Effects of N-methyl-D-aspartate on luteinizing hormone release and Fos-like immunoreactivity in the male White-crowned sparrow (Zonotrichia leucophrys gambelii)

Time course of photo-induced Egr-1 expression in the hypothalamus of a seasonally breeding songbird

Many seasonally-breeding species use daylength to time reproduction. Light-induced release of progonadal hormones involves a complex cascade of responses both inside and outside the brain. In this study, we used induction of early growth response 1 (Egr-1), the protein product of an immediate early gene, to evaluate the time course of such responses in male white-throated sparrows (Zonotrichia albicollis) exposed to a single long day. Induction of Egr-1 in the pars tuberalis began ∼11 h after dawn. This response was followed ∼6 h later by dramatic induction in the tuberal hypothalamus, including in the ependymal cells lining the third ventricle. At approximately the same time, Egr-1 was induced in dopaminergic and vasoactive intestinal peptide neurons in the tuberal hypothalamus and in dopaminergic neurons of the premammillary nucleus. We noted no induction in gonadotropin-releasing hormone (GnRH) neurons until 2 h after dawn the following morning. Overall, our results indicate that Egr-1 responses in GnRH neurons occur rather late during photostimulation, compared with responses in other cell populations, and that such induction may reflect new synthesis related to GnRH depletion rather than stimulation by light cues.

The effect of food restriction on the regulation of gonadotropin-releasing hormone in male house finches (Haemorhous mexicanus)

Seasonal activation of the vertebrate hypothalamic-pituitary-gonadal (HPG) axis and gonadal development is initiated by gonadotropin-releasing hormone-I (GnRH) release from the hypothalamus. In photoperiodic species, the consistent annual change in photoperiod is the primary environmental signal affecting GnRH cell activity, including changes in the synthesis and secretion of this neuropeptide. Non-photoperiodic environmental cues such as energy availability also influence HPG axis activity, but the mechanisms mediating this influence, in particular on the GnRH system, are unclear. Understanding how the neuroendocrine system integrates environmental information is critical in determining the plasticity and adaptability of physiological responses to changing environments. The primary objective of this study was to investigate GnRH-mediated changes in HPG axis activity and gonadal development in response to energy availability in a wild bird. We hypothesized that negative energy balance inhibits HPG axis activity by affecting GnRH secretion. Moderate food restriction for several weeks in male house finches, Haemorhous mexicanus, decreased body condition and inhibited photoinduced testicular growth compared to birds fed ad libitum. Food restriction did not affect plasma luteinizing hormone (LH; a correlate of GnRH release) or plasma testosterone, but it enhanced the plasma LH response to an injection of the glutamatergic agonist, N-methyl-D-aspartate (NMDA). Thus, food restriction may decrease photoinduced HPG axis activation by acting centrally, in particular by attenuating the release of accumulated GnRH stores.

Daily, circadian and seasonal changes of rhodopsin-like encephalic photoreceptor and its involvement in mediating photoperiodic responses of Gambel's white-crowned Sparrow, Zonotrichia leucophrys gambelii

Extra-retinal, non-pineal, encephalic photoreceptors (EP) play important roles in mediating development of the reproductive system by the annual change in day length (photoperiodic gonadal response - PGR) in birds. However, the distribution of rhodopsin-like EPs and their functional daily, circadian and seasonal changes are still unclear in the avian brain. This study identifies two novel groups of rhodopsin-immunoreactive cells in the nucleus paraventricularis magnocellularis (PVN) of the hypothalamus and in the medial basal hypothalamus (MBH) in a seasonally breeding species, Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). In the PVN, rhodopsin-ir cell number showed both daily and circadian changes with more labeled cells apparent in the night phase in photosensitive birds, while only circadian changes were observed involving fewer labeled cells in the night phase in photorefractory birds. Single long day photo-stimulation significantly decreased the rhodopsin-ir cell number only in photosensitive birds, coincident with a rise in plasma levels of luteinizing hormone (LH). In the MBH, rhodopsin-ir cell number did not show daily, circadian or single long day induced changes in either photoperiodic states. But, overall these rhodopsin expressing neurons significantly increased from photosensitive to photorefractory states. In the median eminence (ME), more intense rhodopsin-ir was detected in photorefractory birds compared to photosensitive birds. For expression of GnRH and vasoactive intestinal polypeptide (VIP), seasonal differences were found with opposite relationships, consistent with previous studies. Our results suggest different roles of the two groups of rhodopsin-like EPs in the regulation of PGR in white-crowned sparrows.

