Circadian rhythm in photoperiodic expressions of GnRH-I and GnIH regulating seasonal reproduction in the Eurasian tree sparrow, Passer montanus

https://doi.org/10.1016/j.jphotobiol.2020.111993Get rights and content

Highlights

  • Measurement of day length is critical for timing the life history stages of birds.

  • Photoperiodic responses occur when the light coincides with the photosensitive phase of an entrained circadian rhythm.

  • Failure of such coincidence is read as “short day” with no response.

  • Photoperiodic expressions of GnRH-I and GnIH genes stimulate / inhibit HPG axis and regulate gonadal response.

Abstract

The present study investigates the involvement of circadian rhythm in photoperiodic expressions of GnRH-I and GnIH in the hypothalamus controlling seasonal reproduction in the Eurasian tree sparrow (Passer montanus). Groups of photosensitive birds were exposed for four weeks to resonance light dark cycles comprising of a light phase of 6 h (L) combined with dark phase of different durations (D) such that the period of LD cycles varied by 12 h increments viz. 12- (6 L/6D), 24- (6 L/18D), 36- (6 L/30D), 48- (6 L/42D), 60- (6 L/54D) and 72- (6 L/66D)h. In addition, a control group (C) was maintained under long day length (14 L/10D). Observations, recorded at the beginning and end of experiment, revealed significant testicular growth with corresponding increase in the hypothalamic expression of GnRH-I peptide but low levels of GnIH mRNA and peptide in the birds exposed to resonance cycles of 12, 36 and 60 h which were read as long days. On the other hand, birds experiencing resonance cycles of 24, 48 and 72 h read them as short days wherein they maintained their quiescent gonads and low levels of GnRH-I peptide but exhibited significant increase in GnIH mRNA and peptide expressions. Thus, sparrows responded to resonance light dark cycles differently despite the fact that each of them contained only 6 h of light. These findings suggest that an endogenous circadian rhythm is involved in photoperiodic expressions of above molecules and indicate a shift in their expressions depending upon whether the light falls in the photoinducible or non-photoinducible phase of an endogenous circadian rhythm.

Introduction

The measurement of day length is critical in a photoperiodic species for exact timing of physiological transitions between life history stages [[1], [2], [3]]. Birds have developed precise time-keeping mechanism to design transition through their yearly life cycle which helps them to exploit favorable conditions and survive during the harshest time of the year. The photoperiodic time measurement allows them to anticipate and prepare for the favorable season in advance of its arrival. This has great relevance for maintaining their fitness in the seasonal environments [4]. Since the pioneer discovery of Rowan [5], revealing the importance of day length in control of seasonal reproductive cycles in birds, various attempts have been made to understand the mechanism by which they measure day length to time the physiological preparations for successful reproduction and related seasonal events. Some birds adapt to daily light dark cycle by using their endogenous “clock” to exactly time their physiological and behavioral functions. The interaction of day length with the above clock induces a seasonal response [[6], [7], [8]]. The above endogenous program enables birds in timing switch on (photoinduction) and switch off (photorefractoriness) of their physiological mechanisms. This ensures the occurrence of seasonal events at the most appropriate time of the year when resources in the wild are optimally present and the chances of survival of offspring are maximum. Several studies have shown the participation of a circadian rhythm of photoperiodic photosensitivity (CRPP) in timing initiation and termination of gonadal responses during photoperiodic control of reproductive cycles in some birds [2,[8], [9], [10], [11]]. Bunning [12] formulated that the response of CRPP to light is phase dependent. Further, it is explainable on the basis of an external coincidence model [13] which predicts that a photoperiodic response results due to the coincidence of light with the photosensitive phase or more precisely photoinducible phase of an entrained endogenous circadian rhythm occurring early in subjective night. Light plays dual role in the above model i.e., entrainer as well as inducer [14]. Thus, a photoperiodic response in a long day breeder results when light coincides with the phase of maximum inducibility of the endogenous clock that occurs about 12 h after the sunrise. This is evident in spring and summer months when the photoperiod is read as “long days”. The above coincidence fails to occur during winter months when the light period remains shorter than 12 h and is read as “short day” [15]. However, the short days terminate photorefractoriness by inducing the recovery of the photosensitivity in refractory birds [16].

