The threshold for eliciting the jaw opening reflex in rats is not increased by neonatal capsaicin

Piezo2 Expression in Mechanosensitive Dental Primary Afferent Neurons

Mechanosensitive ion channels have been suggested to be expressed in dental primary afferent (DPA) neurons to transduce the movement of dentinal fluid since the proposal of hydrodynamic theory. Piezo2, a mechanosensitive, rapidly inactivating (RI) ion channel, has been recently identified in dorsal root ganglion (DRG) neurons to mediate tactile transduction. Here, we examined the expression of Piezo2 in DPA neurons by in situ hybridization, single-cell reverse transcriptase polymerase chain reaction, and whole-cell patch-clamp recordings. DPA neurons with Piezo2 messenger RNA (mRNA) or Piezo2-like currents were further characterized based on their neurochemical and electrophysiological properties. Piezo2 mRNA was found mostly in medium- to large-sized DPA neurons, with the majority of these neurons also positive for Nav1.8, CGRP, and NF200, whereas only a minor population was positive for IB4 and peripherin. Whole-cell patch-clamp recordings revealed Piezo2-like, RI currents evoked by mechanical stimulation in a subpopulation of DPA neurons. RI currents were pharmacologically blocked by ruthenium red, a compound known to block Piezo2, and were also reduced by small interfering RNA-mediated Piezo2 knockdown. Piezo2-like currents were observed almost exclusively in IB4-negative DPA neurons, with the current amplitude larger in capsaicin-insensitive DPA neurons than the capsaicin-sensitive population. Our findings show that subpopulation of DPA neurons is indeed mechanically sensitive. Within this subpopulation of mechanosensitive DPA neurons, we have identified the Piezo2 ion channel as a potential transducer for mechanical stimuli, contributing to RI inward currents. Piezo2-positive DPA neurons were characterized as medium- to large-sized neurons with myelinated A-fibers, containing nociceptive peptidergic neurotransmitters.

Differential TRPV1 and TRPV2 channel expression in dental pulp

Hypersensitivity to thermal and mechanical stimuli can occur in painful pulpitis. To explore the neuro-anatomical basis of heat and mechanical sensitivity, we evaluated expression of TRPV1 (heat) and TRPV2 (heat/mechanical) channels in the cell bodies and terminal arborizations of neurons that innervate the dental pulp (DP) and periodontal tissues (PDL). We report that ~50% of trigeminal ganglion (TG) neurons retrogradely labeled from the DP express TRPV2, and this was significantly greater than the general expression of this channel in the TG (15%) and slightly more than what is expressed in the PDL by retrograde labeling (40%). The TRPV1 receptor, however, was less prevalent in neurons innervating the DP than their general expression in the TG (17% vs. 26%) and was more extensively expressed in neurons innervating the PDL (26%). Co-labeling studies showed that 70% of neurons that innervate the DP are myelinated. Approximately 1/3 of the retrogradely labeled neurons from the DP were calcitonin-gene-related-peptide-positive (peptide-expressing), but very few expressed the IB4 marker of non-peptidergic unmyelinated afferents. These findings suggest that the DP has a unique neurochemical innervation with regard to TRP receptor expression, which has significant implications for the mechanisms contributing to odontogenic pain and management strategies.

Response properties of periodontal mechanoreceptors in rats, in vitro

Single unit activities of the inferior alveolar nerve evoked by calibrated von Frey stimuli (1.1, 2.9, 7.8, 11.8, and 17.2mN) on the periodontal ligaments of the mandibular molars or incisors were recorded in an in vitro jaw-nerve preparation of Wistar albino rats. The data of 55 (lower incisor) and 100 (lower molars) units were collected in the present study. Both rapidly (RA) and slowly adapting (SA) type units were found in the incisors, and most of these units were innervated by Abeta fibers. While all the units of the molars were of RA types, the innervated fibers of two-thirds (67/100) of the units have been identified as Adelta fibers. The response patterns of the RA type were subdivided into three types (ON, OFF or ON-OFF type) both in the incisors and the molars. While von Frey thresholds of all incisor units were 11.8 mN except one unit that was 7.8 mN, those of the molars varied from 2.9 to 11.8 mN. In the molars, a majority of afferents innervated the periodontal ligaments of more than one tooth. This study suggests that response properties of periodontal mechanoreceptors are different between the incisors and the molars in rats, suggesting that these receptors have different functions in the regulation of mastication.