GABAergic Neurotransmission in the Premammillary Nucleus of the Turkey Hypothalamus Regulates Reproductive Seasonality and the Onset of Photorefractoriness

BACKGROUND/AIMS

Photoperiod is a major environmental cue in temperate-zone birds which synchronizes breeding with the time of year that offers the optimal environment for offspring survival. Despite continued long photoperiods, these birds eventually become refractory to the stimulating photoperiod and their reproductive systems regress. In this study, we characterized the role of γ-aminobutyric acid (GABA)ergic neurotransmission in modulating the response of the premammillary nucleus (PMM) to a gonad stimulatory photoperiod and the onset of photorefractoriness.

METHODS AND RESULTS

Bilateral ablation of the PMM blocked the light-induced neuroendocrine response from occurring in photosensitive turkeys. Microarray analyses revealed an increase in GABAergic activity in the PMM of photorefractory birds as opposed to photosensitive ones, and this enhanced GABAergic activity appeared to inhibit the photoperiodic signal. Additionally, GABAA and GABAB receptors were expressed by dopamine-melatonin neurons in the PMM, and the administration of the GABA receptor agonist baclofen blocked the photoperiodic reproductive neuroendocrine responses.

CONCLUSIONS

Consistent with the present findings, we propose that the long-sought-after mechanism underlying photorefractoriness is linked to the inhibitory actions of GABA. We suggest that (1) GABAergic interference with photoperiodic entrainment in the PMM initiates the photorefractory state and terminates the annual breeding season in temperate-zone birds, and (2) the PMM is a site of photoreception and photorefractoriness that controls the initiation and termination of avian reproductive seasonality.

Neural and neuroendocrine processing of a non-photic cue in an opportunistically-breeding songbird

Recent studies of the onset of breeding in long-day photoperiodic breeders have focused on the roles of type 2 and 3 iodothyronine deiodinases (DIO2 and DIO3) in the conversion of thyroxine (T4) to triiodothyronine (T3) and subsequent activation of the reproductive axis. It has been hypothesized that an increase in DIO2 and reciprocal decrease in DIO3 causes the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, setting off a reproductive cascade, and that this DIO mechanism for GnRH release is conserved across vertebrate taxa. We sought to test whether social cues that are known to stimulate reproductive behaviors can activate the DIO system to initiate reproduction in a non-photoperiodic bird, the zebra finch (Taeniopygia guttata). Isolation of males and subsequent presentation of females did not increase DIO2 or GnRH expression in the hypothalamus, nor did it decrease gonadotropin-inhibitory hormone (GnIH) or DIO3. Males receiving a female stimulus showed significantly higher mRNA expression and immunoreactive cell count of the immediate early gene early growth response protein 1 (EGR-1) than isolated males, indicating hypothalamic activation in response to a female. Cells immunoreactive for EGR-1 were not co-localized with those immunoreactive for GnRH. Reproductive behaviors (singing, copulation attempts, and overall activity) were significantly higher in males receiving a female stimulus. This study presents a social effect on behavior and EGR-1 expression in the hypothalamus of males in response to females, but more research is needed to determine if the DIO2 system and the GnRH system are responsive to social stimulation in this species.

Neuroendocrine regulation of gonadotropin secretion in seasonally breeding birds

Seasonally breeding birds detect environmental signals, such as light, temperature, food availability, and presence of mates to time reproduction. Hypothalamic neurons integrate external and internal signals, and regulate reproduction by releasing neurohormones to the pituitary gland. The pituitary gland synthesizes and releases gonadotropins which in turn act on the gonads to stimulate gametogenesis and sex steroid secretion. Accordingly, how gonadotropin secretion is controlled by the hypothalamus is key to our understanding of the mechanisms of seasonal reproduction. A hypothalamic neuropeptide, gonadotropin-releasing hormone (GnRH), activates reproduction by stimulating gonadotropin synthesis and release. Another hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH), inhibits gonadotropin synthesis and release directly by acting on the pituitary gland or indirectly by decreasing the activity of GnRH neurons. Therefore, the next step to understand seasonal reproduction is to investigate how the activities of GnRH and GnIH neurons in the hypothalamus and their receptors in the pituitary gland are regulated by external and internal signals. It is possible that locally-produced triiodothyronine resulting from the action of type 2 iodothyronine deiodinase on thyroxine stimulates the release of gonadotropins, perhaps by action on GnRH neurons. The function of GnRH neurons is also regulated by transcription of the GnRH gene. Melatonin, a nocturnal hormone, stimulates the synthesis and release of GnIH and GnIH may therefore regulate a daily rhythm of gonadotropin secretion. GnIH may also temporally suppress gonadotropin secretion when environmental conditions are unfavorable. Environmental and social milieus fluctuate seasonally in the wild. Accordingly, complex interactions of various neuronal and hormonal systems need to be considered if we are to understand the mechanisms underlying seasonal reproduction.

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.

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.

