Seasonal Breeding

Question 1

3 types of rhythms

Answer 1

• ultradian (less than a day)
• circadian (a day)
• intradian (longer than a day)

Question 2

Under constant photoperiod conditions, what happens to endogenous circannual oscillation

Answer 2

Has a period of about 10 months –> ex) mares under constant long days eventually become anestrus anyway

Question 3

what is the current proposed theory on how circannual rhythms work

Answer 3

• “cyclical histogenesis” –> cells/tissues have an approximately annual cycle of synchronized regeneration
• changes in environment are important (photoperiod)

Question 4

what does the central control point for reproduction lie with

Answer 4

altering output of the GnRH pulse generator in the hypothalamus –> changes in brain reflect functional plasticity of neural tissues

Question 5

what does the reason for having seasonality revolve around

Answer 5

giving birth at the time of year most favorable for survival of offspring (spring in temperate climates)

Question 6

where is seasonality more pronounced

Answer 6

the farther a species lives from the equator

Question 7

do females or males exhibit stricter seasonality in reproductive capabilities

Answer 7

females

Question 8

what are the “ultimate factors” in seasonal reproduction

Answer 8

• food availability (food competition, predator pressure)

- lifespan of individuals (shorter lifespan = more opportunistic)

Question 9

proximate factors in seasonality

Answer 9

-cues animals use to time their seasonality –> photoperiod

Question 10

what is the predictor option in seasonal breeders

Answer 10

• animals are planners and use a reliable predictor (photoperiod changes) of a future seasonal change
• conditions may not be optimal when they are fertile, they will be by the end of gestation

Question 11

what else do species have to take into consideration besides gestation length

Answer 11

• how long it will take them to transition from anestrus to fully functional cyclicity
• breeding = spring - (gestation + transition)

Question 12

what is a “critical photoperiod”

Answer 12

length of time that melatonin must be elevated each day to achieve the desired effect

Question 13

what is more important than critical photoperiod

Answer 13

photoperiod history –> 8 hours of light, 16 hours of dark to 12 hours of each

Question 14

what is photorefractory

Answer 14

animals kept under stimulatory photoperiods eventually stop responding to them –> reach end of the endogenous circannual rhythm

Question 15

what is photoinducible

Answer 15

after animals become photorefractory, they may need a period of exposure to non-stimulatory photoperiods before they become responsive to stimulatory ones

Question 16

what does exposure to non-stimulatory photoperiods do

Answer 16

hastens the end of the breeding season before the animal would normally cease activity by becoming photorefractory

Question 17

frequency of GnRH secretion from pulse generator and breeding season association

Answer 17

• high frequency pulses: breeding season (active ovaries from gonadotropin release)
• low frequency pulses: non-breeding season (little gonadotropin release and inactive ovaries)

Question 18

what is the main direct stimulator of GnRH neurons and what does it do

Answer 18

kisspeptin –> GnRH neurons have receptors for kisspeptin and respond to it by releasing GnRH, which disrupts the system that blocks GnRH release

Question 19

GnRH neurons and GnRH secretion in vivo

Answer 19

GnRH neurons don’t secrete much GnRH autonomously and don;t organize themselves to do so synchronously (to give pulse release)

Question 20

do GnRH neurons have receptors for gonadal steroids

Answer 20

no –> can’t directly respond to feedback from estrogen or progesterone

Question 21

how does cyclical histiogenesis alter hypothalamic drive for GnRH secretion (in general)

Answer 21

by altering inputs on the kisspeptin/GnRH system

Question 22

steps in photoperiod control of reproduction (6)

Answer 22

1) retina perceives daylight –> endogenous circadian clock via suprachiasmatic nucleus
2) SCN controls melatonin production by pineal gland (only at night)
3) melatonin controls activity of target cells
4) thyrotropes in pars tuberalis of pituitary respond to melatonin and secrete TSH
5) TSH activates hypothalamus to increase DIO2/decrease DIO3 –> T4 converted to T3
6) T3 is main controller

Question 23

long v short melatonin signals

Answer 23

• long = short day

- short = long day

Question 24

what does T3 control in reproduction

Answer 24

• long day breeders: increases in cell types and alterations in connections that increase KNDy neurons –> increased kisspeptin –> increased GnRH –> cycle
• short day breeders: increase in cell types and connections that inhibit KNDy neurons –> keep GnRH pulse frequency low –> start cycling when they become refractory to input

