Differential Regulation of Intracisternally Injected Angiotensin II-Induced Mechanical Allodynia and Thermal Hyperalgesia in Rats.

The present study examined the underlying mechanisms of mechanical allodynia and thermal hyperalgesia induced by the intracisternal injection of angiotensin (Ang) II. Intracisternal Ang II injection decreased the air puff threshold and head withdrawal latency. To determine the operative receptors for each distinct type of pain behavior, we intracisternally injected Ang II receptor antagonists 2 h after Ang II injection. Losartan, an Ang II type 1 receptor (AT1R) antagonist, alleviated mechanical allodynia. Conversely, PD123319, an Ang II type 1 receptor (AT2R) antagonist, blocked only thermal hyperalgesia. Immunofluorescence analyses revealed the co-localization of AT1R with the astrocyte marker GFAP in the trigeminal subnucleus caudalis and co-localization of AT2R with CGRP-positive neurons in the trigeminal ganglion. Intracisternal pretreatment with minocycline, a microglial inhibitor, did not affect Ang II-induced mechanical allodynia, whereas L-α-aminoadipate, an astrocyte inhibitor, significantly inhibited Ang II-induced mechanical allodynia. Furthermore, subcutaneous pretreatment with botulinum toxin type A significantly alleviated Ang II-induced thermal hyperalgesia, but not Ang II-induced mechanical allodynia. These results indicate that central Ang II-induced nociception is differentially regulated by AT1R and AT2R. Thus, distinct therapeutic targets must be regulated to overcome pain symptoms caused by multiple underlying mechanisms.

TNF-α-Mediated RIPK1 Pathway Participates in the Development of Trigeminal Neuropathic Pain in Rats.

Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) participates in the regulation of cellular stress and inflammatory responses, but its function in neuropathic pain remains poorly understood. This study evaluated the role of RIPK1 in neuropathic pain following inferior alveolar nerve injury. We developed a model using malpositioned dental implants in male Sprague Dawley rats. This model resulted in significant mechanical allodynia and upregulated RIPK1 expression in the trigeminal subnucleus caudalis (TSC). The intracisternal administration of Necrosatin-1 (Nec-1), an RIPK1 inhibitor, blocked the mechanical allodynia produced by inferior alveolar nerve injury The intracisternal administration of recombinant rat tumor necrosis factor-α (rrTNF-α) protein in naive rats produced mechanical allodynia and upregulated RIPK1 expression in the TSC. Moreover, an intracisternal pretreatment with Nec-1 inhibited the mechanical allodynia produced by rrTNF-α protein. Nerve injury caused elevated TNF-α concentration in the TSC and a TNF-α block had anti-allodynic effects, thereby attenuating RIPK1 expression in the TSC. Finally, double immunofluorescence analyses revealed the colocalization of TNF receptor and RIPK1 with astrocytes. Hence, we have identified that astroglial RIPK1, activated by the TNF-α pathway, is a central driver of neuropathic pain and that the TNF-α-mediated RIPK1 pathway is a potential therapeutic target for reducing neuropathic pain following nerve injury.

Mannitol Enhances the Antinociceptive Effects of Diphenhydramine as an Alternative Local Anesthetic.

Mannitol has recently been reported to be effective in enhancing the antinociceptive efficacy of lidocaine. No single study to date, however, has compared diphenhydramine with and without mannitol for nociceptive processing as an alternative local anesthetic. In this study, we examined the antinociceptive efficacy enhancements of diphenhydramine when combined with mannitol. Male Sprague-Dawley rats weighing 230-260 g were used in a hot plate test to evaluate the antinociceptive effects of diphenhydramine. All chemicals were dissolved in isotonic normal saline and administered subcutaneously into the plantar surface of the right hind paw at 10 min before the hot plate test. A subcutaneous injection of 0.5% or 1% diphenhydramine produced significant inhibition of the withdrawal latency time compared with the vehicle treatment. Antinociceptive effects appeared 10 min after the diphenhydramine injections and persisted for over 30 min. The antinociceptive effects of 1% diphenhydramine were not statistically different from those of 1% lidocaine. Although a subcutaneous injection of a 0.5 M mannitol solution alone did not affect the withdrawal latency time, 1% diphenhydramine with 0.5 M mannitol significantly enhanced antinociception. A subcutaneous injection of 1% diphenhydramine with epinephrine (1 : 100,000) solution did not increase the antinociceptive effect of the diphenhydramine. These results suggest that diphenhydramine with mannitol can be used as an alternative local anesthetic.

Early Blockade of EphA4 Pathway Reduces Trigeminal Neuropathic Pain.

BACKGROUND: Although the Eph receptor plays an important role in the development of neuropathic pain following nerve injury, there has been no evidence of the participation of the ephrin A4 receptor (EphA4) in the development of trigeminal neuropathic pain. The present study investigated the role of EphA4 in central nociceptive processing in rats with inferior alveolar nerve injury. MATERIALS AND METHODS: Male Sprague-Dawley rats were used in all our experiments. A rat model for trigeminal neuropathic pain was produced using malpositioned dental implants. The left mandibular second molar was extracted under anesthesia, followed by the placement of a miniature dental implant to injure the inferior alveolar nerve. RESULTS: Our current findings show that nerve injury induced by malpositioned dental implants evokes significant mechanical allodynia and up-regulation of EphA4 expression in the ipsilateral trigeminal subnucleus caudalis. Although daily treatment with EphA4-Fc, an EphA4 antagonist, did not produce prolonged anti-allodynic effects after the chronic neuropathic pain had been already established, an early treatment protocol with repeated EphA4-Fc administration significantly attenuated mechanical allodynia before initiation of chronic neuropathic pain. Finally, we confirmed the participation of the central EphA4 pathway in the development of trigeminal neuropathic pain by reducing EphA4 expression using EphA4 siRNA. This suppression of EphA4 produced significantly prolonged anti-allodynic effects. CONCLUSION: These results suggest that early blockade of central EphA4 signaling provides a new therapeutic target for the treatment of trigeminal neuropathic pain.

