Seasonal differences in the influence of peritonitis on the biosynthetic activity of the pineal gland in young chickens.

The pineal gland synthesizes indoles including melatonin, neurosteroids, and possibly catecholamines. Peritonitis induced in chickens hatched in winter inhibited pineal melatonin biosynthesis, while having the opposite effect during summer. Therefore, the aim of this study was to obtain more information concerning the impact of peritonitis on the pineal gland in chickens hatched in summer. Specifically, we aimed to investigate: 1) the concentration of melatonin (MEL), its substrates, and other indoles; 2) the levels of catecholamines and neurotransmitters such as noradrenaline, dopamine, and their substrates and metabolites; 3) the mRNA levels of genes encoding peptide neurotransmitters, such as vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide, along with their receptors and adrenoceptor alpha 2A; and 4) the mRNA levels of genes encoding elements of the immune system, including pro-inflammatory cytokines interleukin-1β, interleukin-6, interleukin-18, as well as interleukin-6 and interleukin-18 receptors, and Toll-like receptors 4, 15, and 21. Peritonitis was initiated 2 hours before the lights were turned off via intraperitoneal injection of thioglycollate, and 4 hours later, the pineal glands and trunk blood were isolated. High-performance liquid chromatography (HPLC) analysis revealed a significant increase in the levels of 5-hydroxytryptophan (P<0.01), N-acetylserotonin (P<0.05), melatonin (P<0.001), 5- hydroxytryptophol (P<0.05), and 3,4-dihydroxy-L-phenylalanine (P<0.05), along with a decrease in serotonin, dopamine (P<0.001), and noradrenaline (P<0.001) in the pineal glands, following immune system activation. Additionally, RT-qPCR analysis showed a significant increase in the expression of genes encoding cytokines such as interleukin-1β (P<0.001), interleukin-18 (P<0.05), Toll-like receptor 4 (P<0.01), and interleukin-6 receptors (P<0.01), while mRNA level encoding receptors for vasoactive intestinal peptide (P<0.05) and pituitary adenylate cyclase-activating polypeptide (P<0.001) experienced a decrease. These results expand our knowledge about the impact of the immune system activation on the pineal gland, confirming the presence of seasonal differences in this influence and suggesting that these variations may be associated with a shift in energy allocation from immune functions to other traits.