Flashcards in Seasonal Breeding Deck (29):
those that occur at intervals of less than a day
those that occur around a day in length
(body temperature, adrenal activity, sleep/wake cycles)
those that occur at periods of longer than a day
(estrous cycles, migration, pelage changes, hibernation, seasonal reproduction)
those with an annual occurrence
Cyclical histogenesis or histogenesis hypothesis
-cells/tissues have an approximately annual cycle of synchronized regeneration (growth, remodeling, degeneration, and replacement)
-changes in the local environment of the stem cells are considered vital for generation of these circannual rhythms
Why develop a seasonal reproduction?
to give birth at the time of year most favorable for the survival of offspring
Explain change in seasonality and closeness to the equator
species living on the equator may breed continuously while those at higher latitudes will exhibit strict seasonality
Explain change in seasonality between males and females
-females usually exhibit stricter seasonality in their reproductive capacities than males
-males of most species can often breed year round
Name some ultimate factors in seasonality (7)
expected milder environmental temperatures in certain seasons
lifespan of individuals
Name some proximate factors in seasonality (1)
photoperiod (light = photophase, dark = scotophase)
What is the predictor option?
planner species that use a reliable predictor of a future seasonal change as a timing device for maximal reproductive competence
What must a species take into account when considering triggering reproductive competence?
how long it will take them to transition from anestrus to fully functional cyclicity
length of time that melatonin must be elevated each day to achieve the desired effect (melatonin elevated at night)
animal detects changes in day length based on light exposure (8L:16D to 12L:12D = increase in day length)
when an animal is kept under stimulatory photoperiods and they eventually stop responding to them
animals that need exposure to non-stimulatory photoperiods before they become responsive to stimulatory ones
What is the critical event for breeding season vs. non-breeding season?
alteration in the frequency of GnRH secretion from the pulse generator
Describe GnRH secretion in the breeding season vs. non-breeding season of females
Breeding season - high frequency GnRH secretion gives gonadotropin release and active ovaries
Non-breeding season - little synthesis or secretion of GnRH and ovaries are inactive
What stimulates GnRH neurones?
a peptide called kisspeptin
Steps in photoperiod control of reproduction
1. ambient daylength perceived by retina, entrains endogenous circadian clock in SCN
2. SCN controls melatonin production by pineal gland (occurs at night)
3. circulating melatonin levels control activity of target cells
4. thyrotropes of pituitary respond to short melatonin signals (long days) by secreting TSH
5. TSH activates cells in hypothalamus to convert T4 to T3
6. T3 controls tissue histogenesis causing increased GnRH circulation in long-day breeders and decreased GnRH circulation in short-day breeders
Explain the steps in measuring day length
retina projects to suprachiasmatic nucleus (SCN) - SCN projects to superior cervical ganglion (SCG) - SCG innervates pineal gland - pineal gland secretes melatonin
What can stop melatonin secretion?
during hours of darkness, light exposure of sufficient intensity and duration will cause the cessation of melatonin secretion
How does light exposure stop melatonin secretion?
light exposure on retina causes cessation of adrenergic stimulation on N-acetyl transferase, the rate-limiting enzyme in melatonin synthesis
How do melatonin signals act on the pituitary gland to cause TSH release?
thyrotrophys of the pars tuberalis have melatonin receptors - melatonin receptors release TSH in response to short daily melatonin signals
How does TSH act within the hypothalamus?
TSH binds to receptors in hypothalamus - hypothalamus converts T4 to T3 - T3 important in histogenesis
How is the frequency of GnRH pulses inhibited during the non-breeding season? (ewe is example)
-increased sensitivity of the GnRH pulse generator to inhibitory gonadal steroid (estrogen) feedback
-also a steroid independent mechanism at work
Explain the inhibition of GnRH using mechanism 1 (altered sensitivity to steroid feedback)
-GnRH stimulatory kisspeptin system is down-regulated by estrogen feedback
-GnRH inhibitory dopamine system up-regulated by estrogen-sensitive glutaminergic and GABAergic inputs to it
Explain the inhibition of GnRH using mechanism 2 (steroid-independent inhibition of GnRH release)
-isolation of GnRH neurons from capillaries
-Gonadotropin Inhibiting Hormone (GnIH)