Melatonin and the enhancement of immune responses in immature male chickens.

Understanding the role of melatonin in affecting different physiological functions, especially immune responses, is becoming increasingly important in the basic and applied sciences. Enhancing the immune response will result in increasing disease resistance and, therefore, improve production efficiency. The purpose of the present study was to investigate the effects of melatonin, administered during the light or dark period, on BW, feed consumption (FC), and immune responses of immature chickens. Eight-week-old Cornell White Leghorn males were used in this study. The doses of melatonin were 0, 5, 10, 20, and 40 mg/kg BW. Melatonin was administered s.c. every 24 h for 7 consecutive d. The chicks were randomly divided into two groups; one group received injection during the middle of the light period, and the other group received injection during the middle of the dark period. All birds received 16 h light and 8 h darkness during a 24-h period. Body weights were measured before and after melatonin treatment, and FC was also measured. After the seven injections, blood samples were collected from the brachial vein, and total white blood cell (WBC) counts, differential cell counts, and activities of T and B lymphocytes were measured. Body weight was not significantly affected by dose of melatonin or time of injection. Furthermore, melatonin did not significantly affect FC; however, FC was significantly lower in the group that was injected in the dark vs. light period. The WBC counts of birds injected with 40 mg melatonin/kg BW were significantly higher than the WBC counts of saline-injected birds. The heterophil/lymphocyte (H/L) ratios of birds injected during the light period were significantly higher than those of birds injected during the dark period. T- and B-lymphocyte proliferation were significantly higher in birds injected with 40 mg melatonin/kg BW compared to saline-injected birds. These results indicate that melatonin in vivo is important in enhancing not only circulating WBC but also activities of B and T lymphocytes of immature male chickens without adversely affecting BW.