oa Editorial [Hot Topic: The Pineal Hormone Melatonin in Health and Disease (Guest Editors: Charanjit Kaur)]

Melatonin, first discovered by Aaron B Lerner [1], is synthesized by the pineal gland which is a central structure in the circadian system involved in synchronizing the chronobiology of organisms. The pineal gland consists of two different types of cells: the pinealocytes and interstitial cells. The latter are further classified as the astrocytes and microglia/macrophages [2, 3]. Pinealocytes synthesize the neurohormone melatonin [4], a derivative of the amino acid tryptophan. The secretion of melatonin is regulated by the environmental light/dark cycle via the suprachiasmatic nucleus. Besides the pineal gland, synthesis of melatonin also occurs in other cells and organs such as the bone marrow [5], retina [6, 7], platelets [8], the gastrointestinal tract [9], skin [10, 11] and lymphocytes [12]. However, circulating melatonin in mammals has been reported to be largely derived from the pineal gland. Several experimental and clinical studies have supported antioxidant [13-15], anti-inflammatory [16], neuroprotective [17] and anticonvulsant actions of melatonin [18, 19]. Its oncostatic effects against a variety of tumors such as ovarian, endometrial, prostate, breast, intestinal and lung have also been documented [20-23], and melatonin, either alone or in combination with traditional therapy, has been used in the treatment of human cancers [24]. Beneficial effects of melatonin in treating sleep disorders in children and adults [25-28] as well as alleviating a variety of circadian rhythm-related disorders are well documented [29-31]. It is also known to facilitate the differentiation of neural stem cells into neurons [32]. The production of melatonin is known to decline in old age [33] and in conditions such as Alzheimer's disease [34], dementia [35] and stroke [36]. Melatonin has a remarkable therapeutic potential for treatment of many neurodegenerative disorders due to its antioxidant and immune enhancing properties. It has been shown to improve sleep and has been suggested as a useful add-on drug for treating mild cognitive impairment, epilepsy and cancer. Additionally, melatonin has been shown to be useful in the treatment of hypertension. Experimental data has suggested that melatonin reduces complications of traumatic brain injury. Several patents have been filed for using melatonin, its derivatives and analogues in the treatment or prevention of neurodegenerative diseases, sleep disorders and cancers [37-40]. This thematic issue will present reviews on the use of melatonin in neurodegenerative disorders such as Alzheimer's disease, sleep disorders, peripheral nerve regeneration and in cancer from experts. Although, many clinical trials with melatonin are in progress, future studies to determine the treatment regimens and safety of melatonin in clinical practice are necessary along with further in depth research to understand its role in human health and disease. I am grateful to the authors for their contributions to this volume....