Question

Which of the following mechanisms are involved in mediating the increased prolactin secretion during lactation? [1.5 marks]

A . increased progesterone levels

B. increased sensitivity of prolactin negative feedback

C. decreased sensitivity of prolactin negative feedback

D. decreased prolactin gene expression due to the effects of oestrogens during pregnancy

E. increased tyrosine hydroxylase activity and expression

please explain the answer.

Answer 1

ANSWER = OPTION – C

EXPLANATION –

Hormones in our body are tightly regulated and this regulation mainly happens by feedback regulation.

This feedback regulation can be of broadly 2 types;

1. Positive feedback regulation: Sometimes, the action of the hormone on a target tissue causes surplus secretion of hormone. Eg - secretion of luteinizing hormone (LH); action of estrogens on the anterior pituitary cause’s release of LH, before ovulation. This secreted LH further acts on the ovaries to secrete more estrogens, which in turn stimulates the secretion of further LH from anterior pituitary. This action causes a triggered release of LH, also called as ‘LH surge’.

2. Negative feedback regulation: The majority of the hormones secreted by brain are regulated by the negative feedback mechanism. When the levels of a hormone increase in the blood, the hormones inhibit the release of themselves by inhibiting the secretion of releasing & inhibiting hormones from the hypothalamus - which subsequently inhibits the release of hormones from the pituitary gland.

For example, brain-pituitary thyroid axis, brain-pituitary-growth hormone axis etc.

PROLACTIN

• Prolactin is a single chain amino acid polypeptide hormone secreted by the anterior pituitary( 198 amino acid long).
• Prolactin is synthesized in specialized cells in pituitary gland called Lactotrophs. Dopamine, from hypothalamus, inhibits the release of prolactin from pituitary. The concentration of dopamine in hypothalamus is high & is more abundantly present in median eminence where it can be released in the portal system and can easily reach anterior pituitary. Any kind of drugs that decrease dopamine levels in portal circulation stimulate the release of more prolactin while exogenous administration of dopamine inhibits prolactin secretion.
• Release of Prolactin - secretion of prolactin is controlled by Prolactin Inhibiting Hormone (PIH) or dopamine, from hypothalamus, which inhibits the release of prolactin. Other than dopamine, secretion of prolactin is also controlled by other neurotransmitters like TRH and vasoactive intestinal peptide (VIP).There are other factors also which control the release of prolactin like light, sound, smell and stress.
• There is no releasing factor from hypothalamus for lactation & only the anterior pituitary secretes prolactin (PRL) which is the major regulator of lactation in mammals. Prolactin promotes the development of mammary glands and production of milk. However, there is an inhibiting factor for lactation i.e.. dopamine, present in hypothalamus. The prolactin is released by the lactotropes in anterior pituitary in to the general circulation from where prolactin goes to its target cells in the mammary gland and promotes the synthesis and release of milk into the alveoli of mammary glands when stimulated by suckling .Prolactin and oxytocin, together are needed during the period of lactation.

Dopamine is the only molecule which inhibits the release of prolactin from anterior pituitary and since it is present in hypothalamus in high amount, we can call it as the inhibiting factor for prolactin. It is also present in high quantity in median eminence form where it is transported to the anterior pituitary via portal capillary network. There are several factors which contribute to the secretion or inhibition of prolactin from anterior pituitary:

a. Drugs which increase dopamine levels in pituitary portal capillaries influence the PRL secretion.

b. External application of dopamine influences the release of prolactin from anterior pituitary.

                                                           The lactotropes in the anterior pituitary contain receptors for dopamine such that the increase or decrease in the portal dopamine is directly correlated with the changes in secretion of prolactin from anterior pituitary. Though there is no true prolactin releasing hormone in hypothalamus but factors like TRH, vasoactive intestinal peptide (VIP) and oxytocin influence the stimulation of prolactin release.