Trigeminal projections to contralateral dorsal horn: central extent, peripheral origins, and plasticity

Potassium nitrate suppresses hyperactivities of Vc neurons of the model with dentin hypersensitivity

OBJECTIVE

Potassium nitrate (KNO3) suppresses nociception induced by dental hypersensitivity (HYS). We aimed to examine the effects of KNO3 on the neural activity of the trigeminal spinal subnucleus caudalis (Vc) in HYS model rats.

METHODS

KNO3 or vehicle was applied to the exposed dentin of HYS rats for 3 days. c-Fos expression and neuronal activity in the Vc after acetone treatment for cold stimulation were examined to evaluate the effects of KNO3 application on dentin.

RESULTS

The number of c-Fos-immunoreactive cells in the Vc was lower in the group that received KNO3 (KNO3 group) than in the group that received vehicle (control group). Spike firing of Vc neurons in response to cold stimulation of the dentin was recorded before and after KNO3 application to the cavity, and the increased neural activity was effectively suppressed by KNO3 application. Scanning electron microscopy revealed that the dentin tubules were not occluded by deposits in any of the groups.

CONCLUSIONS

KNO3-induced suppression of Vc neuronal activity does not involve physical occlusion of the dentin tubules but likely involves suppression of Aδ or C-fiber activities in the tooth pulp, resulting in the suppression of Vc neuronal activities.

Reduction in calcium responses to whisker stimulation in the primary somatosensory and motor cortices of the model mouse with trigeminal neuropathic pain

OBJECTIVE

Chronic constriction injury (CCI) of the infraorbital nerve induces neuropathic pain, such as allodynia and hyperalgesia, in the orofacial area. However, the changes in the local circuits of the central nervous system following CCI remain unclear. This study aimed to identify the changes following CCI in Thy1-GCaMP6s transgenic mice.

METHODS

Neural activity in the primary somatosensory cortex (S1) and motor cortex (M1) following whisker stimulation was assessed using in vivo Ca2+ imaging. CCI-induced changes in responses were analyzed.

RESULTS

Before CCI, whisker stimulation induced a greater Ca2+ response in the contralateral S1 than in the ipsilateral S1 and contralateral M1. The peak Ca2+ response amplitude in the bilateral S1 and contralateral M1 decreased two days after CCI compared to before CCI. Decreased Ca2+ response amplitude in these regions was observed until four days after CCI. Seven days after CCI, the Ca2+ response amplitude in the contralateral S1 decreased, whereas that in the ipsilateral S1 and contralateral M1 recovered to control levels.

CONCLUSION

These results suggest that neural activity in regions receiving excitatory inputs via corticocortical pathways recovers earlier than in regions receiving thalamocortical inputs. (185/250 words).

Enhancing nerve regeneration in infraorbital nerve injury rat model: effects of vitamin B complex and photobiomodulation

Orofacial nerve injuries may result in temporary or long-term loss of sensory function and decreased quality of life in patients. B vitamins are required for DNA synthesis and the repair and maintenance of phospholipids. In particular, vitamins B1, B6, and B12 are essential for neuronal function. Deficiency in vitamin B complex (VBC) has been linked to increased oxidative stress, inflammation and demyelination. Photobiomodulation (PBM) has antioxidant activity and is neuroprotective. In addition, a growing literature attests to the positive effects of PBM on nerve repair. To assess the effect of PBM and VBC on regenerative process we evaluated the expression of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), myelin basic protein (MBP), laminin and neurofilaments (NFs) using Western blotting to identify regenerative pattern after chronic constriction injury of the infraorbital nerve (CCI IoN) treated by PBM, VBC or its combination. After CCI IoN, the rats were divided into six groups naive, sham, injured (CCI IoN), treated with photobiomodulation (904 nm, 6.23 J/cm2, CCI IoN + PBM), treated with VBC (containing B1, B6 and B12) 5 times, CCI IoN + VBC) and treated with PBM and VBC (CCI IoN + VBC + PBM). The treatments could revert low expression of BDNF, MBP and laminin. Also reverted the higher expression of neurofilaments and enhanced expression of NGF. PBM and VBC could accelerate injured infraorbital nerve repair in rats through reducing the expression of neurofilaments, increasing the expression of BDNF, laminin and MBP and overexpressing NGF. These data support the notion that the use of PBM and VBC may help in the treatment of nerve injuries. This finding has potential clinical applications.

Botulinum toxin type a antinociceptive activity in trigeminal regions involves central transcytosis

OBJECTIVE

Botulinum toxin type A (BoNT-A) provides lasting pain relief in patients with craniofacial pain conditions but the mechanisms of its antinociceptive activity remain unclear. Preclinical research revealed toxin axonal transport to the central afferent terminals, but it is unknown if its central effects involve transsynaptic traffic to the higher-order synapses. To answer this, we examined the contribution of central BoNT-A transcytosis to its action in experimental orofacial pain.

MATERIAL AND METHODS

Male Wistar rats, 3-4 months old, were injected with BoNT-A (7 U/kg) unilaterally into the vibrissal pad. To investigate the possible contribution of toxin's transcytosis, BoNT-A-neutralizing antiserum (5 IU) was applied intracisternally. Antinocicepive BoNT-A action was assessed by duration of nocifensive behaviors and c-Fos activation in the trigeminal nucleus caudalis (TNC) following bilateral or unilateral formalin (2.5%) application into the vibrissal pad. Additionally, cleaved synaptosomal-associated protein of 25 kDa (cl-SNAP-25) immunoreactivity was analyzed in the bilateral TNC.

RESULTS

Unilaterally injected BoNT-A reduced the nocifensive behaviors and bilateral c-Fos activation induced by formalin, which was accompanied by the toxin's enzymatic activity on both sides of the TNC. BoNT-A antinociceptive or enzymatic activities were prevented by the specific neutralizing antitoxin. BoNT-A contralateral action occurred independently from ipsilateral side nociception or contralateral trigeminal nerve-mediated axonal traffic.

CONCLUSION

Herein, we demonstrate that antinociceptive action of pericranially administered BoNT-A involves transsynaptic transport to second order synapses and contralateral trigeminal nociceptive nuclei. These results reveal more complex central toxin activity, necessary to explain its clinical effectiveness in the trigeminal region-related pain states.

Generalized Extension of Referred Trigeminal Pain due to Greater Occipital Nerve Entrapment

We report a very rare case of referred pain caused by greater occipital nerve (GON) entrapment, inducing spontaneous pain in the whole body as well as in the trigeminal nerve region of the face and head. It has already been reported that entrapment of the GON can induce referred pain in the ipsilateral limb as well as the ipsilateral hemiface. A 42-year-old female patient presented with chronic pain in her gums, jaw angle, submandibular region, retro-auricular suboccipital, and temporo-occipital vertex that had been ongoing for four years. As the patient's head pain and facial pain became severe, severe spontaneous pain occurred in the arm, waist, and both lower extremities. This patient's pain in the occipital and neck, spontaneous pain in the face, jaw, and whole body improved with decompression of the GON. Anatomical basis of pain referral to the facial trigeminal area caused by chronic GON entrapment is convergence of nociceptive inflow from high cervical C1-C3 structures and trigeminal orofacial area in the dorsal horn of the cervical spinal cord from the C2 segment up to the medullary dorsal horn (MDH). The major afferent contribution among the suboccipital and high cervical structure is mediated by spinal root C2 that is peripherally represented by the GON. Chronic noxious input from GON entrapment can cause sensitization and hypersensitivity in second order neurons in the trigeminocervical complex (TCC) and MDH in the caudal trigeminal nucleus and high cervical cord. Generalized extension of referred pain due to GON entrapment is thought to involve two possible pathophysiologies. One is the possibility that generalized pain is caused by sensitization of third-order nociceptive neurons in the thalamus. Another speculation is that spontaneous pain may occur throughout the body due to dysfunction of the descending brain stem pain-modulating pathway by sensitization and hyperexcitation of the MDH and trigeminal brainstem sensory nuclear complex (TBSNC).

