Concentration of chicken gonadotropin-releasing hormones I and II in microdissected areas of turkey hen brain during the reproductive cycle

Photoperiodic physiology of summer breeding birds and a search for the role of eye

Photoperiod regulation of gonadal cycles is well studied and documented in both birds and mammals. Change in photoperiod is considered as the most effective and important cue to time the initiation of the annual physiological cycles in birds. Approaching of long days (as observed in summer months), signal long-day breeding birds to initiation reproduction and other related functions. Birds and other non-mammalian vertebrates use the extraocular photoreceptors which may be present in the mediobasal hypothalamus (MBH) or associated regions to measure the photoperiodic time and so are different from mammals where only the eyes are lone photoreceptive organs. The downstream signaling involves thyroid responsive genes playing a crucial role in mediating photoperiodic signals in both birds and mammals. Role of eyes in the avian seasonal cycle has been a questionable issue with evidences both favoring and negating any role. We propose that morphological as well as physiological data argue that retinal photoreceptors can participate in gonadal cycle, at least in the quail and duck. The present review details the studies of photoneuroendocrine control of gonadal axis in birds and review evidences to decipher the role eyes in photoperiodic mediated physiologies in birds.

Photoperiodic regulation of avian physiology: From external coincidence to seasonal reproduction

Seasonal cycles of environmental cues generate variation in the timing of life-history transition events across taxa. It is through the entrainment of internal, endogenous rhythms of organisms to these external, exogenous rhythms in environment, such as cycling temperature and daylight, by which organisms can regulate and time life history transitions. Here, we review the current understanding of how photoperiod both stimulates and terminates seasonal reproduction in birds. The review describes the role of external coincidence timing, the process by which photoperiod is proposed to stimulate reproductive development. Then, the molecular basis of light detection and the photoperiodic regulation of neuroendocrine timing of seasonal reproduction in birds is presented. Current data indicates that vertebrate ancient opsin is the predominant photoreceptor for light detection by the hypothalamus, compared to neuropsin and rhodopsin. The review then connects light detection to well-characterized hypothalamic and pituitary gland molecules involved in the photoperiodic regulation of reproduction. In birds, Gonadotropin-releasing hormone synthesis and release are controlled by photoperiodic cues via thyrotropin-stimulating hormone-β (TSHβ) independent and dependent pathways, respectively. The review then highlights the role of D-box and E-box binding motifs in the promoter regions of photoperiodic genes, in particular Eyes-absent 3, as the key link between circadian clock function and photoperiodic time measurement. Based on the available evidence, the review proposes that at least two molecular programs form the basis for external coincidence timing in birds: photoperiodic responsiveness by TSHβ pathways and endogenous internal timing by gonadotropin synthesis.

Photostimulated expression of type 2 iodothyronine deiodinase mRNA is greatly attenuated in the rostral tuberal hypothalamus of the photorefractory turkey hen

For many temperate-zone avian species, termination of breeding occurs when individuals no longer respond to previously stimulatory day lengths, a condition called photorefractoriness. Long day lengths induce significantly greater expression of c-fos and fos-related antigens (FRAs) in the tuberal hypothalamus of the photosensitive hen than that of the photorefractory hen. The tuber is also a site of photoinducible glial expression of type 2 iodothyronine deiodinase (Dio2), which converts thyroxine into its active form, triiodothyronine (T3). T3 induces withdrawal of glial processes from gonadotrophin-releasing hormone (GnRH) I nerve terminals, which is believed to permit the efficient release of GnRH I into the associated portal vasculature. Using a riboprobe, we tested whether long days induce Dio2 mRNA expression in the turkey tuber and, if so, whether this expression is reduced in photorefractory hens. Long days significantly induced rostral and caudal tuberal hypothalamic Dio2 expression in photosensitive hens. Photorefractory hens had reduced expression of Dio2 with most subjects expressing no detectable mRNA in the rostral tuber and variably attenuated amounts throughout the medial and caudal tuber. We also performed double-label immunohistochemistry to identify co-localisation between FRAs and glial fibrillary acidic protein, a glial marker. FRAs were present in the nuclei of a few astrocytes in the median eminence and infundibular nucleus of the tuber. The temporal and spatial coincidence between FRA and Dio2 expression, their mutual association with glia, and the attenuation of their response during photorefractoriness suggests that the two events are linked and that photorefractoriness involves a reduced capacity for photo-inducible gene expression within glia of the tuberal hypothalamus.

