Rat tooth pulp projections to spinal trigeminal subnucleus caudalis are glutamate-like immunoreactive

Insular cortical descending projections facilitate neuronal responses to noxious but not innoxious stimulation in rat trigeminal spinal subnucleus caudalis

The insular cortex (IC) receives orofacial nociceptive information. Pyramidal neurons in IC layer V send their axons to various brain regions, such as the trigeminal spinal subnucleus caudalis (Sp5C), parabrachial nucleus, and periaqueductal gray. However, little information has been available about the functions of these descending projections from the IC. This study aimed to elucidate the effect of IC → Sp5C on neuronal spike firings responding to noxious and innoxious stimuli to the face of the rat receiving an injection of adeno-associated virus encoding modified channelrhodopsin-2 (ChR2) fused to mCherry under the control of the human synapsin promotor. We classified Sp5C neurons responding to mechanical stimuli into three groups: low-threshold (LT), nociceptive specific (NS), and wide dynamic range (WDR) neurons, which respond to innoxious stimuli (brushing) only, noxious mechanical stimuli (pinching) only, and both noxious and innoxious stimuli, respectively. Neuronal activities of IC neurons were activated by photostimulation (repetitive pulses at 20 Hz for 5 Hz) to the IC that consistently induced action potentials in IC layer V pyramidal neurons. LT neurons showed comparable spike firing rates to brushing the facial skin before and during ChR2 activation induced by photostimulation. In contrast, NS neurons showed an increase in their firing frequency to pinching during ChR2 activation. On the other hand, WDR neurons increased their Sp5C neuronal firing to pinching during ChR2 activation without changing their firing rates to innoxious mechanical stimuli. These results suggest that the IC descending projections facilitate nociception by increasing Sp5C neuronal activities responding to noxious mechanical stimuli.

Descending projections from the insular cortex to the trigeminal spinal subnucleus caudalis facilitate excitatory outputs to the parabrachial nucleus in rats

ABSTRACT

Nociceptive information from the orofacial area projects to the trigeminal spinal subnucleus caudalis (Sp5C) and is then conveyed to several nuclei, including the parabrachial nucleus (PBN). The insular cortex (IC) receives orofacial nociceptive information and sends corticofugal projections to the Sp5C. The Sp5C consists of glutamatergic and GABAergic/glycinergic interneurons that induce excitatory postsynaptic currents and inhibitory postsynaptic currents, respectively, in projection neurons. Therefore, quantification of glutamatergic IC inputs in combination with identifying postsynaptic neuronal subtypes is critical to elucidate IC roles in the regulation of Sp5C activities. We investigated features of synaptic transmission from the IC to glutamatergic and GABAergic/glycinergic Sp5C neurons of laminae I/II using vesicular GABA transporter-Venus transgenic rats that received an injection of adeno-associated virus-channelrhodopsin-2-mCherry into the IC. Selective stimulation of IC axon terminals in Sp5C slice preparations induced monosynaptic excitatory postsynaptic currents in both excitatory glutamatergic and inhibitory GABAergic/glycinergic Sp5C neurons with a comparable amplitude. Paired whole-cell patch-clamp recordings showed that unitary inhibitory postsynaptic currents from inhibitory neurons influencing excitatory neurons, including neurons projecting to the PBN, exhibited a high failure rate and were suppressed by both bicuculline and strychnine, suggesting that excitatory neurons in the Sp5C receive both GABAergic and glycinergic inhibition with low impact. Moreover, selective stimulation of IC axons increased the firing rate at the threshold responses. Finally, we demonstrated that selective stimulation of IC axons in the Sp5C by a chemogenetic approach decreased the thresholds of both mechanical and thermal nociception. Thus, IC projection to the Sp5C is likely to facilitate rather than suppress excitatory outputs from the Sp5C.

Topiramate Inhibits Trigeminovascular Neurons in the Cat

To facilitate understanding the action of antimigraine preventives the effect of topiramate on trigeminocervical activation in the cat was examined. Animals ( n = 7) were anaesthetized and physiologically monitored. The superior sagittal sinus (SSS) was stimulated to produce a model of trigeminovascular nociceptive activation. Cumulative dose-response curves were constructed for the effect of topiramate at doses of 3, 5, 10, 30 and 50 mg/kg on SSS-evoked firing of trigeminocervical neurons. Topiramate reduced SSS evoked firing in a dose-dependent fashion. The maximum effect was seen over 30 min for the cohort taken together. At 3 mg/kg firing was reduced by 36 ± 13% (mean ± SEM) after 15 min. At 5 and 50 mg/kg firing was reduced by 59 ± 6% and 65 ± 14%, respectively, after 30 min. Inhibition of the trigeminocervical complex directly, or neurons that modulate sensory input, are plausible mechanisms for the action of preventives in migraine.

The striatum and pain modulation

The aim of this review was to give a general aspect of the sensorial function of the striatum related to pain modulation, which was intensively studied in our laboratory. We analyse the effect of electrical and chemical stimulation of the striatum on the orofacial pain, especially that produced by tooth pulp stimulation of the lower incisors. We demonstrated specific sites within the nucleus which electrical or chemical stimulation produced inhibition of the nociceptive jaw opening reflex. This analgesic action of the striatum was mediated by activation of its dopamine D(2) receptors and transmitted through the indirect pathways of the basal ganglia and the medullary dorsal reticular nucleus (RVM) to the sensorial nuclei of the trigeminal nerve. Its mechanism of action was by inhibition of the nociceptive response of the second order neurons of the nucleus caudalis of the V par.

Nicotinamide adenine dinucleotide phosphate/neuronal nitric oxide synthase-positive neurons in the trigeminal subnucleus caudalis involved in tooth pulp nociception

Sensory information on facial structures, including teeth pulp, periodontium, and gingiva, is relayed in the trigeminal complex. Tooth pulp inflammation constitutes a common clinical problem, and this peripheral injury can induce neuroplastic changes in trigeminal nociceptive neurons. There is considerable evidence that the trigeminal subnucleus caudalis (Vc) is the principal relay for trigeminal nociceptive information as well as modulation of the painful stimuli. Glutamatergic primary afferents innervating the tooth pulp project to the most superficial laminae of the Vc. N-methyl-D-aspartate receptor stimulation leads to the activation of the enzyme nitric oxide synthase (NOS), which synthesizes the free radical nitric oxide (NO). This enzyme is expressed mainly in lamina II interneurons, and in a small number of cells in lamina I as well as in deep laminae projection neurons of Vc. In the present study, we analyzed the temporal changes in neuronal NOS (nNOS) in Vc local circuitries after unilateral intermediate molar pulp injury. Our results demonstrate that a peripheral dental pulp injury leads to neuroplastic changes in the relative amount and activity of nNOS enzyme. Moreover, after a period of time, the nitrergic system shifts to the initial values, independently of the persistence of inflammation in the pulp tissues.

Purinergic receptors are involved in tooth-pulp evoked nocifensive behavior and brainstem neuronal activity

Background

To evaluate whether P2X receptors are involved in responses to noxious pulp stimulation, the P2X₃ and P2X_2/3 receptor agonist α,β-methyleneATP (α,β-meATP) was applied to the molar tooth pulp and nocifensive behavior and extracellular-signal regulated kinase (ERK) phosphorylation in trigeminal spinal subnucleus caudalis (Vc), trigeminal spinal subnucleus interpolaris (Vi), upper cervical spinal cord (C1/C2) and paratrigeminal nucleus (Pa5) neurons were analyzed in rats.

