Question 9(p.2)

College Answer

Once again candidates are reminded to read and answer the question presented to them as well as take into consideration the proportion of marks allocated. The expectation from the first section of the question was for candidates to only “list” the hormones secreted by the pituitary gland. The main hormones expected were ACTH, TSH, GH, FSH, LH and prolactin anteriorly; and, ADH (vasopressin) and oxytocin posteriorly. Unfortunately, a number of candidates confused which hormones come from which region and therefore were unable to score marks in the second part. The question clearly asked for factors controlling secretion of hormones from the posterior pituitary, therefore, detailed descriptions of the mechanism of action of ADH in the kidney, or of hormones arising from the anterior pituitary did not relate to the question and were not allocated marks. With respect to ADH, most marks were gained by candidates who described not only osmolality and plasma volume, but the relative sensitivity of the pituitary to these and the pre-potent nature of volume over osmolality; also, the influence of other hormonal axes involved in plasma volume regulation. Marks were also available for other inputs including pain, stress, exercise, etc. A brief description of factors influencing oxytocin was also
expected.
Syllabus – N12d
Reference: Gannong p383, Guyton p359

Discussion

List of pituitary hormones:

• Anterior pituitary hormones:
  • Thyroid stimulating hormone (TSH)
  • Corticotropin (ACTH)
  • Pro-opiomelanocortin
  • Growth hormone (GH)
  • Follicle-stimulating hormone (FSH)
  • Luteinising hormone (LH)
  • Prolactin
• Posterior pituitary hormones:
  • Oxytocin
  • Vasopressin

Control of vasopressin secretion:

• Stimulus: 
  • Hypovolemia and hyperosmolarity are the main stimuli
  • Stress, pain, nausea, nicotine and opioids also stimulate release
  • Alcohol inhibits release
• Sensor:  
  • Baroreceptors in the aortic root and carotid bodies sense hypovolemia by detecting changes in blood pressure, responding to ~10% changes in plasma volume 
  • Hypothalamic osmoreceptors in the circumventricular organs (OVLT) detect changes in plasma osmolality, responding to 102% changes in osmolality
  • Defence of volume is prioritised, if there is conflicting input from osmosensors.
• Afferent: 
  • Baroreceptor input is carried though vagus and glossopharyngeal nerves
  • Osmoreceptor input is via fibres from nucleus of the solitary tract and directly from osmoreceptors
  • Connection is to the suparoptic and periventricular hypothalamic nuclei
  • These cell bodies synthesise vasopressin
  • It is secreted from axon terminals in the posterior pituitary

Control of oxytocin secretion:

• Stimulus: 
  • Cyclical release, eg. related to reproductive cycle
  • Stimulation of the breast and nipple (eg. breastfeeding)
  • Central input (emotion,  sexual arousal)
  • Parturition
• Sensor:  
  • Touch and pressure receptors in the breast
  • CNS inputs from higher cortical centres
• Afferent: 
  • Multiple inputs culminate in the  suparoptic and periventricular hypothalamic nuclei, where the cell bodies also synthesise oxytocin
  • Afferent neurotransmission is mainly via glutamate
  • It is secreted from axon terminals in the posterior pituitary
• Efferent processes
  • Oxytocin receptors (Gq protein coupled receptors) are located mainly in the CNS as well as the myometrium of the uterus and the smooth muscle of mammary glands and lactiferous ducts
  • The most physiologically important effects are uterine contraction and smooth muscle contraction of the mammary glands  to stimulate "let down"
  • It is also implicated in social group interaction, sexual arousal, maternal behaviours, and mood regulation

Leng, Gareth, Celine Caquineau, and Nancy Sabatier. "Regulation of oxytocin secretion." Vitamins & hormones 71 (2005): 27-58.

Holmes, Cheryl L., Donald W. Landry, and John T. Granton. "Science review: vasopressin and the cardiovascular system part 2–clinical physiology." Critical care 8.1 (2003): 15.

Lechan, Ronald M., and Roberto Toni. "Functional anatomy of the hypothalamus and pituitary." Endotext [Internet] (2016).

Musumeci, Giuseppe, et al. "A journey through the pituitary gland: Development, structure and function, with emphasis on embryo-foetal and later development." Acta histochemica 117.4-5 (2015): 355-366.

Amar, Arun Paul, and Martin H. Weiss. "Pituitary anatomy and physiology." Neurosurgery Clinics 14.1 (2003): 11-23.