79. MSH and the pineal gland

Question 1

Pineal gland (corpus pineale, CP)

Answer 1

• Pineal gland is an organ located caudo-dorsally from hypothalamus and composed of pinealocytes and glial cells.
• The name: pineal gland to the pine-cone like shape of the organ (Latin: pineas, pine).
• Serotonin and melatonin are synthesized in here. It is relatively independent of the blood-brain barrier. This makes possible for peptides, originating from blood, to influence its functioning.
• Its melatonin production has a circadian rhythm which depends on light exposure. Decreased illumination (darkness) acts positively, while strong illumination (daytime) acts negatively on melatonin production.
• no direct connections with the central nervous system, it is innervated by postganglionic sympathetic nerve fibres.

Pinealocytes are very rich in biogen amines (dopamine, serotonin, histamine) and hypothalamic peptides (GnRH, TRH, somatostatin, neurophysin).

Question 2

Synthesis and release of melatonin

Answer 2

Question 3

Pineal gland – function

Answer 3

Melatonin in the skin has an effect opposite to MSH: it induces concentration of pigment granules and the skin becomes pale. This effect is extremely important in sub-mammalian animal species.

In mammals, three important effects of melatonin can be separated: effects on sexual function, psychic effects, role in defence against free radicals.

Question 4

Influence of sexual cycle

Answer 4

In mammals, the production of melatonin in case of light deprivation is more important: it results in the termination of hypothalamic GnRH and pituitary FSH or LH production.

• Daylight is the inhibiting stimulus of melatonin synthesis. The important role of melatonin, induced by the lack of light exposure, is proven by its estrous cycle initiating effect when administered intracerebrally in the sheep.
• At he same time, melatonin inhibits the estrous cycle of the horse.
• These phenomena refer to the basic role of melatonin in influencing sexual cycle of seasonally estrous animals.
• Illumination does not influence human sexual cycle as much as it does in animals.

Question 5

Human ovulation

Answer 5

It is permissive for ovulation.

-Night time peak of melatonin synthesis is significantly decreased before and during ovulation: inhibition of LH production decreases: ovulation is stimulated.

Question 6

Seasonal emotional fluctuations

Answer 6

In short diurnal periods depression occurs irregularly.

-Lethargy, increased appetite, sleepiness, carbohydrate-hunger.

It is assigned to melatonin overproduction.

Symptoms can be entirely eliminated with phototherapy.

Question 7

MSH

Answer 7

The mostly known effect of this hormone family is the stimulation of pigment granule production and transport along the microtubule system in pigment cells: resulting in the decoloration of the cells

Alpha MSH is formed from ACTH,

Beta MSH is formed by beta lipotropin.

Gamma MSH is cleaved from the N-terminal of POMC.

Question 8

Effect of MSH on the pigment cells

Answer 8

From the nucleus of pigment cells to the periphery there is a microtubule system arranged in a radial symmetry.In resting state, pigment granules are located around the nucleus.

MSH exerts the migration of pigment granules along the microtubule matrix system, which is a process connected to the activation of motoproteins.

The pigment is evenly distributed in the cell, and the cell or tissue becomes darkened. MSH also facilitates pigment synthesis.