Stress and Reproduction

Question 1

What is stress?

Answer 1

• Stress is a state of threat or percieved threat to homeostasis

Question 2

Give an overview of the 2 pathways that are activated in response to stress:

Answer 2

1. The HPA Axis:
   - The brain recieves stress information from the prefrontal cortex and hippocampal regions which causes activation of the paraventricular nucleus in the hypothalamus leading to the release of corticoreleasing hormone from the hypothalamus into the median eminence
   - CRH acts on the anterior pituitary and causes it to release adrenocorticotropin hormone
   - ACTH acts on the adrenal cortex and triggers the release of glucocoritcoids including cortisol
   - Cortisol activates many pathways including a negative feedback loop to inhibit the escalation of the stress response
2. The Fight or Flight Response:
   - The amygdala senses the stress stimulus
   - Via activation of the Stria terminalis the posterior and lateral hypothalamus is activated causing the release of CRH
   - CRH acts on the locus coerelus in the brain stem (as well as other noradrenergic centres) and causes the activation of the sympathetic nervous system
   - Noradrenaline then acts on the adrenal medulla and causes the release of adrenaline which activates the peripheral sympathetic nervous system

Question 3

What is the role of AVP and OXY in the HPA axis?

Answer 3

• These hormones are synthesised in the magnocellular paraventricular nucleus as is released into the hypophyseal portal blood system
• AVP and OXY enhance the secretagogue property of CRH and thus stimulate the release of ACTH from the anterior pituitary

Question 4

What are the central and peripheral functions of a stress response?

Answer 4

Central Functions:

• Raise arousal, alertness and aggression
• Block reproduction, feeding and growth
• Activate counter-regulatory feedback loops

Peripheral:

• Increase respiration
• Increase metabolism
• Increase delivery of nutrients to brain, heart and skeletal muscles
• Inhibit reproduction and growth
• Activate counter-regulatory feedback loops (including immune suppression and suppression of the stress response)

Question 5

How does the HPA Axis Inhibit the HPG Axis and thus reproduction:

Answer 5

1. Stress is percieved by the brain and the paraventricular nucleus of the brain is activated resulting in the synthesis of CRH, AVP and OXY
   - These factors are secreted into the hypophyseal portal blood system and act on the pituitary to increase the secretion of ACTH (and B-endorphin)
   - Cortisol acts to negative inhibit the HPG axis at 3 levels:
2. Hypothalamus- decreases synthesis and release of GnRH
3. Pituitary: Inhibits synthesis and release of FSH and LH
4. Gonads: modulates steroidogenesis and gametogenesis directly
   - B-endorphin also negatively inhibits the HPG axis but it does this only at the level of the hypothalamus

Question 6

How does cortisol inhibit GnRH secretion?

Answer 6

• GnRH neurons do not have cortisol receptors
• Instead cortisol increases the activation of GnIH neurons in the dorsomedial nucleus which therefore increase inhibition of GnRH pulsatility