Effect of Melatonin on Neurobehavioral Functioning in Middle-aged Female Rats

The percentage of Americans over 65 is expected to increase from 56 million to 72 million in 2050. Of importance will be to better understand normal age-related declines in neurocognitive functioning and develop potential interventions that aim to prevent or retard progression to pathological aging (such as Mild Cognitive Impairment or Alzheimer’s disease). Among those diagnosed with these neurodegenerative conditions, women exhibit faster cognitive decline compared to men. Melatonin is an important pineal hormone that not only has chronological properties but has also been found to be a potent neuroprotectant in the brain. Melatonin levels decline with age. Hence, when neurodegenerative conditions are at their peak, melatonin levels are at their lowest. A plethora of animal studies have demonstrated the effectiveness of melatonin in protecting against many central nervous system disorders such as stroke. There is scant information regarding whether melatonin supplementation affects neurobehavioral functioning in normal middle-aged rodents. Hence, the purpose of the present study was to examine whether chronic melatonin administration (50 mg, 90-day pellets implanted s.c.) impacts neurobehavioral functioning in middle-aged (12 months) female rats. Age-matched non-treated middle-aged female rats were also included (sham operated controls). Approximately 44 days following pellet or sham pellet implantation, rats were tested on an open field, object recognition, spatial recognition, Morris Water Maze, and Y-maze task. Melatonin at the dosage used, did not improve spatial or visual memory in middle-aged female rodents. Although melatonin did not affect behavior, further study will need to examine whether melatonin positively affected the brain. Melatonin is known to have neurogenic properties; hence it will be of interest to examine the brain of these animals to determine whether neurogenesis was positively impacted by treatment. As well, older women also show a decline in estrogen with age. Hence, it will be of interest to determine whether combined melatonin and estradiol replacement would affect neurobehavioral functioning.