Lecture Outline #15: The Pineal Gland
I. GENERAL PROPERTIES OF THE PINEAL GLAND
1. Research during the past decade suggests that the pineal gland has more complex physiological functions than originally thought, but all of these functions have not been clearly deciphered.
2. The pineal gland, located behind the hypothalamus, is innervated by sympathetic neurons.
3. This gland synthesizes melatonin from the amino acid tryptophan. (Fig. 20.4 in text, or click here.)
4. Melatonin is secreted mainly during the night, when there is no light. The sequence of events leading to its secretion are somewhat complicated (Fig. 20.7)
(a) in the absence of light, (i.e., the absence of action potentials traveling up the optic nerve) the suprachiasmatic nucleus in the hypothalamus sends impulses through the sympathetic nervous system and secretes norepinephrine (NE) that reacts with receptors on the pinealocytes.
(b) via cAMP-mediated events, serotonin N-acetyltransferase is transcribed and translated to convert serotonin into N-acetylserotonin in the pinealocytes.
(c) N-acetylserotonin is subsequently converted to melatonin by the action of the enzyme HIOMT (hydroxyindole-O-methyltransferase) (better known as acetylserotonin O-methyltransferase).
5. So many potential
functions have been suggested for melatonin in recent years that it is
difficult to itemize and/or confirm all of them. Nevertheless it does appear that
the hormone has biological effects on reproduction and it promotes sleep (and
hibernation).
6. Melatonin also reportedly influences the secretion of ACTH, corticosterone, b-endorphin, prolactin, renin, vasopressin, oxytocin, growth hormone, and luteinizing hormone.
7. Melatonin may also affect the manifestation of psychosis, depression, anxiety and aggression.
8. There are also reports that it acts as an anti-oxidant, delaying aging and reducing cancer???