Corneal nerve changes in herpes zoster ophthalmicus: a prospective longitudinal in vivo confocal microscopy study

PURPOSE

To study the changes in corneal nerves and corneal sensitivity over a 6-month period in patients with herpes zoster ophthalmicus (HZO) compared with healthy subjects.

METHODS

This was a prospective longitudinal study on patients with newly diagnosed HZO. In vivo confocal microscopy (IVCM) corneal nerve parameters and corneal sensitivity were measured and compared between eyes with HZO, contralateral eyes and controls at baseline, 2 and 6 months.

RESULTS

Fifteen subjects with HZO and 15 healthy age and sex matched controls were recruited. HZO eyes revealed a reduction in corneal nerve branch density (CNBD) from baseline to 2 months (9.65 ± 5.75 vs. 5.90 ± 6.87/mm2, p = 0.018), and decreased corneal nerve fibre density (CNFD) at 2 months when compared with control (p = 0.025). However, these differences resolved by 6 months. HZO fellow eyes demonstrated increased corneal nerve fibre area (CNFA), corneal nerve fibre width (CNFW) and corneal nerve fractal dimension (CNFrD) at 2 months compared with baseline (p = 0.025, 0.031, 0.009). There was no change in corneal sensitivity for both HZO affected and HZO fellow eyes from baseline or over time, nor was it different from sensitivity in controls.

CONCLUSION

Corneal denervation was present at 2 months in HZO eyes, with an observed recovery by 6 months. HZO fellow eyes demonstrated increased corneal nerve parameters at 2 months, which could represent a proliferative response to nerve degeneration. IVCM is useful in monitoring corneal nerve changes, and is more sensitive in detecting nerve alterations than esthesiometry.

Tear secretion by Diquafosol suppresses the excitability of trigeminal brainstem nuclear complex neurons by reducing excessive P2Y2 expression in the trigeminal ganglion in dry eye rats

The P2Y₂ receptor agonist, diquafosol sodium, is commonly used to treat the signs and symptoms of dry eye disease (DE) patients. Although diquafosol improves tear film stability, the neural mechanisms underlying the reduction in ocular pain are not well defined. This study determined if repeated application of diquafosol reduces the sensitization of nociceptive neurons in the lower trigeminal brainstem nuclear complex (TBNC) via peripheral P2Y₂ mechanisms in a rat model for DE. Diquafosol was applied to the ocular surface daily for 28 days, starting at day 0 or day 14, after exorbital gland removal. The number of eyeblinks, P2Y₂-immunoreactive neurons in the trigeminal ganglion (TG), and correlates of TBNC neural excitability (i.e., cFos protein and phosphorylated extracellular signal-regulated kinase (pERK) expression) were assessed in male rats. Diquafosol increased spontaneous tear volume and reduced the number of ocular surface-evoked eyeblinks in DE rats. Fluorogold-labeled TG neurons that supply the cornea expressed P2Y₂. The number of P2Y₂-immunoreactive neurons was increased in DE rats and suppressed by diquafosol. Diquafosol also reduced the number of cFos- and pERK-immunoreactive neurons in the TBNC in DE rats. These findings suggest that diquafosol, regardless of late-phase treatment, relieves ocular nociception in DE by reducing peripheral P2Y₂ expression.

The Role of Nervous System and Immune System in Herpes Zoster Ophthalmicus Dissemination and Laterality - Current Views

PURPOSE

Herpes zoster ophthalmicus (HZO) is a sight-threatening condition that is defined as HZ involving the ophthalmic division of the trigeminal nerve. Cases of bilateral HZO in recent literature question the notion of HZO being a strictly unilateral disease. Its pathogenesis is a topic of debate and current literature on VZV dissemination lacks insight into the underlying immunology. This review focuses on novel research in immunology of HZO and aims to formulate hypotheses of spread of lesions through the CNS.

METHODS

A literature search was conducted on Entrez PubMed using the search terms "bilateral" and "herpes zoster ophthalmicus". Articles on ("Immunology" or "immune cells") and "herpes zoster ophthalmicus" were also searched for. Articles published from January 1942 to April 2020 that were in English language were included.

RESULTS

Our findings revealed that hypothesised mechanisms of dissemination causing bilateral ocular disease include transmission from nerves to vessel walls, the synergistic action of the immune and nervous systems through the action of substance P and the von Szily reaction.

CONCLUSIONS

These mechanisms may be investigated using newer models of animal experimentation. It is imperative to define the molecular mechanisms behind VZV transmission to improve methods of identification, treatment, and prevention of HZO.

Unilateral Corneal Insult Also Alters Sensory Nerve Activity in the Contralateral Eye

After the unilateral inflammation or nerve lesion of the ocular surface, the ipsilateral corneal sensory nerve activity is activated and sensitized, evoking ocular discomfort, irritation, and pain referred to the affected eye. Nonetheless, some patients with unilateral ocular inflammation, infection, or surgery also reported discomfort and pain in the contralateral eye. We explored the possibility that such altered sensations in the non-affected eye are due to the changes in their corneal sensory nerve activity in the contralateral, not directly affected eye. To test that hypothesis, we recorded the impulse activity of the corneal mechano- and polymodal nociceptor and cold thermoreceptor nerve terminals in both eyes of guinea pigs, subjected unilaterally to three different experimental conditions (UV-induced photokeratitis, microkeratome corneal surgery, and chronic tear deficiency caused by removal of the main lacrimal gland), and in eyes of naïve animals ex vivo. Overall, after unilateral eye damage, the corneal sensory nerve activity appeared to be also altered in the contralateral eye. Compared with the naïve guinea pigs, animals with unilateral UV-induced mild corneal inflammation, showed on both eyes an inhibition of the spontaneous and stimulus-evoked activity of cold thermoreceptors, and increased activity in nociceptors affecting both the ipsilateral and the contralateral eye. Unilateral microkeratome surgery affected the activity of nociceptors mostly, inducing sensitization in both eyes. The removal of the main lacrimal gland reduced tear volume and increased the cold thermoreceptor activity in both eyes. This is the first direct demonstration that unilateral corneal nerve lesion, especially ocular surface inflammation, functionally affects the activity of the different types of corneal sensory nerves in both the ipsilateral and contralateral eyes. The mechanisms underlying the contralateral affectation of sensory nerves remain to be determined, although available data support the involvement of neuroimmune interactions. The parallel alteration of nerve activity in contralateral eyes has two main implications: a) in the experimental design of both preclinical and clinical studies, where the contralateral eyes cannot be considered as a control; and, b) in the clinical practice, where clinicians must consider the convenience of treating both eyes of patients with unilateral ocular conditions to avoid pain and secondary undesirable effects in the fellow eye.

Microglia and Inhibitory Circuitry in the Medullary Dorsal Horn: Laminar and Time-Dependent Changes in a Trigeminal Model of Neuropathic Pain

Craniofacial neuropathic pain affects millions of people worldwide and is often difficult to treat. Two key mechanisms underlying this condition are a loss of the negative control exerted by inhibitory interneurons and an early microglial reaction. Basic features of these mechanisms, however, are still poorly understood. Using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic pain in mice, we have examined the changes in the expression of GAD, the synthetic enzyme of GABA, and GlyT2, the membrane transporter of glycine, as well as the microgliosis that occur at early (5 days) and late (21 days) stages post-CCI in the medullary and upper spinal dorsal horn. Our results show that CCI-IoN induces a down-regulation of GAD at both postinjury survival times, uniformly across the superficial laminae. The expression of GlyT2 showed a more discrete and heterogeneous reduction due to the basal presence in lamina III of 'patches' of higher expression, interspersed within a less immunoreactive 'matrix', which showed a more substantial reduction in the expression of GlyT2. These patches coincided with foci lacking any perceptible microglial reaction, which stood out against a more diffuse area of strong microgliosis. These findings may provide clues to better understand the neural mechanisms underlying allodynia in neuropathic pain syndromes.

