The role of trigeminal interpolaris-caudalis transition zone in persistent orofacial pain. Int Rev Neurobiol. 2011;97:207-225. [PMID: 21708312 DOI: 10.1016/B978-0-12-385198-7.00008-4]

Variations in touch representation in the hummingbird and zebra finch forebrain

Somatosensation is essential for animals to perceive the external world through touch, allowing them to detect physical contact, temperature, pain, and body position. Studies on rodent vibrissae have highlighted the organization and processing in mammalian somatosensory pathways.1,2 Comparative research across vertebrates is vital for understanding evolutionary influences and ecological specialization on somatosensory systems. Birds, with their diverse morphologies, sensory abilities, and behaviors, serve as ideal models for investigating the evolution of somatosensation. Prior studies have uncovered tactile-responsive areas within the avian telencephalon, particularly in pigeons,3,4,5,6 parrots,7 and finches,8 but variations in somatosensory maps and responses across avian species are not fully understood. This study aims to explore somatotopic organization and neural coding in the telencephalon of Anna's hummingbirds (Calypte anna) and zebra finches (Taeniopygia guttata) by using in vivo extracellular electrophysiology to record activity in response to controlled tactile stimuli on various body regions. These findings reveal unique representations of body regions across distinct forebrain somatosensory nuclei, indicating significant differences in the extent of areas dedicated to certain body surfaces, which may correlate with their behavioral importance.

Divergent changes in PBN excitability in a mouse model of neuropathic pain

The transition from acute to chronic pain involves maladaptive plasticity in central nociceptive pathways. Growing evidence suggests that changes within the parabrachial nucleus (PBN), an important component of the spino-parabrachio-amygdaloid pain pathway, are key contributors to the development and maintenance of chronic pain. In animal models of chronic pain, PBN neurons become sensitive to normally innocuous stimuli and responses to noxious stimuli become amplified and more often produce after-discharges that outlast the stimulus. Using ex vivo slice electrophysiology and two mouse models of neuropathic pain, sciatic cuff and chronic constriction of the infraorbital nerve (CCI-ION), we find that changes in the firing properties of PBN neurons and a shift in inhibitory synaptic transmission may underlie this phenomenon. Compared to PBN neurons from shams, a larger proportion of PBN neurons from mice with a sciatic cuff were spontaneously active at rest, and these same neurons showed increased excitability relative to shams. In contrast, quiescent PBN neurons from cuff mice were less excitable than those from shams. Despite an increase in excitability in a subset of PBN neurons, the presence of after-discharges frequently observed in vivo were largely absent ex vivo in both injury models. However, GABAB-mediated presynaptic inhibition of GABAergic terminals is enhanced in PBN neurons after CCI-ION. These data suggest that the amplified activity of PBN neurons observed in rodent models of chronic pain arise through a combination of changes in firing properties and network excitability.Significance Statement Hyperactivity of neurons in the parabrachial nucleus (PBN) is causally linked to exaggerated pain behaviors in rodent models of chronic pain but the underlying mechanisms remain unknown. Using two mouse models of neuropathic pain, we show the intrinsic properties of PBN neurons are largely unaltered following injury. However, subsets of PBN neurons become more excitable and GABAB receptor mediated suppression of inhibitory terminals is enhanced after injury. Thus, shifts in network excitability may be a contributing factor in injury induced potentiation of PBN activity.

Modulatory Processes in Craniofacial Pain States

Pain is a common symptom associated with many disorders affecting the craniofacial tissues that include the teeth and their supporting structures, the jaw, face and tongue muscles, and the temporomandibular joint. Most acute craniofacial pain states are easily recognized and readily treated, but chronic craniofacial pain states (e.g., temporomandibular disorders [TMD], trigeminal neuropathies, and some headaches) may be especially challenging to manage successfully. This chapter provides an overview of the processes that underlie craniofacial pain, with a focus on the pain-modulatory mechanisms operating in craniofacial tissues and in the central nervous system (CNS), including the role of endogenous chemical processes such as those involving opioids. The chapter outlines in particular findings from preclinical studies that have provided substantial information about the neural as well as nonneural (e.g., glial) processes involved in the initiation, transmission, and modulation of nociceptive signals in the trigeminal system, and also draws attention to their clinical correlates. The increased understanding gained from these preclinical studies of how nociceptive signals can be modulated will contribute to improvements in presently available therapeutic approaches to manage craniofacial pain as well as to the development of novel analgesic approaches.

Dissecting neural circuits from rostral ventromedial medulla to spinal trigeminal nucleus bidirectionally modulating craniofacial mechanical sensitivity

Chronic craniofacial pain is intractable and its mechanisms remain unclarified. The rostral ventromedial medulla (RVM) plays a crucial role in descending pain facilitation and inhibition. It is unclear how the descending circuits from the RVM to spinal trigeminal nucleus (Sp5) are organized to bidirectionally modulate craniofacial nociception. We used viral tracing, in vivo optogenetics, calcium signaling recording, and chemogenetic manipulations to investigate the structure and function of RVM-Sp5 circuits. We found that most RVM neurons projecting to Sp5 were GABAergic or glutamatergic and facilitated or inhibited craniofacial nociception, respectively. Both GABAergic interneurons and glutamatergic projection neurons in Sp5 received RVM inputs: the former were antinociceptive, whereas the latter were pronociceptive. Furthermore, we demonstrated activation of both GABAergic and glutamatergic Sp5 neurons receiving RVM inputs in inflammation- or dysfunction-induced masseter hyperalgesia. Activating GABAergic Sp5 neurons or inhibiting glutamatergic Sp5 neurons that receive RVM projections reversed masseter hyperalgesia. Our study identifies specific cell types and projections of RVM-Sp5 circuits involved in facilitating or inhibiting craniofacial nociception respectively. Selective manipulation of RVM-Sp5 circuits can be used as potential treatment strategy to relieve chronic craniofacial muscle pain.

Naringenin modulates GABA mediated response in a sexdependent manner in substantia gelatinosa neurons of trigeminal subnucleus caudalis in immature mice

The substantia gelatinosa (SG) within the trigeminal subnucleus caudalis (Vc) is recognized as a pivotal site of integrating and modulating afferent fibers carrying orofacial nociceptive information. Although naringenin (4',5,7-thrihydroxyflavanone), a natural bioflavonoid, has been proven to possess various biological effects in the central nervous system (CNS), the activity of naringenin at the orofacial nociceptive site has not been reported yet. In this study, we explored the influence of naringenin on GABA response in SG neurons of Vc using whole-cell patch-clamp technique. The application of GABA in a bath induced two forms of GABA responses: slow and fast. Naringenin enhanced both amplitude and area under curve (AUC) of GABA-mediated responses in 57% (12/21) of tested neurons while decreasing both parameters in 33% (7/21) of neurons. The enhancing or suppressing effect of naringenin on GABA response have been observed, with enhancement occurring when the GABA response was slow, and suppression when it was fast. Furthermore, both the enhancement of slower GABA responses and the suppression of faster GABA responses by naringenin were concentration dependent. Interestingly, the nature of GABA response was also found to be sex-dependent. A majority of SG neurons from juvenile female mice exhibited slower GABA responses, whereas those from juvenile males predominantly displayed faster GABA responses. Taken together, this study indicates that naringenin plays a partial role in modulating orofacial nociception and may hold promise as a therapeutic target for treating orofacial pain, with effects that vary according to sex.

