Casao et al. Reproductive Biology and Endocrinology 2010, 8:59 http://www.rbej.com/content/8/1/59 Open Access RESEARCH © 2010 Casao et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Research Seasonal variations of melatonin in ram seminal plasma are correlated to those of testosterone and antioxidant enzymes Adriana Casao* 1 , Igor Cebrián 2 , Mayra Eoda Asumpção 3 , Rosaura Pérez-Pé 1 , José A Abecia 4 , Fernando Forcada 4 , José A Cebrián-Pérez 1 and Teresa Muiño-Blanco 1 Abstract Background: Some breeds of sheep are highly seasonal in terms of reproductive capability, and these changes are regulated by photoperiod and melatonin secretion. These changes affect the reproductive performance of rams, impairing semen quality and modifying hormonal profiles. Also, the antioxidant defence systems seem to be modulated by melatonin secretion, and shows seasonal variations. The aim of this study was to investigate the presence of melatonin and testosterone in ram seminal plasma and their variations between the breeding and non- breeding seasons. In addition, we analyzed the possible correlations between these hormones and the antioxidant enzyme defence system activity. Methods: Seminal plasma from nine Rasa Aragonesa rams were collected for one year, and their levels of melatonin, testosterone, superoxide dismutase (SOD), glutathione reductase (GRD), glutathione peroxidase (GPX) and catalase (CAT) were measured. Results: All samples presented measurable quantities of hormones and antioxidant enzymes. Both hormones showed monthly variations, with a decrease after the winter solstice and a rise after the summer solstice that reached the maximum levels in October-November, and a marked seasonal variation (P < 0.01) with higher levels in the breeding season. The yearly pattern of GRD and catalase was close to that of melatonin, and GRD showed a significant seasonal variation (P < 0.01) with a higher activity during the breeding season. Linear regression analysis between the studied hormones and antioxidant enzymes showed a significant correlation between melatonin and testosterone, GRD, SOD and catalase. Conclusions: These results show the presence of melatonin and testosterone in ram seminal plasma, and that both hormones have seasonal variations, and support the idea that seasonal variations of fertility in the ram involve interplay between melatonin and the antioxidant defence system. Background Melatonin plays a central role in fine-tuning circadian rhythms [1] and seasonal changes [2] through its daily nocturnal increase in the blood [3]. In seasonally breed- ing mammals that use changes in the photoperiod to time their reproductive cycles, temporal signals to the repro- ductive system are controlled by the daily rhythm in melatonin production [4-7]. Certain breeds of sheep are highly seasonal in terms of reproductive capability, regu- lated by photoperiod and melatonin secretion [8,9]. Therefore, seasonal melatonin variation has been thor- oughly studied in this specie, and although seasonality is less marked in male than in female, changes in testicular volume, hormonal profiles, sexual behaviour and semen * Correspondence: adriana@unizar.es 1 Departamento de Bioquímica y Biología Molecular y Celular, Grupo Biología y Fisiología de la Reproduction (BIOFREZ), Instituto Universitario de Investigation en Ciencias Ambientales de Aragón (IUCA), Facultad de Veterinaria, Universidad de Zaragoza, Spain Full list of author information is available at the end of the article