Hypothalamus to the endocrine system hormones to pituary glands

Hypothalamus is connected to

pituitary gland (hypophysis) via stalk called infundibulum

Pituitary secretes at least

eight major hormones

Pituitary has 2 major lobes

Posterior pituitary: composed of neural tissue that secretes neurohormones

• Posterior lobe, along with infundibulum make up the neurohypophysis

• Anterior pituitary: (adenohypophysis) consists of glandular tissue

Posterior Pituitary lobe is

neural tissue derived from a downgrowth of brain

• Secretes two neurohormones (oxytocin and ADH)

• Hormones are stored in axon terminals in posterior pituitary and are released into blood when neurons fire

Posterior pituitary consists of

axon terminals of neurons from hypothalamic neurons:

• Paraventricular neurons produce oxytocin

• Supraoptic neurons produce antidiuretic hormone (ADH)

Oxytocin and ADH

• Each composed of nine amino acids

• Almost identical but differ in two amino acids

• Both are positive feedback mechanisms

Oxytocin;

• Strong stimulant of uterine contractions released during childbirth

• Also acts as hormonal trigger for milk ejection

• Acts as neurotransmitter in brain

Antidiuretic hormone (ADH) (AKA vasopressin) ;

• Hypothalamus contains osmoreceptors that monitor solute concentrations

• If concentration too high, posterior pituitary triggered to secrete ADH

• Targets kidney tubules to reabsorb more water to inhibit or prevent urine formation

• Release also triggered by pain, low blood pressure, and drugs

• Inhibited by alcohol, diuretics

• High concentrations cause vasoconstriction 

Anterior Pituitary lobe is

glandular tissue derived from an outpocketing of oral mucosa 

Vascularly connected to hypothalamus via hypophyseal portal system consisting of:

• Primary capillary plexus

• Hypophyseal portal veins

• Secondary capillary plexus 

Hypothalamus secretes releasing and inhibiting hormones to

anterior pituitary to regulate hormone secretion

All six hormones are

peptide hormones in the anterior pituitary

All but growth hormone (GH) activate

target cells via cAMP second-messenger system

All but two are tropic hormones (tropins) that regulate secretion of other hormones

• Growth hormone (GH)

• Thyroid-stimulating hormone (TSH) (tropic)

• Adrenocorticotropic hormone (ACTH) (tropic)

• Follicle-stimulating hormone (FSH) (tropic)

• Luteinizing hormone (LH) (tropic)

• Prolactin (PRL)

Growth hormone (GH)

• Also called somatotropin as it is produced by somatotropic cells

• Has direct actions on metabolism and indirect growth-promoting actions

Direct actions on metabolism -

• Glucose-sparing actions decrease rate of cellular glucose uptake and metabolism (anti-insulin effects)

• Triggers liver to break down glycogen into glucose

• Increases blood levels of fatty acids for use as fuel and encourages cellular protein synthesis

Indirect actions on growth:

• GH triggers liver, skeletal muscle, and bone to produce insulin-like growth factors (IGFs)

• IGFs then stimulate:

  • Cellular uptake of nutrients used to synthesize DNA and proteins needed for cell division

  • Formation of collagen and deposition of bone matrix

• GH stimulates most cells to enlarge and divide, but major targets are bone and skeletal muscle

Regulation of secretion -

• GH release or inhibition chiefly regulated by hypothalamic hormones on somatotropic cells

Growth hormone-releasing hormone (GHRH) -

• stimulates GH release 

  • Triggered by low blood GH or glucose, or high amino acid levels

Growth hormone–inhibiting hormone (GHIH) (somatostatin) -

• inhibits release

  • Triggered by increase in GH and IGF levels

• Ghrelin (hunger hormone) also stimulates GH release

• Thyroid-stimulating hormone (TSH) -

• Tropic hormone that is also called thyrotropin as it is produced by thyrotropic cells

• Stimulates normal development and secretory activity of thyroid

• Release triggered by thyrotropin-releasing hormone from hypothalamus

• Inhibited by rising blood levels of thyroid hormones that act on both pituitary and hypothalamus 

  • Also inhibited by GHIH

• Adrenocorticotropic hormone (ACTH) -

• Also called corticotropin as it is secreted by corticotropic cells

  • Precursor to corticotropin is pro-opiomelanocortin

• ACTH stimulates adrenal cortex to release corticosteroids

• Regulation of ACTH release

  • Triggered by hypothalamic corticotropin-releasing hormone (CRH) in daily rhythm

    • Highest levels in morning

  • Internal and external factors that alter release of CRH include fever, hypoglycemia, and stressors

Gonadotropins (FSH and LH)

• Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are secreted by gonadotropic cells of anterior pituitary

• FSH stimulates production of gametes (egg or sperm)

• LH promotes production of gonadal hormones

  • In females, LH helps mature follicles of egg, triggers ovulation and release of estrogen and progesterone

  • In males, LH stimulates production of testosterone

• LH and FSH both are absent from blood in prepubertal boys and girls

• Regulation of gonadotropin release

  • Triggered by gonadotropin-releasing hormone (GnRH) during and after puberty

  • Suppressed by gonadal hormones (feedback)

Prolactin (PRL) -

• Secreted by prolactin cells of anterior pituitary

• Stimulates milk production in females;  role in males not well understood

• Regulation primarily controlled by prolactin-inhibiting hormone (PIH), which is dopamine

• PIH prevents release of PRL until needed, with decreased levels leading to lactation

• Increased estrogen levels stimulate PRL

  • Reason behind breast swelling and tenderness during menstrual cycle

• Blood levels rise toward end of pregnancy

• Suckling stimulates PRL release and promotes continued milk production