Electroacupuncture exerts prolonged analgesic and neuroprotective effects in a persistent dental pain model induced by multiple dental pulp injuries: GABAergic interneurons-astrocytes interaction

Pain within the trigeminal system, particularly dental pain, is poorly understood. This study aimed to determine whether single or multiple dental pulp injuries induce persistent pain, its association with trigeminal central nociceptive pathways and whether electroacupuncture (EA) provides prolonged analgesic and neuroprotective effects in a persistent dental pain model. Models of single dental pulp injury (SDPI) and multiple dental pulp injuries (MDPI) were used to induce trigeminal neuropathic pain. The signs of dental pain-related behavior were assessed using the mechanical head withdrawal threshold (HWT). Immunofluorescence and western blot protocols were used to monitor astrocyte activation, changes in apoptosis-related proteins, and GABAergic interneuron plasticity. SDPI mice exhibited an initial marked decrease in HWT from days one to 14, followed by progressive recovery from days 21 to 42. From days 49 to 70, the HWT increased and returned to the control values. In contrast, MDPI mice showed a persistent decrease in HWT from days one to 70. MDPI increased glial fibrillary acidic protein (GFAP) and decreased glutamine synthetase (GS) and glutamate transporter-1 (GLT1) expression in the Vi/Vc transition zone of the brainstem on day 70, whereas no changes in astrocytic markers were observed on day 70 after SDPI. Increased expression of cleaved cysteine-aspartic protease-3 (cleaved caspase-3) and Bcl-2-associated X protein (Bax), along with decreased B-cell lymphoma/leukemia 2 (Bcl-2), were observed at day 70 after MDPI but not after SDPI. The downregulation of glutamic acid decarboxylase (GAD65) expression was observed on day 70 only after MDPI. The effects of MDPI-induced lower HWT from days one to 70 were attenuated by 12 sessions of EA treatment (days one to 21 after MDPI). Changes in astrocytic GFAP, GS, and GLT-1, along with cleaved caspase-3, Bax, Bcl-2, and GAD65 expression observed 70 days after MDPI, were reversed by EA treatment. The results suggest that persistent dental pain in mice was induced by MDPI but not by SDPI. This effect was associated with trigeminal GABAergic interneuron plasticity along with morphological and functional changes in astrocytes. EA exerts prolonged analgesic and neuroprotective effects that might be associated with the modulation of neuron-glia crosstalk mechanisms.

GFAP-NpHR mediated optogenetic inhibition of trigeminal nucleus caudalis attenuates hypersensitive behaviors and thalamic discharge attributed to infraorbital nerve constriction injury

The significance of hyperactive astrocytes in neuropathic pain is crucial. However, the association between medullary astrocytes and trigeminal neuralgia (TN)-related pain processing is unclear. Here, we examined how optogenetic inhibition of medullary astrocytes in the trigeminal nucleus caudalis (TNC) regulates pain hypersensitivity in an infraorbital nerve (ION) constricted TN model. We used adult Sprague Dawley rats subjected to infraorbital nerve (ION) constriction to mimic TN symptoms, with naive and sham rats serving as controls. For in vivo optogenetic manipulations, rats stereotaxically received AAV8-GFAP-eNpHR3.0-mCherry or AAV8-GFAP-mCherry at the trigeminal nucleus caudalis (TNC). Open field, von Frey, air puff, and acetone tests measured pain behavioral flexibility. In vivo thalamic recordings were obtained simultaneously with optogenetic manipulation in the TNC. Orofacial hyperalgesia and thalamic hyperexcitability were both accompanied by medullary astrocyte hyperactivity, marked by upregulated GFAP. The yellow laser-driven inhibition of TNC astrocytes markedly improved behavioral responses and regulated thalamic neuronal responses. Halorhodopsin-mediated inhibition in medullary astrocytes may modify the nociceptive input transmitted through the trigeminothalamic tract and pain perception. Taken together, these findings imply that this subpopulation in the TNC and its thalamic connections play a significant role in regulating the trigeminal pain circuitry, which might aid in the identification of new therapeutic measures in TN management.

Otological symptoms in patients with rheumatoid arthritis of the temporomandibular joint

OBJECTIVE

The aim of the current study was to describe the pattern of otological symptoms in patients with rheumatoid arthritis (RA), having clinical temporomandibular joint (TMJ) involvement. This issue had not been previously addressed.

METHODS

A questionnaire and examination findings protocol was applied for 141 patients with RA and 141 control subjects.

RESULTS

Otological symptoms (otalgia, hearing loss, tinnitus, and vertigo), all had a significantly higher incidence in RA patients, compared to control subjects (P = .001).

CONCLUSION

The onset and maintenance of otological symptoms in patients with TMJ involvement by RA probably result from peripheral, as well as central nervous system alterations in sensory stimuli programming.

Direct and Indirect Nociceptive Input from the Trigeminal Dorsal Horn to Pain-Modulating Neurons in the Rostral Ventromedial Medulla

The brain is able to amplify or suppress nociceptive signals by means of descending projections to the spinal and trigeminal dorsal horns from the rostral ventromedial medulla (RVM). Two physiologically defined cell classes within RVM, "ON-cells" and "OFF-cells," respectively facilitate and inhibit nociceptive transmission. However, sensory pathways through which nociceptive input drives changes in RVM cell activity are only now being defined. We recently showed that indirect inputs from the dorsal horn via the parabrachial complex (PB) convey nociceptive information to RVM. The purpose of the present study was to determine whether there are also direct dorsal horn inputs to RVM pain-modulating neurons. We focused on the trigeminal dorsal horn, which conveys sensory input from the face and head, and used a combination of single-cell recording with optogenetic activation and inhibition of projections to RVM and PB from the trigeminal interpolaris-caudalis transition zone (Vi/Vc) in male and female rats. We determined that a direct projection from ventral Vi/Vc to RVM carries nociceptive information to RVM pain-modulating neurons. This projection included a GABAergic component, which could contribute to nociceptive inhibition of OFF-cells. This approach also revealed a parallel, indirect, relay of trigeminal information to RVM via PB. Activation of the indirect pathway through PB produced a more sustained response in RVM compared with activation of the direct projection from Vi/Vc. These data demonstrate that a direct trigeminal output conveys nociceptive information to RVM pain-modulating neurons with a parallel indirect pathway through the parabrachial complex.SIGNIFICANCE STATEMENT Rostral ventromedial medulla (RVM) pain-modulating neurons respond to noxious stimulation, which implies that they receive input from pain-transmission circuits. However, the traditional view has been that there is no direct input to RVM pain-modulating neurons from the dorsal horn, and that nociceptive information is carried by indirect pathways. Indeed, we recently showed that noxious information can reach RVM pain-modulating neurons via the parabrachial complex (PB). Using in vivo electrophysiology and optogenetics, the present study identified a direct relay of nociceptive information from the trigeminal dorsal horn to physiologically identified pain-modulating neurons in RVM. Combined tracing and electrophysiology data revealed that the direct projection includes GABAergic neurons. Direct and indirect pathways may play distinct functional roles in recruiting pain-modulating neurons.

Role of Neurotrophins in Orofacial Pain Modulation: A Review of the Latest Discoveries

Orofacial pain represents a multidisciplinary biomedical challenge involving basic and clinical research for which no satisfactory solution has been found. In this regard, trigeminal pain is described as one of the worst pains perceived, leaving the patient with no hope for the future. The aim of this review is to evaluate the latest discoveries on the involvement of neurotrophins in orofacial nociception, describing their role and expression in peripheral tissues, trigeminal ganglion, and trigeminal nucleus considering their double nature as "supporters" of the nervous system and as "promoters" of nociceptive transmission. In order to scan recent literature (last ten years), three independent researchers referred to databases PubMed, Embase, Google Scholar, Scopus, and Web of Science to find original research articles and clinical trials. The researchers selected 33 papers: 29 original research articles and 4 clinical trials. The results obtained by the screening of the selected articles show an interesting trend, in which the precise modulation of neurotrophin signaling could switch neurotrophins from being a "promoter" of pain to their beneficial neurotrophic role of supporting the nerves in their recovery, especially when a structural alteration is present, as in neuropathic pain. In conclusion, neurotrophins could be interesting targets for orofacial pain modulation but more studies are necessary to clarify their role for future application in clinical practice.

Inhibitory Effects of Honokiol on Substantia Gelatinosa Neurons of the Trigeminal Subnucleus Caudalis in Juvenile Mice

Inhibitory neurotransmitters such as gamma-aminobutyric acid (GABA) and glycine are known to be abundant in the substantia gelatinosa (SG) of the trigeminal subnucleus caudalis (Vc). Thus, it has been recognized as an initial synaptic site for regulating orofacial nociceptive stimuli. Honokiol, a principal active ingredient derived from the bark of Magnolia officinalis, has been exploited in traditional remedies with multiple biological effects, including anti-nociception on humans. However, the anti-nociceptive mechanism of honokiol on SG neurons of the Vc remains fully elusive. In this study, effects of honokiol on SG neurons of the Vc in mice were investigated using the whole-cell patch-clamp method. In a concentration-dependent manner, honokiol significantly enhanced frequencies of spontaneous postsynaptic currents (sPSCs) that were independent of action potential generation. Notably, honokiol-induced increase in the frequency of sPSCs was attributed to the release of inhibitory neurotransmitters through both glycinergic and GABAergic pre-synaptic terminals. Furthermore, higher concentration of honokiol induced inward currents that were noticeably attenuated in the presence of picrotoxin (a GABAA receptor antagonist) or strychnine (a glycine receptor antagonist). Honokiol also exhibited potentiation effect on glycine- and GABAA receptor-mediated responses. In inflammatory pain model, the increase in frequency of spontaneous firing on SG neurons induced by formalin was significantly inhibited by the application of honokiol. Altogether, these findings indicate that honokiol might directly affect SG neurons of the Vc to facilitate glycinergic and GABAergic neurotransmissions and modulate nociceptive synaptic transmission against pain. Consequently, the inhibitory effect of honokiol in the central nociceptive system contributes to orofacial pain management.