Recent studies have revealed the mediobasal hypothalamus (MBH) as the site of photoperiodic induction in birds [[17], [18], [19]]. Long photoperiod stimulates synthesis of thyrotropin-stimulating hormone beta (TSH-β) in the pars tuberalis [18] that leads to increase in expression of gene encoding type 2 iodothyronine deiodinase (DIO2). DIO2 is a thyroid hormone activating enzyme that converts T4 to T3 causing an increase in local production of triiodothyronine (T3) [17,20]. T3 further alters the structural arrangement of the gonadotropin releasing hormone-I (GnRH-I) nerve terminals at the median eminence, where the glial end feet that ensheath the terminals retract allowing increase in secretion of GnRH-I [21,22]. GnRH-I further acts on the anterior pituitary to stimulate gonadotropins (luteinizing hormone, LH; follicle-stimulating hormone, FSH) synthesis and release that lead to seasonal changes in reproductive physiology, gonadal growth and functions and behavior in birds. The above physiological cascade is triggered when light falls in a specific time window or photoinducible phase of an endogenous circadian rhythm. In majority of avian species, the annual changes in photoperiod cause marked changes in the GnRH-I secretion [6,[23], [24], [25], [26]]. The increase in day length during spring and summer months progressively engages the photoinducible phase and induces a photoperiodic response. However, under short photoperiod, increased synthesis of type 3 iodothyronine deiodinase (DIO3), a thyroid hormone inactivating enzyme that converts T4 and T3 to inactive metabolites rT3 and T2, respectively [13,18] inhibits GnRH-I synthesis [27] and a photoperiodic response fails to occur. In majority of avian species, the annual changes in photoperiod cause marked changes in the GnRH-I secretion [23,26]. However, recently the invention of another hypothalamic neuropeptide called gonadotropin-inhibitory hormone (GnIH) has challenged the prime role of GnRH-I in control of reproduction [28]. GnIH inhibits synthesis and release of gonadotropins by its direct action on the pituitary gland and also indirectly by decreasing GnRH-I neuronal activity. It has been reported that GnIH acts as neuroendocrine integrator of photoperiodic cue where it integrates external and internal environmental information and regulates gonadotropin secretion to time seasonal reproduction in birds [[29], [30], [31], [32], [33]]. Both GnRH-I and GnIH respond to a variety of environmental signals and act as significant components of the neuronal circuitry in the brain controlling seasonal reproductive responses across avian species.

Despite reasonable research on molecular basis of photoperiodic control of reproduction via HPG axis, the mechanisms by which the expressions of the regulatory molecules are controlled with particular emphasis on the possible role of an endogenous circadian rhythm is not clearly understood and needs further investigations. The cloning of homologues of mammalian circadian clock genes in birds has provided a better way to examine the molecular link between the circadian clock and photoperiodism in birds [34,35]. However, the mechanism of seasonal time measurement still remains unclear, and questions regarding the mechanism by which the circadian clock determines the photoinducible and non-photoinducible phases remain unanswered. Further, due to limited investigations at the mechanistic level, our understanding for the circadian control of molecular mechanisms underlying photoperiodic regulation of reproduction in birds is still in its infancy. Therefore, it is proposed to study the involvement of an endogenous circadian rhythm in photoperiodic expressions of GnRH-I and GnIH regulating seasonal reproduction in a photoperiodic species, the tree sparrow. In our earlier investigations on this species, we have reported that the tree sparrows possess a definite annual reproductive cycle. Gonadal growth in them is triggered by increasing day lengths of spring in March reaching to peak in May. The gonads regress in summer month (June) when the day lengths are still longer than the spring months indicating the onset of photorefractoriness. They are photosensitive and use day length in regulation of their seasonal reproduction [2,8,36]. The initiation of gonadal growth in this species is a long day phenomenon, while the termination of photorefractoriness and recovery of photosensitivity is a short day phenomenon [2,36]. Further, an endogenous circadian rhythm is involved in induction of gonadal growth and consequent increase in plasma levels of gonadal steroids [8,37]. The present study is a step forward to investigate the circadian control of molecular mechanism at the hypothalamic level underlying photoperiodic control of seasonal gonadal cycle in the Eurasian tree sparrow.

Section snippets

Animal Model and Experiment

The Eurasian tree sparrow is a widely distributed avian species occupying different latitudes, including temperate as well as tropical and sub- tropical regions [38]. Its native range expands throughout central and southern Europe, central Asia, and parts of south–east Asia [39]. In India, tree sparrows are plentifully spread in the hilly regions of the North-East India, including Shillong, Meghalaya (Latitude 25°34′N, Longitude 91°53′E) having a maximum height of 1966 feet MSL with predominant

Results

The results are presented in Fig. 1, Fig. 2, Fig. 3, Fig. 4. The birds of control group exposed to long day length (14 L/10D) exhibited significant testicular growth (P < 0.001) confirming their photosensitivity at the time of their exposure to experimental light dark cycles (Fig. 3C). Significant GnRH-I peptide expression (GnRH-I-ir cells number; % cell area; cell area: P < 0.0001 and cell OD: P = 0.0002) in the hypothalamic preoptic area (POA) and the consequent increase in testicular volume

