Availability of aromatic amino acids in the blood plasma differs between lines of laying hens selected for low mortality or production traits

The mechanisms underlying feather pecking (FP) behavior are still unclear. However, neuronal monoamine (serotonin and dopamine) pathways are thought to play a major role. As monoamine precursors, aromatic amino acids (AAA) compete for active transport at the blood brain barrier the central monoamine synthesis is influenced by AAA concentration in the periphery. This study compared the plasma availability of AAA in 2 lines of layers: One selected for low mortality (LM; n = 132), showing low FP and one control line (C), selected for production traits only (n = 132). We expected that plasma levels of AAA are of high individual variability and instable over time, and that most pronounced differences between lines could be related to tyrosine (TYR) level. Blood samples of 132 birds per line, kept in groups of 11, were collected in wk 24 and 29. An AA-profile was received by HPLC of platelet-poor plasma. Data were analyzed using PROC MIXED (SAS 9.3) by line and week and their interactions as fixed effects, pen (line) as random effect and week/bird (pen) as repeated effect. Per line, birds did not differ in PHEN, TYR, TRP or TRP/AAA levels. However, TYR decreased over time (P <0.003) for both lines. An interaction was found between increased PHEN/TYR ratios and age, attributed to lines. Moreover, the line (P <0.05), week (P <0.002) and line × week (P <0.001) interactions in PHEN/TYR ratios were found to be increasing for C birds over time but not for LM birds (P <0.67). Another interaction, attributed to lines and age, could be found in the case of TYR/ TRP ratios, decreasing significantly over time for LM birds (P <0.0001) but increasing in control birds (P <0.0009). These results indicate that there are substantial age-dependent differences in circulating AAA between these 2 lines, which could be based on differences in the metabolic activity of the liver. We therefore suggest that further insights into the contribution of blood AAA to the central synthesis of neurotransmitters are required to gain a better understanding of the mechanisms underlying FP in layers.