Mechanism of Melatonin Stimulation of Growth Hormone Release Via Somatostatin Inhibition

3258 Words Oct 3rd, 2008 14 Pages
Mechanism of melatonin stimulation of growth hormone release via somatostatin inhibition

Joseph Angelo Israel Peguit
University of the Philippines in the Visayas Cebu College
Natural Sciences and Mathematics Division

Abstract

Secretion of growth hormone is affected by two primary hormones: Growth hormone-releasing hormone and somatostatin. The former is the primary stimulant to promote growth hormone secretion while the latter is the primary inhibitor that prevents growth hormone secretion. Many studies have already shown that melatonin stimulates secretion of growth hormone via somatostatin inhibition but its mechanisms remain unknown. I hypothesized that melatonin inhibits somatostatin at transcription level through
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Biosynthesis

The primary production of melatonin occurs in the pineal gland. Synthesis and secretion of melatonin is activated by darkness and suppressed by light. The photic inputs from the retina travel via the retino-hypothalamic tract to the suprachiasmatic nucleus (SCN) of the hypothalamus, then to the superior cervical ganglion and finally to the pineal gland[5,10]. Tryptophan is the main amino acid needed for melatonin production. Melatonin production is a secondary product from serotonin. Serotonin is acted upon by N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) to form melatonin. Among the reactions, NAT is considered to be the rate limiting step in the formation of MT.

Figure 1 Biosynthetic pathway of Melatonin Synthesis (Tan et.al 2006) or [9]

Metabolism

Melatonin must be converted to a more hydrophilic form since it is more of lipophilic than is hydrophilic[7,10]. It is then excreted via the kidneys after conversion[10]. The major catabolism of MT happens in the liver. Figure 2 catabolism of melatonin in Liver (Tan et.al 2006) or [9]

Melatonin Receptors

The receptors for melatonin had already been characterized. The subtype Mel1a dominates the receptors found in the SCN[6]. Though the pattern of receptor distribution varies from species to species but in general mammals contain fewer types of receptors than non-vertebrates [7,8]. The types of receptor expressed are Mel 1a (MT1) and Mel 1C [8].The

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