Schwann cell LRP1 regulates remak bundle ultrastructure and axonal interactions to prevent neuropathic pain.

Trophic support and myelination of axons by Schwann cells in the PNS are essential for normal nerve function. Herein, we show that deletion of the LDL receptor-related protein-1 (LRP1) gene in Schwann cells (scLRP1(-/-)) induces abnormalities in axon myelination and in ensheathment of axons by nonmyelinating Schwann cells in Remak bundles. These anatomical changes in the PNS were associated with mechanical allodynia, even in the absence of nerve injury. In response to crush injury, sciatic nerves in scLRP1(-/-) mice showed accelerated degeneration and Schwann cell death. Remyelinated axons were evident 20 d after crush injury in control mice, yet were largely absent in scLRP1(-/-) mice. In the partial nerve ligation model, scLRP1(-/-) mice demonstrated significantly increased and sustained mechanical allodynia and loss of motor function. Evidence for central sensitization in pain processing included increased p38MAPK activation and activation of microglia in the spinal cord. These studies identify LRP1 as an essential mediator of normal Schwann cell-axonal interactions and as a pivotal regulator of the Schwann cell response to PNS injury in vivo. Mice in which LRP1 is deficient in Schwann cells represent a model for studying how abnormalities in Schwann cell physiology may facilitate and sustain chronic pain.

Proinflammatory-activated trigeminal satellite cells promote neuronal sensitization: relevance for migraine pathology.

BACKGROUND: Migraine is a complex, chronic, painful, neurovascular disorder characterized by episodic activation of the trigeminal system. Increased levels of calcitonin gene-related peptide (CGRP) are found at different levels during migraine attacks. Interestingly, CGRP is also released within the trigeminal ganglia suggesting possible local effects on satellite cells, a specialized type of glia that ensheaths trigeminal neurons. CGRP was shown to enhance satellite-cell production of interleukin 1beta (IL-1beta), while trigeminal neurons express an activity-dependent production of nitric oxide (NO). Thus, in the present study we tested the hypothesis that IL-1beta and NO induce trigeminal satellite cell activation, and that once activated these cells can influence neuronal responses. RESULTS: Primary cultures of rat trigeminal satellite cells isolated from neuronal cultures were characterized in vitro. Cyclooxygenase (COX) expression and activity were taken as a marker of glial pro-inflammatory activation. Most of the experiments were carried out to characterize satellite cell responses to the two different pro-inflammatory stimuli. Subsequently, medium harvested from activated satellite cells was used to test possible modulatory effects of glial factors on trigeminal neuronal activity. IL-1beta and the NO donor diethylenetriamine/nitric oxide (DETA/NO) elevated PGE2 release by satellite cells. The stimulatory effect of IL-1beta was mediated mainly by upregulation of the inducible form of COX enzyme (COX2), while NO increased the constitutive COX activity. Regardless of the activator used, it is relevant that short exposures of trigeminal satellite cells to both activators induced modifications within the cells which led to significant PGE2 production after removal of the pro-inflammatory stimuli. This effect allowed us to harvest medium from activated satellite cells (so-called 'conditioned medium') that did not contain any stimulus, and thus test the effects of glial factors on neuronal activation. Conditioned medium from satellite cells activated by either IL-1beta or NO augmented the evoked release of CGRP by trigeminal neurons. CONCLUSION: These findings indicate that satellite cells contribute to migraine-related neurochemical events and are induced to do so by autocrine/paracrine stimuli (such as IL-1beta and NO). The responsiveness of IL-1beta to CGRP creates the potential for a positive feedback loop and, thus, a plurality of targets for therapeutic intervention in migraine.

Flupirtine antinociception in the rat orofacial formalin test: an analysis of combination therapies with morphine and tramadol.

Combination therapy with two drugs is a straightforward strategy to improve the risk-benefit ratio of analgesic treatments. Flupirtine is a non-opioid analgesic drug acting via the enhancement of so-called M currents, associated to Kv7 potassium channels in the central nervous system. In this study we used the orofacial formalin test as a model of acute inflammatory pain in the rat; putative synergistic interactions between flupirtine and morphine or tramadol, given in various combinations, were investigated. We found that flupirtine exerts antinociception in the second phase of the test, whereas morphine and tramadol induced analgesia both in the first and in the second phase. An isobolographic analysis of data was carried out, showing a synergistic interaction between flupirtine and morphine, as well as between flupirtine and tramadol, in the second phase of the test. Conversely, in the first phase of the test only a single combination of morphine plus flupirtine, but not any of the combinations of tramadol and flupirtine, resulted in a synergistic interaction. Our data clearly indicate that flupirtine enhances in a synergistic manner the acute antinociceptive effects exerted by opioids in this paradigm.

Trigeminal satellite cells express functional calcitonin gene-related peptide receptors, whose activation enhances interleukin-1ß pro-inflammatory effects.

Calcitonin gene-related peptide (CGRP) is the main mediator of trigeminal pain signal. Functional CGRP receptors were detected in trigeminal satellite cells, a specialized type of glia found within the sensory ganglia. CGRP displayed modest pro-inflammatory effects per se on trigeminal satellite cells, while it significantly enhanced IL-1ß actions, increasing the expression and activity of cycloxygenase 2 as well as the expression of the inducible form of nitric oxide synthase and IL-1ß. CGRP effects were reverted by a specific CGRP receptor antagonist and mimicked by elevation of intracellular cAMP levels. CGRP exerted also minor proinflammatory effects on cortical astrocytes.