Results

Genioglossus (GG) muscle activity was evoked by pulpal application of 100 mM α,β-meATP and was significantly larger than GG activity following vehicle (phosphate-buffered saline PBS) application (p < 0.01). The enhanced GG muscle activity following 100 mM α,β-meATP was significantly reduced (p < 0.05) by co-application of 1 mM TNP-ATP (P2X₁, P2X₃ and, P2X_2/3 antagonist). A large number of pERK-LI cells were expressed in the Vc, Vi/Vc, C1/C2 and Pa5 at 5 min following pulpal application of 100 mM α,β-meATP compared to PBS application to the pulp (p < 0.05). The pERK-LI cell expression and GG muscle activity induced by 100 mM α,β-meATP pulpal application were significantly reduced after intrathecal injection of the MAPK/ERK kinase (MEK) inhibitor PD 98059 and by pulpal co-application of 1 mM TNP-ATP (p < 0.05).

Conclusions

The present findings suggest that activation of P2X₃ and P2X_2/3 receptors in the tooth pulp is sufficient to elicit nociceptive behavioral responses and trigeminal brainstem neuronal activity.

Current and prospective pharmacological targets in relation to antimigraine action

Migraine is a recurrent incapacitating neurovascular disorder characterized by unilateral and throbbing headaches associated with photophobia, phonophobia, nausea, and vomiting. Current specific drugs used in the acute treatment of migraine interact with vascular receptors, a fact that has raised concerns about their cardiovascular safety. In the past, alpha-adrenoceptor agonists (ergotamine, dihydroergotamine, isometheptene) were used. The last two decades have witnessed the advent of 5-HT(1B/1D) receptor agonists (sumatriptan and second-generation triptans), which have a well-established efficacy in the acute treatment of migraine. Moreover, current prophylactic treatments of migraine include 5-HT(2) receptor antagonists, Ca(2+) channel blockers, and beta-adrenoceptor antagonists. Despite the progress in migraine research and in view of its complex etiology, this disease still remains underdiagnosed, and available therapies are underused. In this review, we have discussed pharmacological targets in migraine, with special emphasis on compounds acting on 5-HT (5-HT(1-7)), adrenergic (alpha(1), alpha(2,) and beta), calcitonin gene-related peptide (CGRP(1) and CGRP(2)), adenosine (A(1), A(2), and A(3)), glutamate (NMDA, AMPA, kainate, and metabotropic), dopamine, endothelin, and female hormone (estrogen and progesterone) receptors. In addition, we have considered some other targets, including gamma-aminobutyric acid, angiotensin, bradykinin, histamine, and ionotropic receptors, in relation to antimigraine therapy. Finally, the cardiovascular safety of current and prospective antimigraine therapies is touched upon.

Central distribution of nociceptive intradental afferent nerve fibers in the rat

The central distribution of intradental afferent nerve fibers was investigated by combining electron microscopic observations with a selective method for inducing degeneration of the A delta- and C-type afferent fibers. Degenerating terminals were found on the proprioceptive mesencephalic trigeminal neurons and on dendrites in the neuropil of the trigeminal motor nucleus after application of capsaicin to the rat's lower incisor tooth pulp. The results give anatomical evidence of new sites of central projection of intradental A delta- and C-type fibers whereby the nociceptive information from the tooth pulp can affect jaw muscle activity.

New targets in acute migraine treatment

Migraine is a common and highly disabling neurological problem, whose acute treatment was revolutionized by the triptans, serotonin 5-HT1B/1D receptor agonists. Some patients do not respond to triptans, while others are not suitable for them largely because of contraindications based on vascular disease. The exploration of nonvasoconstrictor treatments for acute migraine offers the prospect of dramatic improvements in patient care, as well as important insights into the mechanisms of migraine. Possibilities for such developments include, calcitonin gene-related peptide receptor antagonists, serotonin 5-HT1F and 5-HT1D receptor agonists, glutamate excitatory amino acid receptor antagonists, nitric oxide synthase inhibitors and adenosine A1 receptor agonists. Taken together, the future for migraine and affected patients is bright and promising.

Role of peripheral group I and II metabotropic glutamate receptors in IL-1beta-induced mechanical allodynia in the orofacial area of conscious rats

The present study investigated the role of peripheral group I and II metabotropic glutamate receptors (mGluRs) in interleukin-1beta (IL-1beta)-induced mechanical allodynia in the orofacial area. Experiments were carried out on Sprague-Dawley rats weighing between 230 and 280 g. After subcutaneous administration of 0.01, 0.1, 1, or 10 pg of IL-1beta, we examined withdrawal behavioral responses produced by 10 successive trials of a ramp of air-puffs pressure applied ipsilaterally or contralaterally to the IL-1beta injection site. The thresholds of air puffs were measured 10, 30, 60, 120, or 180 min after 25 microl of IL-1beta was administered through an implanted tube. Subcutaneous injection of IL-1beta produced bilateral mechanical allodynia. While the IL-1beta-induced mechanical allodynia was blocked by pretreatment with an IL-1 receptor antagonist, the IL-1beta-induced mirror-image mechanical allodynia was not blocked by an IL-1 receptor antagonist injected into the contralateral side. Subcutaneous administration of CPCCOEt or LY367385, an mGluR1 antagonist, or MPEP or SIB1893, an mGluR5 antagonist, 10 min prior to injection of IL-1beta abolished IL-1beta-induced mechanical allodynia. Pretreatment with APDC or DCG4, a group II mGluR agonist, blocked the IL-1beta-induced mechanical allodynia. The anti-allodynic effect induced by APDC was inhibited by pretreatment with LY341495, a group II mGluR antagonist. These results suggest that peripheral group I and II mGluRs participate in IL-1beta-induced mechanical allodynia in the orofacial area. Peripheral group I mGluR antagonists blocked the IL-1beta-induced mechanical allodynia, while peripheral group II mGluR agonists produced anti-allodynic effects on IL-1beta-induced mechanical allodynia in the orofacial area of rats.

Can we Develop Neurally Acting Drugs for the Treatment of Migraine?

Serotonin (5-HT)(1B/1D) receptor agonists, which are also known as triptans, represent the most important advance in migraine therapeutics in the four millennia that the condition has been recognized. The vasoconstrictive activity of triptans produced a small clinical penalty in terms of coronary vasoconstriction but also raised an enormous intellectual question: to what extent is migraine a vascular problem? Functional neuroimaging and neurophysiological studies have consistently developed the theme of migraine as a brain disorder and, therefore, demanded that the search for neurally acting antimigraine drugs should be undertaken. The prospect of non-vasoconstrictor acute migraine therapies, potential targets for which are discussed here, offers a real opportunity to patients and provides a therapeutic rationale that places migraine firmly in the brain as a neurological problem, where it undoubtedly belongs.

New targets in the acute treatment of headache

PURPOSE OF REVIEW

The aim of this article is to review recently identified targets for the acute treatment of primary headache disorders.

RECENT FINDINGS

Calcitonin gene-related peptide (CGRP) receptor blockade has been shown to be an effective acute anti-migraine strategy and is a non-vasoconstrictor in terms of the mechanism of action. It is likely that direct blockade of CGRP release by inhibition of trigeminal nerves would be similarly effective in both migraine and cluster headache. Options for acute treatment based on preclinical work and initial clinical studies include: serotonin 5HT1F and 5HT1D receptor agonists, glutamate excitatory amino acid receptor antagonists, nitric oxide synthase inhibitors and adenosine A1 receptor agonists. Proof of principle studies with octreotide, a somatostatin receptor agonist, demonstrated it to be better than placebo in the acute treatment of cluster headache but not in the acute management of migraine.

SUMMARY

The prospect of a non-vasoconstrictor acute migraine therapy offers a real opportunity to patients, and perhaps more importantly, provides a therapeutic rationale to plant migraine and cluster headache firmly in the brain as neurological problems.

