Melatonin is a neurohormone secreted by the pineal gland. Its secretion follows strictly a circadian rhythm with the acrophase occurring during the dark phase and the nadir during the light phase in both nocturnal and diurnal animals.^1,2 Its amino acid precursor is tryptophan which undergoes a stepwise conversion, first to hydroxytryptophan and then to hydroxy tryptamine and eventually to melatonin. Although melatonin is primarily synthesized within the pineal gland, it can be found in the retina and some other sites. In mammals however, a large proportion of the hormone, if not all, found peripherally is believed to be derived from the pineal gland, being that pinealectomy abolishes its synthesis and function.³ Pinealectomy also abolishes the rhythm of melatonin secretion.

Melatonin was first isolated and named in 1958 by Lerner and his colleagues.⁴ McCord and Allen in 1917 first discovered that feeding extracts of cow pineal gland to tadpoles caused lightening of their skin due to contraction of the dark epidermal melanophores, this is believed to be related to the mechanism by which some amphibians and reptiles change the color of their skin.⁵ Studies have also focused on the possible role of melatonin on pigmentation in lower vertebrates and gonadal maturation in mammals.⁶ Melatonin is present in plants and some food crops like rice, corn, tomato and other edible fruits.^7,8 It modulates a number of physiological functions e.g. sleep, circadian, visual, cerebrovascular, reproductive, neuroendocrine, and neuroimmunological functions.⁹ Coupled with its effects on body physiology it also influences behavioral processes like learning and depression.¹⁰ Melatonin receptor binding sites have been found in brain regions associated learning and memory.¹¹ Studies have shown that melatonin reduces recognition time resulting in the facilitation of short-term memory¹² together with its effects on passive and active avoidance learning;¹³ it can be safe to say that melatonin may be beneficial in enhancing learning and memory. Most studies involving melatonin have concentrated on intravenous, intraperitoneal,^14–17 intra-amygdala,⁴ and intra-cerebroventricular routes of administration and those that have used the oral route have studied the effects of melatonin when added to drinking water and not as a bolus as used in our study. Secondly, the duration of most melatonin studies have been for the most part 14 days with most researchers studying acute effects of melatonin.¹⁸ We wanted to examine possible effects of daily oral administration of timed release melatonin when used over a longer duration of time.

The behavioral models used were the open field, the Y maze and the radial arm maze. Mice in respective groups received vehicle (normal saline), standard drug (Scopolamine 0.5 mg/kg i.p. single dose) or one of two doses of melatonin (5 and 10 mg/kg respectively for a period of 30 days). Tests were carried out after the last dose of melatonin. The behavioral tests were conducted in a large quiet room between the hours of 8 a.m. and 9 a.m. At the beginning of the test, each animal was placed in the apparatus and its behavior was videotaped for subsequent analysis.

An assessment of the effects of oral melatonin (TR) on novelty induced behaviors and working memory in mice was made. Melatonin modulates a number of physiological functions, some of which include sleep, immune functions, contraction of smooth muscle and body temperature.²² Its behavioral effects were examined after thirty days of a daily dosing regimen using three behavioral models.

The present study revealed that in the radial arm maze, sub chronic administration of oral timed release formulations of melatonin resulted in a significant increase in working memory compared to the scopolamine induced of memory loss. In comparison to the saline controls, melatonin administered at 10 mg/kg actually led to a decrease in performance. Melatonin also caused a significant reduction in total arm entry at 10 mg/kg whereas at 5 mg/kg the difference was not significant.

The Y-maze is a behavioral model that can be used to examine locomotor activity as well as learning and memory; it assesses hippocampus- dependent navigational behaviors of rodents.²³ The results of the Y maze studies showed an improvement in spatial memory scores in mice that received 5 mg/kg of melatonin when compared to scopolamine control, at 10 mg/kg, no significant improvement was seen.

The neurotransmitter acetylcholine (ACh) is known to be important in learning and memory processes in the hippocampus as evident by association of cholinergic activity with the hippocampus is related with memory. Subnormal activity is associated with spatial memory impairment as shown with drugs that impair cholinergic transmission e.g. scopolamine as used in this study. The radial arm maze is an appetitive motivated task useful in assessing spatial reference as well as spatial working memory performance and factors affecting these processes.²⁴ The radial arm maze is undoubtedly one of the most commonly used maze for assessing working memory in rodent studies,²¹ the two commonly used measures of performance are the number of arms in the maze and memory capacity.

