Differential rostral projections of caudal brainstem neurons receiving trigeminal input after masseter inflammation

• Published in: Journal of comparative neurology (1911) 2003-10, Vol.465 (2), p.220-233
• Main Authors: Ikeda, Tetsuya, Terayama, Ryuji, Jue, Seong-Suk, Sugiyo, Shinichi, Dubner, Ronald, Ren, Ke
• Format: Article
• Language: English
• Subjects: Anesthetics, Local - pharmacology; Animals; Brain Stem - anatomy & histology; Brain Stem - metabolism; Fluorogold; Freund's adjuvant; Freund's Adjuvant - pharmacology; Immunohistochemistry; Inflammation - chemically induced; Lidocaine - pharmacology; Male; Masseter Muscle - drug effects; Masseter Muscle - innervation; Masseter Muscle - pathology; Neural Pathways - anatomy & histology; Neurons - cytology; nucleus submedius; Proto-Oncogene Proteins c-fos - biosynthesis; Proto-Oncogene Proteins c-fos - drug effects; Rats; Skin - drug effects; Skin - innervation; Trigeminal Nerve - anatomy & histology; Trigeminal Nerve - metabolism; trigeminal transition zone
• ISSN: 0021-9967; 1096-9861
• DOI: 10.1002/cne.10836

To understand the functional significance of orofacial injury‐induced neuronal activation, this study examined the rostral projection of caudal brainstem neurons that were activated by masseteric inflammation. Rats were injected with a retrograde tracer, Fluorogold, into the nucleus submedius of the thalamus (Sm), parabrachial nucleus (PB), lateral hypothalamus (LH), or medial ventroposterior thalamic nucleus (VPM) 7 days before injection of an inflammatory agent, complete Freund's adjuvant (CFA), into the masseter muscle. Rats were perfused at 2 hours after inflammation, and brainstem tissues were processed for Fos‐Fluorogold double immunocytochemistry. Although there was no difference in Fos expression among the four groups (n = 4 per site), the rostral projection of Fos‐positive neurons showed dramatic differences. In the ventral portion of the trigeminal subnuclei interpolaris/caudalis (Vi/Vc) transition zone, the percentage of Fos‐positive neurons projecting to the Sm (39.7%) was significantly higher than that projecting to the LH (5.4%) or VPM (5.6%; P < .001). The anesthesia alone also induced Fos expression in ventral Vi/Vc neurons, but these neurons did not project to Sm. In the caudal laminated Vc and dorsal Vi/Vc, the PB was the major site of rostral projection of Fos‐positive neurons. In the caudal ventrolateral medulla and nucleus tractus solitarius, Fos‐positive neurons projected to the Sm, PB, and LH. Most VPM‐projecting neurons examined did not show Fos‐like immunoreactivity after masseter inflammation. These findings emphasize the importance of the trigeminal Vi/Vc transition zone in response to orofacial deep tissue injury. Furthermore, the results differentiate the ventral and dorsal portions of the Vi/Vc transition zone, in that the Sm received projection mainly from activated neurons in the ventral Vi/Vc. The activation of Vi/Vc neurons and associated ascending pathways may facilitate somatoautonomic and somatovisceral integration and descending pain modulation after orofacial deep tissue injury. J. Comp. Neurol. 465:220–233, 2003. © 2003 Wiley‐Liss, Inc.