Peripheral NMDA receptor modulation of jaw muscle electromyographic activity induced by capsaicin injection into the temporomandibular joint of rats

We have previously documented that peripheral N-methyl-d-aspartate (NMDA) receptor mechanisms are involved in nociceptive reflex increases in jaw muscle activity to injection of mustard oil or glutamate into the rat temporomandibular joint (TMJ). The aim of the present study was to determine whether peripheral NMDA receptor mechanisms are also involved in the nociceptive reflex responses in the jaw muscles evoked by injection of the inflammatory irritant and algesic chemical capsaicin into the TMJ. The effects of peripheral injection of NMDA receptor antagonists, MK-801 and APV, on the increases in electromyographic (EMG) activities of digastric and masseter muscles reflexly evoked by capsaicin injection into the TMJ were tested in halothane-anesthetized male rats. The capsaicin injection following pre-injection of vehicle evoked significant increases in EMG activity in both digastric and masseter muscles whereas pre-injection of MK-801 or APV into the TMJ resulted in a significant concentration-related reduction in the magnitude of capsaicin-evoked digastric and masseter EMG activity (ANOVA-on-ranks, P < 0.05). This finding indicates that capsaicin-evoked digastric and masseter EMG activity can be attenuated by pre-injection into the TMJ of NMDA receptor antagonists, and that the activation of peripheral NMDA receptors may be important in the mechanisms whereby capsaicin evokes nociceptive trigeminal responses.

Striatal Inhibition of Nociceptive Responses Evoked in Trigeminal Sensory Neurons by Tooth Pulp Stimulation

The noxious evoked response in trigeminal sensory neurons was studied to address the role of striatum in the control of nociceptive inputs. In urethane-anesthetized rats, the jaw opening reflex (JOR) was produced by suprathreshold stimulation of the tooth pulp and measured as electromyographic response in the digastric muscle, with simultaneous recording of noxious responses in single unit neurons of the spinal trigeminal nucleus pars caudalis (Sp5c). The microinjection of glutamate (80 ηmol/0.5 μl) into striatal JOR inhibitory sites significantly decreased the Aδ and C fiber–mediated–evoked response (53 ± 4.2 and 43.6 ± 6.4% of control value, P < 0.0001) in 92% (31/34) of nociceptive Sp5c neurons. The microinjection of the solvent was ineffective, as was microinjection of glutamate in sites out of the JOR inhibitory ones. In another series of experiments, simultaneous single unit recordings were performed in the motor trigeminal nucleus (Mo5) and the Sp5c nucleus. Microinjection of glutamate decreased the noxious-evoked response in Sp5c and Mo5 neurons in parallel with the JOR, without modifying spontaneous neuronal activity of trigeminal motoneurons ( n = 8 pairs). These results indicate that the striatum could be involved in the modulation of nociceptive inputs and confirm the role of the basal ganglia in the processing of nociceptive information.

Intracisternal NMDA produces analgesia in the orofacial formalin test of freely moving rats

The present study was performed to investigate the antinociceptive response to the intracisternal administration of NMDA in the orofacial area. To achieve this purpose, the effects of NMDA injected intracisternally on the orofacial formalin test were monitored in freely moving rats. We also investigated underlying the mechanisms of NMDA-induced antinociceptive response. Experiments were carried out on 80 male SD rats and surgical procedures were performed under pentobarbital sodium (40 mg/kg, i.p.). Fifty microliters of 5% formalin was applied subcutaneously to the vibrissa pad without any restraining of the animals. For each animal, the number of noxious behavioral responses and the time spent grooming, rubbing, and/or scratching the facial region proximal to the injection site were recorded for nine successive 5-min intervals. The orofacial formalin responses showed two distinct phases separated by a time of relative inactivity. Intracisternal administration of NMDA produced intense scratching behavioral responses with dose related manner. NMDA injected intracisternally 30 min prior to formalin injection, however, inhibited noxious behavioral responses produced by a formalin injection significantly. Pretreatment with naloxone 20 min prior to NMDA injection abolished the inhibition of number of scratches and the duration of scratching produced by the intracisternal injection of NMDA in the late phase. Pretreatment with L-NAME, NO synthesis inhibitor, however, did not affect the antinociceptive response produced by NMDA injected intracisternally. These results suggest that NMDA injected intracisternally produces brief pain behavioral responses and also produces delayed antinociceptive effects in the orofacial formalin test. The opioid pathway seems to be involved in the NMDA-induced antinociception in the orofacial area.

Peripheral glutamate receptors participate in interleukin-1β-induced mechanical allodynia in the orofacial area of rats

The present study was performed to examine peripheral cytokine-induced mechanical allodynia in the orofacial area and to investigate whether peripheral excitatory amino acids participate in the cytokine-induced mechanical allodynia. Experiments were carried out on male Sprague-Dawley rats. After interleukin-1beta (IL-1beta) was applied subcutaneously to the orofacial area, we examined withdrawal responses produced by air puffs applied to the IL-1beta injection site. The threshold of air puffs that produced withdrawal behavioral responses decreased significantly in a dose-dependent manner after injection of IL-1beta. Pretreatment with an IL-1 receptor antagonist abolished the decrease in the threshold of air puffs. Pretreatment with dl-2-amino-5-phosphonvaleric acid, an N-methyl-d-aspartic acid (NMDA) receptor antagonist, did not affect IL-1beta-induced mechanical allodynia. However, pretreatment with 6,7-dinitroquinoxaline-2,3-dione, a non-NMDA receptor antagonist, abolished the decrease in the threshold of air puffs. These results suggest that peripheral cytokine can produce mechanical allodynia in the orofacial area and that excitatory amino acids can modulate IL-1beta-induced mechanical allodynia via non-NMDA receptors.

Co-localization of mu opioid receptor and N-methyl-D-aspartate receptor in the trigeminal dorsal horn

Antagonists acting at the N-methyl-D-aspartate (NMDA) receptor can block the development of tolerance to the analgesic effects of [mu ] opioid receptor (MOR) ligands, such as morphine, and can also enhance the analgesic efficacy of opioids. These findings have led to the hypothesis that interactions between NMDA receptor and MOR ligands may be due to the co-localization of these receptors on neurons in the dorsal horn. We used dual immunogold and immunoperoxidase immunocytochemistry for MOR1 and NMDAR1 to determine the degree of co-localization of these receptors in neurons of the trigeminal dorsal horn. By use of electron microscopy, we found that both receptors were primarily located in dendrites and to a lesser extent in perikarya, axons, axon terminals, and glia. With regard to the degree of co-localization in dendrites, 63% of MOR1-labeled dendrites also contained NMDAR1, whereas 61% of NMDAR1-labeled dendrites also contained MOR1. Most of the dual-labeled profiles (94%) were classified as dendrites, with the remainder being axons, axon terminals, or perikarya. These results suggest that direct interactions between MOR and NMDA receptor ligands are likely mediated through shared dendritic targets in the dorsal horn. Less frequently, we found evidence for modulation of afferents to MOR-containing neurons through presynaptic NMDA receptors.