Effect of herpes simplex keratitis scar location on bilateral corneal nerve alterations: an in vivo confocal microscopy study

Aims

To evaluate the impact of herpes simplex virus (HSV)-induced scar location on bilateral corneal nerve alterations using laser in vivo confocal microscopy (IVCM).

Methods

Central and peripheral corneal subbasal nerve density (CSND) were assessed bilaterally in 39 patients with unilateral HSV-induced corneal scars (21 central scars (CS), 18 peripheral scars (PS)) using IVCM. Results were compared between patients and 24 age-matched controls. CSND was correlated to corneal sensation for all locations.

Results

Overall patients revealed significant decrease of CSND in the central and peripheral cornea (9.13±0.98 and 6.26±0.53 mm/mm2, p<0.001), compared with controls (22.60±0.77 and 9.88±0.49 mm/mm2). CS group showed a decrease in central (8.09±1.30 mm/mm2) and total peripheral nerves (5.15±0.62 mm/mm2) of the affected eyes, whereas PS group demonstrated a decrease in central (10.34±1.48 mm/mm2) and localised peripheral nerves only in the scar area (4.22±0.77 mm/mm2) (all p<0.001). In contralateral eyes, CSND decreased in the central cornea of the CS group (16.88±1.27, p=0.004), and in the peripheral area, mirroring the scar area in the affected eyes of the PS group (7.20±0.87, p=0.032). Corneal sensation significantly decreased in the whole cornea of the affected, but not in contralateral eyes (p<0.001). A positive correlation between CSND and corneal sensation was found in all locations (p<0.001).

Conclusions

Patients with HSV scar demonstrate bilateral CSND decrease as shown by IVCM. CSND and corneal sensation decrease in both central and peripheral cornea in affected eyes, although only in the scar area in PS group. Interestingly, diminishment of CSND was found locally in the contralateral eyes, corresponding and mirroring the scar location in the affected eyes.

Altered Brain Regional Homogeneity Following Contralateral Acupuncture at Quchi (LI 11) and Zusanli (ST 36) in Ischemic Stroke Patients with Left Hemiplegia: An fMRI Study

OBJECTIVE

To study the effect of contralateral acupuncture (CAT) at acupoints of Quchi (LI 11) and Zusanli (ST 36) on the unaffected limbs of ischemic stroke patients with left hemiplegia based on regional homogeneity (ReHo) indices.

METHODS

Ten ischemic stroke patients with left hemiplegia received CAT on right side at LI 11 and ST 36. Functional magnetic resonance imaging (fMRI) was performed before and after acupuncture. A ReHo analytical method was used to compare brain responses of patients before and after CAT operated by REST software.

RESULTS

The stimulation at both LI 11 and ST 36 on the unaffected limbs produced significantly different neural activities. CAT elicited increased ReHo values at the right precentral gyrus and superior frontal gyrus, decreased ReHo value at right superior parietal lobule, left fusiform gyrus and left supplementary motor area.

CONCLUSIONS

Acupuncture at one side could stimulate bilateral regions. CAT could evoke the gyrus which was possibly related to motor recovery from stroke. A promising indicator of neurobiological deficiencies could be represented by ReHo values in post-stroke patients.

Ipsi- and Contralateral Moxibustion Generate Similar Analgesic Effect on Inflammatory Pain

The aim of this study was to investigate whether contralateral moxibustion would generate a similar analgesic effect with ipsilateral moxibustion. Contra- and ipsilateral moxibustion were separately applied to Zusanli (ST36) acupoints of inflammatory pain mice. The analgesic effect was evaluated, respectively, by licking/biting time (LBT) of formalin-induced inflammatory pain and thermal withdrawal latency (TWL) of complete Freund's adjuvant- (CFA-) induced inflammatory pain. For formalin-induced pain, compared with formalin group, the total LBT of ipsi- and contralateral moxibustion reduced in both phase I and phase II, but there was no significant difference between ipsi- and contralateral moxibustion. For CFA-induced inflammatory pain, compared with CFA group, TWL of ipsi- and contra-Moxi groups increased immediately after moxibustion intervention; however there was no obvious difference between ipsi- and contralateral moxibustion at any timepoint. It indicated that contralateral moxibustion had a similar analgesic effect with ipsilateral moxibustion in both formalin- and CFA-induced pain. These results suggest that both ipsi- and contralateral moxibustion could be applied for pain relief.

Innervation of the Nose and Nasal Region of the Rat: Implications for Initiating the Mammalian Diving Response

Most terrestrial animals demonstrate an autonomic reflex that facilitates survival during prolonged submersion under water. This diving response is characterized by bradycardia, apnea and selective increases in peripheral vascular resistance. Stimulation of the nose and nasal passages is thought to be primarily responsible for providing the sensory afferent signals initiating this protective reflex. Consequently, the primary objective of this research was to determine the central terminal projections of nerves innervating the external nose, nasal vestibule and nasal passages of rats. We injected wheat germ agglutinin (WGA) into specific external nasal locations, into the internal nasal passages of rats both with and without intact anterior ethmoidal nerves (AENs), and directly into trigeminal nerves innervating the nose and nasal region. The central terminations of these projections within the medulla were then precisely mapped. Results indicate that the internal nasal branch of the AEN and the nasopalatine nerve, but not the infraorbital nerve (ION), provide primary innervation of the internal nasal passages. The results also suggest afferent fibers from the internal nasal passages, but not external nasal region, project to the medullary dorsal horn (MDH) in an appropriate anatomical way to cause the activation of secondary neurons within the ventral MDH that express Fos protein during diving. We conclude that innervation of the anterior nasal passages by the AEN and nasopalatine nerve is likely to provide the afferent information responsible for the activation of secondary neurons within MDH during voluntary diving in rats.

Antagonism of Transient Receptor Potential Ankyrin Type-1 Channels as a Potential Target for the Treatment of Trigeminal Neuropathic Pain: Study in an Animal Model

Transient receptor potential ankyrin type-1 (TRPA1) channels are known to actively participate in different pain conditions, including trigeminal neuropathic pain, whose clinical treatment is still unsatisfactory. The aim of this study was to evaluate the involvement of TRPA1 channels by means of the antagonist ADM_12 in trigeminal neuropathic pain, in order to identify possible therapeutic targets. A single treatment of ADM_12 in rats 4 weeks after the chronic constriction injury of the infraorbital nerve (IoN-CCI) significantly reduced the mechanical allodynia induced in the IoN-CCI rats. Additionally, ADM_12 was able to abolish the increased levels of TRPA1, calcitonin gene-related peptide (CGRP), substance P (SP), and cytokines gene expression in trigeminal ganglia, cervical spinal cord, and medulla induced in the IoN-CCI rats. By contrast, no significant differences between groups were seen as regards CGRP and SP protein expression in the pars caudalis of the spinal nucleus of the trigeminal nerve. ADM_12 also reduced TRP vanilloid type-1 (TRPV1) gene expression in the same areas after IoN-CCI. Our findings show the involvement of both TRPA1 and TRPV1 channels in trigeminal neuropathic pain, and in particular, in trigeminal mechanical allodynia. Furthermore, they provide grounds for the use of ADM_12 in the treatment of trigeminal neuropathic pain.

Barrelette map formation in the prenatal mouse brainstem

The rodent whiskers are topographically mapped in brainstem sensory nuclei as neuronal modules known as barrelettes. Little is known about how the facial whisker pattern is copied into a brainstem barrelette topographic pattern, which serves as a template for the establishment of thalamic barreloid and, in turn, cortical barrel maps, and how precisely is the whisker pattern mapped in the brainstem during prenatal development. Here, we review recent insights advancing our understanding of the intrinsic and extrinsic patterning mechanisms contributing to establish topographical equivalence between the facial whisker pattern and the mouse brainstem during prenatal development and their relative importance.

