Involvement of descending monoaminergic systems in the transmission of dental pain in the trigeminal nucleus caudalis of the rabbit

Biological sciences related to headache

Headache can occur as a result of activation of pain-sensitive cranial structures, such as the dura mater, vasculature, and the cranial and cervical muscles and ligaments, which are innervated by primary afferent neurons originating from the trigeminal and dorsal root ganglia of the upper cervical spinal nerves. Similar to general nociceptive sensation, C fibers and Adelta fibers are known to play an important role in headache perception. Findings from nerve stimulation studies indicate that C fibers transmit aching, throbbing, or burning pain that builds up slowly, whereas the Adelta fibers conduct sharper initial pain sensation. These primary afferent nerve fibers transmit nociceptive information from the pain-sensitive endings in the cranial structures through the trigeminal and first and second spinal dorsal root ganglia to the brainstem at the pontine level. The nociceptive fibers then project to the central pain-conducting pathways at the spinal trigeminal nucleus. In this chapter, we discuss the anatomy in relation to headache, including the meninges, dural sinuses, blood vessels, sensory ganglia, cranial and neck muscles, and the central pain-conducting pathways.

Involvement of NMDA-nitric oxide pathways in the development of tactile hypersensitivity evoked by the loose-ligation of inferior alveolar nerves in rats

To investigate whether or not NMDA/nitric oxide (NO) pathways in the trigeminal system are involved in the development and/or maintenance of such pathological pain states as the hyperalgesia and allodynia observed after dental surgery, loose-ligation on the left inferior alveolar nerves of rats were performed. The responses to mechanical stimulation were then measured using von Frey filaments. Hypersensitivity to tactile stimulation developed on the ipsilateral side in ligated animals 5 days after surgery and lasted for at least 30 days. In addition, the effects of drugs on these pain states during the period 2-3 weeks following surgery were investigated. As a result, it was observed that tactile hypersensitivity was inhibited by the intraperitoneal (i.p.) administration of both MK-801 hydrogen maleate (0.05-0.1 mg/kg) and N(G)-monomethyl-L-arginine acetate (L-NMMA: 10-100 mg/kg). Still further, NO production and the number of neuronal NO synthase (nNOS)-positive neurons in the trigeminal nucleus caudalis (SpVc) was evaluated. As a result of these experiments, it was found that the changes in NO levels evoked by the intravenous infusions of N-methyl-D-aspartate (NMDA; 10 mg/kg) and MK-801 (0.5 mg/kg) were significantly larger in the loose-ligated rats compared to the sham-operated rats. Moreover, the number of nNOS-positive neurons was found to have increased on the ipsilateral side in layers I/II of the SpVc. These results would suggest that tactile hypersensitivity develops after inferior alveolar nerve injury and that NMDA receptor/NOS/NO production pathways in the SpVc may be involved in the development of such pathophysiological states.

Influence of serotonin receptor antagonists on substance P and serotonin release evoked by tooth pulp stimulation with electro-acupuncture in the trigeminal nucleus cudalis of the rabbit

We studied the effect of NAN-190 (5-HT(1A) antagonist), ketanserin (5-HT(2) antagonist) and ICS 205-930 (5-HT(3) antagonist) on tooth pulp stimulation (TPS)-induced 5-HT release and substance P (SP) release in the superficial layers of the trigeminal nucleus caudalis (SpVc-I,II) in the presence or absence of electro-acupuncture (EAP). TPS slightly increased 5-HT release and significantly increased SP release. In combination with EAP, TPS-induced 5-HT release was remarkably enhanced, whereas SP release was significantly suppressed. Pretreatment with NAN-190 (3.5 mg/kg, i.v.) significantly enhanced the increase in TPS-induced 5-HT release in the presence of EAP. On the other hand, the increase of 5-HT release induced following TPS in the presence of EAP was inhibited by pretreatment with ketanserin (2.5 mg/kg, i.v.) and ICS 205-930 (1 mg/kg, i.v.). When NAN-190 was pre-treated in the animals combined TPS and EAP, the amount of SP release was significantly reduced compared with the absence of this drug. On the other hand, pretreatment with ketanserin and ICS 205-930 reversed the inhibitory effect of EAP on the TPS-generated SP release, especially ICS 205-930, which remarkably enhanced TPS-induced SP release compared with the absence of this drug. On the basis of the obtained results, we concluded that NAN-190 and ICS 205-930 act on EAP-induced analgesia positively and suppressively, respectively, by regulation of TPS-generated SP release through activation of their subtype receptors. On the other hand, ketanserin does not affect TPS-induced 5-HT release and SP release in the presence of EAP.

Serotonin receptor subtypes involved in modulation of electrical acupuncture

We examined the effects of intravenous injection of several serotonin (5-HT) antagonists on the inhibitory action of electro-acupuncture (EAP) against the nociceptive responses in the trigeminal nucleus caudalis in rabbits. The inhibitory effect of EAP was suppressed by pindolol, methysergide and ICS 205-930, whereas NAN-190 and ketanserin amplified the EAP effect. These results suggest that 5-HT1, except 5-HT1A; 5-HT2, except 5-HT2A; and 5-HT3 receptors are positively involved in EAP-induced analgesia, whereas the activation of 5-HT1A and 5-HT2A receptors suppressively act on EAP-induced analgesia.