Orthodontic pain: c-Fos expression in rat brain nuclei after rapid maxillary expansion

BACKGROUND

The aim of this in vivo study was to quantitatively evaluate pain after rapid maxillary expansion (RME) in young rats by analyzing the activation of nociception-related structures, that is, the caudalis, interpolaris, and oralis subnuclei, according to the Fos expression.

METHODS

A total of 65 Wistar rats were assigned to three groups: control group (n = 15) with no treatment, positive control group (n = 25), and experimental group (n = 25) with RME. The experimental animals were euthanized at 6, 12, 24, 48, and 72 hours after RME, and the brain was later carefully collected. Coronal sections through the spinal trigeminal caudalis, spinal trigeminal interpolaris, and spinal trigeminal oralis were cut (thickness of 40 µm) on a cryostat and processed for Fos immunohistochemistry. Images from the sections were captured under light microscopy, and ImageJ software was used to count Fos-like immunoreactive neurons. The Analysis of variance (ANOVA) and Tukey test were used for statistical analysis, and the significance level was set at 5%.

RESULTS

RME induced incisor distalization and opening of the midpalatal suture, as well as neuronal activation of the spinal trigeminal nucleus. The experimental group demonstrated significantly more Fos-positive neurons in subnuclei caudalis and subnuclei interpolaris 6 hours after the maxillary expansion. The Fos immunoreactivity significantly decreased at 12 hours and increased again at 24 and 48 hours (P < 0.001).

CONCLUSIONS

The RME increases the neural activation of brain regions involved in the nociception region, as determined by the Fos expression. The most intense Fos-like immunoreactive expression was detected in the brain 6 hours after the start of the palatal expansion.

Locus coeruleus-noradrenergic modulation of trigeminal pain: Implications for trigeminal neuralgia and psychiatric comorbidities

Trigeminal neuralgia is the most common neuropathic pain involving the craniofacial region. Due to the complex pathophysiology, it is therapeutically difficult to manage. Noradrenaline plays an essential role in the modulation of arousal, attention, cognitive function, stress, and pain. The locus coeruleus, the largest source of noradrenaline in the brain, is involved in the sensory and emotional processing of pain. This review summarizes the knowledge about the involvement of noradrenaline in acute and chronic trigeminal pain conditions and how the activity of the locus coeruleus noradrenergic neurons changes in response to acute and chronic pain conditions and how these changes might be involved in pain-related comorbidities including anxiety, depression, and sleep disturbance.

NMDARs mediate peripheral and central sensitization contributing to chronic orofacial pain

Peripheral and central sensitizations of the trigeminal nervous system are the main mechanisms to promote the development and maintenance of chronic orofacial pain characterized by allodynia, hyperalgesia, and ectopic pain after trigeminal nerve injury or inflammation. Although the pathomechanisms of chronic orofacial pain are complex and not well known, sufficient clinical and preclinical evidence supports the contribution of the N-methyl-D-aspartate receptors (NMDARs, a subclass of ionotropic glutamate receptors) to the trigeminal nociceptive signal processing pathway under various pathological conditions. NMDARs not only have been implicated as a potential mediator of pain-related neuroplasticity in the peripheral nervous system (PNS) but also mediate excitatory synaptic transmission and synaptic plasticity in the central nervous system (CNS). In this review, we focus on the pivotal roles and mechanisms of NMDARs in the trigeminal nervous system under orofacial neuropathic and inflammatory pain. In particular, we summarize the types, components, and distribution of NMDARs in the trigeminal nervous system. Besides, we discuss the regulatory roles of neuron-nonneuronal cell/neuron-neuron communication mediated by NMDARs in the peripheral mechanisms of chronic orofacial pain following neuropathic injury and inflammation. Furthermore, we review the functional roles and mechanisms of NMDARs in the ascending and descending circuits under orofacial neuropathic and inflammatory pain conditions, which contribute to the central sensitization. These findings are not only relevant to understanding the underlying mechanisms, but also shed new light on the targeted therapy of chronic orofacial pain.

Neuronal Activities in the Rostral Ventromedial Medulla Associated with Experimental Occlusal Interference-Induced Orofacial Hyperalgesia

The imbalanced conditions of pronociceptive ON-cells and antinociceptive OFF-cells in the rostral ventromedial medulla (RVM) alter nociceptive transmission and play an important role in the development of chronic pain. This study aimed to explore the neuroplastic mechanisms of the RVM ON-cells and OFF-cells in a male rat model of experimental occlusal interference (EOI)-induced nociceptive behavior reflecting orofacial hyperalgesia and in modified models involving EOI removal at early and later stages. We recorded the mechanical head withdrawal thresholds, orofacial operant behaviors, and the activity of identified RVM ON-cells and OFF-cells in these rats. EOI-induced orofacial hyperalgesia could be relieved by EOI removal around postoperative day 3; this effect could be inhibited by intra-RVM microinjection of the κ-opioid receptor agonist U-69593. EOI removal around postoperative day 8 did not relieve the orofacial hyperalgesia, which could, however, be reversed by intra-RVM microinjection of the NK-1 (neurokinin-1) receptor antagonist L-733060. The activity of ON-cells and OFF-cells did not change during both the initial 3 and 6 d of EOI. When EOI was removed on postoperative day 3, OFF-cell responses decreased, contributing to the reversal of hyperalgesia. When EOI lasted for 8 d or was removed on postoperative day 8, spontaneous activity and stimulus-evoked responses of ON-cell increased, contributing to the maintained hyperalgesia. In contrast, when the EOI lasted for 14 d, OFF-cell responses decreased, possibly participating in the maintenance of hyperalgesia with persistent EOI. Our results reveal that adaptive changes in the RVM were associated with orofacial pain following EOI placement and removal.SIGNIFICANCE STATEMENT A considerable proportion of patients experience chronic orofacial pain throughout life despite the therapies given or removal of potential etiologic factors. However, current therapies lack effectiveness because of limited knowledge of the chronicity mechanisms. Using electrophysiological recording, combined with a behavioral test, we found that the prevailing descending facilitation in the rostral ventromedial medulla (RVM) participates in the maintenance of orofacial hyperalgesia following late removal of nociceptive stimuli, while the prevailing descending inhibition from the RVM may contribute to the reversal of orofacial hyperalgesia following early removal of nociceptive stimuli. Thus, variable clinical outcomes of orofacial pain may be associated with descending modulation, and an optimal window of time may exist in the management of chronic orofacial pain.

Molecular Segmentation of the Spinal Trigeminal Nucleus in the Adult Mouse Brain

The trigeminal column is a hindbrain structure formed by second order sensory neurons that receive afferences from trigeminal primary (ganglionic) nerve fibers. Classical studies subdivide it into the principal sensory trigeminal nucleus located next to the pontine nerve root, and the spinal trigeminal nucleus which in turn consists of oral, interpolar and caudal subnuclei. On the other hand, according to the prosomeric model, this column would be subdivided into segmental units derived from respective rhombomeres. Experimental studies have mapped the principal sensory trigeminal nucleus to pontine rhombomeres (r) r2-r3 in the mouse. The spinal trigeminal nucleus emerges as a plurisegmental formation covering several rhombomeres (r4 to r11 in mice) across pontine, retropontine and medullary hindbrain regions. In the present work we reexamined the issue of rhombomeric vs. classical subdivisions of this column. To this end, we analyzed its subdivisions in an AZIN2-lacZ transgenic mouse, known as a reference model for hindbrain topography, together with transgenic reporter lines for trigeminal fibers. We screened as well for genes differentially expressed along the axial dimension of this structure in the adult and juvenile mouse brain. This analysis yielded genes from multiple functional families that display transverse domains fitting the mentioned rhombomeric map. The spinal trigeminal nucleus thus represents a plurisegmental structure with a series of distinct neuromeric units having unique combinatorial molecular profiles.

