Mechano-chemical nociceptors in the avian trigeminal mucosa.

This study characterised and compared the properties of chemically sensitive nociceptors in the avian nasal and oral epithelia recorded from either microelectrode stabs of the trigeminal ganglion (nasal mucosa) or microdissected twigs of the nasopalatine nerve (palatine mucosa) in urethane anaesthetized hens. A total of 146 slowly and rapidly adapting nasal and palate mechanically sensitive receptors were identified exhibiting von Frey thresholds to mechanical stimulation ranging from 0.2 to 8.3 g (mean 3.21+/-2.28 g, n=39) in the nasal cavity and <0.1 to 15 g (mean 2.17+/-2.69 g, n=62) in the palate. Receptive fields were situated throughout the nasal cavity and palate. A subset of receptors (classified as polymodal nociceptors) exhibited chemical sensitivity when exposed to noxious levels of ammonia gas, and some polymodal afferents also responded to acetic acid vapour or carbon dioxide exposure. Responses to ammonia were characterised by delayed onset, rapid prolonged discharge patterns usually outlasting the chemical stimulation. Clear concentration-response relationships were apparent for chemically receptive afferents in both the nasal and oral cavities, although curve shape varied considerably between receptors. Statistical comparisons of the responses of polymodal nociceptors in each mucosal region revealed no significant difference between their mechanical thresholds, but nasal afferents exhibited lower chemical (ammonia) thresholds and higher maximal firing rates. These response characteristics highlight distinct sensitivities to potentially painful chemical stimulation between epithelial areas. These effects may be due to the inherent properties of the nociceptors in each region but are also likely to reflect the role of the perireceptor environment in mediating peripheral nociception.