Background: Exposure to a high ambient temperature (HT) can cause heat stress, which has a negative impact on physiological
functions. L-tryptophan (L-Trp) as a precursor of serotonergic and kynurenine (Kyn) pathways, has a calmative effect during
different stress statuses. Aims: This study was carried out to determine the influence of intraperitoneal injection of Trp on feeding
behavior, rectal temperature, and some blood parameters in the heat stress condition. Methods: L-tryptophan (25 and 50 mg/kg body
weight, BW) was administered intraperitoneally during either HT (39°C) or control temperature (CT; 31°C) for 5 h whilst fed or
fasted in 7-day-old chicks. Results: L-tryptophan caused elevation in decreased food intake and significantly decreased rectal
temperature during acute heat stress at the dose of 50 mg/kg BW. Rectal temperature reduced in the fasted state at the dose of 50
mg/kg BW, and at the dose of 25 mg/kg BW Trp in the fed state in comparison with the other experimental groups. Reduction of
serum glucose, triglyceride, and corticosterone levels was seen during the fed state. L-tryptophan had a significant reducing effect on
the serum corticosterone level in the fasted state in comparison with the fed state, and also revealed a significant decline at the dose
of 25 mg/kg BW on the elevated serum corticosterone under heat stress. Conclusion: Administration of L-tryptophan leads to increase
cumulative food intake and decrease rectal temperature during heat stress. Also, L-Trp causes to decline increased serum
corticosterone level under heat stress and fasted state. These findings indicated the potential regulator role of Trp to modulate stress
response in heat-exposed chicks.
