Neuroendocrinology

Question 1

Define neuroendocrinology

Answer 1

Branch of life sciences dealing with neurosecretion and the physiological interaction between the CNS and endocrine system.

Question 2

Describe the basic components of the neuroendocrine system

Answer 2

• Two types of neurones in the hypothalamus:
  
  • Parvocellular neurones (small)
  • Magnocellular neurones (large)
• Parvocellular neurones communicate with anterior pituitary via the hypophysial portal system (between hypothalamus and anterior pituitary)
• Magnocellular neurones communicate with the posterior pituitary directly, large neurones that extend their axons to post. pit.
• Both cases, causes release of hormones from the pituitary into the general circulation, where it can have its effect on target tissues expressing the target receptor.
• There are three main axis of the hypothalamus-pituitary:
  
  • Hypothalamic-pit-adrenal axis (HPA)- acts on adrenal gland –> stress
  • Hypothalamic-pit-gonadal axis (HPG) –> acts on gonads –> reproduction
  • Hypothalamic-pit-thyroid axis (HPT) –> thyroid gland –> metabolism

Question 3

How does the hypothalamus control homeostasis?

Answer 3

Hypothalamus controls homeostasis through 3 processes:

1. Behaviour- especially homeostatic seeking behaviour e.g. heat seeking during cold body temp
2. neural control via the ANS
3. Hormonal control via the pituitary gland

Question 4

What are the 6 homeostatic needs controlled by the hypothalamus?

Answer 4

1. Blood pressure and electrolyte composition:
   
   • Plasma osmolarity
   • salt intake
   • thirst
   • vasomotor tone
2. Stress response
   
   • stress hormone release via adrenal gland
3. Growth
   
   • Growth hormone release
4. Reproduction: hormonal control of
   
   • mating
   • pregnancy
   • lactation
5. Body temperature
   
   • metabolic thermoregulation
   • behaviour
6. Energy metabolism
   
   • feeding
   • metabolic rate
   • digestion

Question 5

What are two additional functions of the hypothalamus that have been proposed by studies?

Answer 5

• Control of autonomic NS shown via retrograde tracer injected into the pancreas, tracer seen in the hypothalamus.
• Control of emotional behaviour- hypothalamus is closely related to both the amygdala and the prefrontal cortex- both involved in emotional responses.

Question 6

Describe the structure of the hypothalamus- its different regions- and the main hypothalamic nuclei and their functions.

Answer 6

• Hypothalamus is split into a preoptic region, an anterior or supraoptic region, a middle/tubular region, and a posterior/ mamillary region.
• Main nuclei to know:
  
  • Paraventricular nuclei- located parallel to the third ventricle
  • supraoptic nuclei- located above the optic chiasma
  • arcuate nucleus- located in the posterior/mamillary region
• Paraventricular nucleus:
  
  • Osmoregulation and BP - (AVP)
  • Maternal control (oxytocin)
  • Energy balance (TRH)
  • Stress (CRH)
• Supraoptic nucleus:
  
  • osmoregulation and BP (AVP)
  • maternal control (oxytocin)
• Arcuate nucleus:
  
  • Reproduction (GnRH)
  • Growth (GHRH)
  • Lactation (dopamine)

Question 7

What is the hypothalamic- hypophysial portal system?

Answer 7

• Hypothalamic- hypophysial portal system is a vascular system that connects the hypothalamus to the anterior pituitary
• Formed of 2 capillary beds:
  
  • 1 at the median eminence (capillary bed here) which carries a releasing hormone from hypothalamic parvocellular neurones down to the portal vein to the anterior pituitary
  • Here the releasing hormone (e.g. Growth hormone releasing hormone GHRH) acts on receptors of anterior pituitary neurosecretory cells, causing the release of the downstream hormone (e.g Growth hormone/ GH).
  • Downstream hormone gets released into 2nd capillary bed at anterior pituitary.
  • Hormone gets carried into general circulation by being drained via cavernous sinus.

Question 8

What are the differences between the anterior and posterior pituitary?

Answer 8

Anterior pituitary:

• develops from migration of pharyngeal epithelium
• contains glandular cells
• Receives stimulus from parvocellular neurone - releasing hormone carried via portal system
• Releases hormone after stimulation with a releasing hormone into the capillary bed at anterior pituitary- carried into general circulation

Posterior pituitary:

• Develops directly from neuronal tissue
• contains the axon terminals of Magnocellular neurones
• Hormone release direct from magnocellular neurone terminal