Question

Given below are two statements:
Statement (I) : Progesterone has the inhibitory effect of gonadotropin release.
Statement (II) : Melatonin modulates the frequency of GnRH secretion from hypothalamous.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Statement (I) is false but Statement (II) is true
• B. Both Statement (I) and Statement (II) are false
• C. Statement (I) is true but Statement (II) is false
• D. Both Statement (I) and Statement (II) are true

Hint:

Key endocrine regulations:
- Progesterone \(\rightarrow\) Negative feedback, inhibits GnRH/LH pulse frequency (maintains pregnancy).
- Melatonin \(\rightarrow\) Signals day length, modulating GnRH pulse frequency to coordinate seasonal breeding.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The hypothalamic-pituitary-gonadal (HPG) axis regulates reproductive activity in animals.
This axis is controlled by complex positive and negative feedback loops involving gonadal steroids and environmental cues, such as photoperiod, which are integrated by the pineal hormone melatonin.

Step 2: Detailed Explanation:
Let us analyze both statements.
Statement (I) is correct. Progesterone, the major steroid hormone produced by the corpus luteum during the luteal phase of the estrous cycle or by the placenta during pregnancy, exerts strong negative feedback on the hypothalamus and anterior pituitary.
It inhibits the frequency of Gonadotropin-Releasing Hormone (GnRH) pulses from the hypothalamus.
This suppression of GnRH pulse frequency directly reduces the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary, preventing the preovulatory LH surge and subsequent ovulation during pregnancy or the diestrus phase.
Statement (II) is correct. Melatonin is synthesized and secreted by the pineal gland primarily during hours of darkness.
In seasonal breeders (such as sheep, goats, and horses), melatonin acts as a chemical messenger of photoperiod.
It binds to melatonin receptors in the hypothalamus, modulating the activity of kisspeptin neurons.
These neurons, in turn, regulate the pulse frequency of GnRH secretion.
In short-day breeders (like sheep), high melatonin levels during autumn stimulate GnRH pulse frequency, initiating the breeding season.
In long-day breeders (like horses), low melatonin levels during spring have the stimulatory effect.
Therefore, both Statement (I) and Statement (II) are true.

Step 3: Final Answer:
The correct option is (D).