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 inhibted 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, in response to hypothalamic neural stimulus:
    • ADH
      Short-chain peptide which acts on:
      • 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, structually 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.
Last updated 2018-01-16

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