Pineal Gland

Hi :) In this physiology post we'll investigate the differences in pineal gland function between the lower and higher vertebrates. I'll also describe the pathway for light detection and the subsequent release or synthesis of melatonin and explain the importance of this substance in maintaining circadian rhythm. I'll finish off by explaining how melatonin regulates many aspects of reproduction including the variation in species breeding seasons.

Pineal Gland Function

The pineal gland secretes melatonin, a potent antioxidant which controls the sleep/wake cycle, and is highly innervated by the sympathetic nervous system. It consists of primordial photoreceptive cells and retains light sensitivity in lower vertebrates (such as fish). However, in higher vertebrates it has evolved as a secretory organ. It has no photoreceptivity but responds to light-encoded information which is relayed to the pineal gland.

Pathway of Light to the Pineal Gland:

This pathway is summarised in the flow diagram below:

Light reaches retinal photoreceptors

                                                    ↓ via the retinohypothalamic tract

Suprachiasmatic Nucleus

↓

Sympathetic nerve fibres extend from the superior
cervical ganglion to the pineal gland

↓

Norepinephrine binds to β-adrenergic receptors on pinealocytes

↓

cAMP promotes AANAT synthesis

↓

NAT converts serotonin to melatonin in the hours of darkness.

Melatonin is not stored and instead is released immediately following synthesis. Synthesis is regulated by the enzyme AANAT (this stands for arylalkylamine N-acetyltransferase. We don't need to remember this though!). Reduced light causes the release of norepinephrine from sympathetic nerve terminals in the pinealocytes and this results in increased AANAT synthesis and therefore increased melatonin synthesis and release. 

Melatonin synthesis must be highly regulated to maintain circadian rhythms. A circadian rhythm is any self-sustaining biological process that oscillates approximately every 24 hours. The body determines day length based on the duration of melatonin secretion at night (and not the concentration of melatonin in the body). 

Melatonin and Reproduction

Melatonin influences GnRH secretion (Follicle stimulating hormone and lutenising hormone). Thus the length of daylight can influence gonadotropin secretion, reproductive cycles, gonad size as well as the timing and onset of puberty.

Seasonal Breeders

With these animals, reproduction is timed so that offspring are born at an optimal time of year when food and resources are abundant (spring). Photoperiod alters melatonin secretion by the pineal gland which regulates the release of gondotropin releasing hormone (GnRH) from the hypothalamus. This alters FSH and LH secretion from the anterior pituitary.

Melatonin signals the duration of photoperiod and the interpretation of this by the central nervous system will vary with species. In short-day breeders (sheep and goats) melatonin is progonadotropic while in long-day breeders (horses, rodents, and birds) it is antigonadotropic.

Melatonin may also affect reproduction in males by influencing testis diameter, particularly in short-day breeders. 

That's all for this post, if you have any questions please feel free to ask :)