Radiation treatment decreases bone cancer pain through direct effect on tumor cells

The most used treatment for bone cancer pain is radiation; however, the mechanism responsible for analgesia after irradiation is unknown. The mechanistic influence of a single, localized 10-, 20- or 30-Gy dose of radiation on painful behaviors, osteolysis, histopathology and osteoclast number was evaluated in mice with painful femoral sarcomas. Dramatic reductions in pain behaviors (P < 0.05) and osteolysis (P < 0.0001) were seen in mice irradiated with 20 and 30 Gy. Irradiation reduced the tumor area by more than 75% (P < 0.05) but did not affect osteoclast frequency per mm2 tumor. Treatment with 20 Gy prior to tumor injection had no effect on tumor growth or pain behaviors, suggesting that radiation reduces osteolysis and pain through direct tumor effects. To demonstrate that tumor elimination was responsible for reduction in osteolysis and pain, sarcoma cells containing the suicide gene cytosine deaminase (CD) were inoculated into femora. After onset of bone cancer pain, mice were treated with the prodrug 5-fluorocytosine (5-FC). 5-FC treatment significantly reduced both osteolysis (P < 0.0005) and bone cancer pain (P < 0.05). The findings in this study demonstrate that one mechanism through which radiation decreases bone cancer pain is by direct effects on tumor cells.

Analgesic Efficacy of Bradykinin B1 Antagonists in a Murine Bone Cancer Pain Model

Cancer pain is a significant clinical problem because it is the first symptom of disease in 20% to 50% of all cancer patients, and 75% to 90% of patients with advanced or terminal cancer must cope with chronic pain syndromes related to failed treatment and/or tumor progression. One of the most difficult to treat cancer pains is metastatic invasion of the skeleton that can generate ongoing and bone breakthrough pain, which represents one of the most debilitating cancer-related events. Because bradykinin has been shown to be released in response to tissue injury and plays a significant role in driving acute and chronic inflammatory pain, we focused on bradykinin antagonists in a model of bone cancer pain. In our model of bone cancer, which involves the injection and confinement of 2472 sarcoma cells to the mouse femur, pharmacologic blockade of the bradykinin B1 receptor is effective in reducing pain-related behaviors at both early and advanced stages of bone cancer.

PERSPECTIVE

Bone cancer pain can be severe and difficult to control fully. With a mouse model of bone cancer pain we demonstrate that pharmacologic blockade of the bradykinin B1 receptor is effective in reducing bone cancer pain-related behaviors, suggesting that B1 antagonists might be useful in attenuating bone cancer pain in humans.

A Blocking Antibody to Nerve Growth Factor Attenuates Skeletal Pain Induced by Prostate Tumor Cells Growing in Bone

Prostate cancer is unique in that bone is often the only clinically detectable site of metastasis. Prostate tumors that have metastasized to bone frequently induce bone pain which can be difficult to fully control as it seems to be driven simultaneously by inflammatory, neuropathic, and tumorigenic mechanisms. As nerve growth factor (NGF) has been shown to modulate inflammatory and some neuropathic pain states in animal models, an NGF-sequestering antibody was administered in a prostate model of bone cancer where significant bone formation and bone destruction occur simultaneously in the mouse femur. Administration of a blocking antibody to NGF produced a significant reduction in both early and late stage bone cancer pain-related behaviors that was greater than or equivalent to that achieved with acute administration of 10 or 30 mg/kg of morphine sulfate. In contrast, this therapy did not influence tumor-induced bone remodeling, osteoblast proliferation, osteoclastogenesis, tumor growth, or markers of sensory or sympathetic innervation in the skin or bone. One rather unique aspect of the sensory innervation of bone, that may partially explain the analgesic efficacy of anti-NGF therapy in relieving prostate cancer-induced bone pain, is that nearly all nerve fibers that innervate the bone express trkA and p75, and these are the receptors through which NGF sensitizes and/or activates nociceptors. The present results suggest that anti-NGF therapy may be effective in reducing pain and enhancing the quality of life in patients with prostate tumor-induced bone cancer pain.