Involvement of the NMDA-nitric oxide pathway in the development of hypersensitivity to tactile stimulation in dental injured rats

To investigate mechanisms in pathological pain conditions as the hyperalgesia and allodynia observed after dental surgery, we employed a rat dental-injury model involving the simultaneous pulpectomy to a lower incisor and extraction of an ipsilateral upper incisor. We found that hypersensitivity to tactile stimulation developed on both ipsilateral and contralateral sides in the dental-injured rats 5 days after the surgery and that this lasted for at least 30 days. Recovery from hypersensitivity to tactile stimulation was achieved by the intraperitoneal (i.p.) administration of MK-801 (0.05 mg/kg) or N(G)-monomethyl-L-arginine monoacetate (L-NMMA: 10 - 100 mg/kg), but not attained by N(G)-monomethyl-D-arginine monoacetate (D-NMMA: 100 mg/kg). This recovery effect of L-NMMA (50 mg/kg) was inhibited by pretreatment with L-arginine (600 mg/kg). In the trigeminal nucleus caudalis (SpVc), the changes in nitric oxide (NO) levels invoked by the intravenous (i.v.) administration of N-methyl-D-aspartate (NMDA; 10 mg/kg) were found to be significantly larger in the dental-injured rats than in sham-operated rats. The number of neuronal NO synthase (nNOS)-positive neurons increased in layers I-II and III-IV in the SpVc on both sides of the dental-injured rats. These results suggest that hypersensitivity to tactile stimulation developed following dental injury, and that NMDA receptor/NOS/NO production pathways in the SpVc may be involved in pathological conditions.

Central sensitization of nociceptive neurons in trigeminal subnucleus oralis depends on integrity of subnucleus caudalis

Our recent studies have shown that application to the tooth pulp of the inflammatory irritant mustard oil (MO) produces a prolonged (>40 min) "central sensitization" reflected in neuroplastic changes in the mechanoreceptive field (RF) and response properties of nociceptive brain stem neurons in subnuclei oralis (Vo) and caudalis (Vc) of the trigeminal spinal tract nucleus. In view of the previously demonstrated ascending modulatory influence of Vc on Vo, our aim was to determine whether the Vo neuroplastic changes induced by MO application to the tooth pulp depend on an ascending influence from Vc. In chloralose/urethan-anesthetized rats, MO application to the pulp produced significant increases in Vo nociceptive neuronal orofacial RF size and responses to mechanical noxious stimuli that lasted as long as 40-60 min. These changes were not affected by vehicle (saline) microinjected into Vc at 20 min after MO application, but 0.3 microl of a 5 mM CoCl(2) solution microinjected into the ipsilateral Vc produced a reversible blockade of the MO-induced Vo neuroplastic changes. A similar volume and concentration of CoCl(2) solution injected into subnucleus interpolaris of the trigeminal spinal tract nucleus did not affect the MO-induced neuroplastic changes in Vo. These findings indicate that inflammatory pulp-induced central sensitization in Vo is dependent on the functional integrity of Vc.

Prevalence of missing posterior teeth and intraarticular temporomandibular disorders

STATEMENT OF PROBLEM

The association between missing mandibular posterior teeth and the development of intraarticular temporomandibular disorders (TMDs) remains unclear.

PURPOSE

The purpose of this study was to evaluate the prevalence of missing mandibular posterior teeth and intraarticular TMDs in a mixed population of asymptomatic subjects and symptomatic TMD patients.

MATERIAL AND METHODS

Eighty-two asymptomatic volunteers and 263 symptomatic TMD patients were included in this study. Asymptomatic volunteers completed a subjective questionnaire and underwent clinical examination to document the absence of TMD signs and symptoms. All symptomatic subjects had localized jaw joint pain and pain on movement or when eating. The number of missing mandibular bicuspid and molar teeth (excluding third molars) in each subject was recorded, and magnetic resonance images were made to document the presence or absence of disk displacement in the temporomandibular joints. Subjects were divided into 4 groups: group 1 = asymptomatic, normal magnetic resonance imaging result; group 2 = asymptomatic, disk displacement; group 3 = symptomatic, normal magnetic resonance imaging result; and group 4 = symptomatic, disk displacement. Collected data were analyzed with chi-square tests (P<.05) with no adjustment for multiple comparisons.

RESULTS

A positive association between missing mandibular posterior teeth and the presence of disk displacement was found.

CONCLUSION

The literature does not suggest that replacement of missing posterior teeth prevents the development of TMDs. However, missing mandibular posterior teeth may accelerate the development of degenerative joint disease.

The NMDA receptor antagonist MK-801 reduces Fos-like immunoreactivity within the trigeminocervical complex following superior sagittal sinus stimulation in the cat

Expression of Fos protein is an indicator of neuronal perturbation and is readily observed in the caudal medulla and the spinal cord following trigeminovascular nociceptive activation by electrical stimulation of the superior sagittal sinus (SSS) in the cat. It has been shown in the rat that N-methyl-D-aspartate (NMDA) receptor blockade causes a reduction in Fos protein expression after generalised meningeal irritation. We wished to examine if the same relationship was true in the cat, using the same non-competitive NMDA receptor antagonist MK-801, and a trigeminovascular-specific stimulus. A group of experimental animals underwent stimulation following blinded administration of MK-801 (4 mg/kg i.v.); control animals underwent stimulation minus MK-801, and a non-stimulated control animal underwent surgery alone. The regions examined for Fos-like immunoreactivity were the trigeminal nucleus caudalis (TNC) and its caudal extension into the C(1) and C(2) levels of the upper cervical spinal cord. The Fos-positive cell counts for the three regions (TNC, C(1) and C(2)) were grouped together for analysis. In the control stimulated group a median of 78 (56-99, quartile range, n=4) cells were Fos-positive. In the group treated with MK-801 the median number of Fos-positive cells was reduced to 40 (30-48; P<0.03, n=7). The large reduction that was observed in SSS stimulation-evoked Fos protein expression following the administration of MK-801, taken together with electrophysiological data, indicates a role for glutamate in neurotransmission within the trigeminocervical complex. Understanding glutamatergic mechanisms in the trigeminocervical complex offers mechanistic insight and therapeutic possibilities for primary neurovascular headaches, such as migraine.

Glutamatergic transmission in the trigeminal nucleus assessed with local blood flow

Stimulation of the superior sagittal sinus in humans is pain-producing and in experimental animals leads to excitation of neurons in the caudal trigeminal nucleus and dorsal horns of the C(1/)C(2) cervical spinal cord: the trigeminocervical complex. Neuronal excitation is generally associated with an increase in local blood flow due to flow/metabolism coupling and we have used local blood flow in the trigeminocervical complex to examine the role of N-methyl-D-aspartate (NMDA)-mediated transmission in these neurons. Cats were anaesthetised with alpha-chloralose (60 mg/kg, ip; supplements 20 mg/kg iv) after surgical preparation under halothane (0.5-3%). Animals were paralysed with gallamine triethiodide to prevent possible movement artefact distorting the laser Doppler signals. The superior sagittal sinus was isolated for electrical stimulation (150 V; 250 microsec duration; 0.5, 1, 2, 5, 10 and 20 Hz) and the dorsal surface of the spinal cord exposed at the C(2) level. Blood flow was recorded from the region over the trigeminocervical complex by careful placement of a laser Doppler flow probe. Flow was recorded continuously by an online collection programme and NMDA-mediated transmission modulated by intravenous administration of MK-801 (0.4, 1 and 4 mg/kg, iv) at the stimulation frequency of 5 Hz. Stimulation of the superior sagittal sinus produced a stimulus-locked, frequency-dependent increase in blood flow in the region of the trigeminocervical complex. The mean maximum response was 39+/-4% at 20 Hz. MK-801 had no effect on the resting flow signal but markedly attenuated the SSS-evoked response in a dose-dependent manner. The mean maximum response after 4 mg/kg MK-801 was 13+/-2%. NMDA-mediated transmission is likely to be involved in nociceptive trigeminovascular transmission within the trigeminocervical complex and offers a possible target for both acute and preventative treatment of migraine.