Acute Pain Management

Control of acute pain in oral and maxillofacial surgery is important for patient care and comfort. Oral surgical procedures are associated with tissue injury and inflammation. Acute pain can arise directly from a surgical procedure or from problems such as dental caries, infection, perforation of maxillary sinus, pericoronitis, and jaw fractures. The major factor in acute pain management is deciding on an appropriate intervention and/or analgesics that will provide the best pain relief. Multimodal pain control has taken a leading role in effectively managing acute pain. This article covers the different options available to dental clinicians.

Brainstem Pain-Modulation Circuitry and Its Plasticity in Neuropathic Pain: Insights From Human Brain Imaging Investigations

Acute pain serves as a protective mechanism that alerts us to potential tissue damage and drives a behavioural response that removes us from danger. The neural circuitry critical for mounting this behavioural response is situated within the brainstem and is also crucial for producing analgesic and hyperalgesic responses. In particular, the periaqueductal grey, rostral ventromedial medulla, locus coeruleus and subnucleus reticularis dorsalis are important structures that directly or indirectly modulate nociceptive transmission at the primary nociceptive synapse. Substantial evidence from experimental animal studies suggests that plasticity within this system contributes to the initiation and/or maintenance of chronic neuropathic pain, and may even predispose individuals to developing chronic pain. Indeed, overwhelming evidence indicates that plasticity within this circuitry favours pro-nociception at the primary synapse in neuropathic pain conditions, a process that ultimately contributes to a hyperalgesic state. Although experimental animal investigations have been crucial in our understanding of the anatomy and function of the brainstem pain-modulation circuitry, it is vital to understand this system in acute and chronic pain states in humans so that more effective treatments can be developed. Recent functional MRI studies have identified a key role of this system during various analgesic and hyperalgesic responses including placebo analgesia, offset analgesia, attentional analgesia, conditioned pain modulation, central sensitisation and temporal summation. Moreover, recent MRI investigations have begun to explore brainstem pain-modulation circuitry plasticity in chronic neuropathic pain conditions and have identified altered grey matter volumes and functioning throughout the circuitry. Considering the findings from animal investigations, it is likely that these changes reflect a shift towards pro-nociception that ultimately contributes to the maintenance of neuropathic pain. The purpose of this review is to provide an overview of the human brain imaging investigations that have improved our understanding of the pain-modulation system in acute pain states and in neuropathic conditions. Our interpretation of the findings from these studies is often guided by the existing body of experimental animal literature, in addition to evidence from psychophysical investigations. Overall, understanding the plasticity of this system in human neuropathic pain conditions alongside the existing experimental animal literature will ultimately improve treatment options.

GABA- and Glycine-Mimetic Responses of Linalool on the Substantia Gelatinosa of the Trigeminal Subnucleus Caudalis in Juvenile Mice: Pain Management through Linalool-Mediated Inhibitory Neurotransmission

Linalool, a major odorous constituent in essential oils extracted from lavender, is known to have a wide range of physiological effects on humans including pain management. The substantia gelatinosa (SG) of the trigeminal subnucleus caudalis (Vc) is involved in transmission of orofacial nociceptive responses through thin myelinated A[Formula: see text] and unmyelinated C primary afferent fibers. Up to date, the orofacial antinociceptive mechanism of linalool concerning SG neurons of the Vc has not been completely clarified yet. To fill this knowledge gap, whole-cell patch-clamp technique was used in this study to examine how linalool acted on SG neurons of the Vc in mice. Under a high chloride pipette solution, non-desensitizing and repeatable linalool-induced inward currents were preserved in the presence of tetrodotoxin (a voltage-gated Na[Formula: see text]channel blocker), CNQX (a non-NMDA glutamate receptor antagonist), and DL-AP5 (an NMDA receptor antagonist). However, linalool-induced inward currents were partially suppressed by picrotoxin (a GABA[Formula: see text] receptor antagonist) or strychnine (a glycine receptor antagonist). These responses were almost blocked in the presence of picrotoxin and strychnine. It was also found that linalool exhibited potentiation with GABA- and glycine-induced responses. Taken together, these data show that linalool has GABA- and glycine-mimetic effects, suggesting that it can be a promising target molecule for orofacial pain management by activating inhibitory neurotransmission in the SG area of the Vc.

Brain-Derived Neurotrophic Factor Systemic Response in the Periodontium, Trigeminal Nucleus Caudalis, and Hippocampus Induced by Occlusal Trauma

Occlusal trauma (OT), by causing periodontal tissue damage, can activate and enhance the activity of the peripheral and central nervous system (CNS) neuropeptides. The brain-derived neurotrophic factor (BDNF) gene is activity-dependent and exhibits marked alterations, characterized by protection against injury and repair. Our results show the possible molecular mechanism through which noxious environmental stimuli induce alterations in BDNF activity in the local periodontal tissue, the primary sensory neurons-Vc, and the hippocampus, suggesting systemic impairment. BDNF serves a more positive and enduring trauma protection and repair function in Vc compared to that in local dental tissue.

Glia and Orofacial Pain: Progress and Future Directions

Orofacial pain is a universal predicament, afflicting millions of individuals worldwide. Research on the molecular mechanisms of orofacial pain has predominately focused on the role of neurons underlying nociception. However, aside from neural mechanisms, non-neuronal cells, such as Schwann cells and satellite ganglion cells in the peripheral nervous system, and microglia and astrocytes in the central nervous system, are important players in both peripheral and central processing of pain in the orofacial region. This review highlights recent molecular and cellular findings of the glia involvement and glia–neuron interactions in four common orofacial pain conditions such as headache, dental pulp injury, temporomandibular joint dysfunction/inflammation, and head and neck cancer. We will discuss the remaining questions and future directions on glial involvement in these four orofacial pain conditions.

Microglial activation in the trigeminal spinal subnucleus interpolaris/caudalis modulates orofacial incisional mechanical pain hypersensitivity associated with orofacial injury in infancy

PURPOSE

Infantile tissue injury induces sensory deficits in adulthood. Infantile facial incision (IFI) was reported to cause an enhancement of incision-induced mechanical hypersensitivity in adulthood due to acceleration of the trigeminal ganglion neuronal excitability. However, the effects of IFI on activation of microglia in the spinal trigeminal nucleus and its involvement in facial pain sensitivity is not well known.

METHODS

A facial skin incision was made in the left whisker pad in infant (IFI) and/or adult rats (AFI). Mechanical head withdrawal threshold and microglial activation in the trigeminal spinal nucleus were analyzed.

RESULTS

Mechanical pain hypersensitivity induced by AFI was significantly exacerbated and prolonged by IFI. The number of Iba1-immunoreactive cells in the trigeminal spinal nucleus following AFI was increased by IFI, suggesting that IFI facilitates microglial hyperactivation following AFI. Intraperitoneal administration of minocycline, a microglial activation inhibitor, suppressed the facial incision-induced microglial hyperactivation in the trigeminal spinal nucleus and the exacerbation of the facial mechanical pain hypersensitivity induced by IFI.

CONCLUSION

These results suggest that facial trauma in infants causes hyperactivation of microglia in the trigeminal spinal nucleus following AFI, leading to the prolongation of the facial mechanical pain hypersensitivity.

Altered Brainstem Pain-Modulation Circuitry Connectivity During Spontaneous Pain Intensity Fluctuations

Background

Chronic pain, particularly that following nerve injury, can occur in the absence of external stimuli. Although the ongoing pain is sometimes continuous, in many individuals the intensity of their pain fluctuates. Experimental animal studies have shown that the brainstem contains circuits that modulate nociceptive information at the primary afferent synapse and these circuits are involved in maintaining ongoing continuous neuropathic pain. However, it remains unknown if these circuits are involved in regulating fluctuations of ongoing neuropathic pain in humans.

Methods

We used functional magnetic resonance imaging to determine whether in 19 subjects with painful trigeminal neuropathy, brainstem pain-modulation circuitry function changes according to moment-to-moment fluctuations in spontaneous pain intensity as rated online over a 12-minute period.

