Nociception in Chicken Embryos, Part I: Analysis of Cardiovascular Responses to a Mechanical Noxious Stimulus

SIMPLE SUMMARY: Chicken embryos are frequently not protected by animal welfare laws. However, they are used in various research areas, and male embryos are commonly killed in food production as an alternative to culling day-old chicks. Increasing knowledge regarding the onset of nociception and pain perception in chicken embryos is fundamental for animal welfare protection. The aim of this exploratory study was to further narrow down the period when chicken embryos acquire the capacity for nociception. Therefore, changes in blood pressure and heart rate after the introduction of a noxious stimulus were assessed during the embryonic development of chickens. Embryos from 16 days of incubation onward showed cardiovascular changes after a noxious mechanical stimulus was introduced at the base of the beak, indicating a nociceptive response. ABSTRACT: Although it is assumed that chicken embryos acquire the capacity for nociception while developing in the egg, an exact time point has not yet been specified. The present research was an exploratory study aiming to determine when the capacity of nociception emerges during embryonic development in chickens. Changes in blood pressure and heart rate (HR) in response to a noxious mechanical stimulus at the base of the beak versus a light touch on the beak were examined in chicken embryos between embryonic days (EDs) 7 and 18. Mean arterial pressure (MAP) was the most sensitive parameter for assessing cardiovascular responses. Significant changes in MAP in response to a noxious stimulus were detected in embryos at ED16 to ED18, whereas significant changes in HR were observed at ED17 and ED18. Infiltration anesthesia with the local anesthetic lidocaine significantly reduced the response of MAP on ED18, so the measured cardiovascular changes may be interpreted as nociceptive responses.