Working memory can be defined as short term memory of an object, stimulus, or location used within a testing session, but not necessarily between sessions, this differs from reference memory, which is memory that would typically be acquired following repeated training, and persist long periods of time usually days to months. In the radial arm maze, melatonin caused an improvement in spatial working memory at both doses used when compared to scopolamine control. The effects of melatonin on spatial learning and memory processes have been documented and differ in many respects although quite a number of studies have reported that melatonin given via the oral, intraperitoneal or any route in rodents resulted in improvement in learning and memory processes especially in models of memory loss^25,26 and advocacy on its suitability in the management of Alzheimer’s disease and mild cognitive impairment is ongoing. On the contrary however, some studies have also demonstrated its ability to impair cognition in healthy humans, as evidenced by a deterioration in performance in response and reaction time scores in visual and auditory tasks²⁷ also, impaired performance in a psychomotor digit-symbol substitution test have also been reported.²⁸ Soto Moyano et al²⁹ reported a decline in the visuospatial performance of rats that received exogenous melatonin 30 minutes before radial arm tests.

Some earlier studies had suggested that hypothermic properties of melatonin may result in a reduction of cerebral processing speed,³⁰ affecting cognition indirectly rather than by a direct effect on brain areas that are relevant to cognition. While this theory of ’cerebral freezing’ is rather fascinating to us, we believe that effects of melatonin on memory can be demystified by a meticulous search for an understanding of how it may influence neurotransmitters that are involved in memory processes. The present study shows that melatonin improves memory significantly in comparison to a model of memory loss; while on the contrary, it actually caused a reduction in memory task performance in healthy mice. We believe that melatonin’s effect on memory processes is probably driven through an enhancement of hippocampal cholinergic transmission when there is a pre-existing deficit, however, the influence is not strong enough to fully restore memory functions.

How melatonin exerts these effects on neurobehavior is still subject to continuous study, it has, however, been suggested that the neurobehavioral effects of melatonin use could be secondary to reduced levels of psychomotor vigilance, as revealed by studies that demonstrated increased reaction time scores while evaluating alertness during sustained attention.³¹

The results of the open field test showed a reduction in rearing and grooming behavior while locomotor activity was not significantly different from saline control. The open field is a commonly used behavioral test with horizontal and vertical locomotion being commonly recorded behaviors.³² Both locomotor and rearing activities of rodents are central excitatory behaviors and indicative of the rodent’s explorative ability.¹⁹ A reduction in rearing in rodents point to a central inhibitory effect. The results of this study are consistent with existing literature in that studies have demonstrated that administration of melatonin produced differential behavioral responses that were strongly dependent on experimental design. Glombleck et al reported a reduction in horizontal locomotion in adult golden hamsters after intraperitoneal injection of 30 microgram/kg of melatonin daily for five days.¹⁶ In another study this time by Kopp and collaborators administering 10 mg/kg of melatonin to adult C3H/He mice revealed a significant reduction in horizontal locomotor and rearing;³³ the present study demonstrated a reduction in rearing activity at both 5 and 10 mg/kg of melatonin administered daily over a 30 day period while locomotor activity did not show any significant difference from control, our results however differ from that of Ripple³⁴ whose work on the effects of prenatal exogenous melatonin administration resulted in an increase in locomotor activity and rearing which was blunted by the administration of luzindole; a melatonin receptor antagonist. A reduction in central excitatory behaviors as seen here is in line with a reduced excitatory input such as glutamatergic or an increased inhibitory input such GABAergic or both. Our assumption is that melatonin acts at least by enhancing cortical GABAergic input.

Grooming is a very important and ancient behavior that can be observed in a number of animals.³⁵ It serves a wide range of purposes extending beyond the primary goal of hygiene and body care, to include thermoregulation, chemo-communication, social interaction and stress reduction.³⁶ Grooming is modulated from a number of brain regions, and by neuromediators, hormones and drugs.^37,38 In this study administration of melatonin resulted in a suppression of grooming behavior. It is known that pelage cleaning in rodents increases with exposure to novelty³⁹ and this response is modulated by, central dopaminergic activation via D1 receptors⁴⁰ although inputs from the GABA system has also been implicated in the expression of novelty- induced grooming via its GABA A and GABA B receptors³⁸ this it does by reducing grooming behavior so drugs that inhibit GABA may cause an increase in grooming and vice versa. Melatonin administration over a 12 week period was also reported to have reduced grooming and other mating behaviors in voles⁴¹ and rabbits⁴² and hamsters.^43,44

Exogenously administered oral melatonin is able to affect both central excitation and spatial working memory in mice.