Peter J. Cabot

Immune cell-derived beta-endorphin. Production, release, and control of inflammatory pain in rats.

Localized inflammation of a rats hindpaw elicits an accumulation of beta-endorphin-(END) containing immune cells. We investigated the production, release, and antinociceptive effects of lymphocyte-derived END in relation to cell trafficking. In normal animals, END and proopiomelanocortin mRNA were less abundant in circulating lymphocytes than in those residing in lymph nodes (LN), suggesting that a finite cell population produces END and homes to LN. Inflammation increased proopiomelanocortin mRNA in cells from noninflamed and inflamed LN. However, END content was increased only in inflamed paw tissue and noninflamed LN-immune cells. Accordingly, corticotropin-releasing factor and IL-1beta released significantly more END from noninflamed than from inflamed LN-immune cells. This secretion was receptor specific, calcium dependent, and mimicked by potassium, consistent with vesicular release. Finally, both agents, injected into the inflamed paw, induced analgesia which was blocked by the co-administration of antiserum against END. Together, these findings suggest that END-producing lymphocytes home to inflamed tissue where they secrete END to reduce pain. Afterwards they migrate to the regional LN, depleted of the peptide. Consistent with this notion, immunofluorescence studies of cell suspensions revealed that END is contained predominantly within memory-type T cells. Thus, the immune system is important for the control of inflammatory pain. This has implications for the understanding of pain in immunosuppressed conditions like cancer or AIDS.

Pain control in inflammation governed by selectins

Opioid-containing immune cells migrate preferentially to inflamed sites, where they release β-endorphin which activates peripheral opioid receptors to inhibit pain. Immunocyte recruitment is a multistep, sequential engagement of various adhesion molecules located on immune cells and vascular endothelium. Selectins mediate the initial phase of immunoctye extravasation into inflamed sites. Here we show that anti-selectin treatment abolishes peripheral opioid analgesia elicited either endogenously (by stress) or by corticotropin-releasing factor. This results from a blockade of the infiltration of immunocytes containing β-endorphin and the consequent decrease of the β-endorphin content in the inflamed tissue. These findings indicate that the immune system uses mechanisms of cell migration not only to fight pathogens but also to control pain in injured tissue. Thus, pain is exacerbated by measures inhibiting the immigration of opioid-producing cells or, conversely, analgesia might be conveyed by adhesive interactions that recruit those cells to injured tissue.

The novel N-type calcium channel blocker, AM336, produces potent dose-dependent antinociception after intrathecal dosing in rats and inhibits substance P release in rat spinal cord slices.

&NA; N‐type calcium channels modulate the release of key pro‐nociceptive neurotransmitters such as glutamate and substance P (SP) in the central nervous system. Considerable research interest has focused on the therapeutic potential of the peptidic &ohgr;‐conopeptides, GVIA and MVIIA as novel analgesic agents, due to their potent inhibition of N‐type calcium channels. Recently, the novel peptidic N‐type calcium channel blocker, AM336, was isolated from the venom of the cone snail, Conus catus. Thus, the aims of this study were to (i) document the antinociceptive effects of AM336 (also known as CVID) relative to MVIIA following intrathecal (i.t.) bolus dosing in rats with adjuvant‐induced chronic inflammatory pain of the right hindpaw and to (ii) quantify the inhibitory effects of AM336 relative to MVIIA on K+‐evoked SP release from slices of rat spinal cord. Both AM336 and MVIIA inhibited the K+‐evoked release of the pro‐nociceptive neurotransmitter, SP, from rat spinal cord slices in a concentration‐dependent manner (EC50 values=21.1 and 62.9 nM, respectively), consistent with the antinociceptive actions of &ohgr;‐conopeptides. Following acute i.t. dosing, AM336 evoked dose‐dependent antinociception (ED50≈0.110 nmol) but the doses required to produce side‐effects were an order of magnitude larger than the doses required to produce antinociception. For i.t. doses of MVIIA≤0.07 nmol, dose‐dependent antinociception was also produced (ED50≈0.016 nmol). Unexpectedly, however, i.t. doses of MVIIA>0.07 nmol, produced a dose‐dependent decrease in antinociception but the incidence and severity of the side‐effects continued to increase for all doses of MVIIA investigated, suggesting that dose‐titration with MVIIA in the clinical setting, may be difficult.

Methionine-enkephalin-and Dynorphin A-release from immune cells and control of inflammatory pain

&NA; We have previously shown that &bgr;‐endorphin (END) is contained and released from memory‐type T‐cells within inflamed tissue and that it is capable to control pain (J Clin Invest 100(1) (1997) 142). Methionine‐enkephalin (MET) and Dynorphin‐A (DYN) are endogenous opioids with preference for &dgr;‐ and &kgr;‐opioid receptors, respectively. Both MET and DYN are produced and contained within immune cells. The goal of this study was to determine the release characteristics of MET and DYN in a rat model of localized hindpaw inflammation and to examine the antinociceptive role of MET and DYN in a Freunds adjuvant induced model of inflammatory pain. We found that corticotropin‐releasing factor (CRF) can stimulate the release of both MET and DYN from lymphocytes. This release is dose‐dependent and reversible by the selective CRF antagonist &agr;‐helical‐CRF. Furthermore, CRF (1.5 ng) produces analgesia when injected into the inflamed paw, which is reversible by direct co‐administration of antibodies to MET. Lymphocyte content of MET was 7.0±1.4 ng/million cells, whilst DYN content was ˜30‐fold lower. Both END and DYN, but not MET, were released by IL‐1. Consistently, IL‐1 produced peripheral analgesic effects which were not reversed by antibodies to MET. These results indicate that both MET and DYN play a role in peripheral analgesia but have different characteristics of release. These studies further support a role of the immune system in the control of inflammatory pain. This may be particularly important in patients suffering from compromised immune systems as with cancer and AIDS.

