Developmental changes of melatonin receptor expression in the spleen of the chicken, Gallus domesticus

Melatonin plays an essential role in development and immunoregulation of the avian spleen through its receptors; however, the variations in the expression of the melatonin receptor subtypes in the developing avian spleen are still unclear. The objective of the present study was not only to investigate the distribution patterns and development changes of the expression of the melatonin receptor subtypes (Mel1a, Mel1b and Mel1c) in the chicken spleen but also to identify the correlation between the plasma melatonin concentration and the expression of the melatonin receptor subtypes. The immunohistochemical results indicated that Mel1a was mainly distributed in the red pulp and capillaries, Mel1b was predominantly distributed in the periarterial lymphatic sheath (PALS) and splenic nodule, and Mel1c was widely located in the red pulp, PALS and splenic nodule. From P0 to P21, the mRNA and protein expressions of Mel1a, Mel1b and Mel1c in the spleen were increased (P < 0.05); however, a slight increase in the expression of the three melatonin receptor subtypes was observed after P21 (P > 0.05). Furthermore, the mRNA levels of Mel1b and Mel1c between P0 and P14 raised more quickly than Mel1a. The plasma melatonin concentration increased in an age-dependent manner in the chicken from P0 to P42 (P < 0.05), and this increasing change was linear after P14 (P < 0.05). The melatonin level in the plasma is strongly correlated with the protein expressions of Mel1a (r = 0.938, P = 0.005), Mel1b (r = 0.912, P = 0.011), and Mel1c (r = 0.906, P = 0.012) in the chicken spleen. These results suggest the existence of age-related and region-specific changes in the expression of the melatonin receptor subtypes within the spleen of the chicken, and this characteristic pattern may be involved in the development and functional maturation of the avian spleen.