Yoshiki Imamura

Characterization of heat-hyperalgesia in an experimental trigeminal neuropathy in rats

Abstract Secondary trigeminal neuralgia (STN) follows an injury to the trigeminal nerve or one of its branches. Although rare, this condition results in great suffering and it is notoriously difficult to treat. The experimental analysis of painful neuropathy due to damage to the innervation of the limbs (e.g., the sciatic nerve) has progressed rapidly in recent years, but very few reports have appeared concerning experimental neuropathy in the trigemenial region. We report here an experimental rat model of trigeminal neuropathic pain produced by a chronic constriction injury to the infraorbital nerve (CCI-ION), and on a method that detects heat-evoked pain-related behavior. Rats with the CCI-ION have clear signs of heat-hyperalgesia when stimulated on the snout (the vibrissal pad). The hyperalgesia is seen both ipsi- and contralateral to the side of nerve injury, but is significantly more severe ipsilaterally, and lasts about 12 days.

Astroglia in Medullary Dorsal Horn (Trigeminal Spinal Subnucleus Caudalis) Are Involved in Trigeminal Neuropathic Pain Mechanisms

The aim of this study was to investigate whether astroglia in the medullary dorsal horn (trigeminal spinal subnucleus caudalis; Vc) may be involved in orofacial neuropathic pain following trigeminal nerve injury. The effects of intrathecal administration of the astroglial aconitase inhibitor sodium fluoroacetate (FA) were tested on Vc astroglial hyperactivity [as revealed by glial fibrillary acid protein (GFAP) labeling], nocifensive behavior, Vc extracellular signal-regulated kinase phosphorylation (pERK), and Vc neuronal activity in inferior alveolar nerve-transected (IANX) rats. Compared with sham-control rats, a significant increase occurred in GFAP-positive cells in ipsilateral Vc at postoperative day 7 in IANX rats, which was prevented following FA administration. FA significantly increased the reduced head withdrawal latency to high-intensity heat stimulation of the maxillary whisker pad skin in IANX rats, although it did not significantly affect the reduced escape threshold to low-intensity mechanical stimulation of the whisker skin in IANX rats. FA also significantly reduced the increased number of pERK-like immunoreactive cells in Vc and the enhanced Vc nociceptive neuronal responses following high-intensity skin stimulation that were documented in IANX rats, and glutamine administration restored the enhanced responses. These various findings provide the first documentation that astroglia is involved in the enhanced nociceptive responses of functionally identified Vc nociceptive neurons and in the associated orofacial hyperalgesia following trigeminal nerve injury.

Physiological Mechanisms Of Neuropathic Pain: The Orofacial Region

Neuropathic pain in the orofacial region is the clinical manifestation of trigeminal nerve injury following oral surgeries such as tooth extraction, dental implantation or tooth pulp treatment. Normally non-noxious touching of the facial skin or oral mucosa elicits strong pain named allodynia, and normally noxious stimulation causes intolerable pain named hyperalgesia in the trigeminal neuropathic pain patients. Although the mechanisms underlying trigeminal neuropathic pain have been studied by many researchers, the detailed mechanisms are still unknown. In this chapter, we are focusing on trigeminal neuropathic pain, and describe our recent studies using animal models of trigeminal neuropathic pain. We also present the clinical assessment of trigeminal neuropathic pain patients to develop the appropriate treatment of trigeminal neuropathic pain.

Predictability of Painful Stimulation Modulates Subjective and Physiological Responses

UNLABELLED Clinical observations suggest that the perceived intensity of a painful event increases as the unpredictability of its occurrence increases. We examined the effect of varying stimulus predictability on the Somatosensory Evoked Potential (SEP), Pupil Diameter Response (PDR), Pain Report (PR), and Fear Report (FR) in 25 healthy female volunteers experiencing repeated noxious fingertip shocks. Each volunteer underwent high- and low-stimulus intensities in 4 stimulus patterns defined by stimulus sequence (SEQ) and interstimulus interval (ISI) as follows: A) serial stimulus intensity SEQ with fixed ISI; B) serial stimulus intensity SEQ with varied ISI; C) random stimulus intensity SEQ with fixed ISI; and D) random stimulus intensity SEQ with varied ISI. Results revealed that: (1) lower stimulus predictability led to higher PR and FR, greater PDR magnitude, and greater SEP amplitude; and (2) the 4 dependent measures showed the same response pattern across levels of stimulus predictability. These findings support the hypothesis that lower stimulus predictability is associated with higher reported pain and fear as well as greater physiological arousal. PERSPECTIVE Patients undergoing painful procedures experience more distress when the occurrence of a painful event is unpredictable. Poor predictability increases pain, fear, and associated physiological arousal. Maximizing the predictability of painful events may improve the quality of patient care by minimizing associated levels of pain and fear.