Results

We found that when pain intensity was spontaneously high, connectivity strengths between regions of the brainstem endogenous pain-modulating circuitry-the midbrain periaqueductal gray, rostral ventromedial medulla (RVM), and the spinal trigeminal nucleus (SpV)-were high, and vice-versa (when pain was low, connectivity was low). Additionally, sliding-window connectivity analysis using 50-second windows revealed a significant positive relationship between ongoing pain intensity and RVM-SpV connectivity over the duration of the 12-minute scan.

Conclusion

These data reveal that moment-to-moment changes in brainstem pain-modulation circuitry functioning likely contribute to fluctuations in spontaneous pain intensity in individuals with chronic neuropathic pain.

Altered Brainstem Pain Modulating Circuitry Functional Connectivity in Chronic Painful Temporomandibular Disorder

There is evidence from preclinical models of chronic pain and human psychophysical investigations to suggest that alterations in endogenous brainstem pain-modulation circuit functioning are critical for the initiation and/or maintenance of pain. Whilst preclinical models have begun to explore the functioning of this circuitry in chronic pain, little is known about such functioning in humans with chronic pain. The aim of this investigation was to determine whether individuals with chronic non-neuropathic pain, painful temporomandibular disorders (TMD), display alterations in brainstem pain-modulating circuits. Using resting-state functional magnetic resonance imaging, we performed static and dynamic functional connectivity (FC) analyses to assess ongoing circuit function in 16 TMD and 45 control subjects. We calculated static FC as the correlation of functional magnetic resonance imaging signals between regions over the entire scan and dynamic FC as the correlation of signals in short (50s) windows. Compared with controls, TMD subjects showed significantly greater (static) FC between the rostral ventromedial medulla and both the subnucleus reticularis dorsalis and the region that receives orofacial nociceptive afferents, the spinal trigeminal nucleus. No differences were found in other brainstem pain-modulating regions such as the midbrain periaqueductal gray matter and locus coeruleus. We also identified that TMD subjects experience greater variability in the dynamic functional connections between the rostral ventromedial medulla and both the subnucleus reticularis dorsalis and spinal trigeminal nucleus. These changes may underlie enhanced descending pain-facilitating actions over the region that receives nociceptive afferents, ultimately leading to enhanced nociceptive transmission to higher brain regions and thus contributing to the ongoing perception of pain. PERSPECTIVE: Psychophysical studies suggest that brainstem pain-modulation circuits contribute to the maintenance of chronic pain. We report that individuals with painful TMD display altered static and dynamic FC within the brainstem pain-modulation network. Modifying this circuitry may alter an individual's ongoing pain.

Alternating Hemiplegia of Childhood: gastrointestinal manifestations and correlation with neurological impairments

Background

Alternating Hemiplegia of Childhood (AHC) is caused by mutations of the ATP1A3 gene which is expressed in brain areas that include structures controling autonomic, gastrointestinal, gut motility and GABAergic functions. We aimed to investigate, in a cohort of 44 consecutive AHC patients, two hypotheses: 1) AHC patients frequently manifest gastrointestinal, particularly motility, problems. 2) These problems are often severe and their severity correlates with neurological impairments.

Results

41/44 (93%) exhibited gastrointestinal symptoms requiring medical attention. For these 41 patients, symptoms included constipation (66%), swallowing problems (63%), vomiting (63%), anorexia (46%), diarrhea (44%), nausea (37%), and abdominal pain (22%). Symptoms indicative of dysmotility occurred in 33 (80%). The most common diagnoses were oropharyngeal dysphagia (63%) and gastroesophageal reflux (63%). 16 (39%) required gastrostomy and two fundoplication. Severity of gastrointestinal symptoms correlated with non-paroxysmal neurological disability index, Gross Motor Function Classification System scores, and with the presence/absence of non-gastrointestinal autonomic dysfunction (p = 0.031, 0.043, Spearman correlations and 0.0166 Cramer’s V, respectively) but not with the paroxysmal disability index (p = 0.408).

Conclusions

Most AHC patients have gastrointestinal problems. These are usually severe, most commonly are indicative of dysmotility, often require surgical therapies, and their severity correlates with that of non-paroxysmal CNS manifestations. Our findings should help in management-anticipatory guidance of AHC patients. Furthermore, they are consistent with current understandings of the pathophysiology of AHC and of gastrointestinal dysmotility, both of which involve autonomic and GABAergic dysfunction.

Inhibitory actions of borneol on the substantia gelatinosa neurons of the trigeminal subnucleus caudalis in mice

The substantia gelatinosa (SG) of the trigeminal subnucleus caudalis (Vc) is the first relay site for the orofacial nociceptive inputs via the thin myelinated Aδ and unmyelinated C primary afferent fibers. Borneol, one of the valuable timehonored herbal ingredients in traditional Chinese medicine, is a popular treatment for anxiety, anesthesia, and antinociception. However, to date, little is known as to how borneol acts on the SG neurons of the Vc. To close this gap, the whole-cell patch-clamp technique was applied to elucidate the antinociceptive mechanism responding for the actions of borneol on the SG neurons of the Vc in mice. In the voltage-clamp mode, holding at -60 mV, the borneol-induced non-desensitizing inward currents were not affected by tetrodotoxin, a voltage-gated Na⁺ channel blocker, 6-cyano-7-nitro-quinoxaline-2,3-dione, a non-N-methyl-D-aspartate (NMDA) glutamate receptor antagonist and DL-2-amino-5-phosphonopentanoic acid, an NMDA receptor antagonist. However, borneol-induced inward currents were partially decreased in the presence of picrotoxin, a γ-aminobutyric acid (GABA)_A receptor antagonist, or strychnine, a glycine receptor antagonist, and was almost suppressed in the presence of picrotoxin and strychnine. Though borneol did not show any effect on the glycine-induced inward currents, borneol enhanced GABA-mediated responses. Beside, borneol enhanced the GABA-induced hyperpolarization under the current-clamp mode. Altogether, we suggest that borneol contributes in part toward mediating the inhibitory GABA and glycine transmission on the SG neurons of the Vc and may serve as an herbal therapeutic for orofacial pain ailments.

Involvement of astrocytes activation in orofacial hyperalgesia induced by experimental tooth movement

OBJECTIVE

The study aimed to investigate the involvement of astrocytes in the medullary dorsal horn (MDH) in the orofacial hyperalgesia induced by experimental tooth movement (ETM) and related mechanism.

MATERIALS AND METHODS

Experimental tooth movement was produced with nickel-titanium alloy closed-coil spring fixed between the left maxillary first molar and the left upper incisor. Fluorocitrate was administrated through medullary subarachnoid at 3 days after ETM. Pressure pain threshold (PPT) in masseter cutaneous area was measured. The expression of glial fibrillary acidic protein (GFAP) and c-Fos in MDH was measured using immunofluoroscence staining. The expression of interleukin-1β (IL-1β) and phosphorylated N-methyl-D-aspartic acid (NMDA) receptor subunit NR1 (p-NR1) was measured with Western blotting.

RESULTS

Experimental tooth movement-induced orofacial hyperalgesia from 1 to 9 days as the PPT was significantly reduced (P < .05). Immunofluoroscence staining showed that the expression of c-Fos in MDH was dramatically upregulated at 1 day and 3 days after ETM, while GFAP expression with both immunofluoroscence staining and Western blotting was significantly enhanced at 3 days and 7 days after ETM. Western blotting analysis indicated that the expression of IL-1β and p-NR1 in MDH was significantly enhanced at 3 days after ETM. Furthermore, we found that fluorocitrate administration at 3 days after ETM could markedly suppress the expression of c-Fos, GFAP, IL-1β and p-NR1 and attenuate the reduction of PPT induced by ETM.

CONCLUSION

Astrocyte activation in MDH is involved in the mechanical hyperalgesia, and the subsequent upregulated IL-1β and overexpression of p-NR1 may participate in this process.

