Central sensitization in the trigeminal nucleus caudalis produced by a conjugate of substance P and the A subunit of cholera toxin

Twenty-Three Months Repetitive Transcranial Magnetic Stimulation of the Primary Motor Cortex for Refractory Trigeminal Neuralgia: A Single-Case Study

Treatment refractory or recurrent trigeminal neuralgia (TN) is a severe chronic pain illness. Single-session repetitive transcranial magnetic stimulation (rTMS) has been shown to elicit analgesic effects in several craniofacial pain syndromes, including TN. However, the safety and long-term effect of multi-session rTMS for TN have yet to be fully explored. In this study, we present a case of a patient with medical treatment-refractory TN after microvascular decompression. The patient volunteered to undergo 73 sessions of 10 Hz rTMS over 23 months. Neurovagination was used for precise localization and stimulation of the hand and face representation at the left motor cortex. The numeric pain intensity scores derived using the visual analog scale served as a daily index of treatment efficacy. The patient experienced a significant weekly reduction in pain scores, cumulating in 70.89% overall pain relief. The medication dosages were reduced and then discontinued toward the end of the intervention period. No severe adverse events were reported. From our results, we can conclude that the longitudinal multi-session application of rTMS over the hand and face area of M1 is a safe and effective method for producing long-lasting pain relief in TN. Using rTMS may thus prove helpful as an adjunct to conventional methods for treating pain in TN.

Effects of Oxaliplatin on Facial Sensitivity to Cool Temperatures and TRPM8 Expressing Trigeminal Ganglion Neurons in Mice

The chemotherapeutic agent oxaliplatin is commonly used to treat colorectal cancer. Although effective as a chemotherapeutic, it frequently produces painful peripheral neuropathies. These neuropathies can be divided into an acute sensitivity to cool temperatures in the mouth and face, and chronic neuropathic pain in the limbs and possible numbness. The chronic neuropathy also includes sensitivity to cool temperatures. Neurons that detect cool temperatures are reported to utilize Transient Receptor Potential Cation Channel, Subfamily M, Member 8 (TRPM8). Therefore, we investigated the effects of oxaliplatin on facial nociception to cool temperatures (18°C) in mice and on TRPM8 expressing trigeminal ganglion (TRG) neurons. Paclitaxel, a chemotherapeutic that is used to treat breast cancer, was included for comparison because it produces neuropathies, but acute cool temperature sensitivity in the oral cavity or face is not typically reported. Behavioral testing of facial sensitivity to 18°C indicated no hypersensitivity either acutely or chronically following either chemotherapeutic agent. However, whole cell voltage clamp experiments in TRPM8 expressing TRG neurons indicated that both oxaliplatin and paclitaxel increased Hyperpolarization-Activated Cyclic Nucleotide-Gated channel (HCN), voltage gated sodium channel (Nav), and menthol evoked TRPM8 currents. Voltage gated potassium channel (Kv) currents were not altered. Histological examination of TRPM8 fibers in the skin of the whisker pads demonstrated that the TRPM8 expressing axons and possible Merkel cell-neurite complexes were damaged by oxaliplatin. These findings indicate that oxaliplatin induces a rapid degeneration of TRG neuron axons that express TRPM8, which prevents evoked activation of the sensitized neurons and likely leads to reduced sensitivity to touch and cool temperatures. The changes in HCN, Nav, and TRPM8 currents suggest that spontaneous firing of action potentials may be increased in the deafferented neurons within the ganglion, possibly producing spontaneously induced cooling or nociceptive sensations.

Pain modulated by Bothrops snake venoms: Mechanisms of nociceptive signaling and therapeutic perspectives

Snake venoms are substances mostly composed by proteins and peptides with high biological activity. Local and systemic effects culminate in clinical manifestations induced by these substances. Pain is the most uncomfortable condition, but it has not been well investigated. This review discusses Bothrops snakebite-induced nociception, highlighting molecules involved in the mediation of this process and perspectives in treatment of pain induced by Bothrops snake venoms (B. alternatus, B. asper, B. atrox, B. insularis, B. jararaca, B. pirajai, B. jararacussu, B. lanceolatus, B. leucurus, B. mattogrossensis, B. moojeni). We highlight, the understanding of the nociceptive signaling, especially in snakebite, enables more efficient treatment approaches. Finally, future perspectives for pain treatment concerning snakebite patients are discussed.