Morphine and tumor growth and metastasis

Morphine is an analgesic widely used to alleviate cancer pain. In addition, the perioperative management of pain in cancer surgery patients most often includes opioids. However, there are reports that these drugs may alter cancer recurrence or metastasis. Several mechanisms have been proposed, such as the modulation of the immune response or cellular pathways that control the survival and migratory behavior of cancer cells. The published literature, however, presents some discrepancies, with reports suggesting that opioids may either promote or prevent the spread of cancer. It is of great importance to determine whether opioids, in particular the most widely used, morphine, may increase the risk of metastasis when used in cancer surgery. This review examines the available data on the effects of morphine which influence cancer metastasis or recurrence, including immunomodulation, tumor cell aggressiveness, and angiogenesis, with special emphasis on recently published clinical and laboratory based studies. We further discuss the parameters that may explain the difference between reports on the effects of morphine on cancer.

The μ opioid agonist morphine modulates potentiation of capsaicin-evoked TRPV1 responses through a cyclic AMP-dependent protein kinase A pathway

BackgroundThe vanilloid receptor 1 (TRPV1) is critical in the development of inflammatory hyperalgesia. Several receptors including G-protein coupled prostaglandin receptors have been reported to functionally interact with the TRPV1 through a cAMP-dependent protein kinase A (PKA) pathway to potentiate TRPV1-mediated capsaicin responses. Such regulation may have significance in inflammatory pain. However, few functional receptor interactions that inhibit PKA-mediated potentiation of TRPV1 responses have been described.ResultsIn the present studies we investigated the hypothesis that the μ opioid receptor (MOP) agonist morphine can modulate forskolin-potentiated capsaicin responses through a cAMP-dependent PKA pathway. HEK293 cells were stably transfected with TRPV1 and MOP, and calcium (Ca2+) responses to injection of the TRPV1 agonist capsaicin were monitored in Fluo-3-loaded cells. Pre-treatment with morphine did not inhibit unpotentiated capsaicin-induced Ca2+ responses but significantly altered capsaicin responses potentiated by forskolin. TRPV1-mediated Ca2+ responses potentiated by the direct PKA activator 8-Br-cAMP and the PKC activator Phorbol-12-myristate-13-acetatewere not modulated by morphine.Immunohistochemical studies confirmed that the TRPV1 and MOP are co-expressed on cultured Dorsal Root Ganglion neurones, pointing towards the existence of a functional relationship between the G-protein coupled MOP and nociceptive TRPV1.ConclusionThe results presented here indicate that the opioid receptor agonist morphine acts via inhibition of adenylate cyclase to inhibit PKA-potentiated TRPV1 responses. Targeting of peripheral opioid receptors may therefore have therapeutic potential as an intervention to prevent potentiation of TRPV1 responses through the PKA pathway in inflammation.

Ciguatoxins activate specific cold pain pathways to elicit burning pain from cooling

Ciguatoxins are sodium channel activator toxins that cause ciguatera, the most common form of ichthyosarcotoxism, which presents with peripheral sensory disturbances, including the pathognomonic symptom of cold allodynia which is characterized by intense stabbing and burning pain in response to mild cooling. We show that intraplantar injection of P‐CTX‐1 elicits cold allodynia in mice by targeting specific unmyelinated and myelinated primary sensory neurons. These include both tetrodotoxin‐resistant, TRPA1‐expressing peptidergic C‐fibres and tetrodotoxin‐sensitive A‐fibres. P‐CTX‐1 does not directly open heterologously expressed TRPA1, but when co‐expressed with Nav channels, sodium channel activation by P‐CTX‐1 is sufficient to drive TRPA1‐dependent calcium influx that is responsible for the development of cold allodynia, as evidenced by a large reduction of excitatory effect of P‐CTX‐1 on TRPA1‐deficient nociceptive C‐fibres and of ciguatoxin‐induced cold allodynia in TRPA1‐null mutant mice. Functional MRI studies revealed that ciguatoxin‐induced cold allodynia enhanced the BOLD (Blood Oxygenation Level Dependent) signal, an effect that was blunted in TRPA1‐deficient mice, confirming an important role for TRPA1 in the pathogenesis of cold allodynia.

An animal model of oxaliplatin-induced cold allodynia reveals a crucial role for Nav1.6 in peripheral pain pathways.