Question 25

what is the photoreceptor for mammals

Answer 25

retina

Question 26

how does retina control melatonin secretion

Answer 26

- nerves project via retinohypothalamic tract to SCN - SCN directs fibers to superior cervical ganglion - SCG is sole innervation of pineal gland --> secretes melatonin

Question 27

2 mechanisms by which daily photoperiod is transduced through anatomic pathway to control pineal gland melatonin secretion

Answer 27

- oscillating "clock" within neurons of SCN --> circadian rhythms (alter electrical activity of SCN and therefore pineal gland) - in the dark, light exposure of sufficient intensity and duration will cause cessation of melatonin secretion

Question 28

what happens if you keep an animal under constant darkness

Answer 28

the pineal gland will not secrete melatonin for the whole time --> eventually the circadian pacemaker in the SCN will shut it off due to oscillating activity

Question 29

what happens to the activity of the SCN clock genes under natural conditions

Answer 29

entrained to the light-dark cycle --> SCN stimulates pineal gland to secrete melatonin in the dark

Question 30

how does light exposure stop melatonin secretion

Answer 30

- night: adrenergic input onto pineal gland cells from SCG is increased - stimulates activity of n-acetyl transferase (rate-limiting enzyme in melatonin synthesis) - activity of NAT is 30-70x higher at night --> melatonin synthesis increased - at dawn, light exposure on retina causes cessation of adrenergic stimulation --> levels of NAT and therefore melatonin decrease

Question 31

NAT release in ungulates and primates

Answer 31

NAT is constantly high --> light exposure functions to breakdown the formed NAT enzyme

Question 32

function of pars tuberalis and melatonin

Answer 32

- pars tuberalis contains cells that produce TSH but don't have TRH or T3 receptors --> don't respond to TRH stimulation, not inhibited by T3 - do have melatonin receptors --> respond to short daily melatonin signals by secreting TSH - TSH transported in retrograde manner to hypothalamus

Question 33

thyroid hormone activation locally within hypothalamus

Answer 33

- in hypothalamus, TSH binds to receptors that surround base of 3rd ventricle - alters activity of diodinase enzymes that control conversion of inactive T4 to active T3

Question 34

what do the different dioidinases do

Answer 34

- type II diodinase (DIO2) converts inactive T4 to active T3 | - type III diodinase (DIO3) catabolizes T4 to rT3 and T3 to T2 (end products inactive)

Question 35

relationship between TSH and DIO2

Answer 35

TSH stimulus (long days) causes up-regulation in DIO2

Question 36

2 mechanisms to inhibit frequency of GnRH pulses in ewe non-breeding season

Answer 36

- massively increased sensitivity of the GnRH pulse generator to inhibitory gonadal steroid (estrogen) feedback - steroid independent mechanism because a less severe restruction of GnRH pulse frequency is seen in ovarectomized animals

Question 37

mechanism 1 to inhibit GnRH pulses in non-breeding ewe (most common)

Answer 37

- fewer KNDy neurons in ARC, those present have fewer projections to GnRH neurons - a subset of dopaminergic (DA) neurons become active --> inhibit GnRH secretion - sensory inputs onto DA neurons from glutaminergic neurons are increased (have estrogen receptors) - inhibitory inputs onto DA neurons by GABA neurons are inhibited by estrogen

Question 38

mechanism 2 to inhibit GnRH pulses in non-breeding ewe (not as important)

Answer 38

- subset of kisspeptin neyrons are not estrogen sensitive - birds: isolate terminals of GnRH neurons so GnRH can't be released - gonadotropin inhibiting hormone

Question 39

while sheep are short-day breeders, their annual cycle is really controlled by long days?

Answer 39

- if you keep them under constant short photoperiods, they will become refractory and stop cycling but won't start cycling again - if you give them long-term constant release melatonin implants, the same thing happens - if you keep them under constant long photoperiods, they show robust regular breeding seasons (10 months) - need exposure to long days to initiate inhibition - short days are not necessary for them to have a breeding season

Question 40

opportunistic breeders

Answer 40

- small, short-lived animals that alter reproductive activity based on food availability (if given enough, they will breed year round) - short gestation periods - some respond to the promise of food