Differential effects of trigeminal tractotomy on Adelta- and C-fiber-mediated nociceptive responses

In this study we have tested in the rat, whether trigeminal tractotomy, which deprives the spinal trigeminal nucleus caudalis (Sp5C) of its trigeminal inputs, affected differentially nociceptive responses mediated by C- vs. Adelta-nociceptors from oral and perioral regions. Tractotomy had no effect on the threshold of the jaw opening reflex, induced by incisive pulp stimulation (Adelta-fiber-mediated), but blocked the formalin response (mainly C-fiber-mediated). These results suggest that nociceptive responses mediated by trigeminal C-fibers completely depend on the integrity of the Sp5C, while intraoral sensations triggered Adelta-fibers (especially of dental origin) are primarily processed in the rostral part of the spinal trigeminal nucleus.

Periaqueductal gray matter and nucleus raphe magnus involvement in anterior pretectal nucleus-induced inhibition of jaw-opening reflex in rats

In previous studies we have shown that electrical stimulation of the cortex or anterior pretectal nucleus (APT) inhibits the jaw-opening reflex (JOR). In the present study we investigated whether these effects are mediated by a relay in the periaqueductal gray matter (PAG) or rostroventromedial medulla (RVM). Experiments were performed on chloralose-urethane anesthetized rats. The JOR which was elicited by electrical stimulation of the mandibular incisor tooth was monitored by recording the evoked digastric muscle activity. Conditioning stimulation (20 ms train of 0.2 ms pulses at 400 Hz) was delivered to the facial area of the sensorimotor cortex, APT, PAG or nucleus raphe magnus (NRM) 50 ms prior to the test stimulus to the tooth that evoked the JOR. In addition, the effects of microinjections of glutamate into APT, PAG and NRM on the tooth-evoked JOR were also evaluated. The inhibition of the JOR by electrical and glutamate conditioning stimulation was found to be most potent for activation of the NRM and least potent for the APT. Local anesthetic (2% lidocaine, 0.3-0.6 microliters) block of the PAG could partially, significantly (P less than 0.05) and reversibly reduce both the APT and cortical-induced depression of the JOR. Lidocaine block of the ventromedial pons reversibly reduced the PAG, APT and cortical-induced inhibition of the JOR (P less than 0.05). Lidocaine block of the lateral RVM had powerfully (P less than 0.01) and reversibly reduced the PAG-induced inhibition, but had only a small effect (P less than 0.05) on the APT-induced inhibition and no significant effect on the cortical-induced inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Proportion of unmyelinated axons in rat molar and incisor tooth pulps following neonatal capsaicin treatment and/or sympathectomy

The occurrence of unmyelinated axons was examined ultrastructurally in rat molar and incisor root pulps of normal rats, of neonatally capsaicin-treated rats, of rats subjected to neonatal capsaicin treatment followed by resection of the superior cervical ganglion and of sympathectomized but otherwise normal rats. Following capsaicin treatment the occurrence of unmyelinated pulpal axons was slightly subnormal. Sympathectomy was largely without effect on the population of unmyelinated axons in tooth pulps of capsaicin-treated rats. In normal rats the proportion of unmyelinated axons in molar pulps was not altered by sympathectomy but it caused a slight decrease in the number of unmyelinated incisor pulpal axons. These findings support the view that most of the unmyelinated axons in rat molar and incisor pulps are sensory, that the parent neurons of these axons differ from nociceptive neurons at other sites by being largely resistant to neonatal capsaicin treatment and that very few unmyelinated tooth pulp axons represent postganglionic efferents.

