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The extent to which changes in ultradian and circadian rhythms (URs and CRs) reflect seasonal variations in pineal melatonin secretion was assessed in male Siberian hamsters transferred from long to short day lengths. The period of the locomotor activity UR increased from 2.5 h in long days to 4.5 h in short day lengths, but this and most other features of the short-day ultradian phenotype were unaffected by pinealectomy; only the short-day increase in UR amplitude was counteracted by pineal extirpation. Virtually all UR components were unaffected by gonadectomy or replacement testosterone or estradiol treatment; changes in testicular hormone secretion appear insufficient to account for seasonal fluctuation in URs. Pinealectomy did not affect activity onsets and offsets or phase angles of CR entrainment in short and long day lengths; the duration of nocturnal activity was equivalently longer in short than long days in both pinealectomized and pineal-intact hamsters. CR robustness of pinealectomized hamsters in short days was intermediate between values of long-day and short-day sham-pinealectomized males. Hourly nocturnal locomotor activity was markedly reduced in SD, and this effect was completely reversed by PINx. We conclude that seasonal transitions in UR and CR waveforms controlled by day length are mediated primarily by melatonin-independent mechanisms, with lesser contributions from melatonin-dependent processes. Most seasonal changes in ultradian and circadian rhythms in males of this species are not influenced by gonadal hormones. URs may allow animals to respond appropriately to changing environmental contingencies. In winter reduced activity combined with temporal restructuring of activity to include longer intervals of rest may be adaptive in maintaining body temperature at lower values and down-regulating energy expenditure when above ground temperatures are extremely low.

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.

It has long been known that the avian brain is capable of light detection independently of the eyes. The photoreceptive molecule neuropsin (OPN5) was identified in mammalian and avian brains, and shown to respond to biologically relevant light wavelengths. Whether OPN5 is functionally involved in light detection is unknown. Daylength plays a critical role in regulating the neuroendocrine control of reproduction in birds. The presence of light during a 'photoinducible' phase of the circadian cycle, which occurs 12-16 h after dawn, results in marked changes in hypothalamic gene expression. These changes ultimately control gonadotropin release from the pituitary gland that, in turn, stimulates gonadal development. In this study, we first measured OPN5 expression in the mediobasal hypothalamus (MBH) in border canaries during the photoinducible period in relation to thyrotropin (TSH) β-subunit mRNA expression, which is implicated in the control of avian reproduction. Second, the knockdown of OPN5 via small interfering RNA antisense in the MBH revealed that there is an inhibitory input in the photoinduced regulation of TSHβ mRNA expression. Our data indicate that a decrease in OPN5 mRNA expression is associated with the facilitation in TSHβ mRNA expression in the MBH, a critical step for the light-induced increase in gonadal recrudescence. We hypothesise that the removal of an inhibitory input by OPN5 in birds may be a step that occurs during the photoinducible period. Given the distribution of OPN5 in the brain and periphery, this suggests a possible multifunctional role for light information in regulating other physiological processes.

Animals' lives are typically subdivided into distinct stages, some of which (e.g. breeding) contribute to fitness through enhancing current reproductive success, and some of which (e.g. molting and migration in birds; hibernation in mammals) contribute to fitness through enhancing survival and, therefore, future reproductive opportunities. There is often a trade-off between these two kinds of processes, either because they are temporally incompatible with one another (e.g. migration precludes simultaneous nesting in birds) or because they are energetically incompatible with one another (e.g. successful molting appears to be incompatible with simultaneous nesting in many birds). Consequently, adaptations facilitating appropriate timing and coordination of different life-cycle stages are arguably as important to fitness as are more obvious adaptations such as feeding morphologies and predator avoidance. Mechanisms that facilitate coordination of life-cycle events with the annual cycle of changes in the environment are therefore expected to evolve in response to selection imposed by different environmental challenges. This article focuses on how mechanisms affecting the timing of, and transitions between, life-cycle stages, particularly breeding, have evolved in birds. Through comparative analyses, we show that photorefractoriness and one neuroendocrine correlate of it-plasticity of the gonadotropin releasing hormone system-have evolved in ways that facilitate different degrees of flexibility in timing of the transition from breeding to molting in different environments. We argue that the nature of the mechanistic adaptations will affect the capacity for adaptive adjustments to changing environmental conditions both in the short term (plasticity inherent in individuals) and in the long term (evolutionary responses of populations to selection).