Gonadotropin-releasing hormone-II messenger ribonucleic acid and protein content in the mammalian brain are modulated by food intake

GnRH-II is the most evolutionarily conserved member of the GnRH peptide family. In mammals, GnRH-II has been shown to regulate reproductive and feeding behaviors. In female musk shrews, GnRH-II treatment increases mating behaviors and decreases food intake. Although GnRH-II-containing neurons are known to reside in the midbrain, the neural sites of GnRH-II action are undetermined, as is the degree to which GnRH-II is regulated by energy availability. To determine whether GnRH-II function is affected by changes in food intake, we analyzed the levels of GnRH-II mRNA in the midbrain and GnRH-II protein in numerous target regions. Adult musk shrews were ad libitum fed, food restricted, or food restricted and refed for varying durations. Compared with ad libitum levels, food restriction decreased, and 90 min of refeeding reinstated, GnRH-II mRNA levels in midbrain and GnRH-II peptide in several target areas including the medial habenula and ventromedial nucleus. Refeeding for 90 min also reinstated female sexual behavior in underfed shrews. In male shrews, abundant GnRH-II peptide was present in all sites assayed, including the preoptic area, a region with only low GnRH-II in females. In contrast to females, food restriction did not affect GnRH-II protein in male brains or inhibit their mating behavior. Our results further define the relationship between GnRH-II, energy balance, and reproduction, and suggest that food restriction may inhibit female reproduction by reducing GnRH-II output to several brain nuclei. We postulate that this highly conserved neuropeptide functions similarly in other mammals, including humans, to fine-tune reproductive efforts with periods of sufficient energy resources.

Testosterone induces testicular development but reduces GnRH-I fiber density in the brain of the House Finch, Carpodacus mexicanus

Testosterone (T) in male birds generally inhibits the activity of the hypothalamo-pituitary-gonadal axis, but the androgen can also be gonadostimulatory. The mechanisms responsible for this stimulation are unknown. To address this question, adult male House Finches, Carpodacus mexicanus, held on short day length to inhibit reproductive function, received 20-mm long T-filled Silastic capsules either for 74 days (T+; n=5; Day 1-74) or for 66 days followed by eight days without capsules (T-; n=5), or empty capsules for 74 days (controls, C, n=10). At sacrifice on Day 74, plasma T concentrations were elevated in T+ males and low, and similar in T- and C males. Testes were enlarged in T+ males (136.5+/-57.2mg), small in T- males (16.2+/-9.5 mg), and undeveloped in C males (5.4+/-3.6 mg). The three groups had similar numbers and sizes of hypothalamic gonadotropin-releasing hormone-I immunoreactive (GnRH-I ir) perikarya, but perikaryon GnRH-I ir labelling was lighter in T+ than C males. T+ males also had lower density of GnRH-immunolabelled fibers in the preoptic area and lower intensity of GnRH immunolabelling in the median eminence than C males, suggesting an inhibition of GnRH-I neuronal activity. Plasma luteinizing hormone in the three experimental groups was uniformly low and did not differ. Thus, T administration-induced testicular development was associated with an inhibition of the reproductive neuroendocrine system, demonstrating a direct stimulatory effect of T on testes. Stimulation of testicular function by endogenous T in seasonal breeders may contribute to the rapid gonadal maturation that takes place in response to photostimulation.

Differential effects of testosterone on protein synthesis activity in male and female quail brain

In Japanese quail, testosterone (T) increases the Nissl staining density in the medial preoptic nucleus (POM) in relation to the differential activation by T of copulatory behavior. The effect of T on protein synthesis was quantified here in 97 discrete brain regions by the in vivo autoradiographic (14)C-leucine (Leu) incorporation method in adult gonadectomized male and female quail that had been treated for 4 weeks with T or left without hormone. T activated male sexual behaviors in males but not females. Overall Leu incorporation was increased by T in five brain regions, many of which contain sex steroid receptors such as the POM, archistriatum and lateral hypothalamus. T decreased Leu incorporation in the medial septum. Leu incorporation was higher in males than females in two nuclei but higher in females in three nuclei including the hypothalamic ventromedial nucleus. Significant interactions between effects of T and sex were seen in 13 nuclei: in most nuclei (n=12), T increased Leu incorporation in males but decreased it in females. The POM boundaries were defined by a denser Leu incorporation than the surrounding area and incorporation was increased by T more in males (25%) than in females (6%). These results confirm that protein synthesis in brain areas relevant to the control of sexual behavior can be affected by the sex of the subjects or their endocrine condition and that T can have differential effects in the two sexes. These anabolic changes should reflect the sexually differentiated neurochemical mechanisms mediating behavioral activation.

Photostimulated fos-like immunoreactivity in tuberal hypothalamus of photosensitive vs. photorefractory turkey hens