Endomorphin-1 and endomorphin-2 modulate responses of trigeminal neurons evoked by N-methyl-D-aspartic acid and somatosensory stimuli

The present study investigated the modulation of N-methyl-D-aspartate (NMDA)-evoked and peripheral cutaneous stimulus-evoked responses of trigeminal neurons by endomorphins, endogenous ligands for the mu-opioid receptor. Effects of endomorphins, administered microiontophoretically, were tested on the responses of nociceptive neurons recorded in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in anesthetized rats. Endomorphin-1 and endomorphin-2 predominantly reduced the NMDA-evoked responses, producing an inhibitory effect of 54.1 +/- 2.96% (mean +/- SE; n = 34, P < 0.001) in 92% (34/37) of neurons and 63.6 +/- 3.61% (n = 32, P < 0.001) in 91% (32/35) of neurons, respectively. The inhibitory effect of endomorphins was modality specific; noxious stimulus-evoked responses were reduced more than nonnoxious stimulus-evoked responses. Naloxone applied at iontophoretic current that blocked the inhibitory effect of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin, reduced the peak inhibitory effect of endomorphins on the NMDA- and natural stimulus-evoked responses. We suggest that endomorphins by acting at micro-opioid receptor selectively modulate noxious stimulus-evoked responses in the medullary dorsal horn.

Acute and chronic craniofacial pain: brainstem mechanisms of nociceptive transmission and neuroplasticity, and their clinical correlates

This paper reviews the recent advances in knowledge of brainstem mechanisms related to craniofacial pain. It also draws attention to their clinical implications, and concludes with a brief overview and suggestions for future research directions. It first describes the general organizational features of the trigeminal brainstem sensory nuclear complex (VBSNC), including its input and output properties and intrinsic characteristics that are commensurate with its strategic role as the major brainstem relay of many types of somatosensory information derived from the face and mouth. The VBSNC plays a crucial role in craniofacial nociceptive transmission, as evidenced by clinical, behavioral, morphological, and electrophysiological data that have been especially derived from studies of the relay of cutaneous nociceptive afferent inputs through the subnucleus caudalis of the VBSNC. The recent literature, however, indicates that some fundamental differences exist in the processing of cutaneous vs. other craniofacial nociceptive inputs to the VBSNC, and that rostral components of the VBSNC may also play important roles in some of these processes. Modulatory mechanisms are also highlighted, including the neurochemical substrate by which nociceptive transmission in the VBSNC can be modulated. In addition, the long-term consequences of peripheral injury and inflammation and, in particular, the neuroplastic changes that can be induced in the VBSNC are emphasized in view of the likely role that central sensitization, as well as peripheral sensitization, can play in acute and chronic pain. The recent findings also provide new insights into craniofacial pain behavior and are particularly relevant to many approaches currently in use for the management of pain and to the development of new diagnostic and therapeutic procedures aimed at manipulating peripheral inputs and central processes underlying nociceptive transmission and its control within the VBSNC.

Trigeminovascular nociceptive transmission involves N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptors

Interest in the fundamental mechanisms underlying headache, particularly the pathophysiology of migraine and cluster headache, has lead to the study of the physiology and pharmacology of the trigeminovascular system and its central ramifications. Cats were anaesthetized (60 mg/kg alpha-chloralose, i.p., along with halothane for all surgical procedures) and prepared for physiological monitoring. The animals were placed in a stereotaxic frame and ventilated. A midline craniotomy and C2 laminectomy were performed for access to the superior sagittal sinus and C2 dorsal horn, respectively. The sinus was isolated from the underlying cortex and stimulated electrically after the animals had been paralysed with gallamine (6 mg/kg, i.v.). Units linked to stimulation were recorded with a tungsten-in-glass microelectrode placed in the most caudal part of the trigeminal nucleus, the trigeminocervical complex. Signals from the neurons were amplified, filtered and passed to a microcomputer, where post-stimulus histograms were constructed on-line to analyse the responses to stimulation. Units responded to sagittal sinus stimulation with a typical latency of 8-10 ms. All units studied had a probability of firing of 0.6 or greater. Intravenous injection of the non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (4 mg/kg, i.v.), resulted in a substantial and prolonged blockade of firing of units in the trigeminocervical complex. Similarly, administration of the non-N-methyl-D-aspartate excitatory amino acid receptor blocker, GYKI 52466, lead to a dose-dependent inhibition of trigeminovascular-evoked responses in the trigeminocervical complex. These data demonstrate the participation of both N-methyl-D-aspartate- and non-N-methyl-D-aspartate-mediated mechanisms in transmission within the trigeminocervical complex, and suggest a clear preclinical role of glutamatergic mechanisms in primary headache syndromes, such as migraine and cluster headache.

Orphanin FQ (nociceptin) modulates responses of trigeminal neurons evoked by excitatory amino acids and somatosensory stimuli, and blocks the substance P-induced facilitation of N-methyl-D-aspartate-evoked responses

The present investigation details the modulation of medullary dorsal horn neuron responses to excitatory amino acids and peripheral cutaneous stimuli by orphanin FQ (nociceptin), an endogenous ligand for the opioid receptor-like, receptor. Effects of orphanin FQ, administered microiontophoretically or given intracerebroventricularly, were tested on the responses of nociceptive-specific, wide dynamic range and low threshold neurons recorded in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in anesthetized (urethane or pentobarbital) male rats. Microiontophoretic application of orphanin FQ reduced the N-methyl-D-aspartate-evoked responses in 86% (71/82) of neurons, and the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-evoked responses in 86% (30/35) of neurons. However, orphanin FQ produced a longer lasting inhibitory effect on the N-methyl-D-aspartate-evoked responses relative to the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-evoked responses. The inhibitory effect of orphanin FQ was not modality-specific, responses evoked by noxious as well as non-noxious stimuli were reduced in 22/23 neurons. However, the inhibitory effect was more pronounced on noxious stimulus-evoked responses. Naloxone applied at currents that antagonized the inhibitory effects of selective agonists at mu and kappa opioid receptors failed to inhibit the effects of orphanin FQ. Microiontophoretic co-application of substance P with N-methyl-D-aspartate facilitated the N-methyl-D-aspartate-evoked responses in 52% (26/50) of nociceptive neurons. Orphanin FQ blocked or reduced the substance P-induced facilitation by 86+/-24.4% (n = 14). In order to compare electrophysiological data with previous behavioral observations, effects of orphanin FQ administered intracerebroventricularly were tested on the excitatory amino acid-evoked responses. Orphanin FQ reduced the N-methyl-D-aspartate-evoked responses in 85% (11/13) of neurons whereas the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-evoked responses were facilitated in 69% (9/13) of neurons. We suggest that orphanin FQ produces a predominantly inhibitory effect on, (i) noxious stimuli evoked responses, (ii) excitatory amino acid receptor-mediated transmission and, (iii) the substance P-induced facilitation of the N-methyl-D-aspartate-evoked responses. We conclude that orphanin FQ primarily produced an antinociceptive action at the level of the dorsal horn of the medulla.

Activation of excitatory amino acid receptors in bovine dental pulp evokes the release of iCGRP

The activation of excitatory amino acid (EAA) receptors within the central nervous system is associated with numerous centrally mediated phenomena, including hyperalgesia. However, relatively little is known about the peripheral mechanisms which these receptors may regulate when activated. This research evaluated the hypothesis that EAA receptors in bovine dental pulp activate a population of peptidergic sensory neurons as measured by the release of immunoreactive calcitonin gene-related peptide (iCGRP), a neuropeptide associated with neurogenic inflammation. In vitro superfusion of bovine dental pulp was used to evaluate the regulation of iCGRP secretion by the EAA receptor agonists AMPA, kainate, NMDA, and L-glutamate. Both AMPA and kainate stimulated the release of iCGRP in a concentration-dependent manner (AMPA EC50 = 0.27 +/- 3.3 nM; kainate EC50 = 3.2 +/- 1.1 microM). Pre-treatment and co-administration of the AMPA/kainate receptor antagonist CNQX significantly reduced the iCGRP release evoked by either of these agonists. In contrast, neither NMDA nor L-glutamate induced any consistent changes in iCGRP release. These results suggest that the activation of AMPA and kainate receptors in dental pulp may contribute to peripheral release of vasoactive neuropeptides which mediate a neurogenic component of inflammation.