Central Connections of the Lacrimal Functional Unit

PURPOSE

To study the contribution of each eye to the reflex tear response, after unilateral and bilateral topical anesthesia.

METHOD

A closed-eye, modified Schirmer test was performed bilaterally in 8 normal subjects, in a controlled environment chamber set to 23°C, 45% relative humidity, and 0.08 m/s airflow. Eye drops were instilled into each eye 10 minutes before the Schirmer test. Experiments were as follows: 1) bilateral saline (control), 2) unilateral anesthesia (ipsilateral anesthetic; contralateral saline), and 3) bilateral anesthesia.

RESULTS

There was no difference in between-eye wetting lengths in the saline control eyes (P = 0.394) or the bilaterally anesthetized eyes (P = 0.171). The wetting length was reduced in both eyes after bilateral anesthesia compared with saline controls (P = 0.001; P ≤ 0.0005). After unilateral anesthesia, the wetting length was reduced in the anesthetized eye compared with its saline control by 51.4% (P ≤ 0.0005) and compared with its fellow, unanesthetized eye (P = 0.005). The fellow eye value was also reduced compared with its saline control (P = 0.06).

CONCLUSIONS

The wetting length was reduced by topical anesthesia, when instilled bilaterally and ipsilaterally. The latter response implies an ipsilateral, reflex sensory drive to lacrimal secretion. In the unanesthetized fellow eye, the reduction compared with its saline control was not quite significant. This implies a relative lack of central, sensory, reflex cross-innervation, although the possibility cannot entirely be ruled out. These results are relevant to the possibility of reflex lacrimal compensation from a normal fellow eye, in cases of unilateral corneal anesthesia.

Somatotopy in the Medullary Dorsal Horn As a Basis for Orofacial Reflex Behavior

The somatotopy of the trigeminocervical complex of the rat was defined as a basis for describing circuitry for reflex behaviors directed through the facial motor nucleus. Thus, transganglionic transport of horseradish peroxidase conjugates applied to individual nerves/peripheral receptive fields showed that nerves innervating oropharyngeal structures projected most rostrally, followed by nerves innervating snout, periocular, and then periauricular receptive fields most caudally. Nerves innervating mucosae or glabrous receptive fields terminated densely in laminae I, II, and V of the trigeminocervical complex, while those innervating hairy skin terminated in laminae I-V. Projections to lamina II exhibited the most focused somatotopy when individual cases were compared. Retrograde transport of FluoroGold (FG) deposited into the facial motor nucleus resulted in labeled neurons almost solely in lamina V of the trigeminocervical complex. The distribution of these labeled neurons paralleled the somatotopy of primary afferent fibers, e.g., those labeled after FG injections into a functional group of motoneurons innervating lip musculature were found most rostrally while those labeled after injections into motoneurons innervating snout, periocular and preauricular muscles, respectively, were found at progressively more caudal levels. Anterograde transport of injections of biotinylated dextran amine into lamina V at different rostrocaudal levels of the trigeminocervical complex confirmed the notion that the somatotopy of orofacial sensory fields parallels the musculotopy of facial motor neurons. These data suggest that neurons in lamina V are important interneurons in a simple orofacial reflex circuit consisting of a sensory neuron, interneuron and motor neuron. Moreover, the somatotopy of primary afferent fibers from the head and neck confirms the "onion skin hypothesis" and suggests rostral cervical dermatomes blend seamlessly with "cranial dermatomes." The transition area between subnucleus interpolaris and subnucleus caudalis is addressed while the paratrigeminal nucleus is discussed as an interface between the somatic and visceral nervous systems.

In vivo confocal microscopy detects bilateral changes of corneal immune cells and nerves in unilateral herpes zoster ophthalmicus

PURPOSE

To analyze bilateral corneal immune cell and nerve alterations in patients with unilateral herpes zoster ophthalmicus (HZO) by laser in vivo confocal microscopy (IVCM) and their correlation with corneal sensation and clinical findings.

MATERIALS AND METHODS

This is a prospective, cross-sectional, controlled, single-center study. Twenty-four eyes of 24 HZO patients and their contralateral clinically unaffected eyes and normal controls (n = 24) were included. Laser IVCM (Heidelberg Retina Tomograph/Rostock Cornea Module), corneal esthesiometry (Cochet-Bonnet) were performed. Changes in corneal dendritiform cell (DC) density and morphology, number and length of subbasal nerve fibers and their correlation to corneal sensation, pain, lesion location, disease duration, and number of episodes were analyzed.

RESULTS

HZO-affected and contralateral eyes showed a significant increase in DC influx of the central cornea as compared to controls (147.4 ± 33.9, 120.1 ± 21.2, and 23.0 ± 3.6 cells/mm2; p < 0.0001). In HZO eyes DCs were larger in area (319.4 ± 59.8 μm²; p < 0.001) and number of dendrites (3.5 ± 0.4 n/cell; p = 0.01) as compared to controls (52.2 ± 11.7, and 2.3 ± 0.5). DC density and size showed moderate negative correlation with total nerve length (R = -0.43 and R = -0.57, respectively; all p < 0.001). A higher frequency of nerve beading and activated DCs close to nerve fibers were detected specifically in pain patients.

CONCLUSIONS

Chronic unilateral HZO causes significant bilateral increase in corneal DC density and decrease of the corneal subbasal nerves as compared to controls. Negative correlation was observed for DC density and size to nerve parameters, suggesting interplay between the immune and nervous systems. Patients with chronic pain also showed increased nerve beading and activated DCs.

Do Unilateral Herpetic Stromal Keratitis and Neurotrophic Ulcers Cause Bilateral Dry Eye?

PURPOSE

To evaluate and compare the ocular surface condition in herpetic interstitial stromal keratitis and neurotrophic ulcer groups and their normal fellow eyes.

METHODS

In this observational, cross-sectional case-control study, 85 consecutive patients were included, including 56 cases of treated herpetic interstitial keratitis and 29 patients with neurotrophic ulcers. Fifty-six age- and sex-matched participants were also recruited from a normal population as the control group. We evaluated and scored the subjective and objective measures of dry eye for both eyes of all patients. Then, we compared the score of the groups with one another and also with the control group. The main outcome measures were the discomfort level, visual symptoms of dry eye, conjunctival injection, conjunctival staining, corneal staining, corneal tear signs of dry eye, meibomian gland dysfunction, tear break-up time, Schirmer test score with anesthesia, and tear osmolarity.

RESULTS

The normal fellow eye of the herpetic keratitis group had significantly higher discomfort levels (1.4 ± 0.9 vs. 1.3 ± 0.5, P = 0.003), visual symptoms (1.7 ± 0.8 vs. 1.3 ± 0.7, P = 0.002), tear break-up time (8.3 ± 3.2 vs. 12.1 ± 3.3 seconds, P = 0.003), Schirmer test scores (9.2 ± 3.9 vs. 12.9 ± 3 mm, P = 0.04), and tear osmolarity (9.2 ± 3.9 vs. 12.9 ± 3 mm, P = 0.003) in comparison with normal controls. The normal fellow eyes of the neurotrophic ulcer group had significantly worse values for discomfort level (1.9 ± 0.9 vs. 1.3 ± 0.5, P < 0.001), tear break-up time (7.9 ± 4 vs. 12.1 ± 3.3, P = 0.004), Schirmer test score (8.1 ± 3.9 vs. 12.9 ± 3, P = 0.005), and tear osmolarity (295 ± 9.2 vs. 292.7 ± 5.9, P = 0.02) compared with normal controls.

CONCLUSIONS

Both eyes of patients with neurotrophic ulcer and interstitial herpetic keratitis have a significantly poorer ocular surface condition compared with that of normal controls.

In Vivo Confocal Microscopy Demonstrates Bilateral Loss of Endothelial Cells in Unilateral Herpes Simplex Keratitis

PURPOSE

To report bilateral corneal endothelial cell density (ECD), as well as its correlation with subbasal nerve changes, in patients with unilateral herpes simplex keratitis (HSK).

