Hormones of Hypothalamus

Prolactin, Gonadotropic, Corticotropin, Thyroid Stimulating Hormone and Growth Hormone are hormones of Hypothalamus. It release and regulate functions of Pituitary gland, that’s why it is known as headmaster gland.

The hypothalamus is located adjacent to the third ventricle in the forebrain and is connected by the hypophyseal stalk to the pituitary gland (hypophysis cerebri) immediately inferior to it.

The hypothalamus receives input from various regions of brain, sensory signals from visceral organs and retina. Hypothalamus controls autonomic nervous system, regulates body temperature, thirst, hunger, sexual behaviour and defense reactions like fear and rage.

Hypothalamus forms the critical link between the nervous system and the endocrine system. The hypothalamus regulates both autonomic nervous activity and several aspects of endocrine function. The latter role is fulfilled through its links with the pituitary gland, which secretes a wide range of hormones.

Hypothalamus – monitors the state of the body’s internal environment by analyzing negative feedback (eg. various concentrations of chemicals in the blood) and regulates the pituitary gland.

Secretory neurons in the arcuate, paraventricular, supra optic nuclei and preoptic area of hypothalamus synthesize and secrete seven neurohormones called hypophysiotropins otherwise known as hypothalamus‑releasing hormones/fac­tors (RH or RF) and hypothalamus inhibitory hormones or factors (IH or IF). These hypothalamic hormones pass via the hypophyseal portal system to pituitary gland and regulate the release of all the six hormones of the adenohypophysis. Regulation of the anterior pituitary secretions is mediated by the reciprocal actions of RH or IH of the hypothalamus and the concentrations of hormones of the pituitary gland

Releasing hormones of hypothamus include-

1. Growth Hormone‑ Releasing Hormone (GHRH/ GRH): Increases the release of GH
2. Thyroid Stimulating Hormone– Releasing Hormone (TRH): Increases the release of TSH
3. Corticotropin‑ Releasing Hormone (CRH): Increases the release of ACTH
4. Gonadotropic ‑ Releasing Hormone ( GnRH/LHRH): Increases release of FSH and LH
5. Prolactin– Releasing Hormone (PRH): increases release of prolactin

Inhibitory hormones of hypothamus include-

1. Somatostatin (SRIF/ GHIH)- Inhibits the release of GH.
2. Prolactin Release Inhibitory Hormone (PIH-Dopamine)– Inhibits the prolactin secretion.

GHRH (Somatocrinin or Somatotropin releasing hormone)

GHRH is a hypothalamic peptide hormone containing 44 amino acids. It regulates GH release by its intermittent release into the portal pituitary vein.

Factors that stimulate release of GHRH – estradiol, progesterone, ADH, serotonin, dopamine, a -adrenergics, stress (trauma and fever), deep sleep and exercise.

Factors that inhibit GHRH release – soma­tomedins, increased levels of glucose and fatty acids, b adrenergics, melatonin and morphines.

GH exhibits a negative feed back inhibition to GHRH release. As a galactopoietic agent GHRH causes 10 to 25 % increase in milk yield, total milk fat, protein and lactose when injected to cow and ewes.

TRH (Thyroid Stimulating Hormone – Releasing Hormone)

TRH is a tripeptide and it stimulates release of TSH. TRH also stimulates the secretion of ACTH, PRL, vasopressin and GH. Circulating level of T₃ or T₄ has negative feedback effect on TRH and TSH.

Corticotropin Releasing Hormone (CRH)

Corticotropin Releasing Hormone (CRH) is a 41 amino acid peptide that stimulates ACTH synthesis and release by the corticotroph cells of pituitary gland.

Stress enhances its syn­thesis and release. CRH secretion is influenced by circadian light- darkness cycle. In diurnal animals (cattle, dog, horse) it stimulates ACTH synthesis during morning hours.

Other functions of CRH include stimulating the sympathetic nervous system, decreasing elevated body temperature, and suppressing reproductive functions and sexual activities, suppressing GH release and altering behavior.

Somatostatin (Somatotropion Release – Inhibiting Hormone – SRIF)

Somatostatin is a hypothalamic peptide present in two forms – 14 and 24‑28 amino acids, also present in pancreas.

Potent inhibitor of GH, also inhibits TSH, PRL and ACTH, insulin and glucagon.

Its production is stimulated by GH release from the pituitary, ingestion of carbohydrate, fat, protein, and high level of glucose, amino acids, increased output of glucagon, gastrin, VIP, secretin, CCK, acetylcholine and b adrenergics.

Somatomedin C (from the liver), dopamine, insulin, estradiol and a -adrenergic inhibit the release of somatostatin. Somatostatin has been isolated from brain, pancreas and intestine.

GnRH (Gonadotropic Releasing Hormone) / (LHRH)

GnRH (Gonadotropic Releasing Hormone) is a decapeptide that stimulates the release of both FSH and LH. The frequency of GnRH stimulus is the major regulator of the relative proportions of FSH and LH synthesis and release i.e., less frequent GnRH pulse leads to FSH secretion and more frequent GnRH pulse causes LH secretion.

When estradiol predominates GnRH favors the LH surge to effect ovulation. Under the influence of high progesterone level GnRH induces FSH to stimulate folliculogenesis and steroidogenesis.

Inhibin a glycoprotein hormone from ovarian follicle and testicular seminiferous tubules inhibit GnRH secretion. Prolactin and LH inhibit GnRH via short‑loop negative feed back effect.

GnRH secretion is also influenced by light‑ dark cycle, phero­mones and stress. Dopamine, endorphins and increased levels of ovarian steroids provide inhibitory influence on GnRH release.

In sheep and goat increased production of melatonin during longer dark periods / short day length of autumn initiates GnRH synthesis, thus the puberty in young ones and breeding season in adult. In contrast, lengthening days of spring results in minimal melatonin production and beginning of breeding season in horse.

Prolactin – Inhibiting Hormone (PIH )

Dopamine is the PIH. It also inhibits TSH. Apomorphine and ergot alkaloids inhibit prolactin secretion. Bromocriptine – a long acting dopamine receptor agonist reduces prolactin secretion.