c-Fos induction in the brainstem following electrical stimulation of the trigeminal ganglion of chronically mandibular nerve-transected rats.

Neuronal excitability in the trigeminal sensory nuclei (TSN) changes after nerve transection. We examined the effects of chronic transection of the trigeminal nerve on the c-Fos-immunoreactivity in the TSN induced 2 h after 10 min of electrical stimulation of the trigeminal ganglion (TG) at C-fiber activating condition (1.0 mA, 5 ms, 5 Hz) in urethane-anesthetized rats. In the non-transected control rats, stimulation of the TG induced c-Fos-immunoreactive cells (c-Fos-IR cells) mostly in superficial layers (VcI/II) of the nucleus caudalis (Vc) in its full extent along the dorsomedial-ventrolateral axis, but modestly in the rostral TSN above the obex, the principal, oral, and interpolar nuclei. Three days, 1, 2, or 3 weeks after transection of the inferior alveolar (IAN), infraorbital, or masseteric nerves, the stimulation of the TG induced c-Fos-IR cells in the central terminal fields of the transected nerve in the rostral TSN and magnocellular zone of the Vc. However, the number of c-Fos-IR cells in the VcI/II decreased inside the central terminal fields of the transected nerve and increased outside the fields. These results indicate that transection of the trigeminal nerve increases the excitability of TSN neurons that receive inputs from injured mechanoreceptors and uninjured nociceptors, but decreases it from injured nociceptors. The altered c-Fos responses may imply mechanisms of neuropathic pain seen after nerve injury.

Effects of systemic bicuculline or morphine on formalin-evoked pain-related behaviour and c-Fos expression in trigeminal nuclei after formalin injection into the lip or tongue in rats.

This study examined differences in nociceptive responses between lip and tongue. Formalin-induced pain-related behaviour and c-Fos expression in the trigeminal caudal nucleus (Vc) with/without systemic preadministration of a gamma-aminobutyric acid (GABA) type A receptor antagonist, bicuculline (2 mg/kg, i.p., 10 min before formalin injection) or a micro-opioid receptor agonist, morphine (3 mg/kg, i.p., 10 min before formalin injection) have been studied. Formalin injection into the upper lip induced an immediate pain-related behaviour, mostly face-rubbing behaviour, for 15 min (phase 1, mean +/- SEM/5 min, 81.2 +/- 30.1), followed by a more increased activity for 15 min (phase 2, 205.4 +/- 43.6) and a decline to baseline for next 15 min (phase 3, 63.9 +/- 28.0). Formalin injection into the tongue induced similar amount of pain-related behaviour at phase 1 (67.9 +/- 16.7), followed by similar activity at phase 2 (48.6 +/- 6.2), and lesser behaviour at phase 3 (20.4 +/- 7.6). The behaviour at phase 2 decreased following preadministration of bicuculline or morphine when formalin was injected into the lip (b, 62.5 +/- 14.5; m, 95.8 +/- 10.0) but not into the tongue (b, 31.0 +/- 9.2; m, 77.4 +/- 27.0). A considerable numbers of c-Fos-immunoreactive (IR) cells were induced in the caudal and inter-medio-lateral center of superficial layers of the Vc (VcI/II; mean +/- SEM/section = 225.8 +/- 12.9) and magnocellular zone of the Vc (VcIII/IV; 67.1 +/- 4.7) 2 h after formalin injection into the lip. Much smaller numbers of c-Fos-IR cells were induced in the rostral and dorso-medial one-fourth of the VcI/II (72.6 +/- 3.7) and VcIII/IV (55.6 +/- 6.6) after formalin injection into the tongue. Following preadministration with systemic bicuculline or morphine, the formalin-induced c-Fos-IR cells were decreased more in the VcI/II when formalin was injected into the lip (VcI/II, 102.4 +/- 8.0; VcIII/IV, 32.8 +/- 1.4) than into the tongue (VcI/II, 49.5 +/- 8.1; VcIII/IV, 31.7 +/- 5.3). These results show that the lip is more sensitive to formalin-induced noxious stimulation and regulated more through GABA(A) and micro-opioid receptors than the tongue.

