Ultrastructural analysis of low-threshold mechanoreceptive vibrissa afferent boutons in the cat trigeminal caudal nucleus

Ultrastructural parameters related to synaptic release and their correlation with synaptic connectivity were analyzed in the low-threshold mechanoreceptive vibrissa afferent boutons in laminae III and IV of the trigeminal caudal nucleus (Vc). Rapidly adapting vibrissa afferents were intra-axonally labeled, and quantitative ultrastructural analyses with serial sections were performed on the labeled boutons and their presynaptic endings (p-endings). The volume of the labeled boutons was widely distributed from small to large ones (0.8~12.3 µm3), whereas the p-endings were small and uniform in size. The volume of the labeled boutons was positively correlated with the ultrastructural parameters such as mitochondrial volume (correlation coefficient, r=0.96), active zone area (r=0.82) and apposed surface area (r=0.79). Vesicle density (r=-0.18) showed little correlation to the volume of labeled boutons, suggesting that the total vesicle number of a bouton is proportional to its volume. In addition, the bouton volume was positively correlated with the number of p-endings (r=0.52) and with the number of dendrites postsynaptic to the labeled bouton (r=0.83). These findings suggest that low-threshold mechanoreception conveyed through vibrissa afferents is processed in a bouton size-dependent manner in the Vc, which may contribute to the sensory-motor function of laminae III/IV in Vc

Satellite glial cell P2Y12 receptor in the trigeminal ganglion is involved in lingual neuropathic pain mechanisms in rats

<p>Abstract</p> <p>Background</p> <p>It has been reported that the P2Y₁₂receptor (P2Y₁₂R) is involved in satellite glial cells (SGCs) activation, indicating that P2Y₁₂R expressed in SGCs may play functional roles in orofacial neuropathic pain mechanisms. However, the involvement of P2Y₁₂R in orofacial neuropathic pain mechanisms is still unknown. We therefore studied the reflex to noxious mechanical or heat stimulation of the tongue, P2Y₁₂R and glial fibrillary acidic protein (GFAP) immunohistochemistries in the trigeminal ganglion (TG) in a rat model of unilateral lingual nerve crush (LNC) to evaluate role of P2Y₁₂R in SGC in lingual neuropathic pain.</p> <p>Results</p> <p>The head-withdrawal reflex thresholds to mechanical and heat stimulation of the lateral tongue were significantly decreased in LNC-rats compared to sham-rats. These nocifensive effects were apparent on day 1 after LNC and lasted for 17 days. On days 3, 9, 15 and 21 after LNC, the mean relative number of TG neurons encircled with GFAP-immunoreactive (IR) cells significantly increased in the ophthalmic, maxillary and mandibular branch regions of TG. On day 3 after LNC, P2Y₁₂R expression occurred in GFAP-IR cells but not neuronal nuclei (NeuN)-IR cells (i.e. neurons) in TG. After 3 days of successive administration of the P2Y₁₂R antagonist MRS2395 into TG in LNC-rats, the mean relative number of TG neurons encircled with GFAP-IR cells was significantly decreased coincident with a significant reversal of the lowered head-withdrawal reflex thresholds to mechanical and heat stimulation of the tongue compared to vehicle-injected rats. Furthermore, after 3 days of successive administration of the P2YR agonist 2-MeSADP into the TG in naïve rats, the mean relative number of TG neurons encircled with GFAP-IR cells was significantly increased and head-withdrawal reflex thresholds to mechanical and heat stimulation of the tongue were significantly decreased in a dose-dependent manner compared to vehicle-injected rats.</p> <p>Conclusions</p> <p>The present findings provide the first evidence that the activation of P2Y₁₂R in SGCs of TG following lingual nerve injury is involved in the enhancement of TG neuron activity and nocifensive reflex behavior, resulting in neuropathic pain in the tongue.</p

Calcitonin gene-related peptide promotes cellular changes in trigeminal neurons and glia implicated in peripheral and central sensitization