Preemptive application of QX-314 attenuates trigeminal neuropathic mechanical allodynia in rats.

The aim of the present study was to examine the effects of preemptive analgesia on the development of trigeminal neuropathic pain. For this purpose, mechanical allodynia was evaluated in male Sprague-Dawley rats using chronic constriction injury of the infraorbital nerve (CCI-ION) and perineural application of 2% QX-314 to the infraorbital nerve. CCI-ION produced severe mechanical allodynia, which was maintained until postoperative day (POD) 30. An immediate single application of 2% QX-314 to the infraorbital nerve following CCI-ION significantly reduced neuropathic mechanical allodynia. Immediate double application of QX-314 produced a greater attenuation of mechanical allodynia than a single application of QX-314. Immediate double application of 2% QX-314 reduced the CCI-ION-induced upregulation of GFAP and p-p38 expression in the trigeminal ganglion. The upregulated p-p38 expression was co-localized with NeuN, a neuronal cell marker. We also investigated the role of voltage-gated sodium channels (Navs) in the antinociception produced by preemptive application of QX-314 through analysis of the changes in Nav expression in the trigeminal ganglion following CCI-ION. Preemptive application of QX-314 significantly reduced the upregulation of Nav1.3, 1.7, and 1.9 produced by CCI-ION. These results suggest that long-lasting blockade of the transmission of pain signaling inhibits the development of neuropathic pain through the regulation of Nav isoform expression in the trigeminal ganglion. Importantly, these results provide a potential preemptive therapeutic strategy for the treatment of neuropathic pain after nerve injury.

Age-Related Changes in Immunological Factors and Their Relevance in Allergic Disease Development During Childhood.

PURPOSE: Allergic diseases are triggered by Th2-mediated immune reactions to allergens and orchestrated by various immunological factors, including immune cells and cytokines. Although many reports have suggested that childhood is the critical period in the onset of allergic diseases and aging leads to alter the susceptibility of an individual to allergic diseases, age-related changes in various immunological factors in healthy individuals as well as their difference between healthy and allergic children have not yet been established. METHODS: We investigated the ratio of Th1/Th2 cells and the levels of 22 allergy-related cytokines across all age groups in individuals who were classified as clinically non-atopic and healthy. We also examined their differences between healthy and allergic children to evaluate immunological changes induced by the development of allergic diseases during childhood. RESULTS: The Th1/Th2 ratio rose gradually during the growth period including childhood, reaching peak values in the twenties-thirties age group. Th1/Th2 ratios were significantly lower in allergic children than in healthy controls, whereas 14 of 22 cytokines were significantly higher in allergic children than in healthy controls. On the other hand, there were no differences in Th1/Th2 ratios and cytokines between healthy and allergic adolescents. CONCLUSIONS: In this study, age-related changes in Th1/Th2 ratios were found in normal controls across all age groups, and decreases in Th1/Th2 ratio were observed with increasing of 14 cytokines in allergic children. The results of this study may be helpful as reference values for both monitoring immunological changes according to aging in healthy individuals and distinguishing between normal and allergic subjects in terms of immune cells and soluble factors.

Pulsed radiofrequency application reduced mechanical hypersensitivity and microglial expression in neuropathic pain model.

OBJECTIVE: Pulsed radiofrequency (PRF) procedure has been used in clinical practice for the treatment of chronic neuropathic pain conditions without neuronal damage. The purpose of this study was to investigate the changes in pain response and glial expression after the application of PRF on a dorsal root ganglion (DRG) in a neuropathic pain model. DESIGN: A neuropathic pain model (14 female Sprague-Dawley [SD] rats; 200-250 g) was made by a unilateral L5 spinal nerve ligation (SNL) and transection on the distal side of the ligation. The development of mechanical and cold hypersensitivity on the hindpaw was established postoperative day 9 (POD 9). The rats were then randomly assigned to the PRF (+) and the PRF (-) groups. Furthermore, PRF (2 bursts/s, duration = 20 milliseconds, output voltage = 45 V) was applied on the ipsilateral DRG for 180 seconds, with a maximum temperature of 42°C, at POD 10. Pain behaviors were tested throughout the 12 days after PRF. We also examined the changes of the spinal glial expression by immunohistochemistry. RESULTS: Significant reduction of mechanical hypersensitivity in the PRF (+) group was observed from day 1 after a single PRF procedure and was maintained throughout the following 12 days. Immunoreactivity for OX42 in the ipsilateral dorsal horn also decreased compared with that of the PRF (-) group. However, cold hypersensitivity and glial fibrillary acidic protein (GFAP) immunoreactivity in the dorsal horn was not affected by a PRF procedure. CONCLUSIONS: Our result demonstrated that the mechanical hypersensitivity, induced by L5 SNL, was attenuated by a PRF procedure on the ipsilateral DRG. This analgesic effect may be associated with an attenuation of the microglial activation in the dorsal horn.