Mapping of c-Fos in the trigeminal sensory nucleus following high- and low-intensity afferent stimulation in the rat

Although previous studies have suggested that nociceptive afferents from intra-oral and facial structures are organized differently in the trigeminal sensory nucleus (TSN), more detailed data are needed. The present study aimed to fill this gap, by examining the changes in the expression of c-Fos within the rat TSN following high- and low-intensity electrical stimulation applied to the Gasserian ganglion (GG). A low-intensity stimulus (0.1 mA) induced c-Fos in many neurons in the dorsomedial subdivision (Vodm) of the oral subnucleus (Vo; mean±SEM in a certain segment=163.0±42.7), in the medial part of the dorsomedial subdivision (Vidm) of the interpolar subnucleus (Vi; 120.5±40.1), in the medial corner of the magnocellular zone (VcIII/IV; 47.5±10.5), and in the superficial layers (VcI/II; 1330.0±65.6) along the entire length of the dorsomedial-ventrolateral axis of the caudal subnucleus (Vc). A modest number of Fos-positive neurons were induced in the dorsal principal subnucleus (Vp; 10.0±4.9) and in the lateral VcIII/IV (11.5±1.6). A high-intensity stimulus (1.0 mA) significantly increased the number of Fos-positive neurons in each subdivision compared with the low-intensity stimulus (Vp 32.3±10.8; Vodm 270.3±75.3; Vidm 189.3±38.5; medial VcIII/IV 77.5±18.2; lateral VcIII/IV 24.8±9.3; VcI/II, 2155.8±470.2). At both low- and high-intensity stimulation, the fields where Fos-positive neurons appeared are restricted to the dorsal or dorsomedial subdivisions of the rostral subnuclei, Vp, Vo and Vi, where the main projectional fields of primary afferents from the intraoral structures are found, while Fos-positive neurons were distributed in the entire VcI/II, along the dorsomedial-ventrolateral axis of Vc, where the main projectional fields of primary afferents from the facial skin are found. The threshold to induce c-Fos is, however, different according to the fields. These results suggest that nociceptive processing in the intra-oral region is mediated through the entire length of the rostrocaudal axis of TSN, but is mediated primarily through VcI/II in the facial region.