Diurnal sensitivity in melatonin-induced hematological changes in the male albino rat.

The influence of melatonin on diurnal changes in the hematological profile was investigated in male albino rats. Following treatment with melatonin, under two different experimental protocols, blood samples were collected at 06.00, 12.00, 18.00 and 24.00 h from separate groups of animals for detailed hematological analysis. In experiment 1, melatonin (25 micrograms, s.c.) treatment, once daily at 17.00 h for 4 weeks, led to an alteration in the rhythm of RBC production with a decrease in its counts at 12.00 h. The total WBC counts were increased at 06.00 and 12.00 h and decreased at 18.00 and 24.00 h. In experiment 2, twice-daily administration of melatonin (once at 09.00 h and again at 17.00 h), for 2 weeks, resulted in a decrease in total RBC counts at 06.00 and 24.00 h, whereas at 18.00 h there was an increase, compared to the respective control values. The total WBC counts increased at 06.00, 12.00 and 18.00 h and decreased at 24.00 h. Erythrocyte indices like mean cell volume (MCV), mean cell hemoglobin (MCH) and mean cell hemoglobin concentration (MCHC), in both experiments, correlated with RBC, hemoglobin and hematocrit values. The WBC differential counts in experiment 1 revealed a decrease in total neutrophils at 24.00 h, whereas in experiment 2 there was a general decrease in their number. While in experiment 1 the total lymphocyte number was increased at 06.00 h and decreased at 18.00 and 24.00 h, in experiment 2 it was increased except at 24.00 h. It may be concluded that melatonin has a modulatory role in hemopoiesis and its rhythms. The stimulatory effect of melatonin on WBC supports its purported immunopotentiating action.