Hypothalamus and Pituitary

Describe the control, secretions and functions of the pituitary and the hypothalamus

Hypothalamus

The hypothalamus is a circumventricular organ that regulates a large number of autonomic processes:

• Thermoregulatory
  Integrates thermoreceptor input and controls activity of heat loss and heat gain mechanisms.
• Satiety
  Feelings of hunger are modulated by glucose, CCK, glucagon, and leptin.
• Water balance
  • Contains osmoreceptors which control ADH release from the posterior pituitary
  • Angiotensin II stimulates thirst and ADH release via the subfornical organ and organum vasculosum
• Circadian rhythms
  Balance between anterior and posterior hypothalamic stimulation controls sleep-wake cycle.
• Pituitary control
  • Anterior pituitary by hormone secretion into the long portal vein. Secreted hormones include:
    • GnRH, stimulates FSH and LH release
    • CRH, stimulates ACTH release
    • GHRH, stimulates GH release
    • TRH, stimulates TSH release
    • Somatostatin, inhibits GH and TSH release
    • Dopamine, inhibits prolactin release
  • Posterior pituitary by neuronal innervation
• Behaviour
  Punishment and reward centres.
• Sexual function

Pituitary

The hypothalamic-pituitary axis describes the complex feedback loops between these endocrine organs:

• Short-loop feedback describes negative feedback from the pituitary on the hypothalamus, e.g. GH inhibiting GHRH release
• Long-loop feedback describes negative feedback from a pituitary target gland (i.e. thyroid, adrenal, gonads) on the hypothalamus, e.g. cortisol inhibiting CRH (as well as ACTH) release.
  • These axes are also named with target gland, e.g. hypothalamic-pituitary-adrenal axis

Pituitary Hormones

The pituitary gland secretes eight hormones from two lobes:

• Anterior Pituitary
  Secretes six hormones in response to hypothalamic endocrine stimulus. These are classified as:
  • Stimulating hormones, which act at another gland:
    • ACTH
      Short-chain peptide that stimulates cortisol release from the zona fasciculata. Release is stimulated by CRH, and inhibited by cortisol.
    • TSH
      Glycoprotein that stimulates synthesis and release of T₃ and T₄. Release is stimulated by TRH, and inhibited by T₃.
    • FSH
      Glycoprotein gonadotropin. Release is stimulated by GnRH, and inhibited by circulating sex steroids. Has different effects depending on sex:
      • Females: Stimulates oestrogen synthesis and ovarian follicle development.
      • Males: Stimulates sperm maturation.
    • LH
      Glycoprotein gonadotropin with different effects depending on sex:
      • Females: Rapid increase stimulates ovulation and corpus luteum development.
      • Males: Stimulates testosterone synthesis.
  • Direct acting hormones:
    • GH
      Long-chain peptide released in a pulsatile fashion. Release is stimulated by GHRH and is typically high with exercise, hypoglycaemia, and stress. Release is inhibited by somatostatin and IGF-1. GH has generally anabolic effects:
      • Directly stimulates lipolysis, increasing circulating FFA
      • Indirectly stimulates IGF-1 release, promoting cell growth and development
    • Prolactin
      Long-chain peptide which promotes breast development during gestation, and lactation after delivery.

• Posterior pituitary
  Secretes two hormones:
  • ADH
    Short-chain peptide which is:
    • Released in response to osmoreceptors in the circumventricular organs detecting a change in osmolality
      ADH release is:
      • Reduced when osmolality is <275 mosm.l-1</sup>
      • Increased when osmolality is >290 mOsm.L^-1
    • Effective at:
      • V₁ receptors in vascular smooth muscle, causing vasoconstriction
      • V₂ receptors in kidney collecting ducts to increase water reabsorption, and on endothelium to increase vWF and factor VIII release
      • V₃ receptors in the pituitary to stimulate ACTH release
  • Oxytocin
    Short-chain peptide, structurally similar to ADH, which causes:
    • Uterine contraction
    • Let-down reflex
      Stimulates milk release on suckling.
    • Psychological
      Pair bonding.

References

1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
2. Nickson, C. Vasopressin. LITFL.