phylogeny. The distribution of the neurons that produce this peptide are completely nonoverlapping with any other GnRH forms. Fibers that project from these neurons overlap with GnRH I cells and/or fibers in a few regions, but are primarily divergent. The musk shrew (Suncus murinus) continues to be the most tractable mammalian species to use for studies of the function of GnRH II. The brain of the musk shrew has two GnRH genes (I and II), two GnRH receptors (types-1 and -2), and two different behaviors can be influenced by central infusion of GnRH II, but not by GnRH I; receptivity and feeding. Here, we summarize research on the musk shrew relative to the behavioral functions of GnRH II. First, female musk shrews are continually sexually receptive by virtue of their lack of an ovarian and/or behavioral estrus cycle. This feature of their reproductive ecology may be related to their semi-tropical distribution and their breeding season is highly dependent on changes in the availability of food. When food is not abundant, females stop mating, but brief bouts of feeding reinstate reproductive behavior. Likewise, intake of food is related to GnRH II mRNA and peptide content in the brain; after mild food restriction both decline. When GnRH II is infused centrally, at times when its content is low, it can both enhance receptivity and inhibit food intake. Simultaneous administration of a type-1 antagonist does not change the effect of GnRH II and use of an analog (135-18) that is a specific GnRH II agonist as well as a type-1 antagonist has the same effect as the endogenous GnRH II peptide. We propose that GnRH II plays a critical role by orchestrating the coordination of reproduction with the availability of nutritional support for these activities. Humans are bombarded with copious nutritional opportunities and at present obesity is a larger threat to health in many parts of the world than is under nutrition. It is our hope that understanding neuropeptides such as GnRH II that regulate food intake can ultimately lead to products that may curb appetite and thus decrease obesity and related risks to health.

Effects of social cues on GnRH-I, GnRH-II, and reproductive physiology in female house sparrows (Passer domesticus)

In all vertebrates, at least two forms of gonadotropin-releasing hormone (GnRH) are present: GnRH-I and GnRH-II. GnRH-I directly influences the reproductive axis whereas the function of GnRH-II is less clear. The present experimental objectives were to determine the effect(s) of male social cues on the peripheral and neural responses of female house sparrows (Passer domesticus). We hypothesized that male breeding status would significantly influence the amount of immunoreactive GnRH-II in female house sparrow brains. In order to test this hypothesis, females were caged with a breeding male, a non-breeding male, or caged alone. The presence of breeding males did not significantly influence ovary development, luteinizing hormone, or estradiol levels, but male presence increased female body mass, and male presence and condition interacted to influence ovarian follicle size. Using immunocytochemistry, GnRH-I and GnRH-II immunoreactivity was measured in order to evaluate the neuroendocrine response to breeding status in males. When females were housed with breeding males, there were stable numbers of immunoreactive GnRH-I and -II cells but significantly lower amounts of immunoreactive GnRH-I fibre staining within the preoptic area compared to females housed with non-breeding males. Moreover, immunoreactive GnRH-II fibres in the preoptic area, ventromedial nucleus, and medial septum were significantly greater in females housed alone in chamber with non-breeding males. The data demonstrate that the GnRH system in songbirds is modulated by social context. These finding provide novel insight into the mechanisms involved with regulating avian reproductive physiology.

Distribution of gonadotropin releasing-hormone-II in the house sparrow brain (Passer domesticus)

In songbirds, three isoforms of gonadotropin-releasing hormone (GnRH) have been reported: chicken GnRH-I (GnRH-I); and -II (GnRH-II); and lamprey GnRH-III (lGnRH-III). In galliformes, the distribution of GnRH-I and -II have been extensively studied. GnRH-I cell bodies are located primarily in the preoptic-septal region and primarily project to the median eminence. GnRH-II cell bodies are located in the midbrain and project to various hypothalamic and extrahypothalamic regions. In songbirds, the distributions of only GnRH-I and lGnRH-III have been thoroughly described. Songbirds differ from galliformes in that they possess specialized neural regions for the learning and production of complex vocalizations, the song-control system. In songbirds lGnRH-III was found within many song nuclei. In this study, we investigated the distribution of GnRH-II in the male house sparrow (Passer domesticus) brain. Our objectives were (1) to determine the distribution of GnRH-II in the male songbird brain and (2) to investigate the presence of GnRH-II in song control nuclei. Our study identified immunoreactive magnocellular GnRH-II cell bodies in the oculomotor region of the mesencephalon and parvicellular cells in the lateral hypothalamus. Also, an extensive network of GnRH-II immunoreactive fibres was observed in hypothalamic nuclei and extrahypothalamic regions. Immunoreactive GnRH-II fibres were most abundant in the preoptic area, ventromedial nucleus, hippocampus and the medial septum. Sparse immunoreactivity was also observed in the mesencephalon and diencephalon. GnRH-II was not identified in any of the song-control nuclei. Unlike lGnRH-III, these data suggest that GnRH-II is not directly involved with song learning, perception or production.

