Trigeminal-parabrachial connections: possible pathway for nociception-induced cardiovascular reflex responses

Noxious stimulation of dental nerves elicits marked changes in cardiovascular function. In order to investigate central pathways mediating reflex changes in cardiovascular activity, immunohistochemical localization of cells expressing the immediate-early gene, c-fos, was used to identify central nervous responding to noxious electrical stimulation of mandibular, incisor tooth dentin or chemical (capsaicin) stimulation of tooth pulp in the anesthetized rat. Injections of Fluoro-Gold were made in the lateral parabrachial region to identify efferent projections from the spinal trigeminal nucleus. Electrical and chemical stimulation produced similar patterns of Fos-positive neurons in the spinal trigeminal nucleus: subnuclei caudalis, interpolaris and oralis. Fos-positive neurons were most dense in laminae I and II of the dorsomedial subnucleus caudalis with fewer Fos-positive neurons located in the interpolaris and oralis subnuclei. Sham stimulation of tooth dentin and control vehicle injections into the tooth pulp resulted in either a few weakly stained or no Fos-positive neurons in the spinal trigeminal nucleus. Cell bodies double labeled with Fluro-Gold following injections into the parabrachial region and Fos-protein subsequent to electrical stimulation of incisor tooth were present in all three subnuclei of the spinal trigeminal nucleus. The largest number of Fos-positive neurons with efferent projections to the lateral parabrachial region were located in subnucleus caudalis (32.2 +/- 5.3 S.E.M.) and fewer were located in the interpolaris (0.4 +/- 0.4 S.E.M.) and oralis (19.8 +/- 3.5 S.E.M.) subnuclei. The results demonstrate that nociceptive dental input received by the three subnuclei of the spinal trigeminal nucleus, particularly the subnucleus caudalis, is relayed to the lateral parabrachial nucleus.