Evidence that excitatory amino acid receptors within the temporomandibular joint region are involved in the reflex activation of the jaw muscles

We have previously shown that injection of the inflammatory irritant and small-fiber excitant mustard oil (MO) into the temporomandibular joint (TMJ) region can reflexively induce a prolonged increase in the activity of both digastric and masseter muscles in rats. It is possible that peripheral excitatory amino acid (EAA) receptors play a role in this effect, because MO-evoked increases in jaw muscle activity are attenuated by preapplication of the noncompetitive NMDA receptor antagonist MK-801 into the TMJ region. In the present study the EAA receptor agonists glutamate, NMDA, kainate, and AMPA were applied locally to the TMJ region. Jaw muscle responses similar to those evoked by MO application to the TMJ region were achieved with glutamate, NMDA, AMPA, and kainate. Repeated application of glutamate, NMDA, or AMPA at intervals of 30 min evoked responses in the ipsilateral jaw muscles that were of comparable magnitude. Co-application of the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (0.5 micromol) significantly reduced the magnitude of the glutamate- and NMDA-evoked ipsilateral jaw muscle responses without affecting responses evoked by AMPA. In contrast, co-application of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (1 nmol) significantly reduced the magnitude of the glutamate- and AMPA-evoked ipsilateral jaw muscle responses without affecting responses evoked by NMDA. This evidence suggests that both NMDA and non-NMDA EAA receptor types are located within the TMJ region and may contribute to jaw muscle activity that can be reflexively evoked from the TMJ region.

Central terminals of orofacial primary afferents and NADPH-diaphorase activity in the trigemino-solitary complex of rats

Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity and the central terminal fields of branches of the mandibular and chorda tympani nerves were visualized histochemically at the same time using transganglionic transport of wheat germ agglutinin conjugated with horseradish peroxidase. The blue NADPH-d-positive neurons comprised a sparse network in the dorsomedial spinal trigeminal subnucleus oralis and a dense one in the rostral lateral division of the nucleus of the solitary tract. In the subnucleus caudalis, most labeled neurons were in the superficial zone, and smaller numbers were in the magnocellular zone. The NADPH-d-positive neurons in the subnucleus oralis and the nucleus of the solitary tract overlapped mostly with the transganglionically labeled terminal field from the lingual nerve, partly with the terminal field from the inferior alveolar and chorda tympani nerves, and rarely with the terminal field from the mental nerve. The NADPH-d-positive neurons in the dorsomedial paratrigeminal nucleus and subnucleus caudalis overlapped mostly with the terminal field from the lingual nerve, partly with the terminal field from the inferior alveolar and mental nerves and never with the terminal field from the chorda tympani. A statistically significant reduction in the number of NADPH-d-positive neurons was seen bilaterally in subnucleus oralis and the nucleus of the solitary tract when the lingual nerve was transected. Inflammatory insults to the lingual nerve or tooth pulps significantly increased the number of NADPH-d-positive neurons in subnucleus oralis, the nucleus of the solitary tract, and subnucleus caudalis. These results show that the NO/cyclic GMP system in the trigeminal and solitary nuclei is differentially regulated trans-synaptically by trigeminal afferents depending on the nucleus and sensory modality.

Ultrastructural study of NADPH-d positive neurons in laminae I and II of the rat caudal spinal trigeminal nucleus

The present study investigated the ultrastructure of neurons in the caudal spinal trigeminal nucleus. These neurons which are believed to function as interneurons in the transmission of orofacial nonreflexive nociceptive information, measured 20 microns x 11 microns, and were nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) positive. The reaction product, formazan, was localized in the nuclear envelope, mitochondria, rough endoplasmic reticulum, and multivesicular bodies of these neurons. It was also localized in the membrane of the smooth endoplasmic reticulum at the axon terminal. The neurons were contacted by both axosomatic and axodendritic synapses formed by both NADPH-d positive and NADPH-d negative axon terminals. Two types of NADPH-d positive axon terminals could be recognized. The first was a large terminal containing many stained mitochondria and unstained small round agranular vesicles mixed with some slightly flattened ones. It formed asymmetrical axodendritic synapse. The second type of axon terminals contained pleomorphic synaptic vesicles and formed asymmetrical synapses upon both dendrites and soma. The sources of NADPH-d positive axon terminals were discussed. Most of the unstained axon terminals forming axosomatic and axodendritic synapses with stained cell bodies and dendrites contained flattened vesicles. In addition to the above, complicated synaptic configurations showing NADPH-d positive axoaxonic synapses in relation to NADPH-d negative dendritic spines were also seen in which a NADPH-d negative dendritic spine was completely contacted by a NADPH-d positive bouton which was in turn contacted by another NADPH-d positive bouton.

Excitatory amino release within spinal trigeminal nucleus after mustard oil injection into the temporomandibular joint region of the rat

Inflammation of the temporomandibular joint (TMJ) region evokes pain and hyperalgesia as well as causing persistent changes in the response properties of central trigeminal neurons. To determine if excitatory amino acids have a role in TMJ-induced responses, extracellular concentrations were measured in microdialysate samples from probes positioned in the spinal trigeminal nucleus (Vsp) near the transition region between subnucleus interpolaris and subnucleus caudalis (Vi/Vc) in chloralose-anesthetized rats. Injection of the selective small fiber excitant, mustard oil (20 microliters, 20% solution), into the ipsilateral TMJ region caused a transient (by 10 min) increase in glutamate (from 0.48 +/- 0.16 to 1.94 +/- 0.78 microM, P < 0.005) and aspartate (from 0.29 +/- 0.11 to 1.78 +/- 0.82 microM, P < 0.025) among sites located at the ventrolateral pole of the Vi/Vc transition region (n = 6). Samples from probes located within the ventral Vsp, but outside this Vi/Vc transition region (n = 9), did not show significant changes in amino acid concentrations. Glutamate and aspartate also increased after mustard oil injections into the contralateral TMJ region. Dialysate concentrations of serine and taurine did not change significantly after mustard oil injections. Addition of high potassium (150 mM) to the perfusate solution caused increases in glutamate and aspartate regardless of probe location. The transient and selective release of glutamate and aspartate within the Vi/Vc transition after acute irritation of the TMJ region is consistent with a proposed role for excitatory amino acids in mediating noxious sensory input from deep orofacial structures. Together with previous reports of c-fos expression, these results suggest that neurons within the ventrolateral portion of the Vi/Vc transition may serve as a relay site for the integration of sensory or reflex responses to acute inflammation of the TMJ region.

