Background: Environmental signals noticeably affect the brain function and development, especially in the early life. This study was conducted to assess the effects of dark rearing and melatonin on LTP induction of hippocampal responses.

Materials & Methods: In vivo experiments were carried out on 2 groups (n=32) of 45 days old male Wistar rats kept in standard 12 hour light/dark condition (Light Reared-LR) or in complete darkness (Dark Reared-DR) since birth throughout the study. Each group, in turn, was divided into control and melatonin treated groups. Stimulating the Schaffer collaterals of CA3 area of hippocampus, field potentials were recorded in the CA1 area. Thirty minutes after the stability of the responses the hippocampus was perfused by either saline (control) or 2 µg melatonin (drug treated) and recordings continued for 30 further minutes. Then, the tetanic stimulation was applied to the Schaffer collaterals and the field potentials were pooled for 120 minutes post-tetanus in the LR animals.

Results: Tetanic stimulation of the Schaffer collaterals resulted in LTP in both control LR and DR rats, with more pronounced amplitude and maintenance in the former. Melatonin influenced potentiation of the field responses differently. Whereas melatonin showed a negligible effect on LTP of the DR treated group, it markedly depressed amplitude of post-tetanic responses.

Conclusion: It seems that melatonin deteriorates LTP in the hippocampal responses, considerably in the LR animals. The weak effect of the drug in the DR rats can be related to lower sensitivity of melatonin receptors due to the increased melatonin serum levels in the dark reared rats.