Trigeminal satellite cells modulate neuronal responses to triptans: relevance for migraine therapy.

In the present paper, we have further developed an in vitro model to study neuronal-glial interaction at trigeminal level by characterizing the effects of conditioned medium (CM) collected from activated primary cultures of satellite glial cells (SGCs) on calcitonin gene-related peptide (CGRP) release from rat trigeminal neurons. Moreover, we investigated whether such release is inhibited by a clinically relevant anti-migraine drug, sumatriptan. CM effects were tested on trigeminal neuronal cultures in different conditions of activation and at different time points. Long-term exposures of trigeminal neurons to CM increased directly neuronal CGRP release, which was further enhanced by the exposure to capsaicin. In this framework, the anti-migraine drug sumatriptan was able to inhibit the evoked CGRP release from naïve trigeminal neuron cultures, as well as from trigeminal cultures pre-exposed for 30 min to CM. On the contrary, sumatriptan failed to inhibit evoked CGRP release from trigeminal neurons after prolonged (4 and 8 h) pre-exposures to CM. These findings were confirmed in co-culture experiments (neurons and SGCs), where activation of SGCs or a bradykinin priming were used. Our data demonstrate that SGCs activation could influence neuronal excitability, and that this event affects the neuronal responses to triptans.

Peripheral antinociceptive effects of low doses of naloxone in an in vivo and in vitro model of trigeminal nociception.

Naloxone has been used to antagonize opioid effects for many years, even though at low doses it can exert antinociceptive effects. This 'paradoxical' analgesia has been detected after systemic administration of naloxone given alone or in combination with opioid drugs. In the present study, we investigated possible peripheral antinociceptive effects of low doses of naloxone using both an in vivo and in vitro model of trigeminal nociception. Low doses of naloxone injected locally into the rat wiskerpad elicited antinociceptive activity in the rat orofacial formalin test. The block of primary afferents with local administration of capsaicin suggested that naloxone acts both directly on sensory neurons and indirectly, by modulating the inflammatory component of the second phase of formalin test. Naloxone analgesia is maintained in rats made tolerant to the mu-receptor agonist DAMGO, suggesting the involvement of delta- and kappa-opioid receptors. Subsequently, the effects of very low doses of naloxone were tested in primary cultures of rat trigeminal neurons activated with bradykinin, in order to elucidate the mechanisms of action underlying naloxone antinociceptive effects. Naloxone inhibited bradykinin-evoked CGRP release in two different experimental paradigms, i.e. primed and unprimed cultures, acting at the level of delta- and kappa-opioids receptors. These results suggest that low doses of naloxone can directly modulate the activation of the trigeminal neurons by modulating the activity of specific opioid receptors, and this effect may be clinically relevant in combined therapies where an increased analgesic effect is sought through the potentiation of peripheral mechanisms.

Antinociceptive activity of buprenorphine and lumiracoxib in the rat orofacial formalin test: a combination analysis study.

Combination of two or more analgesics is widely used for the treatment of moderate and severe pain syndromes, allowing usage of lower doses of each compound and thereby limiting side effects; there is currently a large interest in investigating the potential advantages of combinations between opioids and non-steroidal inflammatory drugs (NSAIDs), coxibs in particular. The rat orofacial formalin test is a useful pre-clinical model of inflammatory trigeminal pain for evaluating antinociceptive activity of analgesics and their combinations. Injection of formalin in the rat wiskerpad induces a stereotyped response (rubbing), consisting of two distinct phases: a first 'phasic' phase and a second 'tonic' phase. In this work we tested a partial agonist to mu-opioid receptors, buprenorphine, and a selective cyclo-oxygenase-2 inhibitor, lumiracoxib, each of which given i.p. either alone or in combination. Buprenorphine reduced nociception both in the first and in the second phase, whereas lumiracoxib induced antinociception in the second phase only. The interaction between the two drugs was assessed through isobolographic analysis after combined administration at a fixed dose ratio. Such combination produced a dose-dependent antinociceptive effect in both phases. We observed a statistical difference between the theoretical and the experimental ED(50), which indicated synergistic interaction in the second phase. Concerning the first phase, we assumed that the antinociceptive effects were almost completely to be attributed to buprenorphine, since lumiracoxib was ineffective when administered alone. However, we found an unexpected difference between the theoretical and experimental ED(50), suggesting synergism in the first phase as well.

Buprenorphine inhibits bradykinin-induced release of calcitonin gene-related peptide from rat trigeminal neurons via both mu-opioid and nociceptin/orphanin peptide receptors.

In this study we used the dual opioid and nociceptin/orphanin peptide (NOP) agonist buprenorphine to investigate the relative contributions of opioid and NOP systems in regulating bradykinin-stimulated calcitonin-gene related peptide (CGRP) release from primary cultures of neonatal rat trigeminal neurons. We found that: bradykinin stimulates CGRP secretion either by a direct effect or after applying so-called "bradykinin-priming" protocol. In both cases, buprenorphine was able to inhibit bradykinin-stimulated CGRP secretion; however, inhibition was mediated by NOP receptors when buprenorphine was added to the incubation medium along with bradykinin, whereas it appeared to be mediated by mu-opioid receptors in bradykinin priming experiments. Bradykinin treatments also caused an increase in neuronal prostaglandin production; prostanoids appeared to be involved in the stimulatory effects of bradykinin as well as in buprenorphine inhibition, through apparently unrelated mechanisms.