Nociception in Chicken Embryos, Part II: Embryonal Development of Electroencephalic Neuronal Activity In Ovo as a Prerequisite for Nociception

SIMPLE SUMMARY: Even today, we do not know from which point the chicken embryo is able to process and feel pain. This is of special interest as worldwide, millions of male embryos are killed before hatching. This work aimed to examine when during the development of the embryo the brain shows normal activity, based on EEG recordings. The data strongly suggest developmental day 13 as the earliest embryonal stage being able to process pain. These results may support legislative processes establishing updated laws on animal welfare. ABSTRACT: Chicken culling has been forbidden in Germany since 2022; male/female selection and male elimination must be brought to an embryonic status prior to the onset of nociception. The present study evaluated the ontogenetic point at which noxious stimuli could potentially be perceived/processed in the brain in ovo. EEG recordings from randomized hyperpallial brain sites were recorded in ovo and noxious stimuli were applied. Temporal and spectral analyses of the EEG were performed. The onset of physiological neuronal signals could be determined at developmental day 13. ERP/ERSP/ITC analysis did not reveal phase-locked nociceptive responses. Although no central nociceptive responses were documented, adequate EEG responses to noxious stimuli from other brain areas cannot be excluded. The extreme stress impact on the embryo during the recording may overwrite the perception of noniceptive stimuli. The results suggest developmental day 13 as the earliest embryonal stage being able to receive and process nociceptive stimuli.