Properties of nociceptive and non-nociceptive neurons in trigeminal subnucleus oralis of the rat.

Recent studies have provided evidence suggesting the involvement of rostral components of the V brainstem complex such as trigeminal (V) subnucleus oralis in orofacial pain mechanisms. Since there has been no detailed investigation of the possible existence of nociceptive oralis neurons in the rat to substantiate this recent evidence, the present study was initiated to determine if neurons responsive to noxious orofacial stimuli were present in subnucleus oralis and to characterize their functional properties. In anesthetized rats, recordings were made of the extracellular activity of single neurons functionally characterized as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR) or nociceptive-specific (NS) neurons. The 342 LTM neurons responded only to light mechanical stimulation of orofacial tissues. The mechanoreceptive field of the LTM neurons included the intraoral region in 28% and was localized to the adjacent perioral area in 65%. For 95% the field was localized within one V division. Responses evoked in LTM neurons by electrical stimulation of the orofacial mechanoreceptive field revealed A fiber afferent inputs but no activity that could be attributed to C fiber afferent inputs. The 72 nociceptive neurons included 52 WDR neurons which responded to light (e.g. tactile) as well as noxious (e.g. heavy pressure; pinch) mechanical stimulation of perioral cutaneous and intraoral structures, and 20 NS neurons which responded exclusively to noxious mechanical stimuli. They also differed from the LTM neurons in that 36% of the WDR and 20% of the NS neurons had a mechanoreceptive field involving more than one V division. However, in accordance with our findings for the LTM neurons, the majority of WDR and NS neurons had a mechanoreceptive field involving the intraoral and perioral representations of the mandibular and/or maxillary divisions; those neurons having a mandibular field which especially included intraoral structures predominated in the dorsomedial zone of subnucleus oralis whereas those with a perioral mechanoreceptive field which particularly involved the maxillary division were concentrated in the ventrolateral zone of oralis. In contrast to the LTM neurons, 57% of the WDR and 67% of the NS neurons showed evidence of electrically evoked C fiber as well as A fiber afferent inputs from their mechanoreceptive field. We also noted suppression of the electrically evoked responses by heating of the tail or pinching of the paw. This effect was considered to be a reflection of diffuse noxious inhibitory controls, and was seen in NS as well as WDR neurons; most, but not all, of these neurons received A fiber as well as C fiber orofacial afferent inputs.(ABSTRACT TRUNCATED AT 400 WORDS)