"Desensitization of Nociceptive and Serotonergic Thermoregulatory Responses by Capsaicin in Chicken: Role of Capsaicin-Sensitive Subtype-1 Vanilloid Receptor-Independent Mechanisms"

Background: It is well known that chicken is insensitive to pain burning sensation of capsaicin (main ingredient of hot chilli peppers) this may be due to insensitivity of transient receptors membrane potential of vanilloid subtype-1 (TRPV1) of chicken to capsaicin. So actions of capsaicin in chicken could be mediated through TRPV1-independent pathways. Serotonin (5-HT) acts as an excitatory transmitter in the heat loss effectors pathways producing hypothermia in mammals and birds. 

The aim of the present study was to investigate the involvement of capsaicin sensitive TRPV1-independent mechanisms in nociceptive and thermoregulatory responses after exposure to mild heat stress and administration of 5-hydroxytryptamine (5-HT) as regards to its ability to induce a hyperthermia in chicken.

Methods: we induced desensitization to TRPV1-independent mechanisms by two times administration of capsaicin (10 mg/kg, i.v.) in newly hatched chicks. Thermoregulatory responses to administration of capsaicin, serotonin (5-HT), and exposure to heat (Heat escape test in thermal cages at 42 °C for 50 min), and nociceptive responses (number of eye wipes/30 s and duration (s) of sustained eye blinks to intraocular administration of ammonia were measured. In addition the preference to CAP was measured in two-bottle preference test. 

Main Results revealed that an administration of 5-HT produced a significant dose-dependent hypothermia in young chicks lasted up to 1 h. This hypothermic effect of 5-HT (10 mg/kg) was not observed in chicks that had been pretreated with capsaicin. Further, capsaicin pretreated-chicks showed impaired tolerance to heat as indicated by increased escape % from heat chamber (42 °C), and reduced nociceptive response to intraocular administration of 0.1% ammonia solution in terms of decreased number of eye wipes/30 sec to 3.20 ± 1.03 compared with control chicks 8.60 ± 1.51, and reduced duration (s) of sustained eye blinks to 14.10 ± 6.78 s in CAP-pretreated chicks compared to vehicle injected control (43.10 ± 5.36 s). Further, CAP-treated group showed a tendency for more consumption of capsaicin in the compared to vehicle injected control group 

Conclusion: These data may indicate the thermoregulatory responses to 5-HT, and heat and nociceptive responses to ammonia solution are likely to be mediated by capsaicin-sensitive TRPV1-independent mechanisms in chicken.