METHODS

Thirty-six eyes of 36 patients with corneal scarring caused by HSK, as well as their respective contralateral clinically unaffected eyes, were prospectively studied and compared with 26 eyes of 26 healthy volunteers. In vivo confocal microscopy and corneal sensation of the central cornea were performed bilaterally in all patients and in one random eye of controls. The ECD and subbasal corneal nerve density, including the lengths of total nerves, main trunks, and branches were evaluated and correlated to central corneal sensation.

RESULTS

The ECD was significantly lower in eyes affected with HSK than in controls (2304 ± 578 vs. 2940 ± 370 cells/mm(2), P < 0.0001). Surprisingly, lower ECD was also detected in contralateral clinically unaffected eyes (2548 ± 423), compared to controls (P = 0.02). Both affected and contralateral eyes showed decrease in total nerve length, compared to controls (10.0 ± 6.3 vs. 17.6 ± 6.3 vs. 21.9 ± 4.3 mm/mm2, respectively; P < 0.05 for all). The ECD correlated positively with total nerve length (r = 0.39, P = 0.0009) and with corneal sensation (r = 0.31, P = 0.009).

CONCLUSIONS

In vivo confocal microscopy findings demonstrated alterations in corneal ECD in both affected and clinically unaffected contralateral eyes of patients with unilateral HSK. Moreover, the positive significant correlation between the ECD and the subbasal nerve density may suggest a potential link between corneal innervation and corneal endothelial cell homeostasis.

Synaptic ultrastructure changes in trigeminocervical complex posttrigeminal nerve injury

Trigeminal nerves collecting sensory information from the orofacial area synapse on second-order neurons in the dorsal horn of subnucleus caudalis and cervical C1/C2 spinal cord (Vc/C2, or trigeminocervical complex), which is critical for sensory information processing. Injury to the trigeminal nerves may cause maladaptive changes in synaptic connectivity that plays an important role in chronic pain development. Here we examined whether injury to the infraorbital nerve, a branch of the trigeminal nerves, led to synaptic ultrastructural changes when the injured animals have developed neuropathic pain states. Transmission electron microscopy was used to examine synaptic profiles in Vc/C2 at 3 weeks postinjury, corresponding to the time of peak behavioral hypersensitivity following chronic constriction injury to the infraorbital nerve (CCI-ION). Using established criteria, synaptic profiles were classified as associated with excitatory (R-), inhibitory (F-), and primary afferent (C-) terminals. Each type was counted within the superficial dorsal horn of the Vc/C2 and the means from each rat were compared between sham and injured animals; synaptic contact length was also measured. The overall analysis indicates that rats with orofacial pain states had increased numbers and decreased mean synaptic length of R-profiles within the Vc/C2 superficial dorsal horn (lamina I) 3 weeks post-CCI-ION. Increases in the number of excitatory synapses in the superficial dorsal horn of Vc/C2 could lead to enhanced activation of nociceptive pathways, contributing to the development of orofacial pain states.

Autonomic responses to lateralized cold pressor and facial cooling tasks

Asymmetry in central nervous system (CNS) control of autonomic nervous system (ANS) activity, a widely debated topic, was investigated via lateralized presentation of two ANS challenges: cold pressor, which elicits primarily sympathetic activation, and facial cooling, a predominantly vagal task. Seventy-three university students (37 female) engaged in these tasks while cardiovascular and electrodermal measures were acquired. Compared to right-side cold pressor, left cold pressor elicited generally larger cardiac, blood pressure, and skin conductance responses, but did not evoke asymmetric changes in heart rate variability. Facial cooling elicited significant increases in vagally mediated heart rate variability, but they were also not lateralized. These findings are consistent with reports of right hemisphere dominance in sympathetic regulation, but indicate that CNS vagal control is relatively symmetric. These results are framed in terms of polyvagal theory and neurovisceral integration two influential models of CNS-ANS integration in the service of adaptive environmental engagement.

Involvement of astroglial glutamate-glutamine shuttle in modulation of the jaw-opening reflex following infraorbital nerve injury

To evaluate the mechanisms underlying orofacial motor dysfunction associated with trigeminal nerve injury, we studied the astroglial cell activation following chronic constriction injury (CCI) of the infraorbital nerve (ION) immunohistochemically, nocifensive behavior in ION-CCI rats, and the effect of the glutamine synthase (GS) blocker methionine sulfoximine (MSO) on the jaw-opening reflex (JOR), and also studied whether glutamate-glutamine shuttle mechanism is involved in orofacial motor dysfunction. GFAP-immunoreactive (IR) cells were observed in the trigeminal motor nucleus (motV) 3 and 14 days after ION-CCI, and the nocifensive behavior and JOR amplitude were also strongly enhanced at these times. The number of GS- and GFAP-IR cells was also significantly higher in ION-CCI rats on day 7. The amplitude and duration of the JOR were strongly suppressed after MSO microinjection (m.i.) into the motV compared with that before MSO administration in ION-CCI rats. After MSO administration, the JOR amplitude was strongly suppressed, and the duration of the JOR was shortened. Forty minutes after m.i. of glutamine, the JOR amplitude was gradually returned to the control level and the strongest attenuation of the suppressive effect of MSO was observed at 180 min after glutamine m.i. In addition, glutamine also attenuated the MSO effect on the JOR duration, and the JOR duration was extended and returned to the control level thereafter. The present findings suggest that astroglial glutamate-glutamine shuttle in the motV is involved in the modulation of excitability of the trigeminal motoneurons affecting the enhancement of various jaw reflexes associated with trigeminal nerve injury.

Morphology and connections of intratrigeminal cells and axons in the macaque monkey

Trigeminal primary afferent fibers have small receptive fields and discrete submodalities, but second order trigeminal neurons often display larger receptive fields with complex, multimodal responses. Moreover, while most large caliber afferents terminate exclusively in the principal trigeminal nucleus, and pars caudalis (sVc) of the spinal trigeminal nucleus receives almost exclusively small caliber afferents, the characteristics of second order neurons do not always reflect this dichotomy. These surprising characteristics may be due to a network of intratrigeminal connections modifying primary afferent contributions. This study characterizes the distribution and morphology of intratrigeminal cells and axons in a macaque monkeys. Tracer injections centered in the principal nucleus (pV) and adjacent pars oralis retrogradely labeled neurons bilaterally in pars interpolaris (sVi), but only ipsilaterally, in sVc. Labeled axons terminated contralaterally within sVi and caudalis. Features of the intratrigeminal cells in ipsilateral sVc suggest that both nociceptive and non-nociceptive neurons project to principalis. A commissural projection to contralateral principalis was also revealed. Injections into sVc labeled cells and terminals in pV and pars oralis on both sides, indicating the presence of bilateral reciprocal connections. Labeled terminals and cells were also present bilaterally in sVi and in contralateral sVc. Interpolaris injections produced labeling patterns similar to those of sVc. Thus, the rostral and caudal poles of the macaque trigeminal complex are richly interconnected by ipsilateral ascending and descending connections providing an anatomical substrate for complex analysis of oro-facial stimuli. Sparser reciprocal crossed intratrigeminal connections may be important for conjugate reflex movements, such as the corneal blink reflex.

Unilateral herpes zoster ophthalmicus results in bilateral corneal nerve alteration: an in vivo confocal microscopy study

PURPOSE

Herpes zoster ophthalmicus (HZO), thought to be a unilateral disease, results in loss of corneal sensation, leading to neurotrophic keratopathy. This study aimed to analyze bilateral corneal nerve changes in patients with HZO by in vivo confocal microscopy (IVCM) and their correlation with corneal sensation as a measure of nerve function.

DESIGN

Prospective, cross-sectional, controlled, single-center study.

PARTICIPANTS

Twenty-seven eyes with the diagnosis of HZO and their contralateral clinically unaffected eyes were studied and compared with normal controls (n = 15).