Fractalkine Signaling in Microglia Contributes to Ectopic Orofacial Pain following Trapezius Muscle Inflammation

Fractalkine (FKN) signaling is involved in mechanical allodynia in the facial skin following trapezius muscle inflammation. Complete Freunds adjuvant (CFA) injection into the trapezius muscle produced mechanical allodynia in the ipsilateral facial skin that was not associated with facial skin inflammation and resulted in FKN but not FKN receptor (CX3CR1) expression, and microglial activation was enhanced in trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord (C1–C2). Intra-cisterna magna anti-CX3CR1 or anti-interleukin (IL)-1β neutralizing antibody administration decreased the enhanced excitability of Vc and C1–C2 neurons in CFA-injected rats, whereas intra-cisterna magna FKN administration induced microglial activation and mechanical allodynia in the facial skin. IL-1β expression and p38 mitogen-activated protein kinase phosphorylation were enhanced in activated microglia after CFA injection. The excitability of neurons whose receptive fields was located in the facial skin was significantly enhanced in CFA-injected rats, and the number of cells expressing phosphorylated extracellular signal-regulated kinase (pERK) following noxious mechanical stimulation of the facial skin was significantly increased in Vc and C1–C2. We also observed mechanical allodynia of the trapezius muscle as well as microglial activation and increased pERK expression in C2–C6 after noxious stimulation of the trapezius muscle in facial skin-inflamed rats. These findings suggest that FKN expression was enhanced in Vc and C1–C2 or C2–C6 following trapezius muscle or facial skin inflammation, microglia are activated via FKN signaling, IL-1β is released from the activated microglia, and the excitability of neurons in Vc and C1–C2 or C2-C6 is enhanced, resulting in the ectopic mechanical allodynia.

Mechanisms involved in extraterritorial facial pain following cervical spinal nerve injury in rats.

BackgroundThe aim of this study is to clarify the neural mechanisms underlying orofacial pain abnormalities after cervical spinal nerve injury. Nocifensive behavior, phosphorylated extracellular signal-regulated kinase (pERK) expression and astroglial cell activation in the trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal dorsal horn (C1-C2) neurons were analyzed in rats with upper cervical spinal nerve transection (CNX).ResultsThe head withdrawal threshold to mechanical stimulation of the lateral facial skin and head withdrawal latency to heating of the lateral facial skin were significantly lower and shorter respectively in CNX rats compared to Sham rats. These nocifensive effects were apparent within 1 day after CNX and lasted for more than 21 days. The numbers of pERK-like immunoreactive (LI) cells in superficial laminae of Vc and C1-C2 were significantly larger in CNX rats compared to Sham rats following noxious and non-noxious mechanical or thermal stimulation of the lateral facial skin at day 7 after CNX. Two peaks of pERK-LI cells were observed in Vc and C1-C2 following mechanical and heat stimulation of the lateral face. The number of pERK-LI cells in C1-C2 was intensity-dependent and increased when the mechanical and heat stimulations of the face were increased. The decrements of head withdrawal latency to heat and head withdrawal threshold to mechanical stimulation were reversed during intrathecal (i.t.) administration of MAPK/ERK kinase 1/2 inhibitor PD98059. The area of activated astroglial cells was significantly higher in CNX rats (at day 7 after CNX). The heat and mechanical nocifensive behaviors were significantly depressed and the number of pERK-LI cells in Vc and C1-C2 following noxious and non-noxious mechanical stimulation of the face was also significantly decreased following i.t. administration of the astroglial inhibitor fluoroacetate.ConclusionsThe present findings have demonstrated that mechanical allodynia and thermal hyperalgesia occur in the lateral facial skin after CNX and also suggest that ERK phosphorylation of Vc and C1-C2 neurons and astroglial cell activation are involved in orofacial extraterritorial pain following cervical nerve injury.

