Responses of trigeminal subnucleus oralis nociceptive neurones to subcutaneous formalin in the rat

Abstract

Extracellular recordings of 33 single nociceptive neurones of the trigeminal subnucleus oralis (SNO) were made in rats under halothane nitrous oxide anaesthesia. These neurones were tested for their responses to a s.c. injection of formalin in their receptive field. Such a chemical noxious stimulation is known to induce a biphasic response of nociceptive dorsal horn neurones, the second period of which would be due to inflammation. Twenty-three neurones were characterized as nociceptive non-specific (NnS) and 10 as nociceptive specific neurones (NS). Formalin activated both SNO NS and NnS neurones, but, when they responded, NS neurones ( n = 5) showed only the first phase of activity while NnS neurones showed either one ( n = 13) or two phases ( n = 6). Biphasic responses were most often observed for NnS neurones with Aδ- and C-fibre inputs. These results indicate that the time-course similarity between the behavioural and the neuronal responses to formalin exists only for some SNO convergent neurones and that therefore the SNO does not seem to be very involved in the inflammatory component of the pain caused by formalin.