Seasonal and individual variation in response to GnRH challenge in male dark-eyed juncos (Junco hyemalis)

Concentrations of gonadal steroids such as testosterone (T) often vary widely in natural populations, but the causes and particularly the consistency of this variation is relatively unexplored. In breeding males of a wild population of the dark-eyed junco (Junco hyemalis), we investigated seasonal and individual variation in circulating T during two breeding seasons by measuring the responsiveness of the HPG axis to a standardized injection of gonadotropin-releasing hormone (GnRH). Individuals were bled prior to and 30min after injection. Pre- and post-challenge levels of T were measured using EIA. Many subjects were sampled repeatedly across multiple breeding stages. Plasma T concentrations nearly doubled in response to GnRH during early spring, but showed significantly smaller increases in later breeding stages. When controlling for seasonal variation in response to challenge, we also found repeatable differences among individuals, indicating individual consistency in the release of T in response to a standardized stimulus. These seasonal and individual differences may arise from comparable variation in responsiveness of the pituitary or a decline in gonadal sensitivity to downstream gonadotropins. In contrast, pre-challenge T showed almost no seasonal changes and did not differ consistently among individuals. To our knowledge, this is the first demonstration of individual repeatability of short-term hormonal changes in a wild population. Such repeatability suggests that hormonal plasticity might evolve in response to changing selection pressures.

Simultaneous pituitary-gonadal recrudescence in two Corsican populations of male blue tits with asynchronous breeding dates

Animal populations living in geographically variable environments respond to different selection pressures. The adaptive character of the responses to environmental information determines the degree of synchrony of the breeding period with local optimal conditions. An example is provided by two populations of Mediterranean blue tits (Parus caeruleus) in Corsica, breeding in different habitats, with a 1-month difference in the onset of egg laying. This difference in the onset of lay is supposed to be adaptive because, although chicks from both populations are raised mostly on caterpillars, the timing of the appearance of caterpillars is earlier for populations of tits associated with deciduous oak trees than those associated with evergreen oak trees. Here, we show that, despite the difference in the timing of egg laying, males from these two populations start seasonal hypothalamo-hypophysial-testicular development at approximately the same time, in late winter. Specifically, the vernal recrudescence of brain GnRH-I perikarya and fibers, testes volume and song activity began around the same dates and proceeded at the same pace in late winter in both populations. Plasma testosterone and LH levels displayed seasonal variations that were shifted by less than 2 weeks compared to the 1-month difference in egg laying periods. We hypothesize that the strong selection pressures on these two populations to adapt the timing of their breeding seasons to their local environment may have acted mostly on the female egg laying dates, and not so much on the initiation and rate of seasonal recrudescence of the hypothalamo-hypophysial-testicular activity in males.

Control of annual reproductive cycle in the subtropical house sparrow (Passer domesticus): evidence for conservation of photoperiodic control mechanisms in birds

Background

In many birds, day length (=photoperiod) regulates reproductive cycle. The photoperiodic environment varies between different seasons and latitudes. As a consequence, species at different latitudes may have evolved separate photoperiodic strategies or modified them as per their adaptive need. We studied this using house sparrow as a model since it is found worldwide and is widely investigated. In particular, we examined whether photoperiodism in house sparrows (Passer domesticus) at 27°N, 81°E shared features with those exhibited by its conspecifics at high latitudes.

Results

Initial experiment described in the wild and captive conditions the gonad development and molt (only in captives) cycles over a 12-month period. Both male and female sparrows had similar seasonal cycles, linked with annual variations in day length; this suggested that seasonal reproduction in house sparrows was under the photoperiodic control. However, a slower testis and attenuated follicular growth among captives indicated that other (supplementary) factors are also involved in controlling the reproductive cycle. Next experiment examined if sparrows underwent seasonal variations in their response to stimulatory effects of long day lengths. When birds were transferred every month over a period of 1 year to 16 hours light:8 hours darkness (16L:8D) for 17–26 weeks, there was indeed a time-of-year effect on the growth-regression cycle of gonads. The final experiment investigated response of house sparrows to a variety of light-dark (LD) cycles. In the first set, sparrows were exposed for 31 weeks to photoperiods that were close to what they receive in between the period from sunrise to sunset at this latitude: 9L:15D (close to shortest day length in December), 12L:12D (equinox, in March and September) 15L:9D (close to longest day length in June). They underwent testicular growth and regression and molt in 12L and 15L photoperiods, but not in 9L photoperiod. In the second set, sparrows were exposed for 17 weeks to photoperiods with light periods extending to different duration of the daily photosensitivity rhythm (e.g. 2L:22D, 6L:18D, 10L:14D, 14L:10D, 18L:6D and 22L:2D). Interestingly, a slow and small testicular response occurred under 2L and 10L photoperiods; 6L:18D was non-inductive. On the other hand, 14L, 18L and 22L photoperiods produced testicular growth and subsequent regression response as is typical of a long day photostimulation.

Conclusion

Subtropical house sparrows exhibit photoperiodic responses similar to that is reported for its population living at high latitudes. This may suggest the conservation of the photoperiodic control mechanisms in birds evolved over a long period of time, as a physiological strategy in a temporally changing environment ensuring reproduction at the best suited time of the year.