Discussion

The results obtained from the present experiment clearly suggest that the tree sparrows utilize a photosensitive rhythm with a period of about 24 h to regulate photoperiodic expressions of GnRH-I and GnIH genes in control of their reproductive responses (Fig. 1, Fig. 2, Fig. 3, Fig. 4). The resonance cycles of 12, 36 and 60 h acted as long days by inducing significant testicular growth as a consequence of increased hypothalamic expression of GnRH-I and maintenance of decreased levels of GnIH

Conclusions

The “resonance” model has been previously documented in some avian species. The present study is a step forward in understanding the mechanistic details of the regulation of important components of neuroendocrine circuitry regulating photoperiodic responses in the trees sparrow. It provides further an unequivocal evidence for an external-coincidence circadian mechanism in controlling seasonal gonadal cycle mediated by altered expression of opposing stimulatory/inhibitory hypothalamic

Declaration of Competing Interest

The authors declares that there is no conflict of interest.

Acknowledgement

This work was financially supported by grant from the Department of Biotechnology, New Delhi, India. We are grateful to Dr. Henryk F. Urbanski, Oregon Health and Sciences University, Portland, Oregon, USA and Dr. K Tsutsui, Laboratory of Integrative Brain Sciences, Department of Biology, Waseda University, Tokyo, Japan for providing the rabbit anti- GnRH antibody and anti-quail GnIH serum, respectively.

References (60)

  • M. Paul et al.

    Tracking the seasons: the internal calendars of vertebrates

    Philos. Trans. R. Soc. Lond. Ser. B Biol. Sci.

    (2008)
  • A.S. Dixit et al.

    Photoperiod as a proximate factor in control of seasonality in the subtropical male Tree Sparrow, Passer montanus

    Front. Zool.

    (2011)
  • N. Perfito et al.

    Anticipating spring: wild populations of great tits (Parus major) differ in expression of key genes for photoperiodic time measurement

    PLoS One

    (2012)
  • W.E. Bradshaw et al.

    Adaptation to temperate climates

    Evolution

    (2004)
  • W. Rowan

    Relation of light to bird migration and developmental changes

    Nature

    (1925)
  • A. Dawson et al.

    Photoperiodic control of seasonality in birds

    J. Biol. Rhythm.

    (2001)
  • V. Kumar et al.

    The bird clock: a complex, multi-oscillatory and highly diversified system

    Biol. Rhythm. Res.

    (2004)
  • J. Partecke et al.

    Differences in the timing of reproduction between urban and forest European blackbirds (Turdus merula): result of phenotypic flexibility or genetic differences?

    Proc. R. Soc. Lond. B

    (2004)
  • V. Kumar

    Photoperiodism in higher vertebrates: an adaptive strategy in temporal environment

    Ind. J. Exp. Biol.

    (1997)
  • S. Rani et al.

    Time course of sensitivity of the photoinducible phase to light in the Redheaded Bunting, Emberiza bruniceps

    Biol. Rhythm. Res.

    (1999)
  • E. Bunning

    Die endogene Tagesrhythmikals Grundlage der photoperiodische Reaktion

    Ber. Deut. Bot. Ges.

    (1936)
  • C.S. Pittendrigh et al.

    The entrainment of circadian oscillations by light and their role as photoperiodic clocks

    Am. Nat.

    (1964)
  • C.S. Pittendrigh

    The circadian oscillation in Drosophila Pseudoobscura pupae: a model for the photoperiodic clock

    Z. Pflanzenphysiol.

    (1966)
  • V. Kumar et al.

    The nature of photoperiodic clock in vertebrates

    Proc. Zool. Soc. Calcutta J. B. S. Haldane Comm.

    (1993)
  • P.D. Tewary et al.

    Photoperiodic control of ovarian cycle in rose finch, Carpodacus erythrinus

    J. Exp. Zool.

    (1983)
  • T. Yoshimura et al.

    Light-induced hormone conversion of T4 to T3 regulates photoperiodic response of gonads in birds

    Nature

    (2003)
  • N. Nakao et al.

    Thyrotrophin in the pars tuberalis triggers photoperiodic response

    Nature

    (2008)
  • A. Rastogi et al.

    Neural correlates of migration: activation of hypothalamic clock (s) in and out of migratory state in the blackheaded bunting (Emberiza melanocephala)

    PLoS One

    (2013)
  • T. Watanabe et al.

    Hypothalamic expression of thyroid hormone activating and -inactivating enzyme genes in relation to photorefractoriness in birds and mammals

    Am. J. Physiol. Regul. Integr. Comp. Physiol.

    (2007)
  • T. Yamamura et al.

    Seasonal morphological changes in the neuro-glial interaction between gonadotropin releasing hormone nerve terminals and glial end feet in Japanese quail

    Endocrinology

    (2004)
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