<p>Abstract</p> <p>Background</p> <p>Calcitonin gene-related peptide (CGRP), a neuropeptide released from trigeminal nerves, is implicated in the underlying pathology of temporomandibular joint disorder (TMD). Elevated levels of CGRP in the joint capsule correlate with inflammation and pain. CGRP mediates neurogenic inflammation in peripheral tissues by increasing blood flow, recruiting immune cells, and activating sensory neurons. The goal of this study was to investigate the capability of CGRP to promote peripheral and central sensitization in a model of TMD.</p> <p>Results</p> <p>Temporal changes in protein expression in trigeminal ganglia and spinal trigeminal nucleus were determined by immunohistochemistry following injection of CGRP in the temporomandibular joint (TMJ) capsule of male Sprague-Dawley rats. CGRP stimulated expression of the active forms of the MAP kinases p38 and ERK, and PKA in trigeminal ganglia at 2 and 24 hours. CGRP also caused a sustained increase in the expression of c-Fos neurons in the spinal trigeminal nucleus. In contrast, levels of P2X₃in spinal neurons were only significantly elevated at 2 hours in response to CGRP. In addition, CGRP stimulated expression of GFAP in astrocytes and OX-42 in microglia at 2 and 24 hours post injection.</p> <p>Conclusions</p> <p>Our results demonstrate that an elevated level of CGRP in the joint, which is associated with TMD, stimulate neuronal and glial expression of proteins implicated in the development of peripheral and central sensitization. Based on our findings, we propose that inhibition of CGRP-mediated activation of trigeminal neurons and glial cells with selective non-peptide CGRP receptor antagonists would be beneficial in the treatment of TMD.</p

Mechanisms involved in an increment of multimodal excitability of medullary and upper cervical dorsal horn neurons following cutaneous capsaicin treatment

<p>Abstract</p> <p>Background</p> <p>In order to evaluate mechanisms that may underlie the sensitization of trigeminal spinal subnucleus caudalis (Vc; the medullary dorsal horn) and upper cervical spinal cord (C1-C2) nociceptive neurons to heat, cold and mechanical stimuli following topical capsaicin treatment of the facial skin, nocifensive behaviors as well as phosphorylation of extracellular regulated-kinase (pERK) in Vc and C1-C2 neurons were studied in rats.</p> <p>Results</p> <p>Compared to vehicle application, capsaicin application to the lateral facial skin produced 1 hour later a flare in the skin, and also induced significantly greater nocifensive behaviors to heat, cold or mechanical stimulus of the lateral facial skin. The intrathecal (i.t.) injection of the MEK inhibitor PD98059 markedly attenuated the nocifensive behaviors to these stimuli in capsaicin-treated rats. Moreover, the number of pERK-like immunoreactive (pERK-LI) cells in Vc and C1-C2 was significantly larger following the heat, cold and mechanical stimuli in capsaicin-treated rats compared with vehicle-treated rats. The number of pERK-LI cells gradually increased following progressive increases in the heat or mechanical stimulus intensity and following progressive decrease in the cold stimulus. The ERK phosphorylation in Vc and C1-C2 neurons was strongly inhibited after subcutaneous injection of the capsaicin antagonist capsazepine in capsaicin-treated rats.</p> <p>Conclusion</p> <p>The present findings revealed that capsaicin treatment of the lateral facial skin causes an enhancement of ERK phosphorylation in Vc and C1-C2 neurons as well as induces nocifensive behavior to heat, cold and mechanical simulation of the capsaicin-treated skin. The findings suggest that TRPV1 receptor mechanisms in rat facial skin influence nociceptive responses to noxious cutaneous thermal and mechanical stimuli by inducing neuroplastic changes in Vc and C1-C2 neurons that involve in the MAP kinase cascade.</p

Central connections of the lingual tonsillar branch of the glossopharyngeal nerve and the superior laryngeal nerve in lamb

