ELSEVIER Neuroscience Research 23 (1995) 415-418 Rapid communication Trigeminoparabrachial projection neurons showing substance P receptor-like immunoreactivity in the rat Yu-Qiang Ding’, Masahiko Takada, Ryuichi Shigemoto, Noboru Mizuno* Department of Morphological Brain Science, Faculty of Medicine. Kyoto University, Yoshida-Konoe-cho. Sakyo-ku. Kyoro 606-01. Japan Received 19 July 1995; accepted I I August 1995 Abstract By means of substance P receptor (SPR) immunofluorescence histochemistry combined with Fluoro-Gold fluorescent retrograde labeling, SPR-like immunoreactive neurons in the caudal subnucleus of the spinal trigeminal nucleus of the rat were observed to send their axons to the nucleus of Kiilliker-Fuse and ventrolateral part of the lateral parabrachial nucleus bilaterally with a clear ipsilateral dominance. These neurons were distributed mainly in lamina I, and additionally in lamina III. Keywords: Substance P receptor; Parabrachial nucleus; Nucleus of Kiilliker-Fuse; Spinal trigeminal nucleus; Nociception; Rat The spinal trigeminal nucleus of the rat sends many projection fibers to the parabrachial region including the parabrachial nucleus and nucleus of K6lliker-Fuse (KF) (Cechetto et al., 1985; Standaert et al., 1986; Ma and Peschanski, 1988; Bernard et al., 1989; Slugg and Light, 1994; Feil and Herbert, 1995).Sincethe spinal tri- geminal nucleus is the primary relay site of orofacial pain information to higher levels of the central nervous system (for reviews, see Darian-Smith, 1966; Brodal, 1981; Dubner and Bennett, 1983; Sessle, 1987), and since the parabrachial region of the rat contains neurons re- sponsive to noxious stimulation of the orofacial struc- tures (Bernard and Besson, 1990; Bullitt, 1990), the trigeminoparabrachial projections are considered to be implicated in the mediation of nociceptive information from the orofacial structures. On the other hand, many of the primary afferent fibers related to nociception contain substance P (SP) (for review, seeOtsuka and Yoshioka, 1993),and both l Corresponding author, Tel.: +81 75 753 4334; Fax: +81 75 753 4340. ’ On leave from the Department of Anatomy, The Fourth Military Medical University, Xi’an, People’s Republic of China. axons with SP-like immunoreactivity (SP-LI) and neu- ronal cell bodies with SP receptor-like immunoreactivity (SPR-LI) are distributed in the spinal trigeminal nucleus of the rat (Ribeiro-da-Silva et al., 1989; Nakaya et al., 1994;Brown et al., 1995;for further review, seeOtsuka and Yoshioka, 1993).Someof these SPR-LI neurons in the spinal trigeminal nucleus of the rat may receive nociceptive primary afferent fibers with SP-LI and send their axons to the parabrachial region. In our previous study (Ding et al., 1995),someof spi- nal neurons with SPR-LI were observed to send their axons to the parabrachial region in the rat by means of SPR immunofluorescence histochemistry combined with Fluoro-Gold fluorescent retrograde labeling. Thus, the present study was attempted, in turn, to examine the distribution of trigeminoparabrachial projection neu- rons with SPR-LI in the spinal trigeminal nucleus of the rat by using the same method. Thirteen adult male Wistar rats weighing 350-450 g were used for the present study. The distribution of SPR-LI neurons in the spinal trigeminal nucleus was examined in four rats. The rats were anesthetizeddeeply with sodium pentobarbital(50 mg/kg body wt., i.p.) and 016%0102/95/.W9.50 0 1995 Elsevier Science Ireland Ltd. All rights reserved SSDI 0168-0102(95)00961-R