Summary Cold allodynia elicited by local intraplantar injection of the chemotherapeutic agent oxaliplatin is mediated through Nav1.6‐expressing peripheral sensory fibres. Activation of Nav1.6 alone elicits only mechanical allodynia and spontaneous pain, but when combined with inhibition of Kv channels, profound cold allodynia develops. ABSTRACT Cold allodynia, pain in response to cooling, occurs during or within hours of oxaliplatin infusion and is thought to arise from a direct effect of oxaliplatin on peripheral sensory neurons. To characterize the pathophysiological mechanisms underlying acute oxaliplatin‐induced cold allodynia, we established a new intraplantar oxaliplatin mouse model that rapidly developed long‐lasting cold allodynia mediated entirely through tetrodotoxin‐sensitive Nav pathways. Using selective inhibitors and knockout animals, we found that Nav1.6 was the key isoform involved, while thermosensitive transient receptor potential channels were not involved. Consistent with a crucial role for delayed‐rectifier potassium channels in excitability in response to cold, intraplantar administration of the K+‐channel blocker 4‐aminopyridine mimicked oxaliplatin‐induced cold allodynia and was also inhibited by Nav1.6 blockers. Intraplantar injection of the Nav1.6 activator Cn2 elicited spontaneous pain, mechanical allodynia, and enhanced 4‐aminopyridine‐induced cold allodynia. These findings provide behavioural evidence for a crucial role of Nav1.6 in multiple peripheral pain pathways including cold allodynia.

Non-Stimulated, Agonist-Stimulated and Store-Operated Ca2+ Influx in MDA-MB-468 Breast Cancer Cells and the Effect of EGF-Induced EMT on Calcium Entry

In addition to their well-defined roles in replenishing depleted endoplasmic reticulum (ER) Ca2+ reserves, molecular components of the store-operated Ca2+ entry pathway regulate breast cancer metastasis. A process implicated in cancer metastasis that describes the conversion to a more invasive phenotype is epithelial-mesenchymal transition (EMT). In this study we show that EGF-induced EMT in MDA-MB-468 breast cancer cells is associated with a reduction in agonist-stimulated and store-operated Ca2+ influx, and that MDA-MB-468 cells prior to EMT induction have a high level of non-stimulated Ca2+ influx. The potential roles for specific Ca2+ channels in these pathways were assessed by siRNA-mediated silencing of ORAI1 and transient receptor potential canonical type 1 (TRPC1) channels in MDA-MB-468 breast cancer cells. Non-stimulated, agonist-stimulated and store-operated Ca2+ influx were significantly inhibited with ORAI1 silencing. TRPC1 knockdown attenuated non-stimulated Ca2+ influx in a manner dependent on Ca2+ influx via ORAI1. TRPC1 silencing was also associated with reduced ERK1/2 phosphorylation and changes in the rate of Ca2+ release from the ER associated with the inhibition of the sarco/endoplasmic reticulum Ca2+-ATPase (time to peak [Ca2+]CYT = 188.7±34.6 s (TRPC1 siRNA) versus 124.0±9.5 s (non-targeting siRNA); P<0.05). These studies indicate that EMT in MDA-MB-468 breast cancer cells is associated with a pronounced remodeling of Ca2+ influx, which may be due to altered ORAI1 and/or TRPC1 channel function. Our findings also suggest that TRPC1 channels in MDA-MB-468 cells contribute to ORAI1-mediated Ca2+ influx in non-stimulated cells.

Rapid, Opioid-sensitive Mechanisms Involved in Transient Receptor Potential Vanilloid 1 Sensitization

TRPV1 is a nociceptive, Ca2+-selective ion channel involved in the development of several painful conditions. Sensitization of TRPV1 responses by cAMP-dependent PKA crucially contributes to the development of inflammatory hyperalgesia. However, the pathways involved in potentiation of TRPV1 responses by cAMP-dependent PKA remain largely unknown. Using HEK cells stably expressing TRPV1 and the μ opioid receptor, we demonstrated that treatment with the adenylate cyclase activator forskolin significantly increased the multimeric TRPV1 species. Pretreatment with the μ opioid receptor agonist morphine reversed this increased TRPV1 multimerization. FRET analysis revealed that treatment with forskolin did not cause multimerization of pre-existing TRPV1 monomers on the plasma membrane and that intracellular pools of TRPV1 exist mostly as monomers in this model. This suggests that increased TRPV1 multimerization occurred from an intracellular store of inactive TRPV1 monomers. Treatment with forskolin also caused an increase in TRPV1 expression on the plasma membrane not resulting from increased TRPV1 expression, and this rapid TRPV1 translocation was inhibited by treatment with morphine. Thus, potentiation of TRPV1 responses by cAMP-dependent PKA involves plasma membrane insertion of functional TRPV1 multimers formed from an intracellular store of inactive TRPV1 monomers. This potentiation occurs rapidly and can be dynamically modulated by activation of the μ opioid receptor under conditions where cAMP levels are raised, such as with inflammation. Increased translocation and multimerization of TRPV1 channels provide a cellular mechanism for finetuning of nociceptive responses that allow for rapid modulation of TRPV1 responses independent of transcriptional changes.