Photorefractoriness in commercial turkey hens can be viewed as a failure of previously sexually stimulatory photoperiods to maintain egg production via activation of cGnRH I neurons, but the neural locus of photorefractoriness, i.e., where in the brain failure occurs, is not known. We used a c-fos antiserum that detects c-Fos and Fos-related antigens to characterize Fos-like immunoreactivity (FLI) as a measure of neuronal activation. FLI was measured in somatically mature, photosensitive hens (held on short photoperiods [8L:16D] for at least 10 weeks) before (non-photostimulated-photosensitive group) and after 48 h of exposure to long photoperiods (16L:8D; photostimulated-photosensitive group). We also measured FLI in hens that had become photorefractory, transferred to short photoperiods for 1 week--an insufficient time period to reverse photorefractoriness--and then exposed to long photoperiods for 48 h (photostimulated-photorefractory group). FLI was nearly absent in the tuberal hypothalamus of non-photostimulated-photosensitive hens but FLI was abundant in photostimulated-photosensitive hens. FLI was greatly reduced (P<0.01) in the rostral tuberal hypothalamus of photostimulated-photorefractory hens. All hens showed variable extra-tuberal FLI in locations associated with stress, e.g., paraventricular nucleus, lateral septal area, and nucleus taenia. Double-label fluorescence immunohistochemistry with c-fos antiserum and anti-Neu-N, a neuron-specific protein, showed that a substantial fraction of tuberal FLI-positive cells in photostimulated-photosensitive hens were neuronal. These results implicate neurons in the rostral tuberal hypothalamus as a potential neural locus of photorefractoriness, as FLI in this region appears coupled with cGnRH I activation in photostimulated-photosensitive but not photostimulated-photorefractory turkey hens.

Photostimulation affects gonadotropin-releasing hormone immunoreactivity and activates a distinct neuron population in the hypothalamus of the mallard

To reveal central mechanisms that transduce photic stimuli to sexually related neuroendocrine changes, Fra-2-ir, an inducible immediate-early gene marker of neuronal activation has been consecutively localized with that of GnRH-I in the brain of mallards that were triggered by artificial light at the photosensitive phase of the reproductive cycle. Strong neuronal activation was found in the POM and PVN in response to 1x or 4x 20 h light exposure that was accompanied with an increase of GnRH-ir in the hypothalamus and a dramatic depletion of GnRH-ir from terminals in the median eminence. The Fra-2 and GnRH-ir profiles, however, were not co-localized in any region at any phase of photostimulation. These results demonstrate profound changes of GnRH-ir in the hypothalamus and reveal a distinct, photoresponsive cell population in the anterior hypothalamic area of the mallard.

Changes in brain gonadotropin-releasing hormone- and vasoactive intestinal polypeptide-like immunoreactivity accompanying reestablishment of photosensitivity in male dark-eyed juncos (Junco hyemalis)

In seasonally breeding, photoperiodic birds, the development of photorefractoriness is associated with decreased brain expression of gonadotropin-releasing hormone-like immunoreactivity (GnRH-li ir) and increased expression of vasoactive intestinal polypeptide-like immunoreactivity (VIP-li ir). Dissipation of photorefractoriness and reestablishment of photosensitivity are associated with increased GnRH-li ir brain production, but concurrent changes in VIP-li ir expression have not been investigated. To address this question, we compared the expression of VIP-li ir in the infundibulum (INF) of adult male dark-eyed juncos (Junco hyemalis) that were made photorefractory (PR) by prolonged exposure to long days with that of birds that were not photostimulated (PS), but had regained photosensitivity by exposure to short days for 5 (short-term-PS, ST-PS) or 13 (long-term-PS, LT-PS) consecutive months. Photosensitive males had smaller INF VIP-li ir cell bodies than PR males, but the numbers of INF VIP-li ir cells were independent of photoperiodic condition. Changes in infundibular VIP-li ir were correlated with changes in preoptic area (POA) GnRH-li expression. Specifically, photosensitive males had more and larger POA GnRH-li ir cells and more GnRH-li ir fibers in this region than PR males. Further, LT-PS males had more GnRH-li ir POA fibers and larger testes than ST-PS juncos. Thus, induction of photorefractoriness is associated with increased VIP and decreased GnRH brain expression whereas dissipation of photorefractoriness concurs with decreased VIP and increased GnRH brain expression. These results suggest a physiological role for VIP in the control of changes in GnRH expression as a function of the photosensitive condition.

Control of luteinizing hormone and prolactin secretion in birds

In birds, the secretion of luteinizing hormone (LH) and prolactin is controlled by the releasing hormones, gonadotrophin releasing hormone-I (GnRH-I) and vasoactive intestinal polypeptide (VIP), respectively. The secretion of LH is depressed and the secretion of prolactin is at its highest, during incubation/brooding young and during the development of reproductive photorefractoriness. In incubating domestic chickens, decreased LH and increased prolactin secretion are correlated with decreased hypothalamic GnRH-I mRNA and increased hypothalamic VIP mRNA. Increased plasma prolactin contributes to the suppression of LH secretion in incubating poultry, possibly acting at the levels of the hypothalamus and the anterior pituitary gland. During the development of photorefractoriness, decreased plasma LH and increased plasma prolactin are correlated with decreased hypothalamic GnRH-I and increased hypothalamic VIP. In birds exposed to seasonal changes in daylength, the seasonally maximal concentrations of plasma prolactin associated with the development of photorefractoriness can be explained, in part, by the saturation daylength for photoinduced prolactin release being reached in late spring/mid summer. During the development of photorefractoriness, high concentrations of plasma prolactin play a role in ensuring the rapid regression of the gonads but are not essential for the maintenance of the condition.