minocycline and Pain--Intractable

Burn injury induces severe pain that can be refractory to existing pharmacotherapies. The underlying mechanism of burn pain remains unclear. We previously established an animal model and reported that unilateral burn injury induces chronic and bilateral mechanical allodynia, which is associated with central sensitization and microglial activation in the spinal cord dorsal horn. Modulation of the activity of microglia and p38 mitogen-activated protein kinase (MAPK) has been shown to ameliorate neuropathic pain in several nerve-injury pain models. In the present study, we show in this rat model that daily treatment with the microglial inhibitor minocycline (10 mg/kg), administered at the time of burn injury and for 7 days thereafter, significantly attenuates ipsilateral and contralateral allodynia as assessed up to 1 month following burn injury. These sensory changes are paralleled by significant suppression of evoked hyperexcitability of dorsal-horn neurons and of the expression of phosphorylated p38 (phospho-p38) in OX42+ microglial cells within the dorsal horn. Our results suggest that modulation of inflammation at early times after burn injury may have long-lasting effects, attenuating central neuropathic mechanisms which contribute to pain after burn injury.. We demonstrate, in a rodent model of burn-associated pain, that the microglial inhibitor minocycline, delivered at the time of burn injury and for 1 week thereafter, has long-lasting effects, attenuating microglial activation and neuronal hyperresponsiveness in the dorsal horns, and ameliorating allodynia for at least 1 month.

Topics: Analgesics; Animals; Burns; Disease Models, Animal; Hyperalgesia; Inflammation Mediators; Male; Microglia; Minocycline; Pain, Intractable; Rats; Rats, Sprague-Dawley; Treatment Outcome

Post-herpetic neuralgia (PHN) is a devastating complication of shingles. The treatment of PHN with traditional pharmaceutical agents has various side effects. Therefore, the treatment of intractable PHN is often very time consuming, mainly because the available treatments often lead to intolerable side effects before the efficient dose can be reached. Opioids such as morphine and oxycodone are the most widely used drugs for the alleviation for severe chronic pain. A number of high quality studies demonstrated that opioids are effective in relieving neuropathic pain including PHN. Yet concerns of misuse, abuse and tolerance of opioids have, however, severely influenced their contribution to neuropathic pain, especially the tolerance that resulted in a loss of drug effect or the necessity for escalating doses to produce pain relief. The glia cells, particularly microglia and astrocytes are thought to play an important role in central sensitization. It is known that activated microglia cells produce NO, cytokines, and cyclooxygenase. All of these chemicals regulate synaptic transmissions in the central nervous system. Additionally, glia modulations showed antiallodynic and antihyperalgesic properties in various experimental pain models. Minocycline, a semisynthetic, second-generation tetracycline can potently inhibit microglial activation and proliferation. Also, the growing body of recent evidence indicates that minocycline attenuates morphine tolerance in neuropathic mice with a mechanism related to microglia. The combination of morphine and minocycline has synergetic effect. This can prevent the development of intractable PHN and attenuate morphine antinociceptive tolerance and further improve the efficacy of morphine and therefore reducing its dosage and side effects. We thereby hypothesize that the combination of morphine and minocycline may produce a duel effect of morphine antinociceptive and minocycline selectively inhibiting the activation of microglia.

Topics: Drug Synergism; Drug Therapy, Combination; Humans; Microglia; Minocycline; Morphine; Neuralgia, Postherpetic; Pain, Intractable

There is increasing evidence that spinal glial cells play an important role in chronic pain states. However, so far no data on the role of microglia in muscle pain are available. The aim of the present study was to investigate the involvement of spinal microglial cells in chronic muscle pain. In a rat model of chronic muscle inflammation (injection of complete Freund s adjuvant into the gastrocnemius-soleus muscle) alterations of microglia were visualized with quantitative OX-42 immunohistochemistry in the dorsal horn of the segments L4 and L5 12 days after induction of inflammation. In behavioural experiments the influence of chronic intrathecally applied minocycline - a specific microglia inhibitor - or an antibody against tumour necrosis factor-alpha (TNF-alpha; a cytokine released from microglia) on pain-related behaviour was investigated after 1, 3, 6, and 12 days. The immunhistochemical data show that in the deep laminae of the spinal dorsal horn microglial cells reacted with morphological changes to the muscle inflammation. Following inflammation, the mean boundary length surrounding the OX-42 immunostained area was significantly shorter. This indicates that microglial cells were activated by the myositis and withdrew their processes. Chronic intrathecal administration of minocycline or anti TNF-alpha with an osmotic mini-pump largely normalised the inflammation-induced changes in spontaneous exploratory behaviour and attenuated the hypersensitivity to mechanical stimulation. Both the immunohistochemical and behavioural data show that spinal microglial cells are involved in nociceptive processes in the cause of a chronic muscle inflammation.

Topics: Animals; Anti-Bacterial Agents; Antibodies; Biomarkers; CD11b Antigen; Cell Shape; Chronic Disease; Disease Models, Animal; Exploratory Behavior; Freund's Adjuvant; Hyperalgesia; Immunohistochemistry; Male; Microglia; Minocycline; Myositis; Nociceptors; Pain Threshold; Pain, Intractable; Posterior Horn Cells; Rats; Rats, Sprague-Dawley; Tumor Necrosis Factor-alpha