Hypothalamus And Its Hormones

 

Hypothalamus is an ovoid mass located at the base of the brain. It is composed of

many bilaterally paired nuclei (Cell bodies within CNS are called Nuclei). These Nuclei

are separated by the medial portion of the Hypothalamus, the Third Ventricle of the brain. The

pituitary Gland lies below Hypothalamus in a bony depression of Sphenoid bone, called as

“Sella Tunica”.

The hypothalamus is a small region located within the brain that controls many bodily

functions, including eating and drinking, sexual functions and behaviors, blood pressure and

heart rate, body temperature maintenance, the sleep-wake cycle, and emotional states (e.g.,

fear, pain, anger, and pleasure). Hypothalamic hormones play a pivotal role in the regulation of

many of those functions. Because the hypothalamus is part of the central nervous system, the

hypothalamic hormones actually are produced by nerve cells (i.e., neurons). In addition,

because signals from other neurons can modulate the release of hypothalamic hormones, the

hypothalamus serves as the major link between the nervous and endocrine systems.

The hypothalamic hormones are released into blood vessels that connect the

hypothalamus and the pituitary gland (i.e., the hypothalamic-hypophyseal portal system).

Because they generally promote or inhibit the release of hormones from the pituitary gland,

hypothalamic hormones are commonly called releasing or inhibiting hormones.

The major releasing and inhibiting hormones include the following:

• Corticotropin-releasing hormone (CRH), which is part of the hormone system regulating

carbohydrate, protein, and fat metabolism as well as sodium and water balance in the body

• Gonadotropin-releasing hormone (GnRH), which helps control sexual and reproductive

functions, including pregnancy and lactation (i.e., milk production)

• Thyrotropin-releasing hormone (TRH), which is part of the hormone system controlling the

metabolic processes of all cells and which contributes to the hormonal regulation of lactation

• Growth hormone-releasing hormone (GHRH), which is an essential component of the

system promoting the organism’s growth

• Somatostatin, which also affects bone and muscle growth but has the opposite effect as that

of GHRH

• Dopamine, a substance that functions primarily as a neurotransmitter but also has some

hormonal effects, such as repressing lactation until it is needed after childbirth.

Hypothalamus secretes a number of hormones that regulate various body functions.

However, the hormones of Reproductive importance secreted by the hypothalamus are Oxytocin

and GnRH.

1. Oxytocin Hormone

Oxytocin is synthesized in the Supra-Optic and Para-Ventricular nuclei of

Hypothalamus. It is transported from the hypothalamus to the Posterior Pituitary gland

(Neurohypo-physis) in small vesicles enclosed by a membrane, and is stored there.

- It is released from the Neurohypophysis in response to neuroendocrine reflexes.

- Oxytocin is also produced in the Corpus Luteum (CL) of the ewe, cow, and women.

- Chemically, oxytocin is a Polypeptide, comprising of 08 Amino Acids (Octapeptide).

- Half-life of Oxytocin is very short, about 5-6 minutes.

Actions in the Body

It causes contractions of uterine muscles, and thus plays a major role in Parturition.

The responsiveness of smooth muscles to oxytocin is potentiated (increased) by estrogens

(A priming effect).

- It causes contractions in the oviduct and thus involved in the transport of sperm and ovum

into the oviduct.

- In males, it may help in sperm transport through the male reproductive tract.

- It causes contractions in the myoepithelial cells (Smooth muscle cells) of alveoli of

mammary glands, which results in milk let down.

- Exogenous oxytocin has a luteolytic effect in cows, ewe, and goats.

- Ovarian oxytocin stimulates the release of PGF2α from the endometrium.

- In birds, Vasotocin (hypothalamic hormones) has oxytocin like actions; it causes

contractions of the shell gland and vagina of the hen to induce oviposition.

Clinical Applications

It is used to aid delivery if a female has been in labor for an extended period due to a lack

of uterine contractions (uterine inertia). The fetus should be in a normal position.

- It is also used for expulsion of Retained Placenta (RFM).

- It is also used to control the re-occurrence of uterine prolapse.

- In Animal production, it is used for Milk Let down in dairy animals.

2. Gonadotropin-Releasing Hormone (GnRH)

GnRH is synthesized in the cranial part of the hypothalamus (Arcuate nucleus) and stored in the

Median Eminance.

- Chemically, GnRH is a polypeptide, having 10 amino acids (Decapeptide).

- Its Half-life is 6-7 minutes.

Release of GnRH

- It is secreted Episodically (In pulses after every 02 hours).

- Its secretion is controlled through the Feedback mechanism by gonadal steroids

(Estrogens in the female and testosterone in the male).

- Epinephrine and Nor-epinephrine increase GnRH pulse frequency.

- Opioids (e.g. β – Endorphin) and Dopamine reduce GnRH pulse frequency, or

inhibit release from the Median Eminance.

- In sheep & Goats, Melatonin released from the Pineal Gland also stimulates

secretion of GnRH.

- Secretion of GnRH is increased during coitus in Induced Ovulators (Cat &

Camel), followed by Pre-ovulatory LH surge after 15 minutes.

- Pheromones also influence the hypothalamic output of GnRH. Pheromones are

air-borne or water-borne chemical exciters (or inhibitors). They stimulate

Olfactory Bulb and Light track cycles that act on Retina.

Actions of GnRH

- It stimulated the secretion of pituitary gonadotropins (FSH and LH).

- It induces the release of FSH when progesterone prevails, and LH when Estradiol

predominates.

Clinical Uses of GnRH

- It is used to induce ovarian activity in post-partum cows and buffaloes.

- It can also be used for the treatment of cystic ovaries.

- It can also be used for the induction of estrus in an-estrus cases.