METHODS

In vivo confocal microscopy (Confoscan 4; Nidek Technologies, Gamagori, Japan) and corneal esthesiometry (Cochet-Bonnet; Luneau Ophthalmologie, Chartres, France) of the central cornea were performed bilaterally in all patients and controls. Patients were grouped into normal (>5.5 cm), mild (>2.5-5.5 cm), and severe (<2.5 cm) loss of sensation.

MAIN OUTCOME MEASURES

Changes in corneal nerve density, total nerve number, main nerve trunks, branching, and tortuosity were evaluated after IVCM and were correlated to corneal sensation, disease duration, and number of recurrences.

RESULTS

Eyes with herpes zoster ophthalmicus had a significant (P<0.001) decrease in total nerve length (595.8±358.1 vs. 2258.4±989.0 μm/frame), total number of nerves (5.4±2.8 vs. 13.1±3.8), number of main nerve trunks (2.3±1.1 vs. 4.7±1.2), and number of nerve branches (3.2±2.3 vs. 8.4±3.7) as compared with controls. In the contralateral clinically unaffected eyes, total nerve length (1053.1±441.4 μm/frame), total number of nerves (8.3±2.9), and main nerve trunks (3.1±1.0) also were decreased significantly as compared with controls (P<0.01). Reduced nerve density, total nerve count, main trunks, and tortuosity was correlated significantly with corneal sensation across all subgroups (P<0.001).

CONCLUSIONS

Patients with unilateral HZO demonstrated a profound and significant bilateral loss of the corneal nerve plexus as compared with controls, demonstrating bilateral changes in a clinically unilateral disease. Loss of corneal sensation strongly correlated with subbasal nerve plexus alterations as shown by IVCM.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Central action of peripherally applied botulinum toxin type A on pain and dural protein extravasation in rat model of trigeminal neuropathy

BACKGROUND

Infraorbital nerve constriction (IoNC) is an experimental model of trigeminal neuropathy. We investigated if IoNC is accompanied by dural extravasation and if botulinum toxin type A (BoNT/A) can reduce pain and dural extravasation in this model.

METHODOLOGY/PRINCIPAL FINDINGS

Rats which developed mechanical allodynia 14 days after the IoNC were injected with BoNT/A (3.5 U/kg) into vibrissal pad. Allodynia was tested by von Frey filaments and dural extravasation was measured as colorimetric absorbance of Evans blue-plasma protein complexes. Presence of dural extravasation was also examined in orofacial formalin-induced pain. Unilateral IoNC, as well as formalin injection, produced bilateral dural extravasation. Single unilateral BoNT/A injection bilaterally reduced IoNC induced dural extravasation, as well as allodynia (lasting more than 2 weeks). Similarly, BoNT/A reduced formalin-induced pain and dural extravasation. Effects of BoNT/A on pain and dural extravasation in IoNC model were dependent on axonal transport through sensory neurons, as evidenced by colchicine injections (5 mM, 2 µl) into the trigeminal ganglion completely preventing BoNT/A effects.

CONCLUSIONS/SIGNIFICANCE

Two different types of pain, IoNC and formalin, are accompanied by dural extravasation. The lasting effect of a unilateral injection of BoNT/A in experimental animals suggests that BoNT/A might have a long-term beneficial effect in craniofacial pain associated with dural neurogenic inflammation. Bilateral effects of BoNT/A and dependence on retrograde axonal transport suggest a central site of its action.

Somatotopic direct projections from orofacial areas of primary somatosensory cortex to pons and medulla, especially to trigeminal sensory nuclear complex, in rats

The primary somatosensory cortex (S1) projects to the thalamus and brainstem somatosensory nuclei and modulates somatosensory information ascending to the S1 itself. However, the projections from the S1 to the brainstem second-order somatosensory neuron pools have not been fully studied. To address this in rats, we first revealed the somatotopic representation of orofacial areas in the S1 by recording cortical surface potentials evoked by stimulation of the lingual, mental, infraorbital, and frontal nerves. We then examined the morphology of descending projections from the electrophysiologically defined orofacial S1 areas to the pons and medulla after injections of an anterograde tracer, biotinylated dextranamine (BDA), into the orofacial S1 areas. BDA-labeled axon terminals were seen mostly in the trigeminal sensory nuclear complex (TSNC) and had a strong contralateral predominance. They also showed a somatotopic arrangement in dorsoventral and superficial-deep directions within almost all rostrocaudal TSNC levels, and in a rostrocaudal direction within the trigeminal caudal subnucleus. In the principal nucleus (Vp) or oral subnucleus (Vo) of TSNC, the BDA-labeled axon terminals showed a somatotopic arrangement closely matched to that of the electrophysiologically defined projection sites of orofacial primary afferents; these projection sites were marked by injections of a retrograde tracer, Fluorogold (FG), into the Vp or Vo. The FG injections labeled a large number of S1 neurons, with a strong contralateral predominance, in a somatotopic manner, which corresponded to that presented in the electrophysiologically defined orofacial S1 areas. The present results suggest that the orofacial S1 projections to somatotopically matched regions of trigeminal second-order somatosensory neuron pools may allow the orofacial S1 to accurately modulate orofacial somatosensory transmission to higher brain centers including the orofacial S1 itself.

Behavioral and immunohistochemical evidence for central antinociceptive activity of botulinum toxin A

Botulinum toxin A (BTX-A) is approved for treatment of different cholinergic hyperactivity disorders, and, recently, migraine headache. Although suggested to act only locally, novel observations demonstrated bilateral reduction of pain after unilateral toxin injection, and proposed retrograde axonal transport, presumably in sensory neurons. However, up to now, axonal transport of BTX-A from periphery to CNS was identified only in motoneurons, but with unknown significance. We assessed the effects of low doses of BTX-A injected into the rat whisker pad (3.5 U/kg) or into the sensory trigeminal ganglion (1 U/kg) on formalin-induced facial pain. Axonal transport was prevented by colchicine injection into the trigeminal ganglion (5 mM, 2 μl). To find the possible site of action of axonally transported BTX-A, we employed immunohistochemical labeling of BTX-A-truncated synaptosomal-associated protein 25 (SNAP-25) in medullary dorsal horn of trigeminal nucleus caudalis after toxin injection into the whisker pad. Both peripheral and intraganglionic BTX-A reduce phase II of formalin-induced pain. Antinociceptive effect of BTX-A was prevented completely by colchicine. BTX-A-truncated SNAP-25 in medullary dorsal horn (spinal trigeminal nucleus) was evident 3 days following the peripheral treatment, even with low dose applied (3.5 U/kg). Presented data provide the first evidence that axonal transport of BTX-A, obligatory for its antinociceptive effects, occurs via sensory neurons and is directed to sensory nociceptive nuclei in the CNS.

Peripheral purinergic receptor modulation influences the trigeminal ganglia nitroxidergic system in an experimental murine model of inflammatory orofacial pain

ATP plays an important role as an endogenous pain mediator generating and/or modulating pain signaling from the periphery to the central nervous system. The aim of this study was to analyze the role of peripheral purinergic receptors in modulation of the nitroxidergic system at a trigeminal ganglia level by monitoring changes in nitric oxide synthase isoforms. We also evaluated Fos-positive neurons in brainstem (spinal trigeminal nucleus) and pain-related behavior. We found that local administration of the P2 purinergic receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) decreased face-rubbing activity, nitric oxide synthase isoform expression in trigeminal ganglia, and Fos expression in spinal trigeminal nucleus after subcutaneous injection of formalin. These results suggest a role for peripheral P2 purinergic receptors in orofacial pain transmission through modulation of the nitroxidergic system. .

