Neuroendocrine and the Pituitary Gland

Diagram showing intracrine, autocrine, paracrine, endocrine, neuroendocrine

Where is the hypothalamus located?

At the base of the brain

What is the role of the hypothalamic pituitary axis?

• The hypothalamus detects stimuli

• It secretes multiple hormones to control widespread homeostatic functions

• Uses the pituitary gland as an output organ (initiates response)

What is the pituitary gland also known as?

Hypophysis

Name the 2 nuclei in the hypothalamus

• Paraventricular nucleus

• Suprachiasmatic nucleus

What are the functions of the paraventricular nucleus (PVN) of the hypothalamus?

• Neuroendocrine role: Secretes CRH and TRH

• Integrates sympathetic and parasympathetic output to regulate the autonomic nervous system

What are the key functions of the suprachiasmatic nucleus (SCN) of the hypothalamus?

• Suprachiasmatic nucleus receives light input from the retina and synchronises circadian rhythms with the light dark cycle

What is the key function of the hypothalamus?

Hypothalamus regulates sleep-wake cycle, feeding behaviour, and body temperatures

How does the hypothalamus control the anterior pituitary?

The hypothalamus indirectly controls the anterior pituitary

The hypothalamus releases regulatory hormones which act on the anterior pituitary to control hormone secretion

What does the anterior pituitary release?

• Regulatory hormones

• Primary hormones that act on target endocrine cells

How does the hypothalamus control the posterior pituitary?

• It controls it directly

• Primary hormones are produced in the hypothalamus

Which hormones are associated with the posterior pituitary and where are they mainly produced?

• Oxytocin is mainly produced in the supraoptic nucleus

• ADH is mainly produced in the paraventricular nucleus

What are hypothalamic releasing factors?

Hormones produced by the hypothalamus that stimulate the anterior pituitary to release specific hormones

Name the main hypothalamic releasing factors

• CRF

• TRH

• GHRH

• GnRH

• PRF

What do the following releasing factors stimulate?
CRF

TRH

GHRH

GnRH

PRF

CRF stimulates ACTH release.
TRH stimulates TSH release.
GHRH stimulates growth hormone release.
GnRH stimulates LH and FSH release.
PRF stimulates prolactin release.

What is the anatomy of the pituitary gland?

What is the difference between trophic and primary hormones of the anterior pituitary?

• Trophic hormones act on other endocrine glands

• Primary hormones act directly on target tissues

Which anterior pituitary hormones are trophic?

ACTH

TSH

FSH

LH

Which anterior pituitary hormone stimulates the adrenal cortex?

ACTH

Which anterior pituitary hormone stimulates the thyroidal gland?

TSH

Which anterior pituitary hormones regulate reproductive function?

FSH and LH

What are the main functions of growth hormone and prolactin?

Growth hormone stimulates growth and metabolism, while prolactin stimulates milk production

How does the hypothalamus control hormone secretion from the anterior pituitary?

Hypothalamic neurones release hypophysiotropic hormones into capillaries in the median eminence

These travel through the hypothalamo-pituitary portal vessels to the anterior pituitary, where they regulate hormone secretion from pituitary cells into the blood stream.

Diagram of the hypothalamic-pituitary axis

What are the effects of growth hormone (a.k.a somatotrophin, somatotropin)

• Increase cell size, number, and differentiation

• Stimulate protein synthesis

• Stimulate fat utilization

• Alter carbohydrate metabolism

What is IGF-1 and what are its key roles and properties?

Insulin-like growth factors (IGFs) are polypeptide hormones mainly produced by the liver in response to growth hormone

• IGF-1 mediates many effects of GH

• Both IGF-1 and GH act via cell surface receptor tyrosine kinases

• IGF-1 has a long half life compared to GH

• IGF-1 is important for bone growth and overall tissue growth

Where do IGFs act?

At a receptor tyrosine kinase

When is GH released?

In response to growth hormone releasing hormone (GHRH)

When is GH release decreased?

By growth hormone-inhibiting hormone (GHIH)

What releases GHRH?

Arcuate nucleus

Where is somatostatin released from?

Periventricular nucleus

Why is growth hormone important?

GH stimulates growth and metabolism, largely by increasing protein synthesis and acting via IGF-1 to promote tissue and bone growth

What happens in GH deficiency?

Reduced protein synthesis and impaired growth and metabolism due to lack of GH/IGF-1 action.

What is pituitary dwarfism?

Short stature caused by GH deficiency, often due to general anterior pituitary dysfunction or specific GH deficiency.

Can dwarfism occur even if GH levels are normal?

Yes - if there is a hereditary IGF-1 deficiency, GH is present but cannot stimulate growth effectively.

What is the effect of GH decline after adolescence?

Accelerated aging-like effects, including reduced protein synthesis and tissue maintenance.

What is gigantism?

Excess GH secretion during childhood (before epiphyseal plates close), usually due to a pituitary tumour, causing excessive linear growth.

What is acromegaly?

Excess GH secretion after adolescence (after epiphyseal closure), usually from a pituitary tumour, causing enlarged hands, feet, and facial features.

What are the indirect effects of GH?

• Growth

• Bone

• Protein synthesis

What are the direct effects of GH?

• Increased lipolysis

• Increased blood glucose

IGF-1 is produced mainly by liver in response to GH. It mediates long term growth effects. How does it regulate RH?

IGF-1 is produced mainly by the liver in response to GH and mediates the long-term growth effects of GH.

What stimulates GH release?

GHRH, sleep, exercise, stress, and low blood glucose.

What inhibits GH release?

Somatostatin (GHIH) and negative feedback from GH and IGF-1.