The Evolution of Neuroscience as a Research Field Relevant to Dentistry

The field of neuroscience did not exist as such when the Journal of Dental Research was founded 100 y ago. It has emerged as an important scientific field relevant to dentistry in view of the many neurally based functions manifested in the orofacial area (e.g., pain, taste, chewing, swallowing, salivation). This article reviews many of the novel insights that have been gained through neuroscience research into the neural basis of these functions and their clinical relevance to the diagnosis and management of pain and sensorimotor disorders. These include the neural pathways and brain circuitry underlying each of these functions and the role of nonneural as well as neural processes and their "plasticity" in modulating these functions and allowing for adaptation to tissue injury and pain and for learning or rehabilitation of orofacial functions.

Differential activation of ascending noxious pathways associated with trigeminal nerve injury

Trigeminal spinal subnucleus caudalis (Vc) neurons that project to the ventral posteromedial thalamic nucleus (VPM) and parabrachial nucleus (PBN) are critical for orofacial pain processing. We hypothesized that persistent trigeminal nerve injury differentially alters the proportion of Vc neurons that project to VPM and PBN in a modality-specific manner. Neuroanatomical approaches were used to quantify the number of Vc neurons projecting to VPM or PBN after chronic constriction injury of the infraorbital nerve (ION-CCI) and subsequent upper-lip stimulation. Male rats received injections of retrograde tracer fluorogold into the contralateral VPM or PBN on day 7 after ION-CCI, and at 3 days after that, either capsaicin injection or noxious mechanical stimulation was applied to the upper lip ipsilateral to nerve injury. Infraorbital nerve chronic constriction injury rats displayed greater forelimb wiping to capsaicin injection and mechanical allodynia of the lip than sham rats. Total cell counts for phosphorylated extracellular signal-regulated kinase-immunoreactive (pERK-IR) neurons after capsaicin or mechanical lip stimuli were higher in ION-CCI than sham rats as was the percentage of pERK-IR PBN projection neurons. However, the percentage of pERK-IR VPM projection neurons was also greater in ION-CCI than sham rats after capsaicin but not mechanical lip stimuli. The present findings suggest that persistent trigeminal nerve injury increases the number of Vc neurons activated by capsaicin or mechanical lip stimuli. By contrast, trigeminal nerve injury modifies the proportion of Vc nociceptive neurons projecting to VPM and PBN in a stimulus modality-specific manner and may reflect differential involvement of ascending pain pathways receiving C fiber and mechanosensitive afferents.

Neural Pathways of Craniofacial Muscle Pain: Implications for Novel Treatments

Craniofacial muscle pain is highly prevalent in temporomandibular disorders but is difficult to treat. Enhanced understanding of neurobiology unique to craniofacial muscle pain should lead to the development of novel mechanism-based treatments. Herein, we review recent studies to summarize neural pathways of craniofacial muscle pain. Nociceptive afferents in craniofacial muscles are predominantly peptidergic afferents enriched with TRPV1. Signals from peripheral glutamate receptors converge onto TRPV1, leading to mechanical hyperalgesia. Further studies are needed to clarify whether hyperalgesic priming in nonpeptidergic afferents or repeated acid injections also affect craniofacial muscle pain. Within trigeminal ganglia, afferents innervating craniofacial muscles interact with surrounding satellite glia, which enhances the sensitivity of the inflamed neurons as well as nearby uninjured afferents, resulting in hyperalgesia and ectopic pain originating from adjacent orofacial tissues. Craniofacial muscle afferents project to a wide area within the trigeminal nucleus complex, and central sensitization of medullary dorsal horn neurons is a critical factor in muscle hyperalgesia related to ectopic pain and emotional stress. Second-order neurons project rostrally to pathways associated with affective pain, such as parabrachial nucleus and medial thalamic nucleus, as well as sensory-discriminative pain, such as ventral posteromedial thalamic nuclei. Abnormal endogenous pain modulation can also contribute to chronic muscle pain. Descending serotonergic circuits from the rostral ventromedial medulla facilitate activation of second-order neurons in the trigeminal nucleus complex, which leads to the maintenance of mechanical hyperalgesia of inflamed masseter muscle. Patients with temporomandibular disorders exhibit altered brain networks in widespread cortical and subcortical regions. Recent development of methods for neural circuit manipulation allows silencing of specific hyperactive neural circuits. Chemogenetic silencing of TRPV1-expressing afferents or rostral ventromedial medulla neurons attenuates hyperalgesia during masseter inflammation. It is likely, therefore, that further delineation of neural circuits mediating craniofacial muscle hyperalgesia potentially enhances treatment of chronic muscle pain conditions.

Photobiomodulation and B vitamins administration produces antinociception in an orofacial pain model through the modulation of glial cells and cytokines expression

Chronic constriction injury (CCI) of infraorbital nerve (IoN) results in whisker pad mechanical allodynia in rats and activation glial cells contributing to the development of orofacial pain. Whisker pad mechanical allodynia (von Frey stimuli) was tested pre and postoperatively and conducted during the treatment time. Photobiomodulation (PBM) and vitamins B complex (VBC) has been demonstrated therapeutic efficacy in ameliorate neuropathic pain. The aim of this study was to evaluate the antinociceptive effect of PBM, VBC or the combined treatment VBC ​+ ​PBM on orofacial pain due to CCI-IoN. Behavioral and molecular approaches were used to analyses nociception, cellular and neurochemical alterations. CCI-IoN caused mechanical allodynia and cellular alterations including increased expression of glial fibrillary acid protein (GFAP) and ionized calcium binding adaptor molecule 1 (Iba-1), administration of VBC (B1/B6/B12 at 180/180/1.8 ​mg/kg, s.c., 5 times all long 10 sessions) and PBM therapy (904 ​nm, power of 75Wpico, average power of 0.0434 ​W, pulse frequency of 9500 ​Hz, area of the beam 0.13 ​cm², 18 ​s duration, energy density 6 ​J/cm², with an energy per point of 0.78 ​J for 10 sessions) or their combination presented improvement of the nociceptive behavior and decreased expression of GFAP and Iba-1. Additionally, CCI-IoN rats exhibited an upregulation of IL1β, IL6 and TNF-α expression and all treatments prevented this upregulation and also increased IL10 expression. Overall, the present results highlight the pain reliever effect of VBC or PBM alone or in combination, through the modulation of glial cells and cytokines expression in the spinal trigeminal nucleus of rats.

Peripheral and Central Mechanisms of Persistent Orofacial Pain

Neuroplastic changes in the neuronal networks involving the trigeminal ganglion (TG), trigeminal spinal subnucleus caudalis (Vc), and upper cervical spinal cord (C1/C2) are considered the mechanisms underlying the ectopic orofacial hypersensitivity associated with trigeminal nerve injury or orofacial inflammation. It has been reported that peripheral nerve injury causes injury discharges in the TG neurons, and a barrage of action potentials is generated in TG neurons and conveyed to the Vc and C1/C2 after trigeminal nerve injury. Long after trigeminal nerve injury, various molecules are produced in the TG neurons, and these molecules are released from the soma of TG neurons and are transported to the central and peripheral terminals of TG neurons. These changes within the TG cause neuroplastic changes in TG neurons and they become sensitized. The neuronal activity of TG neurons is further accelerated, and Vc and C1/C2 neurons are also sensitized. In addition to this cascade, non-neuronal glial cells are also involved in the enhancement of the neuronal activity of TG, Vc, and C1/C2 neurons. Satellite glial cells and macrophages are activated in the TG after trigeminal nerve injury and orofacial inflammation. Microglial cells and astrocytes are also activated in the Vc and C1/C2 regions. It is considered that functional interaction between non-neuronal cells and neurons in the TG, Vc, and C1/C2 regions is a key mechanism involved in the enhancement of neuronal excitability after nerve injury or inflammation. In this article, the detailed mechanisms underlying ectopic orofacial hyperalgesia associated with trigeminal nerve injury and orofacial inflammation are addressed.