Neuronal disinhibition in the trigeminal nucleus caudalis in a model of chronic neuropathic pain

The mechanisms underlying neuropathic facial pain syndromes are incompletely understood. We used a unilateral chronic constriction injury of the rat infraorbital nerve (CCI-IoN) as a facial neuropathic model. Pain-related behavior of the CCI-IoN animals was tested at 8, 15 and 26 days after surgery (dps). The response threshold to mechanical stimulation with von Frey hairs on the injured side was reduced at 15 and 26 dps, indicating the presence of allodynia. We performed unitary recordings in the caudalis division of the spinal trigeminal nucleus (Sp5C) at 8 or 26 dps, and examined spontaneous activity and responses to mechanical and thermal stimulation of the vibrissal pad. Neurons were identified as wide dynamic range (WDR) or low-threshold mechanoreceptive (LTM) according to their response to tactile and/or noxious stimulation. Following CCI-IoN, WDR neurons, but not LTM neurons, increased their spontaneous activity at 8 and 26 dps, and both types of Sp5C neurons increased their responses to tactile stimuli. In addition, the on-off tactile response in neurons recorded after CCI-IoN was followed by afterdischarges that were not observed in control cases. Compared with controls, the response inhibition observed during paired-pulse stimulation was reduced after CCI-IoN. Immunohistochemical studies showed an overall decrease in GAD65 immunoreactivity in Sp5C at 26 dps, most marked in laminae I and II, suggesting that following CCI-IoN the inhibitory circuits in the sensory trigeminal nuclei are depressed. Consequently, our results strongly suggest that disinhibition of Sp5C neurons plays a relevant role in the appearance of allodynia after CCI-IoN.

Modulation of paratrigeminal nociceptive neurons following temporomandibular joint inflammation in rats

To evaluate the involvement of paratrigeminal nucleus (Pa5) nociceptive neurons in temporomandibular joint (TMJ) inflammation-induced pain and its autonomic correlates, we conducted behavioral, single unit recording and Fos immunohistochemical studies in anesthetized rats. Nocifensive behaviors to mechanical, heat or cold stimulation of the lateral face over the TMJ region were significantly enhanced in the TMJ-inflamed rats for 10-14 days after injection of complete Freund's adjuvant (CFA) into the TMJ and gradually decreased at the end of the 14-day observation period. Lowering of the nocifensive threshold in TMJ-inflamed rats lasted longer in vagus nerve-transected rats than vagus nerve-intact rats. A large number of Fos-like immunoreactive (LI) cells were observed in the Pa5, and half of them were retrogradely labeled with Fluorogold (FG) injected into the parabrachial nucleus. Background activity of Pa5 wide dynamic range and nociceptive specific neurons was significantly higher in the TMJ-inflamed rats when compared with controls. Responses to mechanical stimuli were significantly higher in NS neurons in the TMJ-inflamed rats. All thermal responsive Pa5 neurons were exclusively sensitive to cold and the response to cold was significantly higher in the TMJ-inflamed rats compared with control rats. Vagus nerve stimulation significantly decreased responses to mechanical and cold stimuli as well as the background activity in TMJ-treated rats but not in TMJ-untreated rats. The present findings suggest that populations of Pa5 neurons are nociceptive and involved in TMJ inflammation-induced pain as well as in autonomic processes related to TMJ pain.

Central nervous system networks involved in the processing of meningeal and cutaneous inputs from the ophthalmic branch of the trigeminal nerve in the rat

This study analysed the organization of central nervous system networks involved in the processing of meningeal inputs in the male, Sprague-Dawley rat. We injected the anterograde tracer, biotin dextran, into areas of the medullary trigeminal nucleus caudalis (Sp5C), which receive inputs from the ophthalmic division of the trigeminal nerve. Double-labelling immunohistochemical studies were then performed to compare calcitonin gene-related peptide (CGRP) or serotonin 1D (5HT1(D)) receptor distributions in the areas innervated by Sp5C neurons. Dense, topographically organized intratrigeminal connections were observed. Sp5C neurons projected to the commissural subnucleus of the solitary tract, A5 cell group region/superior salivatory nucleus, lateral periaqueductal grey matter, inferior colliculus and parabrachial nuclei. Trigeminothalamic afferents were restricted to the posterior group and ventroposteromedial thalamic nuclei. Some of these areas are also immunoreactive for 5HT1(D) and CGRP and thus remain potential central targets of triptan molecules and other antimigraine drugs.

Intracisternal administration of mitogen-activated protein kinase inhibitors reduced mechanical allodynia following chronic constriction injury of infraorbital nerve in rats

The present study investigated the role of mitogen-activated protein kinase (MAPK) in orofacial neuropathic pain following chronic constriction injury of the infraorbital nerve (ION-CCI). Experiments were carried out on male Sprague-Dawley rats weighing between 200 and 230 g. The ION was separated from adhering tissue, and two ligatures (5-0 chromic gut) were tied loosely around it. We examined the air-puff thresholds (mechanical allodynia), scores of pinprick (mechanical hyperalgesia), and face grooming frequency for acetone application (hypersensitivity for cold stimulation) - 3, 3, 6, 9, 12, 15, 20, 25, 30, and 40 days after surgery. ION-CCI produced mechanical allodynia, hyperalgesia, and cold hypersensitivity. We investigated whether administration of MAPKs inhibitors blocks ION-CCI-induced mechanical allodynia. Intracisternal administration with PD98059 or SB203580, a MEK inhibitor or a p38 MAPK inhibitor, respectively, significantly inhibited ION-CCI-induced mechanical allodynia in the orofacial area. These results indicate that the ION-CCI produced behavioral alterations in the orofacial area and those central MAPKs pathways contribute to orofacial neuropathic pain. Our findings suggest that MAPKs inhibitors have a potential role in treatment for orofacial neuropathic pain.

Effects of traditional “Juci” (contralateral acupuncture) on orofacial nociceptive behavior in the rat

PURPOSE

"Juci", one of the traditional acupuncture techniques, means contralateral acupuncture; i.e., implanting a needle into an acupoint to treat a given disease or disorder, but on the side of the body opposite to the diseased side. The aim of this study was: (1) to assess acupuncture effects on formalin-induced nociceptive behavior in the orofacial region in the rat, and (2) to evaluate the efficacy of Juci in the orofacial formalin test.

METHODS

Forty-four adult male Wistar rats were used in the present study. A 1.0% formalin solution (25 microl s.c., diluted in saline) was injected into the right upper lip. The rats were randomly assigned to five groups. (1) The control group (n = 9), which received formalin injection without acupuncture pretreatment; (2) the ipsilateral Ho-ku (see note below) acupuncture group (n = 10); (3) the contralateral Ho-ku acupuncture group (n = 11); (4) the acupuncture plus naloxone group (n = 9), where intraperitoneal naloxone (1.0 mgxkg(-1)) was injected immediately before acupuncture pretreatment; and (5) the sham acupuncture group (n = 5). "Ho-ku" is the term used for the "Large Intestine 4" acupoint, located between the first and second metacarpal bones.

RESULTS

The injection of formalin produced the characteristic biphasic behavioral response. Acupuncture significantly inhibited the response in the early and late phases. Naloxone significantly reversed these effects. There were no statistically significant differences between the ipsilateral and Juci acupuncture groups. Sham acupuncture did not exert any significant effect on the formalin-induced behavior.

CONCLUSION

Our results showed that the degree of effectiveness of Juci was similar to that of the ipsilateral acupuncture technique. Therefore, the Juci technique is also useful for the treatment of orofacial pain.