Pharmacological Characterization of Orofacial Nociception in Female Rats Following Nitroglycerin Administration

Rodent models of human disease can be valuable for understanding the mechanisms of a disease and for identifying novel therapies. However, it is critical that these models be vetted prior to committing resources to developing novel therapeutics. Failure to confirm the model can lead to significant losses in time and resources. One model used for migraine headache is to administer nitroglycerin to rodents. Nitroglycerin is known to produce migraine-like pain in humans and is presumed to do the same in rodents. It is not known, however, if the mechanism for nitroglycerin headaches involves the same pathological processes as migraine. In the absence of known mechanisms, it becomes imperative that the model not only translates into successful clinical trials but also successfully reverse translates by demonstrating efficacy of current therapeutics. In this study female rats were given nitroglycerin and nociception was evaluated in OPADs. Estrous was not monitored. Based on the ED₅₀ of nitroglycerin a dose of 10 mg/kg was used for experiments. Sumatriptan, caffeine, buprenorphine and morphine were administered to evaluate the reverse translatability of the model. We found that nitroglycerin did not produce mechanical allodynia in the face of the rats, which is reported to be a consequence of migraine in humans. Nitroglycerin reduced the animals’ participation in the assay. The reduced activity was verified using an assay to measure exploratory behavior. Furthermore, the effects of nitroglycerin were not reversed or prevented by agents that are effective acute therapies for migraine. Two interesting findings from this study, however, were that morphine and nitroglycerin interact to increase the rats’ tolerance of mechanical stimuli on their faces, and they work in concert to slow down the central motor pattern generator for licking on the reward bottle. These interactions suggest that nitroglycerin generated nitric oxide and mu opioid receptors interact with the same neuronal circuits in an additive manner. The interaction of nitroglycerin and morphine on sensory and motor circuits deserves additional examination. In conclusion, based on the results of this study the use of nitroglycerin at these doses in naïve female rats is not recommended as a model for migraine headaches.

Ocular surface inflammation induces de novo expression of substance P in the trigeminal primary afferents with large cell bodies

We evaluated the changes in substance P (SP)-expressing trigeminal neurons (TNs) innervating the cornea following ocular surface inflammation. Ocular surface inflammation was induced in Sprague-Dawley rats using 0.1% benzalkonium chloride (BAK). The corneal staining score, corneal epithelial apoptosis, conjunctival goblet cells, and density of corneal subbasal nerve plexus (SNP) were assessed, and the mRNA levels of SP, interleukin (IL)-1β, IL-6, and tumour necrosis factor-α were measured in corneas and ipsilateral trigeminal ganglia (TG). SP-immunoreactivity (IR) was measured in corneal intraepithelial nerves and TNs. The cell size of corneal TNs in the TG was calculated. All parameters were observed immediately (BAK group), at 1 week (1 w group), and 2 months (2 m group) after 2 weeks of BAK application. BAK caused an increase in the corneal staining score and the number of apoptotic cells, loss of conjunctival goblet cells, reduced density of corneal SNP, and upregulated expression of SP and inflammatory cytokines in both the cornea and TG in the BAK group but those changes were not observed in the 2 m group. On the other hand, SP-IR% and mean cell size of corneal TNs increased significantly in the BAK, 1 w, and 2 m groups, compared to the control. Our data suggest that following ocular surface inflammation, large-sized corneal TNs which normally do not express SP, expressed it and this phenotype switching lasted even after the inflammation disappeared. Long-lasting phenotypic switch, as well as changes in the expression level of certain molecules should be addressed in future studies on the mechanism of corneal neuropathic pain.

