Expression of c-Fos-like immunoreactive neurons in the supratrigeminal region in the rat following noxious stimulation of the orofacial tissues Mineo Watanabe a, * , Eiji Tanaka a , Mika Nishi a , Tatsunori Iwabe a , Yukiko Hattori a , Setsuko Suemune b , Kazuo Tanne a a Department of Orthodontics and Craniofacial Developmental Biology, Division of Cervico-Gnathostomatology, Hiroshima University Graduate School of Biomedical Sciences, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8553, Japan b Department of Oral Growth and Developmental Biology, Division of Clinical Medical Science, Hiroshima University Graduate School of Biomedical Sciences, Hiroshima, Japan Received 26 August 2002; received in revised form 30 September 2002; accepted 3 October 2002 Abstract This study was conducted to investigate immunohistochemically the expression of c-Fos in neurons around the trigeminal motor nucleus following application of mechanical force to a tooth, a mechanical pinch to the tongue, and paraformaldehyde injection into the periodontal ligament and masseter muscle. Following the application of all of these stimuli except pinching the tongue, a small number of c-Fos-like immunoreactive (Fos-LI) neurons were found in the ipsilateral supratrigeminal region. c-Fos expression was also detected in numerous neurons in the superficial laminae of the subnucleus caudalis in all experimental animals. Induction of c-Fos in the subnucleus caudalis and the supratrigem- inal region indicates that Fos-LI neurons in the supratrigeminal region are involved in the processing of intraoral nociceptive signals. q 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Immunohistochemically; c-Fos; Noxious stimulation; Intraoral tissues; Supratrigeminal region; Subnucleus caudalis The reticular region around the trigeminal motor nucleus (Vmo) is divided into the supratrigeminal region (a reticular zone capping the trigeminal nucleus dorsally and rostro- dorsally; Vsup), the intertrigeminal region (a narrow reticu- lar zone interposed between the principal sensory trigeminal nucleus (Vp) and Vmo; Vint), and the lateral tegmental regions close to the medial border of Vmo. It has also been shown that the Vsup and Vint contain commissural inter neurons connecting to masticatory motoneurons [11,12]. Intra-axonal injection of horseradish peroxidase (HRP) revealed that neurons in the Vsup receive input from the trigeminal mesencephalic nucleus (Vme) neurons arising from either muscle spindles in the masseter muscle or periodontal ligament (PDL) mechanoreceptors [3]. Furthermore, according to anatomical studies in which HRP or peroxidase-wheat germ agglutinin conjugate (HRP-WGA) was injected into the Gasserian ganglion (GG), Vsup was found to receive intraoral primary afferent innervation from the GG [10]. The above tracers are internalized and transported by all the primary afferent neurons, however, they can not distin- guish primary neurons with distinct morphological or biochemical characteristics. On the other hand, the calcito- nin gene-related peptide (CGRP), a natural marker for small-to-medium neurons, was used to investigate the central projection of small nociceptive trigeminal neurons [16]. Dense CGRP-like immunoreactivity was found in the rat Vsup [1], although CGRP is not an exclusive marker for unmyelinated primary neurons because it is also present in fine myelinated axons [20]. Thus, the discrimination between unmyelinated and myelinated trigeminal primary neurons in the Vsup is still unclear. The Proto-oncogene c-fos is one of the immediate early genes and induces c-Fos protein in nuclei of the central nervous system. In the study of pain, c-Fos has widely been used as a marker for analyzing nociceptive pathways in the spinal cord and the trigeminal nuclear complex (TNC) [4,17–19]. Neuroscience Letters 335 (2002) 99–102 0304-3940/02/$ - see front matter q 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(02)01163-1 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 181-82-257-5686; fax: 181-82- 257-5687 E-mail address: mineo@hiroshima-u.ac.jp (M. Watanabe).