The Distribution of Acetylcholinesterase Reactivity and Substance P, Serotonin and Met-Enkephalin Immunoreactivity in the Myelencephalon of Normal Chickens (Gallus Gallus Domesticus) and After Debeaking

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Abstract
The distribution of substance P-, met-enkephalin- and serotonin-immunoreactivity (IR) in the lower brainstem of hens is described using the PAP-method. Sections stained for acetyl- cholinesterase activity were used to provide a general description of the area. Serotonergic cells were observed in the locus ceruleus and nucleus subceruleus, the n. linearis caudalis, n. tegmenti ventralis, n. raphes and ventral reticular formation. Serotonergic varicosities sur- round the cells in several motor nuclei and cell groups in the reticular formation; scarce serotonergic elements were found in the subnucleus interpolaris of the descending trigeminal system and in layers III and IV of the subnucleus caudalis and dorsal horn, a moderate density of varicosities in layer II. Layers I and II contain substance P-IR and enkephalinergic varicosi- ties. Other regions with dense enkephalinergic varicosities are the dorsolateral parvocellular reticular formation, including the dorsal part of n. centralis medullae oblongatae, n. solitarius and n. cuneatus et gracilis. Substance P-IR is largely the same as enkephalin-IR. No distinct differences are found between normal adult chickens and animals that were debeaked at ages between 1 day and 6 weeks. As for the long-term detrimental effects of debeaking, sensory deprivation (loss of tactile sense) and heightened vulnerability to damage of the beak may be more important factors than long-lasting changes of the nociceptive system.

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