Peripheral and central substance P expression in rat CFA-induced TMJ synovitis pain

Synovitis contributes to temporomandibular joint (TMJ) pain, nevertheless, the detailed nociceptive mechanism remains unclear. In this study, a rat model of TMJ synovitis was induced by intra-articular injection with complete Freund’s adjuvant (CFA). After CFA-induced synovitis, pain behaviors were observed. Then, TMJ, trigeminal ganglion, and trigeminal nucleus caudalis (TNC) tissues were collected, and immunohistochemistry was used to detect the expression of substance P (SP) and protein gene product 9.5 (PGP9.5) in the synovium tissue. Furthermore, the gene expression level of SP and PGP9.5 in synovium was detected by reverse transcription-polymerase chain reaction (RT-PCR). Afterwards, the expression of SP in the trigeminal ganglion and TNC and c-fos in the TNC was detected by immunohistochemistry. Compared with the control group, the expression of SP and PGP9.5 nerve fibers density and gene levels of them in the synovium tissue were significantly increased in CFA-induced TMJ synovitis rats. Similarly, SP expression in the trigeminal ganglion and TNC, and c-fos expression in the TNC were also obviously increased in CFA-induced TMJ synovitis rats. Collectively, CFA-induced rat TMJ synovitis resulted in obvious pain. This nociceptive reaction could be attributed to the augmented quantity of SP and PGP9.5 positive-stained nerve fibers distributed in the inflammatory synovium as well as enhanced SP expression in the trigeminal ganglion and TNC tissue. c-fos expression in the rat TNC illustrates CFA-induced TMJ synovitis can evoke the acute pain.

Sleep-inducing effect of substance P-cholera toxin A subunit in mice

Evidence indicates that the neuropeptide substance P (SP) can act through neurokinin receptors to alter sleep and/or non-rapid eye movement (NREM) sleep slow-wave activity. Consequently, drugs acting on SP receptors could potentially be used as a novel treatment for sleep-related disorders. In the present study, we used SP conjugated with cholera toxin A subunit (SP-CTA), which enhances its duration of activity on SP receptor-expressing cells, to determine the effects of selectively activating SP receptor-expressing brain cells on sleep regulation in mice. Herein, we found that intracerebroventricular administration of SP-CTA enhanced amounts of NREM sleep which was highly fragmented. This result suggests that the activation of SP receptor-expressing cells in the brain can produce not only arousal effects as shown in previous studies but also sleep-inducing effects.

Sex differences in mouse Transient Receptor Potential Cation Channel, Subfamily M, Member 8 expressing trigeminal ganglion neurons

The detection of cool temperatures is thought to be mediated by primary afferent neurons that express the cool temperature sensing protein Transient Receptor Potential Cation Channel, Subfamily M, Member 8 (TRPM8). Using mice, this study tested the hypothesis that sex differences in sensitivity to cool temperatures were mediated by differences in neurons that express TRPM8. Ion currents from TRPM8 expressing trigeminal ganglion (TRG) neurons in females demonstrated larger hyperpolarization-activated cyclic nucleotide-gated currents (Ih) than male neurons at both 30° and 18°C. Additionally, female neurons' voltage gated potassium currents (Ik) were suppressed by cooling, whereas male Ik was not significantly affected. At the holding potential tested (-60mV) TRPM8 currents were not visibly activated in either sex by cooling. Modeling the effect of Ih and Ik on membrane potentials demonstrated that at 30° the membrane potential in both sexes is unstable. At 18°, female TRPM8 TRG neurons develop a large oscillating pattern in their membrane potential, whereas male neurons become highly stable. These findings suggest that the differences in Ih and Ik in the TRPM8 TRG neurons of male and female mice likely leads to greater sensitivity of female mice to the cool temperature. This hypothesis was confirmed in an operant reward/conflict assay. Female mice contacted an 18°C surface for approximately half the time that males contacted the cool surface. At 33° and 10°C male and female mice contacted the stimulus for similar amounts of time. These data suggest that sex differences in the functioning of Ih and Ik in TRPM8 expressing primary afferent neurons leads to differences in cool temperature sensitivity.