TRPM8 Mediates Hyperosmotic Stimuli-Induced Nociception in Dental Afferents

Hyperosmolar sweet foods onto exposed tooth dentin evoke sudden and intense dental pain, called dentin hypersensitivity. However, it remains unclear how hyperosmolar stimuli excite dental primary afferent (DPA) neurons and thereby lead to dentin hypersensitivity. This study elucidated whether TRPM8, which is well known as a cold temperature- or menthol-activated receptor, additionally mediates nociception in response to hyperosmolar stimuli in adult mouse DPA neurons, which are identified by a fluorescent retrograde tracer: DiI. Single-cell reverse transcription polymerase chain reaction revealed that TRPM8 was expressed in subsets of DPA neurons and that TRPM8 was highly colocalized with TRPV1 and Piezo2. Immunohistochemical analysis also confirmed TRPM8 expression in DPA neurons. By using Fura-2-based calcium imaging, application of hyperosmolar sucrose solutions elicited calcium transients in subsets of the trigeminal ganglion neurons, which was significantly abolished by a selective TRPM8 antagonist: N-(3-Aminopropyl)-2-[(3-methylphenyl)methoxy]-N-(2-thienylmethyl)benzamide (AMTB) hydrochloride. When we further examined changes of c-fos expression (a neuronal activation marker) in the spinal trigeminal nucleus after hyperosmolar stimulation onto exposed tooth dentin, c-fos mRNA and protein expression were increased and were also significantly reduced by AMTB, especially in the spinal trigeminal interpolaris-caudalis transition zone (Vi/Vc). Taken together, our results provide strong evidence that TRPM8 expressed in DPA neurons might mediate dental pain as a hyperosmosensor in adult mice.

Orofacial Movements Involve Parallel Corticobulbar Projections from Motor Cortex to Trigeminal Premotor Nuclei

How do neurons in orofacial motor cortex (MCtx) orchestrate behaviors? We show that focal activation of MCtx corticobulbar neurons evokes behaviorally relevant concurrent movements of the forelimb, jaw, nose, and vibrissae. The projections from different locations in MCtx form gradients of boutons across premotor nuclei spinal trigeminal pars oralis (SpVO) and interpolaris rostralis (SpVIr). Furthermore, retrograde viral tracing from muscles that control orofacial actions shows that these premotor nuclei segregate their outputs. In the most dramatic case, both SpVO and SpVIr are premotor to forelimb and vibrissa muscles, while only SpVO is premotor to jaw muscles. Functional confirmation of the superimposed control by MCtx was obtained through selective optogenetic activation of corticobulbar neurons on the basis of their preferential projections to SpVO versus SpVIr. We conclude that neighboring projection neurons in orofacial MCtx form parallel pathways to distinct pools of trigeminal premotor neurons that coordinate motor actions into a behavior.

Examining the relationship and clinical management between traumatic brain injury and pain in military and civilian populations

In this review, we discuss the comorbidity of traumatic brain injury (TBI) and pain among civilians and military members, the common causes of pain resulting from TBI, and offer insight about the therapeutic management of TBI symptoms and pain. Traumatic brain injury (TBI) is a debilitating health problem and one of the most common post-TBI symptoms is pain, which can contribute to psychological issues such as Post-traumatic stress disorder (PTSD) and depression. Headache pain appears to be the most common type of pain that results from TBI, yet pain can also be more widespread. Managing TBI symptoms and pain simultaneously is difficult because extensive randomized control and clinical studies assessing the effectiveness of therapeutic approaches are lacking. Pharmacological agents such as antidepressants and Triptans and nonpharmacological therapies such as cognitive rehabilitation and physical therapies are commonly used yet it is unknown how effective these therapies are in the long-term. A combination of pharmacological and non-pharmacological therapies is often more effective for managing TBI symptoms and pain than either treatment alone. However, future research is needed to determine the most therapeutic approaches for managing the comorbidity of pain and TBI symptoms in the long term. This review offers suggestions for such future studies.

Role of medullary astroglial glutamine synthesis in tooth pulp hypersensitivity associated with frequent masseter muscle contraction

Background

The mechanisms underlying tooth pulp hypersensitivity associated with masseter muscle hyperalgesia remain largely underinvestigated. In the present study, we aimed to determine whether masseter muscle contraction induced by daily electrical stimulation influences the mechanical head-withdrawal threshold and genioglossus electromyography activity caused by the application of capsaicin to the upper first molar tooth pulp. We further investigated whether astroglial glutamine synthesis is involved in first molar tooth pulp hypersensitivity associated with masseter muscle contraction.

Methods

The first molar tooth pulp was treated with capsaicin or vehicle in masseter muscle contraction or sham rats, following which the astroglial glutamine synthetase inhibitor methionine sulfoximine or Phosphate buffered saline (PBS) was applied. Astroglial activation was assessed via immunohistochemistry.

Results

The mechanical head-withdrawal threshold of the ipsilateral masseter muscle was significantly decreased in masseter muscle contraction rats than in sham rats. Genioglossus electromyography activity was significantly higher in masseter muscle contraction rats than sham rats. Glial fibrillary acidic protein-immunoreactive cell density was significantly higher in masseter muscle contraction rats than in sham rats. Administration of methionine sulfoximine induced no significant changes in the density of glial fibrillary acidic protein-immunoreactive cells relative to PBS treatment. However, mechanical head-withdrawal threshold was significantly higher in masseter muscle contraction rats than PBS-treated rats after methionine sulfoximine administration. Genioglossus electromyography activity following first molar tooth pulp capsaicin treatment was significantly lower in methionine sulfoximine-treated rats than in PBS-treated rats. In the ipsilateral region, the total number of phosphorylated extracellular signal-regulated protein kinase immunoreactive cells in the medullary dorsal horn was significantly smaller upon first molar tooth pulp capsaicin application in methionine sulfoximine-treated rats than in PBS-treated rats.

Conclusions

Our results suggest that masseter muscle contraction induces astroglial activation, and that this activation spreads from caudal to the obex in the medullary dorsal horn, resulting in enhanced neuronal excitability associated with astroglial glutamine synthesis in medullary dorsal horn neurons receiving inputs from the tooth pulp. These findings provide significant insight into the mechanisms underlying tooth pulp hypersensitivity associated with masseter muscle contraction.

Dural stimulation in rats causes brain-derived neurotrophic factor-dependent priming to subthreshold stimuli including a migraine trigger

Migraine is one of the most common and most disabling disorders. Between attacks, migraine patients are otherwise normal but are sensitized to nonnoxious events known as triggers. The purpose of these studies was to investigate whether a headache-like event causes sensitization, or priming, to subsequent subthreshold events. Interleukin-6 (IL-6) was applied to the rat cranial dura mater which produced cutaneous facial and hind paw allodynia that lasted 24 hours. At 72 hours, IL-6-treated rats developed allodynia in response to dural stimulation with either a pH 6.8 or pH 7.0 solution and to a systemic nitric oxide (NO) donor, a well-known migraine trigger. Vehicle-treated rats did not respond to either pH stimulus or to the NO donor, demonstrating that IL-6 exposure primes rats to subthreshold stimuli. Inhibitors of brain-derived neurotrophic factor (BDNF) signaling given either systemically or intracisternally 24 hours after IL-6 eliminated responses to dural pH stimulation at 72 hours. Additionally, intracisternal administration of BDNF without previous dural stimulation produced allodynia and once resolved, animals were primed to dural pH 6.8/pH 7.0 and a systemic NO donor. Finally, hind paw IL-6 produced paw allodynia but not priming to paw injection of pH 7.0 at 72 hours demonstrating differences in priming depending on location. These data indicate that afferent input from the meninges produces BDNF-dependent priming of the dural nociceptive system. This primed state mimics the interictal period of migraine where attacks can be triggered by normally nonnoxious events and suggests that BDNF-dependent plasticity may contribute to migraine.

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.

Late-onset jaw and teeth pain mimicking trigeminal neuralgia associated with chronic vagal nerve stimulation: case series and review of the literature

BACKGROUND

Vagal nerve stimulation (VNS) for refractory epilepsy is well established. Trigeminal neuralgia itself is a common disease in adults, and thus, late-onset pain in the trigeminal region under VNS, which is extremely rare, may not be recognized as caused by VNS.

CASE PRESENTATION

Two patients with drug-resistant symptomatic epilepsy treated with chronic VNS experienced stimulation-related pain in the lower and upper jaw and teeth on the side of stimulation. No evidence of local spread of the stimulation current was present. The pain started with a delay of years after device implantation and weeks after the last increase in the pacing parameters. At the time of onset, the pain was not recognized as VNS-related, leading to extensive examinations. The trigeminal neuralgia-like pain resolved after adjustment of the stimulation current intensity. In one of the patients, the pain disappeared within one to two days following every epileptic seizure. To our knowledge, this is the first case report of late-onset trigeminal pain under VNS revealing a direct link between epileptogenic and pain processes.