Bone cancer pain: from model to mechanism to therapy

Pain is the cancer-related event that is most disruptive to the cancer patient's quality of life. Although bone cancer pain is one of the most severe and common of the chronic pains that accompany breast, prostate, and lung cancers, relatively little is known about the mechanisms that generate and maintain this pain. Recently, we developed a mouse model of bone cancer pain. Ten days following tumor implantation into the intramedullary space of the femur, significant bone destruction and bone cancer pain-related behaviors were observed and progressed in severity over time. A critical question is how closely this model mirrors human bone cancer pain. In a recent publication, we show that, as in humans, pain-related behaviors are diminished by systemic morphine administration in a dose-dependent fashion that is naloxone-reversible. Humans suffering from bone cancer pain generally require significantly higher doses of morphine as compared to individuals with inflammatory pain and in the mouse model the doses of morphine required to block bone cancer pain-related behaviors were 10 times that required to block peak inflammatory pain behaviors of comparable magnitude induced by hindpaw injection of complete Freund's adjuvant (CFA; 1-3 mg/kg). As these animals were treated acutely, there was not time for morphine tolerance to develop and the rightward shift in analgesic efficacy observed in bone cancer pain versus inflammatory pain suggests a fundamental difference in the underlying mechanisms that generate bone cancer versus inflammatory pain. These results indicate that this model will be useful in defining drug therapies that are targeted for complex bone cancer pain syndromes.

Anti-NGF therapy profoundly reduces bone cancer pain and the accompanying increase in markers of peripheral and central sensitization

Bone cancer pain can be difficult to control, as it appears to be driven simultaneously by inflammatory, neuropathic and tumorigenic mechanisms. As nerve growth factor (NGF) has been shown to modulate inflammatory and neuropathic pain states, we focused on a novel NGF sequestering antibody and demonstrated that two administrations of this therapy in a mouse model of bone cancer pain produces a profound reduction in both ongoing and movement-evoked bone cancer pain-related behaviors that was greater than that achieved with acute administration of 10 or 30 mg/kg of morphine. This therapy also reduced several neurochemical changes associated with peripheral and central sensitization in the dorsal root ganglion and spinal cord, whereas the therapy did not influence disease progression or markers of sensory or sympathetic innervation in the skin or bone. Mechanistically, the great majority of sensory fibers that innervate the bone are CGRP/TrkA expressing fibers, and if the sensitization and activation of these fibers is blocked by anti-NGF therapy there would not be another population of nociceptors, such as the non-peptidergic IB4/RET-IR nerve fibers, to take their place in signaling nociceptive events.

Tumor-induced injury of primary afferent sensory nerve fibers in bone cancer pain

Bone is the most common site of chronic pain in patients with metastatic cancer. What remains unclear are the mechanisms that generate this pain and why bone cancer pain can be so severe and refractory to treatment with opioids. Here we show that following injection and confinement of NCTC 2472 osteolytic tumor cells within the mouse femur, tumor cells sensitize and injure the unmyelinated and myelinated sensory fibers that innervate the marrow and mineralized bone. This tumor-induced injury of sensory nerve fibers is accompanied by an increase in ongoing and movement-evoked pain behaviors, an upregulation of activating transcription factor 3 (ATF3) and galanin by sensory neurons that innervate the tumor-bearing femur, upregulation of glial fibrillary acidic protein (GFAP) and hypertrophy of satellite cells surrounding sensory neuron cell bodies within the ipsilateral dorsal root ganglia (DRG), and macrophage infiltration of the DRG ipsilateral to the tumor-bearing femur. Similar neurochemical changes have been described following peripheral nerve injury and in other non-cancerous neuropathic pain states. Chronic treatment with gabapentin did not influence tumor growth, tumor-induced bone destruction or the tumor-induced neurochemical reorganization that occurs in sensory neurons or the spinal cord, but it did attenuate both ongoing and movement-evoked bone cancer-related pain behaviors. These results suggest that even when the tumor is confined within the bone, a component of bone cancer pain is due to tumor-induced injury to primary afferent nerve fibers that innervate the tumor-bearing bone. Tumor-derived, inflammatory, and neuropathic mechanisms may therefore be simultaneously driving this chronic pain state.

