Preventive and curative effects of radon inhalation on chronic constriction injury-induced neuropathic pain in mice

A New Potential Antidepressant: Dexmedetomidine Alleviates Neuropathic Pain-Induced Depression by Increasing Neurogenesis in the Hippocampus

INTRODUCTION

Studies have suggested dexmedetomidine (DEX) as a potential antidepressant. However, no relevant research exists on its effects and mechanisms in curing depression caused by chronic pain. Therefore, an understanding of DEX's role in depressive disorders proposes new approaches for antidepressant treatment.

METHODS

In this study, C57Bl/6 mice (n = 80) were divided into sham (n = 8) and chronic constrictive injury (CCI, n = 72) groups. The CCI group was further divided into six subgroups: CCI + normal saline (NS), CCI + DEX6.25, CCI + DEX12.5, CCI + DEX25, CCI + DEX50, and CCI + DEX100. Fourteen days after CCI, mice that did not develop a depressive phenotype were excluded through sucrose preference test (SPT), forced swimming test (FST), paw thermal withdrawal latency (PTWL), and serum corticosterone (CORT). Subsequently, mice in the sham group were administered 0.1 mL/10 g NS once daily. However, mice in the CCI subgroups were administered NS (0.1 mL/10 g), DEX (6.25 µg/kg), DEX (12.5 µg/kg), DEX (25 µg/kg), DEX (50 µg/kg), and DEX (100 µg/kg) intraperitoneally once daily for 1 week, respectively. Afterward, bromodeoxyuridine (BrdU) was injected intraperitoneally once daily as well for 3 consecutive days before sampling, following BrdU- and doublecortex (DCX)-positive cell detection in the hippocampus through immunofluorescence.

RESULTS

The success rate of the chronic pain-depression (CPD) model was 62.5%. As observed, DEX dose-dependently affected sucrose preferences during the SPT and immobility time during FST. Results also showed that 25 µg/kg DEX had the best promotion effect during increased sucrose preference and reduced immobility time. Moreover, although DEX improved PTWL and serum CORT, no improvement over the DEX 25 µg/kg treatment was observed. Compared to the sham group, the percentage of BrdU+ and DCX+ cells was also significantly lower in the CCI + NS group. Besides, DEX dose-dependently affected cell proliferation and neuronal differentiation. Additionally, the percentage of BrdU+ and DCX+ cells in the dentate gyrus (DG) region of the hippocampus was highest in the CCI + DEX25 group.

CONCLUSION

Therefore, DEX dose-dependently alleviates depression induced by chronic pain through neurogenesis promotion in the DG region of the hippocampus.

Confirmation of efficacy, elucidation of mechanism, and new search for indications of radon therapy

Indications of radon therapy include various diseases related to respiratory, painful, digestive, chronic degenerative, senile, etc. derived from reactive oxygen species, but most are based on empirical prescriptions. For this reason, we have evaluated the relation between the biological response caused by radon and the tissue/organ absorbed dose more quantitatively, and have promoted the elucidation of mechanisms related to the indication and searching newly. As a result, as a mechanism, a series of moderate physiological stimulative effects accompanying a small amount of oxidative stress by radon inhalation are being elucidated. That is, hyperfunction of anti-oxidation/immune regulation/damage repair, promotion of anti-inflammation/circulating metabolism/hormone secretion, induction of apoptosis/heat shock protein, etc. Also, new indications include inflammatory/neuropathic pain, hepatic/renal injury, colitis, type 1 diabetes, complication kidney injury, hyperuricemia, transient cerebral ischemia, and inflammatory edema. Furthermore, we examined the combined antioxidant effect of radon inhalation and antioxidants or therapeutic agents. As a result, it was clear that any combination treatment could enhance the suppression effect of disease. It can be expected that radon therapy can be used effectively by applying it in addition to usual treatment, since reduction in its dosage can also be expected by concomitant use for drugs with strong side effects.

Balneotherapy decreases mechanical hyperalgesia by reversing BDNF and NOS2 immunocontent in spinal cord of mice with neuropathic pain

In the last decades, balneotherapy or thermalism has been used for health promotion and in the treatment of inflammatory and chronic processes. We found that balneotherapy reduced mechanical hyperalgesia, as well the increase of BDNF and NOS2 levels in the spinal cord, while increased BDNF and NOS1 in the paw. The data presented herein demonstrated for the first time in a murine model of neuropathic pain, the analgesic effect of balneotherapy with the water from the natural springs of Santo Amaro da Imperatriz-Brazil. Nevertheless, future clinical trials should be conducted to test the effectiveness of balneotherapy in neuropathic pain patients.