N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor antagonism reduces Fos-like immunoreactivity in central trigeminal neurons after corneal stimulation in the rat

The role of glutamate receptors in processing noxious sensory input from the cornea was assessed in barbiturate-anesthetized rats. Animals were treated with selective antagonists for N-methyl-D-aspartate or non-N-methyl-D-aspartate receptor subtypes prior to application of mustard oil to the corneal surface. Neural activation was estimated from the number of neurons that produced Fos, the protein product of the immediate early gene, c-fos, as detected by immunocytochemistry. Fos-positive neurons were found at two distinct regions of the spinal trigeminal nucleus: the subnucleus interpolaris/caudalis transition and the subnucleus caudalis/upper cervical cord transition. The number of Fos-positive neurons was reduced dose-dependently by the competitive N-methyl-D-aspartate receptor antagonist, 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (0.08-8 nmol, i.c.v.), or by the non-N-methyl-D-aspartate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (2.5-250 nmol, i.c.v.). The greatest reduction in Fos-positive cells was seen at the subnucleus caudalis/upper cervical cord transition after blockade of either receptor subtype. Combined blockade of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors did not cause a further reduction in the number of Fos-positive neurons than was seen after the highest dose of either antagonist alone. Peripheral or central administration of the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester, had no effect on the number of Fos-positive neurons after corneal stimulation. These results suggest that corneal input to neurons at the subnucleus caudalis/upper cervical cord transition, and to a lesser extent, at the subnucleus interpolaris/subnucleus caudalis transition depends on excitatory amino acid transmission. Both N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptor subtypes, but not the formation of nitric oxide, contribute to the processing of acute corneal stimuli by central trigeminal neurons.

Opioids modulate N-methyl-D-aspartic acid (NMDA)-evoked responses of neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis)

Extracellular single unit recordings were made from 74 neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis). N-methyl-D-aspartic acid (NMDA) excited nociceptive as well as non-nociceptive neurons. NMDA receptor antagonist, DL-2-Amino-5-Phosphonovaleric acid (AP-5), blocked the NMDA-evoked excitation. Microiontophoretic application of a selective mu-opioid receptor agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO), reduced the NMDA-evoked responses of 100% of nociceptive specific (NS), 93% of wide dynamic range (WDR) and 86% of low threshold (LT) neurons in the superficial and deeper dorsal horn of the medulla. In contrast, application of a selective delta 1-opioid receptor agonist, [D-Pen2,5]enkephalin (DPDPE), reduced the NMDA-evoked responses of 90% of NS neurons, 72% of WDR neurons and 67% of LT neurons in the superficial and deeper dorsal horn of the medulla. DPDPE also produced excitatory or biphasic effects. The inhibitory actions of DAMGO and DPDPE were reversed by naloxone and/or 7-benzylidenenaltrexone (BNTX), mu- and delta 1-receptor antagonists. It is concluded that mu- and delta-opioid receptor agonists produce a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive and non-nociceptive neurons in the medullary dorsal horn.

Electron microscopy of immunoreactivity patterns for glutamate and gamma-aminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (Subnucleus Caudalis)

We studied the ultrastructure of the synaptic organization in the feline spinal trigeminal nucleus, emphasizing specific neurotransmitter patterns within lamina II of the pars caudalis/medullary dorsal horn. Normal adults were perfused, and Vibratome sections from pars caudalis were processed for electron microscopy. Ultrathin sections were reacted with antibodies for the excitatory neurotransmitter glutamate (Glu) and for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by using postembedding immunogold techniques. Both single- and double-labeled preparations were examined. Results with single labeling show that Glu-immunoreactive terminals have round synaptic vesicles and form asymmetric synaptic contacts onto dendrites. GABA-immunoreactive axon terminals and vesicle-containing dendrites have pleomorphic vesicles, and the axon terminals form symmetric contacts onto dendrites and other axons. Double labeling on a single section shows glomeruli with central Glu-immunoreactive terminals that are presynaptic to dendrites, including GABA+ vesicle-containing dendrites. These Glu+ terminals are also postsynaptic to GABA+ axon terminals, and these GABA-immunoreactive terminals may also be presynaptic to the GABA+ vesicle-containing dendrites. Quantitative analyses confirm the specificity of the Glu and GABA immunoreactivities seen in the various glomerular profiles. The results suggest that a subpopulation of Glu-immunoreactive primary afferents (excitatory) may be under the direct synaptic influence of a GABA-immunoreactive intrinsic pathway (inhibitory) by both presynaptic and postsynaptic mechanisms.

The NMDA receptor antagonist MK-801 reduces Fos-like immunoreactivity in central trigeminal neurons and blocks select endocrine and autonomic responses to corneal stimulation in the rat

The N-methyl-D-aspartate (NMDA) receptor is implicated in multiple aspects of pain processing by the central nervous system. However, the role of NMDA receptors in the endocrine and autonomic aspects of nociception remains uncertain. The present study examined the influence of the NMDA receptor antagonist, MK-801 (0.02-2.0 mg/kg, intracarotid), on the adrenal and autonomic responses to corneal stimulation (mustard oil, 20% sol.) in barbiturate-anesthetized rats. Fos-like immunoreactivity (Fos-LI) evoked by corneal stimulation was quantified within the spinal trigeminal nucleus (Vsp) of MK-801 pretreated animals to assess activation of central trigeminal neurons. Corneal stimulation-evoked increases in the plasma concentrations of adrenocorticotropin (ACTH), epinephrine and norepinephrine were reduced dose-dependently by MK-801. Plasma ACTH also increased after moderate hemorrhage, a response that was not affected by MK-801. MK-801 did not reduce the magnitude of corneal stimulation-evoked increases in arterial pressure and heart rate; however, prestimulus arterial pressure was reduced by drug treatment. Fos-LI was distributed bimodally within the ipsilateral caudal Vsp: one peak of Fos-LI in the subnucleus interpolaris/caudalis transition region and a second peak within the superficial laminae of the subnucleus caudalis/upper cervical cord transition region. The magnitude of both peaks of Fos-LI was reduced dose-dependently by MK-801. These results indicate a significant contribution from NMDA receptors in control of select endocrine and autonomic responses that accompany trigeminal nociception and in activation of central trigeminal neurons that process corneal nociceptive input.

A medullary dorsal horn relay for the cardiorespiratory responses evoked by stimulation of the nasal mucosa in the muskrat Ondatra zibethicus: evidence for excitatory amino acid transmission

Stimulation of the upper respiratory tract, including the nasal mucosa, with water, vaporous irritants, or gases, induces a collation of several cardiorespiratory responses including an apnea and bradycardia and often some change in arterial blood pressure. Since the nasal mucosa is innervated by branches of the trigeminal nerve, it implies that some part of the trigeminal system within the central nervous system mediates the autonomic responses induced by nasal stimulation. In the present study, respirations, heart rate and arterial pressure were monitored in muskrats anesthetized with a mixture of chloralose-urethane. We induced a bradycardia and apnea by stimulating the nasal mucosa of muskrats with brief (5 s) transnasal application of vapors of ammonia hydroxide. In an effort to determine the central site where the trigeminal mediation of the cardiorespiratory responses occurs, small nanoliter injections of 2% lidocaine were made bilaterally into the subnucleus caudalis of the spinal trigeminal nucleus (referred to as the medullary dorsal horn) to determine if the responses could be blocked. The responses could be blocked when the lidocaine injections on both sides were placed in the rostral, ventral parts of the medullary dorsal horn, but persisted when the injections were placed elsewhere. Since lidocaine blocks both neurons and fibers of passage, nanoliter injections of kynurenate, a general excitatory amino acid antagonist, were used in a similar paradigm to circumvent the problem of blocking only fibers of passage.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA R1 mRNA distribution in motor and thalamic-projecting sensory neurons in the rat spinal cord and brain stem

The N-methyl-D-aspartate (NMDA) receptor is important in both sensory and motor neurotransmission. In this study we examine NMDA R1 mRNA hybridization signal over individual sensory and motor neurons in the spinal cord and brain stem. A significantly greater quantity of NMDA R1 mRNA was present in motor neurons of the lumbar spinal cord and hypoglossal nucleus compared to thalamic projecting sensory neurons in the spinal cord dorsal horn, the spinal trigeminal nucleus pars caudalis and the cuneate and gracile nuclei. No significant difference in the quantity of NMDA R1 mRNA was observed between sensory neurons known to relay predominantly nociceptive information (trigeminothalamic and spinothalamic tract neurons) and that relay predominantly touch and proprioceptive information (dorsal column neurons).

