Abstract
For more than a half century the hormone melatonin has been associated with vertebrate reproduction, particularly in the context of seasonal breeding. This association is due in large measure to the fact that melatonin secretion from the pineal gland into the peripheral circulation is a nocturnal event whose duration is reflective of night length, which of course becomes progressively longer during winter months and correspondingly shorter during the summer months. The nocturnal plasma melatonin signal is conserved in essentially all vertebrates and is accessed not just for reproductive rhythms, but for seasonal cycles of metabolic activities, immune functions, and behavioral expression. A vast literature on melatonin and vertebrate biology has accrued over the past 60 years since melatonin's discovery, including the broad topic of animal reproduction, which is far beyond the scope of this human-focused review. Although modern humans in the industrialized world appear in general to have little remaining reproductive seasonality, the relationships between melatonin and human reproduction continue to attract widespread scientific attention. The purpose of this chapter is to draw attention to some newer developments in the field, especially those with relevance to human fertility and reproductive medicine. As the vast majority of studies have focused on the female reproductive system, a discussion of the potential impact of melatonin on human male fertility will be left for others.
Highlights
BRIEF INTRODUCTIONBy virtue of being a small, amphiphilic, indoleamine molecule, melatonin (5-methoxy-N-acetyl tryptamine) is synthesized de novo from serotonin (5-hydroxy-tryptamine), a highly dispersed biologically active molecule in its own right
There is far little substantiated information on the potential association between clinical syndromes involving the reproductive system and melatonin deficits, or melatonin receptor polymorphisms
Melatonin and/or novel analogs of melatonin will eventually find their place in the armamentarium of reproductive medicine in ways that we probably can’t even imagine
Summary
By virtue of being a small, amphiphilic, indoleamine molecule, melatonin (5-methoxy-N-acetyl tryptamine) is synthesized de novo from serotonin (5-hydroxy-tryptamine), a highly dispersed biologically active molecule in its own right. Melatonin and Human Reproduction expression systems and ex vivo) are in the nanomolar range [cf [7]], whereas many if not most experimental protocols have employed very pharmacological concentrations to achieve significant effects Another point to consider is that plasma melatonin levels are physiologically elevated for many hours at nighttime, protocols often expose tissues or cells to only very short melatonin treatments, which may be physiologically irrelevant. The nocturnal melatonin signal duration is of major importance for physiological seasonality, the circadian phasing of the melatonin signal has important ramifications for general circadian functions, including body temperature, endocrine rhythms, and sleep [10] This is in part due to fact that melatonin receptors are expressed in the SCN and can mediate phaseshifting feedback effects of melatonin. The goal of this chapter is stimulate future research into the relationship of melatonin to human reproductive function in anticipation of generating novel diagnostic and therapeutic tools to improve human health and fertility
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