Activation of Antioxidative Functions by Radon Inhalation Enhances the Mitigation Effects of Pregabalin on Chronic Constriction Injury-Induced Neuropathic Pain in Mice

Radon inhalation brings pain relief for chronic constriction injury- (CCI-) induced neuropathic pain in mice due to the activation of antioxidative functions, which is different from the mechanism of the pregabalin effect. In this study, we assessed whether a combination of radon inhalation and pregabalin administration is more effective against neuropathic pain than radon or pregabalin only. Mice were treated with inhaled radon at a concentration of 1,000 Bq/m(3) for 24 hours and pregabalin administration after CCI surgery. In mice treated with pregabalin at a dose of 3 mg/kg weight, the 50% paw withdrawal threshold of mice treated with pregabalin or radon and pregabalin was significantly increased, suggesting pain relief. The therapeutic effects of radon inhalation or the combined effects of radon and pregabalin (3 mg/kg weight) were almost equivalent to treatment with pregabalin at a dose of 1.4 mg/kg weight or 4.1 mg/kg weight, respectively. Radon inhalation and the combination of radon and pregabalin increased antioxidant associated substances in the paw. The antioxidant substances increased much more in radon inhalation than in pregabalin administration. These findings suggested that the activation of antioxidative functions by radon inhalation enhances the pain relief of pregabalin and that this combined effect is probably an additive effect.

Radon inhalation protects against transient global cerebral ischemic injury in gerbils

Although brain disorders are not the main indication for radon therapy, our previous study suggested that radon inhalation therapy might mitigate brain disorders. In this study, we assessed whether radon inhalation protects against transient global cerebral ischemic injury in gerbils. Gerbils were treated with inhaled radon at a concentration of 2,000 Bq/m(3) for 24 h. After radon inhalation, transient global cerebral ischemia was induced by bilateral occlusion of the common carotid artery. Results showed that transient global cerebral ischemia induced neuronal damage in hippocampal CA1, and the number of damaged neurons was significantly increased compared with control. However, radon treatment inhibited ischemic damage. Superoxide dismutase (SOD) activity in the radon-treated gerbil brain was significantly higher than that in sham-operated gerbils. These findings suggested that radon inhalation activates antioxidative function, especially SOD, thereby inhibiting transient global cerebral ischemic injury in gerbils.

Antinociceptive effects of radon inhalation on formalin-induced inflammatory pain in mice

Radon therapy is clinically useful for the treatment of inflammatory diseases. The mechanisms of pain relief remain to be fully elucidated. In this study, we investigated the antinociceptive effects of radon inhalation in a mouse model of formalin-induced inflammatory pain. Immediately, after radon inhalation at a concentration of background level (ca. 19 Bq/m(3)), 1,000 or 2,000 Bq/m(3) for 24 h, 1.35 % formalin (0.5 % formaldehyde in saline, 20 μl) was subcutaneously injected into the hind paw of mice, and we measured licking response time. Radon inhalation inhibited the second phase of response in formalin test. Formalin administration induced nociception and increased tumor necrosis factor alpha (TNF-α) and nitric oxide (NO) levels in serum and leukocyte migration in paws. Concurrently, formalin injection decreased antioxidative functions. Radon inhalation produced antinociceptive effects, i.e., lowered serum TNF-α and NO levels, and restored antioxidative functions. The results showed that radon inhalation inhibited formalin-induced inflammatory pain.

Inhibitory effects of pretreatment with radon on acute alcohol-induced hepatopathy in mice

We previously reported that radon inhalation activates antioxidative functions in the liver and inhibits carbon tetrachloride-induced hepatopathy in mice. In addition, it has been reported that reactive oxygen species contribute to alcohol-induced hepatopathy. In this study, we examined the inhibitory effects of radon inhalation on acute alcohol-induced hepatopathy in mice. C57BL/6J mice were subjected to intraperitoneal injection of 50% alcohol (5 g/kg bodyweight) after inhaling approximately 4000 Bq/m³ radon for 24 h. Alcohol administration significantly increased the activities of glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT) in serum, and the levels of triglyceride and lipid peroxide in the liver, suggesting acute alcohol-induced hepatopathy. Radon inhalation activated antioxidative functions in the liver. Furthermore, pretreatment with radon inhibited the depression of hepatic functions and antioxidative functions. These findings suggested that radon inhalation activated antioxidative functions in the liver and inhibited acute alcohol-induced hepatopathy in mice.