The nociceptive jaw-opening reflex: evidence for alpha 2 adrenoceptor involvement

The ability of alpha adrenoceptor agonists to modulate the tooth pulp stimulation evoked (TPS) jaw-opening reflex (JOR) was investigated in rats and rabbits. Low doses of clonidine (6.25-50 micrograms/kg, IV) significantly increased dEMG thresholds. These effects were antagonized by alpha 2 adrenoceptor antagonists (e.g., yohimbine), but not by alpha 1 adrenoceptor antagonists (e.g., prazosin) or mu receptor antagonists (e.g., naloxone). Polar alpha 2 adrenoceptor agonists (e.g., ST-91 and 4-hydroxyclonidine) that cross the blood brain barrier (BBB) poorly and lipophilic alpha 1 adrenoceptor agonists (e.g., ST-587) that cross the BBB easily were without affect on the TPS-JOR. Structures of the peripheral efferent neurocircuitry of the JOR (e.g., the digastric muscle and the neuromuscular junction of the digastric muscle and its motor nerve, the mylohyoid) were shown not to be active sites of clonidine's effect on the TPS-JOR. Treatment with phentolamine (an alpha adrenoceptor antagonist that poorly crosses the BBB) completely poorly crosses the BBB) completely antagonized clonidine's initial transient cardiovascular (pressor) effect without altering its TPS-JOR effects. Pretreatment with reserpine (a catecholamine depleting agent) failed to alter clonidine's affects on the TPS-JOR. Our studies suggest that alpha 2 adrenoceptors potently modulate the TPS-JOR and such modulation may be important in understanding trigeminal neuronal circuitries that partake in pain processing.

Evaluation of the analgesic effects of capsaicin using a new rat model for tonic pain

An animal model for tonic pain has been produced by means of injecting monosodium urate crystals into a knee joint of rat hind paws in order to evaluate the analgesic effects of various drugs and analgesic methods and to elucidate the physiology of tonic pain. This model allows for stable and long-term behavioural changes due to tonic pain followed by complete recovery without tissue damages. It is advantageous in allowing for objective and quantitative evaluation of the effects of analgesics and should prove useful in research on pain and the development of pain therapy techniques. Using this model, the analgesic effects of capsaicin was evaluated when administered to the neonatal rat or locally to peripheral nerves. In both cases, significant analgesic effects were obtained.

Response of serotonin-containing neurons in nucleus raphe magnus to morphine, noxious stimuli, and periaqueductal gray stimulation in freely moving cats

Extracellular single-unit recordings were made in nucleus raphe magnus in unanesthetized, unrestrained cats. Discharge of serotonergic neurons in this region was increased when animals were aroused by noxious stimuli such as pinch and radiant heating of the tail, but these cells were not specifically nociceptive. Peristimulus time histograms indicated that stimulation in the periaqueductal gray was excitatory but alveolar nerve stimulation at a noxious current intensity was no more effective than nonnoxious nerve stimulation in activating serotonergic unit discharge: Similarly, stressful treatments such as physical restraint increased the discharge of some serotonergic neurons, but these cells were activated during any period of behavioral arousal whether or not arousal was the result of aversive treatment. Injection of Formalin into the paw produced pain lasting about 30 min without increasing serotonergic unit discharge above rates observed during undisturbed active waking behavior. The activity of serotonergic neurons was not increased by an analgesic dose of morphine (2 mg/kg, i.p.). These results then are not consistent with the hypothesis that morphine analgesia depends on activation of serotonergic neurons in nucleus raphe magnus or that these cells are specifically involved in modulation of nociception. These neurons may, however, be involved in nociceptive control within the context of a general modulation of sensorimotor processes by serotonin in the central nervous system. We did observe neurochemically unidentified neurons in the medulla whose discharge was more specifically activated by aversive stimuli and also by morphine. It is possible that these neurons are more directly involved in the mediation of opiate and/or stress-induced analgesia.