The Brain as an Endocrine Organ

Question 1

Describe the hypothalamus

Answer 1

• Main hormone control system of the body
• Is an outcrop of the forebrain (diencephalon)
• Across different mammalian lineages, forebrain structures are enlarged as a result of increasing encephalisation and increased complexity, but the hypothalamus in relation to our body remains the same size
• The hypothalamus is located above the pituitary
• Divided into 3 zones – lateral, medial and periventricular
• The periventricular zone receives inputs from other zones, the brain stem and the telencephalon
• Neurosecretory cells in the periventricular zone secrete hormones into the blood
• Other periventricular cells control the autonomic nervous system

Question 2

Describe the anterior pituitary

Answer 2

• Connected to the hypothalamus by the hypohaseal portal system
• The anterior pituitary is the glandular tissue of the pituitary

Pulsing reproductive system:
• The decapeptide GnRH is produced in pulses within the brain
• Acts on the anterior pituitary, stimulates the gonadotrophs to produce LH and FSH
• FSH stimulated by slow GnRH pulses → 1 pulse every 2 to 3 hours
• LH stimulated by fast GnRH pulses → more than 1 pulse an hour
• FSH and LH act on the gonads to produce steroid hormone
• FSH and LH are composed of a common Gn-a subunit an then unique FSH-b and LH-b subunits

Question 3

Describe GnRH neurons

Answer 3

• Born from stem cells in the nasal placode
• Unsual in that they are not found in a cluster – they are widely dispersed, around 100um apart and scattered in a continuum throughout the basal forebrain
• Approx 1000 in mammals
• Poor interconnections between each other so don’t cross-talk effectively – they are regulated by other neurons

Question 4

Describe the posterior pituitary

Answer 4

• Of nervous origin (white in colour)
• Has neurosecretory cells which project from the hypothalamus into the capillaries in the base of the posterior pituitary (as opposed to into a portal system first) and they secrete oxytocin and vasopressin
• Oxytocin and vasopressin target the pituitary and are secreted into blood vessels to have actions in peripheral organs, but they can also have direct action on brain behavior! → important take home message