Glutamatergic stimulation of luteinising hormone secretion in relatively refractory male songbirds

Seasonal breeding in two Sonoran desert passerines, the Cassin's (Aimophila cassinii) and Rufous-crowned (Aimophila ruficeps) Sparrows is thought to be terminated by the development of a decrease in responsiveness to photostimulation, a condition known as relative photorefractoriness. It was predicted that the development of relative refractoriness is a consequence of a decrease in gonadotrophin-releasing hormone (GnRH) synthesis and associated stores of releasable GnRH. This hypothesis was tested by determining the luteinising hormone (LH) responses to the excitatory amino acid glutamate agonist N-methyl-D,L-aspartate (NMA) in males of the two species subjected to photomanipulations aimed at generating five groups: Fully photosensitive with undeveloped testes on short days (8L : 16D); fully photosensitive with developed testes on 13L : 11D; relatively photorefractory with regressed testes on 13L : 11D, and groups with developed testes held on 15L : 9D or 16L : 8D. LH release was stimulated in the Cassin's Sparrow by NMA most in the 8L group; to a lesser, but similar extent in the two 13L groups; and not at all in the 15L and 16L groups. LH release was not stimulated by NMA in any of the photoperiodic regimes in the Rufous-crowned Sparrow. In both species, NMA induced Fos-like immunoreactivity in the anterior and basal hypothalamus, but not in GnRH cell bodies. It is concluded that the development of relative photorefractoriness in Cassin's Sparrows is a consequence of reduced GnRH synthesis, reflected in a reduction in releasable GnRH. The lack of LH response of the Rufous-crowned Sparrows to NMA administration may be a consequence of high responsiveness to handling stress.

Seasonal-like growth and regression of the avian song control system: neural and behavioral plasticity in adult male Gambel's white-crowned sparrows

Birdsong is regulated by a series of discrete brain nuclei known as the song control system. In seasonally-breeding male songbirds, seasonal changes in steroid sex hormones regulate the structure and electrophysiology of song control system neurons, resulting in dramatic changes in singing behavior. Male songbirds can be brought into the laboratory, where circulating levels of steroid hormone and photoperiod can be abruptly manipulated, providing controlled conditions under which rapid "seasonal-like" changes in behavior and morphology can be carefully studied. In this mini-review, we discuss the steroidal and cellular mechanisms underlying seasonal-like growth and regression of the song control system in adult male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), and its impact on song behavior. Specifically, we discuss recent advances concerning: (1) the role of androgen and estrogen receptors in inducing seasonal-like growth of the song control system; (2) how photoperiod modulates the time course of testosterone-induced growth of the song control system; (3) how bilateral intracerebral infusion of androgen and estrogen receptor antagonists near the song control nucleus HVC prevents seasonal-like increases in song stereotypy but not song rate; and (4) the steroidal and cellular mechanisms that mediate rapid regression of the song control system. Throughout this mini-review we compare data collected from white-crowned sparrows to that from other songbird species. We conclude by outlining avenues of future research.

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.

Activity of the hypothalamic-pituitary-gonadal axis differs between behavioral phenotypes in female white-throated sparrows (Zonotrichia albicollis)

The white-throated sparrow (Zonotrichia albicollis) lends itself particularly well to investigations of neuroendocrine mechanisms of social behavior because of a behavioral polymorphism that correlates with a plumage phenotype. Roughly half of the individuals of this species exhibit a white stripe (WS) on the crown and engage in a more aggressive strategy, whereas the other half exhibit a tan stripe (TS) and assume a more parental strategy. These behavioral differences are mirrored by hormonal and neuroendocrine differences; for example, males of the WS morph have higher plasma testosterone than do TS males, and females of the TS morph have higher plasma luteinizing hormone than females of the WS morph. These differences suggest that the regulation of the hypothalamic-pituitary-gonadal (HPG) axis may differ according to morph. In this study, we compared HPG axis activity at each level by measuring (1) the number, size, and staining intensity of GnRH immunoreactive (ir) neurons; (2) plasma LH; and (3) plasma estradiol (E2) in females. We found that TS females had more GnRH-ir neurons in the septo-preoptic area of the hypothalamus than did WS females, and GnRH-ir neuronal cell bodies were larger in the WS than the TS females. There was no morph difference in the intensity of GnRH labeling. TS females had higher plasma LH, which is consistent with a previous report, and higher plasma E2. We hypothesize that the differences in GnRH-ir cell number and size are related to differences in LH and E2 secretion, and may be relevant to polymorphic social behavior.

