Interaction between the immune system and pineal gland seems to be bidirectional. The feedback effect of an inflammatory response on the pineal gland is poorly understood, however. This study was performed to analyse the pattern of mRNA expression of receptors belonging to the Toll-like receptors (TLR) family in the pineal gland of ewes under different photoperiod conditions. TLRs are involved in the initiation of the innate immune response because they recognize different pathogen-associated molecular patterns (PAMPs). The experiment was carried out on adult Blackface ewes during long-day (LD, 18:6, June; n = 6) and short-day (SD, 6:18, December; n = 6) periods. The animals were euthanized 5 h after sunset (at 21:00 during SD and 2:00 during LD). The mean concentration of circulating melatonin in the last hour of the experiment was higher (P < 0.05) during the SD (270 ± 15 pg· ml–1) compared with the LD period (108 ± 5 pg · ml–1). The expression of genes encoding each TLR was differentiated. TLR3 had the highest mRNA expression, followed by TLRs 1, 2, 5, 7, 9, 8, 4, 10 and 6. Gene expression also varied for each TLR and was influenced by the photoperiod and/or melatonin. The gene whose expression varied the most was TLR10 (coefficient of variation, 42%) during the LD period, but during the SD period, the expression of TLR6 varied most (coefficient of variation, 42%). The study showed that the cells of the pineal gland express transcripts for all ten members of the TLR family, therefore, the functioning of this gland may be affected by number of PAMPs recognized by these receptors.
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