Elimination of neurokinin-1 receptor neurons in caudal nucleus reverses the effects of systemic bicuculline on c-Fos expression in rat trigeminal sensory nucleus: I. High intensity electrical stimulation of the trigeminal ganglion.

Although neurokinin-1 receptor (NK-1)-bearing neurons are distributed in lamina I of the trigeminal caudal nucleus (Vc) and constitute major projection neurons, little is known about their fundamental role(s) in nociceptive processing. This study examines the effect of intra cisterna magna injection of substance P (SP) conjugated to saporin (SP-Sap; 5 microM, 5 microl) [with/without systemic administration of bicuculline] on c-Fos expression in the trigeminal sensory nucleus (TSN) induced 2 h after 10 min repetitive electrical stimulation of the trigeminal ganglion (TG) at high intensity (1.0 mA, 5 Hz, 5 ms) in the urethane-anesthetized rat. In the SP-Sap-treated rats, the numbers of NK-1-immunopositive neurons in laminae I and III of the Vc decreased compared with rats similarly pretreated with saline (Sal; 5 microl) or blank-saporin (Bl-Sap; 5 microM, 5 microl). In Sal- or Bl-Sap-treated controls, high intensity stimulation induced c-Fos expression in neurons throughout the full extent of ipsilateral superficial layers of the Vc (VcI/II), magnocellular zone of the Vc (VcIII/IV) and the dorsal or dorsomedial subdivisions of the rostral TSN above the obex (trigeminal principal, oral (Vo) and interpolar nuclei). Preadministration of bicuculline (2 mg/kg, i.p.) decreased the numbers of c-Fos-immunopositive neurons in the VcI/II, VcIII/IV and Vo in Sal- or Bl-Sap-treated controls. In contrast, high intensity stimulation induced less c-Fos-immunopositive neurons in the VcI/II and Vo of rats treated with SP-Sap compared with those in Sal- or Bl-Sap-treated controls. In SP-Sap-treated rats preadministered with bicuculline, the numbers of c-Fos-immunopositive neurons in the VcI/II and Vo were increased compared with the SP-Sap-treated rats preadministered with Sal. These results suggest that NK-1-immunopositive neurons in laminae I and III of Vc play a pivotal role in the nociceptive specific processing in the TSN through GABA(A) receptors.

Effects of intrathecal c-fos antisense oligodeoxynucleotide on adjuvant-induced thermal hyperalgesia.

We examined the effects of intrathecally preadministered injections of a phosphorothioate analog of c-fos antisense and mismatch oligodeoxynucleotides (ODNs) on the withdrawal latency to a thermal stimulus following unilateral injection of complete Freund's adjuvant (CFA) into the hind footpad of rats. Pretreatment with the c-fos antisense ODN significantly decreased the CFA-induced expression of c-Fos protein dose-dependently in ipsilateral laminae I/II (LI/II) of the dorsal horn (mean +/- SEM per section: 10 nM ODN, 43.9+/-1.3; 25 nM ODN, 19.4+/-4.1) compared with pretreatment with the mismatch ODN (63.6+/-2.9; 60.6+/-4.0) or saline (56.6+/-5.5). Animals pre-treated with 25 nM of the c-fos antisense ODN significantly increased the withdrawal latency to the noxious thermal stimulation (63.0-70.5%; compared with contralateral to the CFA injection) compared with animals pretreated with mismatch ODN (28.5-42.6%) or saline (26.4-45.3%) from 0 to 5 h after unilateral injection of CFA into the hind footpad. Pretreatment with 10 nM antisense ODN had a less significant effect. These results indicate that the expression of CFA-induced c-Fos in the dorsal horn might facilitate thermal nociception.

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 rostro-caudal axis of TSN, but is mediated primarily through VcI/II in the facial region.