Afferent and efferent central connections of the lingual-tonsillar branch of the glossopharyngeal nerve (LT-IX) and the superior laryngeal nerve (SLN) in the lamb were traced with horseradish peroxidase (HRP) histochemistry. After entering the brainstem, most LT-IX and SLN afferent fibers turned caudally in the solitary tract (ST). Some afferent fibers of LT-IX terminated in the medial nucleus of the solitary tract slightly caudal to their level of entry. The remaining fibers projected to the dorsolateral, ventrolateral, and interstitial areas of the nucleus of the solitary tract (NST) at the level of the area postrema. Superior laryngeal nerve afferent fibers terminated extensively in the medial and ventral NST at levels near the rostral pole of the area postrema. Further caudal, near the level of obex, SLN afferent terminations were concentrated in the region ventrolateral to the ST and in the interstitial NST. The caudal extent of LT-IX and the rostral extent of SLN terminals projected to similar levels of the NST, but only a relatively small proportion of the total projections overlapped. Lingualtonsillar and SLN fibers also coursed rostrally to terminate in the caudal pons within and medial to the dorsomedial principal sensory trigeminal nucleus. Other labeled afferent fibers traveled caudally in the dorsal spinal trigeminal tract to terminate in the dorsal two-thirds of the spinal trigeminal nucleus at the level of obex. Large numbers of labeled cells with fibers in the LT-IX or SLN were located in the ipsilateral rostral nucleus ambiguus and surrounding reticular formation. Fewer labeled cells were observed in the inferior salivatory nucleus following HRP application to either the LTIX or SLN. The LT-IX and SLN projections to areas of the NST associated with upper airway functions, like swallowing and respiration, suggest an important role for these two nerves in the initiation and control of airway reflexes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50028/1/902450404_ftp.pd

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

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

Alteration of primary afferent activity following inferior alveolar nerve transection in rats

<p>Abstract</p> <p>Background</p> <p>In order to evaluate the neural mechanisms underlying the abnormal facial pain that may develop following regeneration of the injured inferior alveolar nerve (IAN), the properties of the IAN innervated in the mental region were analyzed.</p> <p>Results</p> <p>Fluorogold (FG) injection into the mental region 14 days after IAN transection showed massive labeling of trigeminal ganglion (TG). The escape threshold to mechanical stimulation of the mental skin was significantly lower (i.e. mechanical allodynia) at 11-14 days after IAN transection than before surgery. The background activity, mechanically evoked responses and afterdischarges of IAN Aδ-fibers were significantly higher in IAN-transected rats than naive. The small/medium diameter TG neurons showed an increase in both tetrodotoxin (TTX)-resistant (TTX-R) and -sensitive (TTX-S) sodium currents (<it>I</it>_Na) and decrease in total potassium current, transient current (<it>I</it>_A) and sustained current (<it>I</it>_K) in IAN-transected rats. The amplitude, overshoot amplitude and number of action potentials evoked by the depolarizing pulses after 1 μM TTX administration in TG neurons were significantly higher, whereas the threshold current to elicit spikes was smaller in IAN-transected rats than naive. Resting membrane potential was significantly smaller in IAN-transected rats than that of naive.</p> <p>Conclusions</p> <p>These data suggest that the increase in both TTX-S <it>I</it>_Naand TTX-R <it>I</it>_Naand the decrease in <it>I</it>_Aand <it>I</it>_kin small/medium TG neurons in IAN-transected rats are involved in the activation of spike generation, resulting in hyperexcitability of Aδ-IAN fibers innervating the mental region after IAN transection.</p

The Role of Cutaneous Receptors in the Menton Tap Silent Period

The neuromuscular mechanisms of the electromyographic silent period are poorly understood but clinically significant. Our purpose was to isolate the contribution of cutaneous afferents from the chin to the latency and the duration of the EMG silent period. Under the conditions of our experiment, the afferents of the mental nerve have no observable influence on the latency or the duration of the silent period educed by a menton tap.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66492/2/10.1177_00220345790580011001.pd