Operant assays for assessing pain in preclinical rodent models: highlights from an orofacial assay

Despite an immense investment of resources, pain remains at epidemic proportions. Given this, there has been an increased effort toward appraising the process by which new painkillers are developed, focusing specifically on why so few analgesics make it from the benchside to the bedside. The use of behavioral assays and animal modeling for the preclinical stages of analgesic development is being reexamined to determine whether they are truly relevant, meaningful, and predictive. Consequently, there is a strengthening consensus that the traditional reflex-based assays upon which several decades of preclinical pain research has been based are inadequate. Thus, investigators have recently turned to the development of new preclinical assays with improved face, content, and predictive validity. In this regard, operant pain assays show considerable promise, as they are more sensitive, present better validity, and, importantly, better encompass the psychological and affective dimensions of pain that trouble human pain sufferers. Here, we briefly compare and contrast reflex assays with operant assays, and we introduce a particular operant orofacial pain assay used in a variety of experiments to emphasize how operant pain assays can be applied to preclinical studies of pain.

Anti-nociceptive effect of a conjugate of substance P and light chain of botulinum neurotoxin type A

Neuropathic pain is a debilitating condition resulting from damage to sensory transmission pathways in the peripheral and central nervous system. A potential new way of treating chronic neuropathic pain is to target specific pain-processing neurons based on their expression of particular receptor molecules. We hypothesized that a toxin-neuropeptide conjugate would alter pain by first being taken up by specific receptors for the neuropeptide expressed on the neuronal cells. Then, once inside the cell the toxin would inhibit the neurons' activity without killing the neurons, thereby providing pain relief without lesioning the nervous system. In an effort to inactivate the nociceptive neurons in the trigeminal nucleus caudalis in mice, we targeted the NK1 receptor (NK1R) using substance P (SP). The catalytically active light chain of botulinum neurotoxin type A (LC/A) was conjugated with SP. Our results indicate that the conjugate BoNT/A-LC:SP is internalized in cultured NK1R-expressing neurons and also cleaves the target of botulinum toxin, a component-docking motif necessary for release of neurotransmitters called SNAP-25. The conjugate was next tested in a murine model of Taxol-induced neuropathic pain. An intracisternal injection of BoNT/A-LC:SP decreased thermal hyperalgesia as measured by the operant orofacial nociception assay. These findings indicate that conjugates of the light chain of botulinum toxin are extremely promising agents for use in suppressing neuronal activity for extended time periods, and that BoNT/A-LC:SP may be a useful agent for treating chronic pain.

Characterization of bilateral trigeminal constriction injury using an operant facial pain assay

In order to better understand and treat neuropathic pain, scientific study must use methods that can assess pain processing at the cortical level where pain is truly perceived. Operant behavior paradigms can accomplish this. We used an operant task to evaluate changes following chronic constriction injury to the trigeminal nerves. We also relate these behavioral changes to immunohistochemistry of transient receptor potential channels vanilloid 1 and melastatin 8 (TRPV1 and TRPM8) in the trigeminal ganglia. Following nerve injury, successful performance of the operant task was reduced and aversive behaviors were observed with 10 and 37 °C stimulation, indicating cold allodynia and mechanical allodynia respectively. In contrast, while aversive behaviors were observed with 48 °C stimulation, successful performance of the operant task was not substantially hindered following injury. These behavioral changes were accompanied by an increase in TRPV1 positive cells and an increased intensity of TRPM8 staining at 2 weeks post-injury, when cold allodynia is maximal. These findings suggest that the incorporation of operant behavioral assessment in the study of pain may provide insight into the relationship among peripheral changes, motivational drive, and pain. Understanding this relationship will allow us to better treat and prevent chronic neuropathic pain.

Paroxetine engenders analgesic effects through inhibition of p38 phosphorylation in a rat migraine model

In this study, a model of migraine was established by electrical stimulation of the superior sagittal sinus in rats. These rats were then treated orally with paroxetine at doses of 2.5, 5, or 10 mg/kg per day for 14 days. Following treatment, mechanical withdrawal thresholds were significantly higher, extracellular concentrations of 5-hydroxytryptamine in the periaqueductal grey matter and nucleus reticularis gigantocellularis were higher, and the expression of phosphorylated p38 in the trigeminal nucleus caudalis was lower. Our experimental findings suggest that paroxetine has analgesic effects in a rat migraine model, which are mediated by inhibition of p38 phosphorylation.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).