Morphine administered in the substantia gelatinosa of the spinal trigeminal nucleus caudalis inhibits nociceptive activities in the spinal trigeminal nucleus oralis

The present study investigates the effects of morphine microinjection into the spinal trigeminal nucleus caudalis (Sp5C) or the spinal trigeminal nucleus oralis (Sp5O) on C-fiber-evoked activities of Sp5O convergent neurons, after supramaximal percutaneous electrical stimulation in halothane-anesthetized rats. When it was microinjected into the Sp5O, morphine (2.5 microg in 0. 25 microl) never depressed the C-fiber-evoked responses of Sp5O convergent neurons (n = 13), whereas these neurons were responsive to the inhibitory effects of systemic morphine (6 mg/kg, i.v.) in a naloxone-reversible manner. On the contrary, morphine microinjected into the Sp5C produced a naloxone-reversible inhibition of the C-fiber-evoked responses of Sp5O neurons (n = 14). The magnitude and the time course of this effect varied according to the location of the injection sites. After microinjection into the superficial laminae (n = 7), a strong depressive effect of morphine (7 +/- 5% of control) on the C-fiber-evoked responses was apparent as soon as 5 min after the injection and could always be reversed by naloxone, administered either intravenously (0.4 mg/kg) or locally (2.5 microg in 0.6 microl) at the same site as morphine. After microinjection into deeper laminae (V-VI), a significant depressive effect (34 +/- 5% of control) of morphine could be detected only 20 min after the injection and was reversed only by intravenous administration of naloxone. These results suggest that morphine exerts its antinociceptive action on Sp5O convergent neurons by blocking the C-fiber inputs that relay in the Sp5C substantia gelatinosa. The mechanisms that underlie the activation of Sp5O convergent neurons by C-fibers and the inhibition of C-fiber-evoked responses of Sp5O convergent neurons by morphine microinjected into the Sp5C are discussed.

A possible involvement of monoaminergic and opioidergic systems in the analgesia induced by electro-acupuncture in rabbits

To elucidate mechanisms involved in analgesia induced by effects of electro-acupuncture (EAP), effects of EAP on evoked potentials and release of substance P (SP) following tooth pulp stimulation (ST) in the superficial layers of the trigeminal nucleus caudalis (Vc-I-II) were studied in the rabbit. The potentials evoked by ST were composed of two main components with conduction velocity of ca. 30 m/sec (fast component) and ca. 12 m/sec (late component). The late component was significantly inhibited by morphine (10 mg/kg, i.v.) or CP-96,345 (5 mg/kg, i.v.), an SP antagonist. This inhibitory effect of morphine was antagonized by naloxone (1 mg/kg, i.v.) or methysergide (5 mg/kg, i.v.). In addition, the late component was significantly inhibited by EAP, which was observed in ca. 70% of the rabbits examined. This EAP-induced inhibitory effect was antagonized by naloxone (1 mg/kg, i.v.) or methysergide (5 mg/kg, i.v.), but not by prazosin (5 mg/kg, i.v.) and yohimbine (1 mg/kg, i.v.). The stimulus-evoked SP release was inhibited by EAP, which was significantly antagonized by pretreatment with naloxone (1 mg/kg, i.v.) or methysergide (5 mg/kg, i.v.). These results suggest that one of the mechanisms of analgesia induced by EAP is due to inhibition of the stimulus-evoked SP release in the Vc-I-II through activation of the descending serotonergic systems linking up with opioidergic systems.

Non-steroidal anti-inflammatory drugs and spinal nociceptive processing

Non-steroidal anti-inflammatory drugs have a direct action on spinal nociceptive processing in vivo with a relative order of potency which correlates with their capacity as inhibitors of cyclooxygenase activity. However, recent clinical surveys and new in vivo evidence strongly suggest that for some of these agents, centrally mediated analgesia may also be achieved by additional mechanisms, which are independent of prostaglandin synthesis inhibition. In this review we explore the likelihood for such mechanisms following an extensive survey of existing data. The implications of these mechanisms are discussed in the light of our current understanding of spinal nociceptive processing.

Influence of electro-acupuncture on the release of substance P and the potential evoked by tooth pulp stimulation in the trigeminal nucleus caudalis of the rabbit

The effects of electro-acupuncture (EAP) on the release of substance P (SP) and the responses evoked by tooth pulp stimulation (ST) in superficial layers of the trigeminal nucleus caudalis (Vc-I,II) were studied in rabbits. ST evoked increase in release of immunoreactive SP (iSP). This increase was inhibited by EAP in 9 of 13 animals. The potentials evoked by ST were composed of two main components with latency times of ca 4.3 msec and ca. 9.4 msec. The latter component, reflecting the excitation of A delta fibers, was significantly inhibited by CP-96,345 (3 mg/kg, i.v.), an SP antagonist. EAP also inhibited the latter component in 8 of 11 animals. These results suggest that one of the mechanisms of analgesia induced by EAP is inhibition of stimulus-evoked SP release in the Vc-I,II.

Serotonin inhibits release of substance P evoked by tooth pulp stimulation in trigeminal nucleus caudalis in rabbits

The influence of the serotonin (5-HT) system on the release of immunoreactive substance P after electrical stimulation of the lower incisor pulp was examined by perfusion of the superficial layers of the subnucleus caudalis of the brain stem trigeminal sensory nuclear complex of rabbits in situ. Increased release of immunoreactive substance P was observed after electrical stimulation of the pulp at 40 V. Stimulation of the nucleus raphe magnus significantly increased the release of 5-HT and completely inhibited the release of immunoreactive substance P, evoked by stimulation of the tooth pulp. Local application of 5-HT (10(-6) M) inhibited the release of immunoreactive substance P induced by stimulation and this inhibition was antagonized by methysergide (10(-4) M) applied concomitantly to the superficial layers of the trigeminal nucleus. These results suggest a functional interaction between substance P and 5-HT in the superficial layers of the trigeminal nucleus for regulation of transmission of dental pain.