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.

Estradiol facilitates the expression of male sexual behavior in Japanese quail within a few minutes. These rapid behavioral effects of estradiol could result from rapid changes in its local production in the preoptic area by aromatase, the enzyme converting testosterone into estradiol. Alternatively, aromatase activity may remain constant but fluctuations of local estradiol production could arise from rapid changes in the concentration of the enzymatic substrate, namely testosterone. Rapid increases of circulating testosterone levels have been observed in males of various species following social encounters. Surprisingly, in quail, the interaction with a female seems to result in a decrease in circulating testosterone levels. However, in that study conducted in quail, the samples were collected at intervals longer than the recently observed rapid effects of estradiol on sexual behavior. In the present study we investigated whether plasma testosterone concentrations fluctuate on a shorter time-frame. Eleven male were tested 5 min before and 5, 15 or 30 min after being allowed to have visual access to a female or to copulate with a female for 5 min. Both types of interactions resulted in a significant decline in circulating testosterone levels at latencies as short as 5 min. These data demonstrate that the decrease in testosterone levels is initiated shortly after sexual encounters. Because visual interactions with a female did not result in a rapid increase in testosterone concentrations, these findings rule out the possibility that a rapid rise in circulating testosterone levels participates in the rapid increase in brain estrogen synthesis and its facilitatory effects on copulatory behavior.

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.

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.

Vertebrates exhibit several forms of gonadotropin-releasing hormone (GnRH). In birds, chicken GnRH-I (cGnRH-I) is the primary hypophysiotropic form that regulates the hypothalamo-pituitary-gonad (HPG) axis. In seasonally breeding songbirds, cGnRH-I immunoreactivity is primarily in the preoptic septo-infidibular regions and varies greatly between the breeding and non-breeding season. Chicken GnRH-II (cGnRH-II) is more widely distributed throughout the avian brain, but cGnRH-II immunoreactivity has been found in the median eminence of quail. Thus, cGnRH-II may function in addition to cGnRH-I in the regulation of the HPG axis. If so, we predicted that cGnRH-II immunoreactivity should exhibit seasonal and age related variation similar to that observed for cGnRH-I. In this study, we compared hypothalamic immunoreactivity of cGnRH-I and -II (ir-cGnRH-I and ir-cGnRH-II) between breeding and non-breeding adult and juvenile house sparrows (Passer domesticus). We used immunocytochemistry with highly specific antibodies developed for quail to examine brains of free-living house sparrows collected in the breeding and non-breeding season. Results showed a greater number of ir-cGnRH-I cell bodies and beaded fibres in breeding adult birds compared to non-breeding adult and juvenile birds. Moreover, there were significantly more ir-cGnRH-II fibres in the preoptic area of breeding adult birds compared to non-breeding adult and juvenile birds. These data suggest that cGnRH-II undergoes seasonal- and age-related changes similar to cGnRH-I and implicate a role in regulating reproduction.

Background Production of contextually appropriate social behaviors involves integrated activity across many brain regions. Many songbird species produce complex vocalizations called ‘songs’ that serve to attract potential mates, defend territories, and/or maintain flock cohesion. There are a series of discrete interconnect brain regions that are essential for the successful production of song. The probability and intensity of singing behavior is influenced by the reproductive state. The objectives of this study were to examine the broad changes in gene expression in brain regions that control song production with a brain region that governs the reproductive state. Results We show using microarray cDNA analysis that two discrete brain systems that are both involved in governing singing behavior show markedly different gene expression profiles. We found that cortical and basal ganglia-like brain regions that control the socio-motor production of song in birds exhibit a categorical switch in gene expression that was dependent on their reproductive state. This pattern is in stark contrast to the pattern of expression observed in a hypothalamic brain region that governs the neuroendocrine control of reproduction. Subsequent gene ontology analysis revealed marked variation in the functional categories of active genes dependent on reproductive state and anatomical localization. HVC, one cortical-like structure, displayed significant gene expression changes associated with microtubule and neurofilament cytoskeleton organization, MAP kinase activity, and steroid hormone receptor complex activity. The transitions observed in the preoptic area, a nucleus that governs the motivation to engage in singing, exhibited variation in functional categories that included thyroid hormone receptor activity, epigenetic and angiogenetic processes. Conclusions These findings highlight the importance of considering the temporal patterns of gene expression across several brain regions when engaging in social behaviors.