Distribution of nitric oxide synthase-immunoreactive interneurons in the spinal trigeminal nucleus

The spinal trigeminal nucleus is involved in the transmission of orofacial sensory information. Neither the distribution of the neuromessenger, nitric oxide, within the trigeminal system nor the possible relationship of this simple gas with trigeminothalamic neurons has been carefully studied. Using immunocytochemical (against nitric oxide synthase) and histochemical (NADPH-diaphorase staining) techniques, we have found that nitric oxide neurons and processes are more prominent in the nucleus caudalis and the dorsomedial aspect of the nucleus oralis than in other spinal trigeminal regions. To study the relationship of nitric oxide to trigeminothalamic neurons and intertrigeminal interneurons of the spinal trigeminal nucleus, spinal trigeminal neurons were retrogradely labeled with fluorogold by thalamic injections or by injections into the junction of the nucleus interpolaris and nucleus caudalis. Medullary sections were subsequently processed with NADPH-diaphorase histochemistry. None of the diaphorase-stained neurons in the spinal trigeminal nucleus was found to contain fluorogold; however, some diaphorase-stained processes were found in close proximity to trigeminothalamic neurons. Following spinal trigeminal nucleus injections, many diaphorase-stained neurons were found to contain fluorogold, especially in the nucleus caudalis, suggesting that nitric oxide-containing neurons in the spinal trigeminal nucleus are intertrigeminal interneurons. Collectively, these data indicate that nitric oxide is most prominent in interneurons located in nucleus caudalis and that these interneurons give rise to processes that appose trigeminothalamic neurons, raising the possibility that they may indirectly influence orofacial nociceptive processing at the level of the spinal trigeminal nucleus via nitric oxide production.

NMDA receptor mRNA expression in NOS-containing neurons in the spinal trigeminal nucleus of the rat

The spinal trigeminal nucleus (STN) is involved in the transmission of orofacial sensory information. Nitric oxide (NO), an important neuromessenger, and the glutamate receptor subtype, NMDA NR1, have been implicated in nociception in the STN. However, the anatomical relationship of NO and NMDA NR1 has not been investigated within this nucleus. Using both immunocytochemical (against NO synthase; NOS) and in situ hybridization studies of NMDA NR1 receptor mRNA, we found that NOS-containing neurons in the STN expressed more mRNA for NR1 than did non-NOS-containing neurons in the STN. These data suggest that NMDA activation may lead to NO production in the STN and is consistent with previous studies, implicating both NMDA and NO in nociception.

Expression of mRNA for the N-methyl-D-aspartate (NMDAR1) receptor by the enteric neurons of the rat

In this study, in situ hybridization techniques were employed to map the distribution of enteric neurons which express mRNA for the glutamate N-methyl-D-aspartate receptor 1 (NMDAR1). We hybridized tissue sections from the stomach, duodenum, ileum and descending colon of adult rats with a 1.43-kB riboprobe cleaved from a clone of the NMDA receptor. Enteric neurons expressing the mRNA were found in both myenteric and submucosal ganglia at each of the sampling sites. Possible functions of NMDA receptors on enteric neurons are discussed.

Microinjections of glutamate within trigeminal subnucleus interpolaris alters adrenal and autonomic function in the cat

The influence of rostral portions of the trigeminal sensory complex on adrenal and autonomic function was assessed by microinjections of L-glutamate (500 or 5 mM, 100 nl) directed at subnucleus interpolaris (Vi) or at the nucleus principalis/subnucleus oralis level (Vp/Vo) in chloralose-anesthetized cats. Microinjections of glutamate (500 mM) within Vi evoked prompt (by +1 min) dose-related increases in the adrenal secretion of epinephrine (+11.4 +/- 2.5 ng/min, P < 0.001), adrenal blood flow (+0.19 +/- 0.06 ml/min, P < 0.05), mean arterial pressure (+6.6 +/- 3.0 mmHg, P < 0.025) and heart rate (+8.0 +/- 2.7 beats/min, P < 0.01, n = 16). Microinjections of lower doses of L-glutamate (5 mM, n = 7) within Vi had no effect. Microinjections of 500 mM glutamate within VP/Vo (n = 15) or within the spinal trigeminal tract (n = 13) had no consistent effect on adrenal or autonomic function. Plasma concentrations of ACTH were not altered significantly by glutamate regardless of dose or of the site of injection. The results suggest that local release of glutamate within Vi, but not within Vp/Vo, influences adrenal and autonomic function. Together with previous results obtained after injections of glutamate within subnucleus caudalis, these data indicate that glutaminergic input to both Vi and to more caudal portions of the spinal trigeminal nucleus contribute to the control of autonomic function such as that which often accompanies trigeminal nociception.

Microinfusions of excitatory amino acid antagonists into the trigeminal sensory complex antagonize the jaw opening reflex in freely moving rats

Microinfusions of EAA antagonists (APV 0.1 microliter 25 mM, gamma-DGG 0.1 microliter 50 mM, CNQX 0.1 microliter 50 mM, ketamine 0.1 microliter 0.2 M) were performed in freely moving rats while recording the long latency jaw opening reflex (JOR) elicited by electrical stimulation of the dental pulp. NMDA and non-NMDA antagonists were applied in the trigeminal sensory complex at the termination of dental pulp afferents. The selective NMDA antagonist APV strongly reduced the amplitude of the polysynaptic JOR. gamma-DGG and ketamine, which are broader spectrum NMDA antagonists, showed similar effects with some variations in their kinetics. CNQX, an antagonist for non-NMDA receptor subtypes, failed to affect the JOR. The results suggest that long latency JOR requires activation of NMDA receptors, while the early component elicited by periodontal afferents does not. These NMDA-receptors could belong either to JOR interneurons activated by tooth pulp afferents or to digastric motoneurons, receiving the inputs through a polysynaptic pathway. Recent anatomical results favour the first hypothesis while not excluding the second.

Glutamate-immunoreactive terminals synapse on primate spinothalamic tract cells

Glutamate has been shown to excite spinothalamic tract (STT) neurons and has been localized to primary afferent neurons, spinal cord projection neurons, and interneurons in the spinal cord dorsal horn. The likelihood that glutamate-immunoreactive (GLU-IR) terminals directly innervate STT neurons was investigated. For these studies three lamina IV or V STT cells in the lumbar spinal cords of three monkeys (Macaca fascicularis) were identified electrophysiologically and characterized. Two were identified as high threshold neurons and one as a wide dynamic range neuron. Following intracellular injection of the cells with HRP and reaction to give the cells a Golgi-like appearance, the tissues were processed for electron microscopy. Postembedding immunogold methods with antibodies specific for glutamate were used to identify GLU-IR terminals apposing the somata and dendrites of the STT neurons, including dendrites that extended into laminae IV and III. The GLU-IR terminals were numerous and constituted a mean of 46% of the population counted that appose the STT soma and 50% of the profiles apposing the dendrites. Fifty-four percent of the somatic and 50% of the dendritic surface length was contacted by GLU-IR terminals. Most terminals contained round clear vesicles and some contained a variable number of large dense core vesicles. For one of the three cells examined it was determined that 45% of the terminals apposing the soma were GLU-IR and 30% of the terminals were gamma aminobutyric acid-immunoreactive (GABA-IR). In an additional monkey, a lamina I cell retrogradely labeled from the ventral posterolateral nucleus of the thalamus was found to be ensheathed in glial processes.(ABSTRACT TRUNCATED AT 250 WORDS)