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Photoperiod reproduction

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Photoperiod reproduction

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    1. Photoperiod & reproduction

    2. Overview Seasonal changes in physiology The “Photo-neuroendocrine System” Melatonin biosynthesis Regulation by photoperiod Melatonin target tissues Hypothalamus, pituitary pars tuberalis “Photorefractoriness” and circannual rhythms

    3. Seasonal Rhythms

    4. Seasonal Physiological Rhythms Photoperiodic regulation of seasonal physiology in lab conditions

    5. Seasonal Reproduction and Prolactin / Moulting rhythms are synchronsed differently in diferent mammals

    6. Timing of Seasonal Reproduction

    7. Circannual Rhythms Occur in long-lived seasonal species (e.g. sheep) Occur in constant photoperiod (i.e. endogenous control) Equatorial species Melatonin target tissues? Photoperiod entrains circannual rhythms to annual cycle

    8. Seasonal Changes in Gonadotrophin Releasing Hormone (GnRH) Secretion

    9. The “Photo-neuroendocrine System” Pineal melatonin is a key hormonal clock output Melatonin is synthesised and secreted at night Rhythmic melatonin secretion is driven by a poly-synaptic output pathway from the suprachiasmatic nuclei (SCN) Release of noradrenaline (NA) stimulates pineal

    10. The Photo-neuroendocrine System: Pineal Regulation by Photoperiod

    11. Melatonin Biosynthesis Synthesised from tryptophan Key rate-limiting enzyme in melatonin biosynthesis is arylalkylamine-N-acetyltransferase (AA-NAT) AA-NAT activity is stimulated by noradrenergic stimulation of the pineal gland (output from the SCN)

    12. Species Differences in Mammalian AA-NAT Regulation

    13. How does Changing Photoperiod Regulate Physiology? In mammals, Melatonin is essential for photoperiodic time measurement Removal of pineal blocks responses Melatonin infusion experiments have been done to determine how changing melatonin secretion controls reproductive activation

    14. Timed Infusion Experiments in PinX Hamsters

    15. Decoding the Melatonin Signal Importance of melatonin signal parameters investigated in pinealectomised hamsters Carefully controlled melatonin signals imposed by timed injections/infusions Key parameter is melatonin signal duration Long duration melatonin: short day response Short duration melatonin: long day response

    16. Where & how does melatonin act?

    17. Receptor “autoradiography”

    18. Melatonin Receptors Major mammalian subtypes: MT1 and MT2 (previously Mel1a and Mel1b) G-protein-coupled receptors MT1 receptors implicated in seasonal neuroendocrinology Strong expression Absence of functional MT2 in hamsters Melatonin receptors expressed in many neuroendocrine tissues

    19. Melatonin Target Sites in the Brain Most species express melatonin receptors in multiple brain structures Much inter-species variation Two major candidate sites for effects on reproductive activation 1 - Mediobasal hypothalamus (MBH) 2 - Pars tuberalis (PT)

    20. The Case for the MBH

    21. Mediobasal Hypothalamus Lesion (hamster)

    22. Mediobasal Hypothalamus Lesion (sheep)

    23. Summary of Mediobasal Hypothalamus Lesion Studies Attenuate effect of photoperiod (and melatonin infusion) on reproductive status and body weight No obvious effect on prolactin secretion Is prolactin regulated via a separate melatonin target site (e.g. the PT)?

    24. The Case for the PT

    25. Receptor “autoradiography”

    26. ?TSH is photoperiodically regulated in the sheep PT NB SYSTEMIC TSH LEVELS DO NOT CHANGENB SYSTEMIC TSH LEVELS DO NOT CHANGE

    27. Functional TSH-receptors in the sheep neuroendocrine system

    28. Photoperiodic DIO2 regulation explains springtime need for T4 in the sheep basal hypothalamus

    29. ?TSH and DIO2 are both induced in first 10 days following transfer to Long Days

    30. TSH rapidly induces DIO2 expression in sheep hypothalamus

    33. Paul et al (2008) Phil Trans Roy SocB 363:341 Morgan & Hazlerigg (2008) J Neuroendocrinol 20:820 Hanon et al (2008) Current Biology 18:1147 Hazlerigg & Loudon (2008) Current Biology 18:R795 Selected Further Reading

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