CONCLUSION

A painless interval between the last change of the pacing parameters and trigeminal pain can lead to the erroneous interpretation that this is a typical trigeminal neuralgia. The lack of its recognition as a side effect of VNS can lead to unnecessary examinations and delayed adjustment of stimulation parameters. In patients with signs of late-onset trigeminal pain under VNS with normal electrode impedance and no evidence of local current spread, the replacement of the VNS lead does not seem to be beneficial. A review of the literature on VNS side effects including pain and device malfunctions was undertaken.

Mechanisms of craniofacial pain

Aim To provide an overview of mechanisms underlying craniofacial pain; to highlight peripheral and central adaptations that may promote chronification of pain in craniofacial pain states such as migraine and temporomandibular disorders (TMD). Background Pain is a common symptom associated with disorders involving craniofacial tissues including the teeth and their supporting structures, the temporomandibular joint and the muscles of the head. Most acute painful craniofacial conditions are easily recognized and well managed, but others, especially those that are chronic (e.g., migraine, TMD and trigeminal neuropathies), present clinical challenges. Preclinical studies have provided substantial information about the anatomical and physiological mechanisms related to the initiation and modulation of nociceptive signals in the trigeminal system. While knowledge of the mechanisms underlying chronic craniofacial pain remains limited, both clinical and preclinical investigations suggest that changes in afferent inputs to the brain as well as in brain structure and modulatory pathways occur in chronic pain. Collectively, these changes result in amplification of nociception that promotes and sustains craniofacial chronic pain states. Conclusions The increased understanding gained of the physiological and pathological processing of nociception in the trigeminal system has provided new perspectives for the mechanistic understanding of acute craniofacial pain conditions and the peripheral and central adaptations that are related to pain chronification. Such knowledge may contribute to improvements in currently available treatments as well as to the development of novel analgesic therapies.

Supradural inflammatory soup in awake and freely moving rats induces facial allodynia that is blocked by putative immune modulators

Facial allodynia is a migraine symptom that is generally considered to represent a pivotal point in migraine progression. Treatment before development of facial allodynia tends to be more successful than treatment afterwards. As such, understanding the underlying mechanisms of facial allodynia may lead to a better understanding of the mechanisms underlying migraine. Migraine facial allodynia is modeled by applying inflammatory soup (histamine, bradykinin, serotonin, prostaglandin E2) over the dura. Whether glial and/or immune activation contributes to such pain is unknown. Here we tested if trigeminal nucleus caudalis (Sp5C) glial and/or immune cells are activated following supradural inflammatory soup, and if putative glial/immune inhibitors suppress the consequent facial allodynia. Inflammatory soup was administered via bilateral indwelling supradural catheters in freely moving rats, inducing robust and reliable facial allodynia. Gene expression for microglial/macrophage activation markers, interleukin-1β, and tumor necrosis factor-α increased following inflammatory soup along with robust expression of facial allodynia. This provided the basis for pursuing studies of the behavioral effects of 3 diverse immunomodulatory drugs on facial allodynia. Pretreatment with either of two compounds broadly used as putative glial/immune inhibitors (minocycline, ibudilast) prevented the development of facial allodynia, as did treatment after supradural inflammatory soup but prior to the expression of facial allodynia. Lastly, the toll-like receptor 4 (TLR4) antagonist (+)-naltrexone likewise blocked development of facial allodynia after supradural inflammatory soup. Taken together, these exploratory data support that activated glia and/or immune cells may drive the development of facial allodynia in response to supradural inflammatory soup in unanesthetized male rats.

Assessment of risk factors for oro-facial pain and recent developments in classification: implications for management

Oro-facial pain research has during the last decades provided important novel insights into the basic underlying mechanisms, the need for standardised diagnostic procedures and classification systems, and multiple treatment options for successful rehabilitation of the patient in pain. Notwithstanding the significant progress in our knowledge spanning from molecules to chair, there may also be limitations in our ability to integrate and interpret the tremendous amount of new data and information, in particular in terms of the clinical implications and overriding conceptual models for oro-facial pain. The aim of the present narrative review is to briefly summarise some of the current thoughts on oro-facial pain mechanisms and recent attempts to identify biomarkers and risk factors leading to the proposal of a new risk assessment diagram for oro-facial pain (RADOP) and a provocative new concept based on stochastic variation between multiple risk factors. Finally, the implications for novel management strategies will briefly be discussed.

The dorsal spinal cord and hindbrain: From developmental mechanisms to functional circuits

Neurons of the dorsal hindbrain and spinal cord are central in receiving, processing and relaying sensory perception and participate in the coordination of sensory-motor output. Numerous cellular and molecular mechanisms that underlie neuronal development in both regions of the nervous system are shared. We discuss here the mechanisms that generate neuronal diversity in the dorsal spinal cord and hindbrain, and emphasize similarities in patterning and neuronal specification. Insight into the developmental mechanisms has provided tools that can help to assign functions to small subpopulations of neurons. Hence, novel information on how mechanosensory or pain sensation is encoded under normal and neuropathic conditions has already emerged. Such studies show that the complex neuronal circuits that control perception of somatosensory and viscerosensory stimuli are becoming amenable to investigations.

Atypical facial pain in multiple sclerosis caused by spinal cord seizures: a case report and review of the literature

Background

Pain is a very commonly reported symptom and often drives patients to seek medical attention; however, it can prove a very difficult diagnostic conundrum and even more challenging to treat effectively. Accurately determining the primary pain generator is key, as certain conditions have efficacious medical and surgical treatments. We present a rare case of a man with multiple sclerosis presenting with spinal cord seizures causing dermatomal pain. While pain has been reported in the context of motor symptoms attributed to spinal cord seizures in a small number of spinal cord conditions, this case represents the first report of pain exclusively associated with spinal cord demyelination in multiple sclerosis.

Case presentation

We present the case of a 60-year-old Caucasian male patient with multiple sclerosis who reported a 5-year history of progressive pain in his left retroauricular region and superior left shoulder. He described this pain as sharp, episodic, and unrelenting and he was referred for consideration for surgical treatment of trigeminal neuralgia. He had no evidence of trigeminal nerve root pathology on magnetic resonance imaging, but did show dorsolateral spinal cord demyelination at the C3–4 level. His symptoms therefore represent an unusual presentation of spinal cord seizures.

Conclusions

Spinal cord seizures are rarely reported in multiple sclerosis and typically present with focal motor seizures. These have been reported to present with cramping dysesthesia and pruritus, though rarely with primary pain. Knowledge of uncommon pain presentations is critical for the increasing number of primary care physicians caring for patients with such chronic neurological diseases as it will guide management and referral patterns. This knowledge is also important for the treating neurologists and neurosurgeons. Neurosurgical intervention for trigeminal neuralgia poses considerable surgical risk, and it should be avoided where possible. Identifying the primary pain generator is, therefore, critical for accurate diagnosis and management.

Development of regional specificity of spinal and medullary dorsal horn neurons

Extensive studies have focused on the development and regionalization of neurons in the central nervous system (CNS). Many genes, which play crucial roles in the development of CNS neurons, have been identified. By using the technique “direct reprogramming”, neurons can be produced from multiple cell sources such as fibroblasts. However, understanding the region-specific regulation of neurons in the CNS is still one of the biggest challenges in the research field of neuroscience. Neurons located in the trigeminal subnucleus caudalis (Vc) and in the spinal dorsal horn (SDH) play crucial roles in pain and sensorimotor functions in the orofacial and other somatic body regions, respectively. Anatomically, Vc represents the most caudal component of the trigeminal system, and is contiguous with SDH. This review is focused on recent data dealing with the regional specificity involved in the development of neurons in Vc and SDH.

Corneomandibular reflex: Anatomical basis

Corneomandibular reflex is a pathological phenomenon evident in cases of severe brainstem damage. It is considered to be a pathological exteroceptive reflex, associated with precentro bulbar tract lesions. The sign is useful in distinguishing central neurological injuries to metabolic disorders in acutely comatose patients, localizing lesions to the upper brainstem area, determining the depth of coma and its evolution, providing evidence of uncal or transtentorial herniation in acute cerebral hemisphere lesions, and it is a marker of supraspinal level impairment in amyotrophic lateral sclerosis and multiple sclerosis. This sign was evident in a patient with severe brain damage. We discuss the literature findings and its relevance in prognosis establishment.