Rapid neuroendocrine responses to auditory courtship signals

In many species, courtship signals enhance reproductive function in the receiver. How these social signals are processed by the brain, particularly how they induce an endocrine response, is not well understood. Songbirds provide an ideal model in which to study this phenomenon because of the large existing literature on both their auditory neurobiology and the control of their reproductive physiology by environmental cues. To date, all of the relevant studies on songbirds have involved measuring the effects of male vocalizations on ovarian function over a period of weeks, a time course that precludes detailed analysis of the neuroendocrine mechanisms operating during song perception. We played recordings of conspecific male song to laboratory-housed female white-throated sparrows and quantified the resulting rapid changes in LH as well as the induction of the immediate early gene Egr-1 in the GnRH system and mediobasal hypothalamus (MBH). Hearing song for 42 min induced LH release and Egr-1 expression in the MBH, but did not alter Egr-1 expression in GnRH neurons. The time course of LH release and the pattern of Egr-1 expression together suggest that song acts as a trigger to induce GnRH release in a manner resembling photostimulation. The Egr-1 response in the MBH was qualitatively distinguishable from the responses to either photostimulation or pharmacologically induced LH release but seemed to involve overlapping neuronal populations. Song-induced Egr-1 expression in the MBH was correlated with the expression in midbrain and forebrain auditory centers, further supporting a role for the MBH in processing social information.

A test of absolute photorefractoriness and photo-induced neural plasticity of song-control regions in black-capped chickadees (Poecile atricapillus)

We tested whether male and female black-capped chickadees, Poecile atricapillus (L., 1766), were absolutely photorefractory according to Hamner's (1968) criteria of (i) spontaneous regression of gonads during prolonged long-day exposure and (ii) no subsequent recrudescence of gonads in response to constant light. We initially exposed black-capped chickadees to constant long-day photoperiods. Gonads regressed spontaneously, demonstrating that the birds met Hamner's first criterion for absolute photorefractoriness. Once their gonads fully regressed and the birds were in advanced prebasic moult, we exposed them to one of three photoperiods for an additional 2 weeks: constant light (24 h L), short days (8 h L), and controls (15 h L). Constant light challenge had no effect on gonadal condition or rate of moult, confirming that the birds met Hamner's second criterion for absolute refractoriness. We also compared volumes of song-control nuclei in the three groups and found that males overall had larger HVC, robust nucleus of arcopallium, and area X than females, but that longer days (24 h L) did not increase volumes and that shorter days (8 h L) did not decrease volumes compared with controls (15 h L). These data support the inference that black-capped chickadees do indeed become absolutely photorefractory, and that photorefractoriness precludes photo-induced plasticity of the song-control system.

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.

The effects of a single long photoperiod on induction and dissipation of reproductive photorefractoriness in European starlings

In many birds, long photoperiods stimulate gonadal maturation but also cause photorefractoriness, leading to gonadal regression. While much is known about the neuroendocrinology of photostimulation, little is known about photorefractoriness, partly due to lack of an experimental model. This study aimed to develop a model to test whether a single long photoperiod (LP) initiates the mechanism leading to photorefractoriness. It made use of the fact that in castrated European starlings, luteinizing hormone (LH) is low in photorefractory birds and high in photosensitive birds. In the first experiment, groups of castrated photorefractory birds were transferred from a long to a short photoperiod and then exposed to one LP every 5, 10, 20, 30, 40, or 60 days. In birds exposed to one LP every 5 days, LH stayed low. In birds exposed to one LP every 10 days, each LP caused a pulse in LH, but mean LH remained low. One LP every 20 days increased LH 6 days later, followed by a decrease at 11 days and then a further increase, so that overall, LH increased slowly. In birds exposed to one LP every 30, 40, or 60 days, LH increased at the same rate as in short photoperiod controls. Each LP did not cause a significant increase in LH, but did cause a decrease. A second experiment examined the changes in LH following a LP in more detail. Castrated starlings had been exposed to one LP every 14 days for 16 weeks. On the day of the final LP, LH values were midway between photorefractory and photosensitive values. The LP caused an increase in LH from the second day to a peak after 7 days. Thereafter, LH declined to initial values after 14 days, followed, in the absence of further LP, by a second increase to photosensitive values. These results suggest that the mechanisms causing photostimulation and photorefractoriness are both initiated during the first long photoperiod. That gonadal maturation precedes regression must reflect the relative rates at which the two processes reach completion.

Central nervous system regulation of gonadal development in the avian male

Many avian species, particularly domesticated ones used for egg and meat production, are photoperiodic. Research conducted over the past several years has revealed the neural components, neurotransmitters, neuromodulators, and gonadotropins that play an important role in responding to changes in day length. An ultimate effect of the neuroendocrine response of birds to light is gonadal development and production of fertilized eggs and young for the next generation. The main purpose of this paper is to address the major neural systems that have been shown to affect reproductive function in males. Potential areas of research that would help elucidate the mechanism of neural activation of gonadal function are suggested.