MELATONIN, GESTATION AND FETAL DEVELOPMENT Haldar C and Yadav R Pineal Research Lab, Department of Zoology, Banaras Hindu University, Varanasi – 221005, India. Correspondence to be addressed to: Prof. C. Haldar, Ph.D. Email - chaldar@bhu.ac.in, chaldar2001@yahoo.com SUMMARY The pineal gland, the transducer of several environmental cues through its hormone, melatonin, is known to play a critical role in the reproduction of several seasonally breeding mammals. The role of pineal gland in the control of male reproduction is already established beyond doubt. The logical hypothesis that the pineal gland would be involved in the aspects of female reproduction, particularly gestation, embryonic development and fetal growth, has been tested in a few seasonally breeding mammals and this paper reviews the outcome as of now. Evidence for establishment of an inverse relationship before pregnancy followed by moderately high activity with a positive correlation during pregnancy and a sudden increase in pineal activity during parturition has been produced, which suggests that high melatonin level has something to do with parturition or early abortion. The pineal gland perhaps maintains the normal physiology during gestation and post-parturition periods. This is further reflected in the prolactin levels of fetal sheep which is modulated by the photoperiod experienced by the mother during gestation. Maternal photoperiodic exposures during gestation and lactation periods alter the neonatal growth and sexual matu- ration of the Indian palm squirrel via the maternal transfer of photoperiodic information through the pineal gland, as aspect known as “pineal programming”. The primary source of melatonin for the fetus is the maternal pineal gland, which is trans- ferred across the placenta during conception and, later after parturition, through the milk, which suggests that a maternally generated melatonin rhythm would be expressed in fetal cerebrospinal fluid as well as in the circulation. Further, maternal melatonin is necessary for normal somatic growth and postnatal development of reproductive organs of the offspring. Melatonin ultimately affects reproductive activity by modulating hypothalamic neuroendocrine circuits whose activity is necessary for gonadal function. The influence of melatonin on reproductive development begins during the prenatal period and extends into the postnatal life. Photoperiodic information, mediated via the pineal gland may be important for maintaining gestational physiology as well as postpartum recovery in female rodents. Further, melatonin has been shown to play adaptive role in the maintenance of delayed embryonic development in bats. Correlation between a developmental decline in melatonin levels and the timing of puberty in humans led to the speculation that melatonin regulates the timing of puberty in the human. These studies on the influence of melatonin on gestation indicate two major inferences viz., melatonin could be one of the major hormones establishing homeostasis during gestation since any disturbance in the level of melatonin causes abortion, and melatonin is one of the hormones of great adaptive significance for embryonic diapause. Key words: Embryonic growth, gestation, melatonon, photoperiod, pineal gland, puberty. INTRODUCTION Reproductive function in mammals consists of an intricate interplay of hormonal events that are respon- sible for the development and maturation of gametes, puberty, major events of the reproductive cycle (i.e., ovu- lation and sexual receptivity) and preparation of the uterus for the possible implantation of embryos. The pineal gland is known to play a critical role in the reproduction of several seasonally breeding mam- malian species (1). The seasonality of reproduction is usually enforced by a dependence on the day length per- ceived, which in turn regulates the synthesis of melato- nin, the pineal hormone. The daily duration and the level of melatonin regulate the secretion of gonadotropins / gonadal steroids. They regulate the breeding in long day breeders such as squirrels, ferrets, voles, hamsters etc. (2-5). In the broadest sense, then, melatonin influences reproduction by restricting the season of conception enabling the offspring to be born under the most favor- able environmental conditions. The role of pineal gland in the control of male reproduction has been quite clearly established. However, its role in female reproduction, especially during gesta- tion, is not clear, although the females allocate the maxi- mum energy towards the begetting and bear most of the cost of reproduction. The pineal gland of the female pro- vides indications of higher activity, with a higher circulat- ing level of melatonin during the reproductive period (6). Therefore, it is conceivable that the pineal gland is in- volved in the aspects of female reproduction, particularly gestation, embryonic development and fetal growth. Un- til a little more than a decade ago there was no informa- tion on i) the influence of pineal gland on gestation, ii) the influence of pregnancy on the metabolism and function of pineal, and iii) the interrelationship between maternal and fetal pineal glands. 1 JER : 2006 (1) : 1 - 12