Acta Vet. Brno 2007, 76: 371-377

Effect of a Short-Term and Long-Term Melatonin Administration on Mammary Carcinogenesis in Female Sprague-Dawley Rats Influenced by Repeated Psychoemotional Stress

M. Kassayová1, E. Adámeková2, B. Bojková1, P. Kubatka3, I. Ahlers1, E. Ahlersová1

1Department of Animal Physiology, Institute of Biological and Ecological Sciences, Faculty of Science, P. J. Šafárik University, Košice, Slovak Republic
2Abbott Laboratories Slovakia, Bratislava, Slovak Republic
3Department of Pharmacology, Jessenius Faculty of Medicine in Martin, Comenius University, Slovak Republic

Received November 13, 2006
Accepted July 9, 2007

The aim of this study was to evaluate the effect of melatonin (MEL) on N-methyl-N-nitrosourea (NMU)-induced mammary carcinogenesis in female Sprague-Dawley rats exposed to repeated psychoemotional stress - immobilization in boxes. NMU was applied intraperitoneally in two doses each of 50 mg/kg b.w. between 40 - 50 postnatal days. Melatonin was administered in drinking water at a concentration of 4 μg/ml daily from 15:00 h to 8:00 h. The application was initiated 5 days prior to the fi rst NMU dose and lasted 15 days, i.e. during the promotion phase of tumour development, or long-term until the end of the experiment (week 20). Immobilization (2 h per day) began on the third day after the second carcinogen application and lasted for 7 consecutive days. Short-term MEL administration to immobilized animals increased incidence by 22%, decreased tumour frequency per animal by 26% and reduced tumour volume gain (by 21%) when compared to the immobilized group without MEL application. Decreased frequency per animal by 28% and more than a 40% decrease in tumour volume gain and cumulative volume were the most pronounced changes in the animals drinking MEL until the end of the experiment. Long-term MEL administration reduced the number and size of mammary tumours more markedly than its short-term administration. Melatonin decreased certain attributes of mammary carcinogenesis in female rats influenced by psychoemotional stress.