Mechanisms involved in modulation of trigeminal primary afferent activity in rats with peripheral mononeuropathy

In order to clarify the mechanisms underlying the changes in primary afferent neurons in trigeminal neuropathic pain, a chronic constriction nerve injury model of the infraorbital nerve (ION-CCI) was developed in rats. Mechanical allodynia was observed at 3 days after ION-CCI and lasted more than 14 days. Single-unit activities were recorded from the ION of anesthetized rats. C-, Abeta- and Adelta-units were identified on the basis of their conduction velocity. Adelta-units were frequently encountered at a later period after ION-CCI. The highest Adelta-spontaneous activity was recorded at 3 days after ION-CCI and progressively decreased after that, but spontaneous activity was still higher at 14 days after ION-CCI than that of naïve rats. Mechanical-evoked responses of Adelta-units were also highest at 3 days after ION-CCI and then gradually decreased. In consideration of these data, patch-clamp recordings were performed on medium to large size neurons of the dissociated trigeminal ganglion (TRG). Patch-clamp recordings revealed that the IK (sustained) and IA (transient) in rats with ION-CCI were significantly smaller than those of naïve rats, and correlated with an increase in duration of repolarization phase and a decrease in duration of depolarization phase, respectively. The hyperpolarization-activated current (Ih) was significantly larger in TRG neurons of rats with ION-CCI as compared with those of naïve rats. The present results suggest that Ih, IK and IA in Adelta-afferent neurons in TRG are significantly involved in the changes in afferent spontaneous activity and mechanically evoked activity that accompany mechanical allodynia produced by trigeminal nerve injury.

Selective distribution and function of primary afferent nociceptive inputs from deep muscle tissue to the brainstem trigeminal transition zone

Orofacial injury activates two distinct regions in the spinal trigeminal complex, the subnuclei interpolaris/caudalis (Vi/Vc) transition zone and the laminated Vc, or medullary dorsal horn (MDH). Studies suggest that the Vi/Vc transition zone plays an important role in processing orofacial deep input. To test this hypothesis, we employed a double-tracing strategy to compare central projections of primary afferent neurons that innervate the masseter muscle and the overlying skin. Different tracers were injected either centrally (Fluoro-Gold: ventral Vi/Vc, or MDH) or peripherally (wheat germ agglutinin-conjugated horseradish peroxidase or cholera toxin B: masseter or overlying skin) in the same rat. Trigeminal ganglion tissue sections were processed for single or double immunohistochemistry. The double labeling of ganglion neurons indicates their site of peripheral and central innervations. A population of small to medium-sized neurons was doubly labeled after injections of the tracers into the masseter-Vi/Vc, masseter-MDH, or the skin-MDH. However, only a few double-labeled neurons were occasionally observed after injections of the tracers into the skin-Vi/Vc. Injection of an N-methyl-D-aspartate receptor antagonist, AP-5, into the Vi/Vc and MDH attenuated masseter inflammatory hyperalgesia. In contrast, hyperalgesia after inflammation of the skin overlying the masseter was attenuated by injection of AP-5 into the MDH but not Vi/Vc. These results indicate that while both masseter and cutaneous inputs project to the MDH, masseter afferents provide an additional input to the Vi/Vc. These findings provide further evidence to support a role of the trigeminal transition zone in response to orofacial deep injury.

Trigeminovestibular and trigeminospinal pathways in rats: retrograde tracing compared with glutamic acid decarboxylase and glutamate immunohistochemistry

This study identified neurons in the sensory trigeminal complex with connections to the medial (MVN), inferior (IVN), lateral (LVN), and superior (SVN) vestibular nuclei or the spinal cord. Trigeminovestibular and trigeminospinal neurons were localized by injection of retrograde tracers. Immunohistochemical processing revealed gamma-aminobutyric acid (GABA)- and glutamate-containing neurons in these two populations. Trigeminovestibular neurons projecting to the MVN and the IVN were in the caudal principal nucleus (5P), pars oralis (5o), interpolaris (5i), and caudalis (5c) and scattered throughout the rostral 5P. Projections were bilateral to the IVN, with an ipsilateral dominance to the MVN, except from the rostral 5P, which was contralateral. Neurons projecting to the LVN were numerous in the ventral caudal 5P and the 5o and less abundant in the rostral 5P, 5i, and 5c. Our results suggested that only 5P and 5o project to the dorsal LVN. Neurons projecting to the SVN were in the dorsal 5P, 5o, and 5i but not in 5c. Trigeminospinal neurons were mainly in the ventral 5o and 5i and in the lateral 5c, rarely or never in 5P. Among trigeminovestibular neurons, most of the somas were immunoreactive for glutamate, but some reacted for GABA. Among trigeminospinal neurons, the number of somas immunoreactive for each of the two amino acids was similar. Trigeminal terminals were observed in contact with vestibulospinal neurons in the IVN and LVN, giving evidence of a trigeminovestibulospinal pathway. Therefore, inhibitory and excitatory facial inputs may contribute through trigeminospinal or trigeminovestibulospinal pathways to the control of head/neck movements.

Trigeminal transition zone/rostral ventromedial medulla connections and facilitation of orofacial hyperalgesia after masseter inflammation in rats

Recent studies have implicated a role for the trigeminal interpolaris/caudalis (Vi/Vc) transition zone in response to orofacial injury. Using combined neuronal tracing and Fos protein immunocytochemistry, we investigated functional connections between the Vi/Vc transition zone and rostral ventromedial medulla (RVM), a key structure in descending pain modulation. Rats were injected with a retrograde tracer, FluoroGold, into the RVM 7 days before injection of an inflammatory agent, complete Freund's adjuvant, into the masseter muscle and perfused at 2 hours postinflammation. A population of neurons in the ventral Vi/Vc overlapping with caudal ventrolateral medulla, and lamina V of the trigeminal subnucleus caudalis (Vc), exhibited FluoroGold/Fos double staining, suggesting the activation of the trigeminal-RVM pathway after inflammation. No double-labeled neurons were found in the dorsal Vi/Vc and laminae I-IV of Vc. Injection of an anterograde tracer, Phaseolus vulgaris leucoagglutinin, into the RVM resulted in labeling profiles overlapped with the region that showed FluoroGold/Fos double labeling, suggesting reciprocal connections between RVM and Vi/Vc. Lesions of Vc with a soma-selective neurotoxin, ibotenic acid, significantly reduced inflammation-induced Fos expression as well as the number of FluoroGold/Fos double-labeled neurons in the ventral Vi/Vc (P<0.05). Compared with control rats, lesions of the RVM (n=6) or Vi/Vc (n=6) with ibotenic acid led to the elimination or attenuation of masseter hyperalgesia/allodynia developed after masseter inflammation (P<0.05-0.01). The present study demonstrates reciprocal connections between the ventral Vi/Vc transition zone and RVM. The Vi/Vc-RVM pathway is activated after orofacial deep tissue injury and plays a critical role in facilitating orofacial hyperalgesia.

Heterosynaptic long-term depression of craniofacial nociception: divergent effects on pain perception and blink reflex in man

Noxious low-frequency stimulation (LFS) of presynaptic nerve fibers induces long-term depression (LTD) of synaptic transmission. In vitro studies suggest a sole homosynaptic effect. Consequently, the present study addressed the hypothesis that LTD of craniofacial nociception in man is mediated by a homosynaptic mechanism. Nociceptive supraorbital afferents were excited by electric pulses via a concentric electrode in ten healthy volunteers. The electrically evoked bilateral blink reflex (BR) was recorded from both orbicularis oculi muscles by surface electrodes. The BR was evoked in blocks of ten electric stimuli each (0.1 Hz) with an interblock interval of 8 min. Conditioning noxious LFS (1 Hz, 20 min) was applied via concentric electrode either to the same site as BR test stimuli (ipsilateral) or to the corresponding contralateral forehead area (contralateral). LFS and test stimulus intensities corresponded to about threefold the pain threshold. After three baseline stimulus blocks, either conditioning ipsilateral or contralateral LFS were applied or stimulation was interrupted for 20 min as a control task. Afterwards, test stimulation blocks were continued for 40 min. Each volunteer participated in all three sessions on different days. Noxious LFS induced LTD of the BR independently from the side of conditioning stimulation. Pain perception decreased after ipsilateral LFS but not after contralateral LFS. The bilateral effect of noxious LFS on the BR provides evidence for heterosynaptic LTD based on bilateral projections of supraorbital nerve afferents onto spinal trigeminal nuclei. The divergent effect on pain perception may be due to a preferential contralateral projection of nociceptive afferents onto reflex interneurons but not onto trigeminothalamic projection neurons.