Effects of midazolam on pain sensations in the face

OBJECTIVES This study investigated the effects of midazolam, a sedative, on tactile and pain sensations on the skin of the chin. STUDY DESIGN Thirty-seven volunteers were segregated into four groups; the first group was the control group; the second to fourth groups were administered 0.025 mg/kg, 0.05 mg/kg, and 0.075 mg/kg of midazolam, respectively, as a bolus injection. All volunteers reclined in a dental chair for the experiment. Tactile and pain sensations were determined over time after injection of midazolam, the former using von Fray thread, the latter using an esthesiometer. RESULTS Thresholds of tactile sensitivity and of pain were statistically significantly different from control values at 10 minutes after injection of midazolam in the 0.05 mg/kg group and in the 0.075 mg/kg group. CONCLUSION Although 0.025 mg/kg of midazolam produced sedation, at least 0.05 mg/kg of this agent was required to alter the thresholds for perception of tactile and painful stimulation.

Immune and Endocrine Function in Patients With Burning Mouth Syndrome

Objectives:Research suggests that varied etiologic factors are responsible for burning mouth syndrome (BMS). We examined the role of immune and endocrine function in the pathology of BMS. Methods:We conducted a case-control study to evaluate immune (lymphocyte subpopulations) and endocrine (hypothalamus-pituitary-adrenal axis and sympathetic-adrenomedullary system) function in 47 female BMS patients and 47 age-matched female controls presenting at an university clinic. Psychological state was assessed with the Zung Self-Rating Depression Scale and Taylor Manifest Anxiety Scale. Results:BMS patients were significantly more anxious than controls (P=0.011). Plasma adrenaline level was significantly lower (P=0.020) in BMS patients than in controls, and linear regression analysis of all patients combined revealed that depression level was significantly positively associated with plasma noradrenaline and cortisol levels (P=0.002 and 0.001, respectively). However, as compared with controls, BMS patients had a significantly lower CD8(+) cell count (P<0.001) and a significantly higher CD4/CD8 ratio (P=0.002). Discriminant analysis revealed that CD8(+) cell count and CD4/CD8 ratio were independent variables that distinguished BMS patients from controls. Discussion:The immunoendocrine system is substantially involved, and may have a key role, in the mechanism of chronic pain in BMS patients. Immune function was significantly and specifically suppressed in BMS, although the hypothalamic-pituitary-adrenal axis and sympathetic nervous system were predominantly activated by psychological stress that was not specific to BMS.

Interaction of IL-1β and P2X(3) receptor in pathologic masseter muscle pain.

The exact mechanism underlying chronic masseter muscle pain, a conspicuous symptom in temporomandibular disorder, remains unclear. We investigated whether expression of P2X3 receptor (P2X3R) is involved in mechanical hyperalgesia after contraction of masseter muscle (CMM). As compared with sham rats, the head-withdrawal threshold (HWT) to mechanical pressure stimulation of masseter muscle (MM) (but not after similar stimulation of facial skin) was significantly lower, and IL-1β level was significantly higher, in CMM rats on day 7 after CMM. The mean percentage of FG-labeled P2X3R-positive neurons was significantly increased in TG following successive IL-1β injections into the MM for 7 days. Successive administration of an IL-1β receptor-antagonist into the MM attenuated the increase of P2X3-IR cells in the TG. ATP release from MM after 300-g pressure stimulation of MM was also significantly enhanced after CMM. Administration into MM of the selective P2X3,2/3 receptor antagonist A-317491 attenuated the decrement of HWT in CMM rats. A significant increase in HWT was also observed at 30 min after A-317491 (60 µg) injection in IL-1β-injected rats. These findings suggest that P2X3R expression associated with enhanced IL-1β expression and ATP release in MM has a possible important role in MM mechanical hyperalgesia after excessive muscular contraction.

Modulation of formalin-evoked hyperalgesia by intrathecal N-type Ca channel and protein kinase C inhibitor in the rat.

Abstract1. ω-CgTx attenuated formalin-evoked biphasic flinches, while PKC inhibitor (STU) attenuated phase 2 and was reversed by PDBu.2. ω-CgTx and STU suppressed the increase in CSF-glutamate after formalin injection.3. Morphine completely suppressed both increased flinching and CSF glutamate release.4. Thus, ω-CgTx (N-type Ca channels) may regulate neurotransmitter release evoked by C fiber activation and the formalin-evoked hyperalgesia may possibly be provoked as a result of PKC activation elicited by both presynaptic neurotransmitter release and activation of NMDA receptors in the spinal neurons.