Pathophysiology of bone cancer pain

The most common cancers, such as those affecting the breast, prostate, and lung have a strong predilection to metastasize to bone. Bone metastasis frequently results in pain, pathologic fractures, hypercalcemia, and spinal cord compression. Pain can have a devastating effect on the quality of life in advanced cancer patients and is a serious complication of cancer. Although significant advances are being made in cancer treatment and diagnosis, the basic neurobiology of bone cancer pain is poorly understood. New insights into the mechanisms that induce cancer pain now are coming from animal models. Chemicals derived from tumor cells, inflammatory cells, and cells derived from bone appear to be involved simultaneously in driving this frequently difficult-to-control pain state. Understanding the mechanisms involved in the pathophysiology of bone cancer pain will improve both our ability to provide mechanism-based therapies and the quality of life of cancer patients.

Bone cancer pain: the effects of the bisphosphonate alendronate on pain, skeletal remodeling, tumor growth and tumor necrosis

Patients with metastatic breast, lung or prostate cancer frequently have significant bone cancer pain. In the present report we address, in a single in vivo mouse model, the effects the bisphosphonate alendronate has on bone cancer pain, bone remodeling and tumor growth and necrosis. Following injection and confinement of green fluorescent protein-transfected murine osteolytic tumor cells into the marrow space of the femur of male C3H/HeJ mice, alendronate was administered chronically from the time the tumor was established until the bone cancer pain became severe. Alendronate therapy reduced ongoing and movement-evoked bone cancer pain, bone destruction and the destruction of sensory nerve fibers that innervate the bone. Whereas, alendronate treatment did not change viable tumor burden, both tumor growth and tumor necrosis increased. These data emphasize that it is essential to utilize a model where pain, skeletal remodeling and tumor growth can be simultaneously assessed, as each of these can significantly impact patient quality of life and survival.

Spinal neurons that express NK-1 receptors modulate descending controls that project through the dorsolateral funiculus

Selective ablation of spinal neurons possessing substance P receptors (NK-1 receptors) using the selective cytotoxin conjugate substance P-saporin (SP-SAP) decreases hyperalgesia and central sensitization. The mechanisms by which NK-1 expressing neurons modulate the excitability of other dorsal horn neurons are unclear. Because the majority of NK-1 expressing spinal neurons project rostrally, it is possible that they are part of a spinal-supraspinal circuitry that contributes to descending modulation of excitability of spinal nociceptive neurons. We therefore determined whether ablation of spinal neurons that possess the NK-1 receptor altered descending systems that travel via the dorsolateral funiculus (DLF). Spontaneous activity and responses of dorsal horn neurons evoked by mechanical (von Frey monofilaments) and heat (35-51 degrees C) stimuli were determined before and after transection of the DLF and were compared in rats pretreated with intrathecal application of vehicle or SP-SAP. In vehicle-treated rats, transection of the DLF caused a 233% increase in mean spontaneous activity of neurons and enhanced their responses to mechanical and heat stimuli, whereas these increases in excitation were blocked in rats pretreated with SP-SAP. Importantly, SP-SAP alone had no effect on spontaneous or evoked activity in the absence of DLF transection. These results demonstrate that spinal neurons expressing the NK-1 receptor appear to play a pivotal role in regulating descending systems that modulate activity of nociceptive dorsal horn neurons.

Bone cancer pain and the role of RANKL/OPG

Cancer-induced bone diseases are common and can have a devastating impact at the end of life. One of the most difficult sequelae of cancer is metastases to the skeleton, an event that results in bone destruction and bone cancer pain. Bone cancer pain is usually progressive as the disease advances, and is particularly difficult to treat. Recently, experimental models of bone cancer pain have been developed and have provided seminal insight in understanding the pathophysiology of bone cancer pain. Animal models of bone cancer provided the finding that bone destruction (osteolysis) is associated with pain, and it has been determined that cancer-induced osteolysis is mediated by osteoclasts. Having established that RANK ligand contributed to cancer-induced osteoclastogenesis, it was determined that disruption of the RANKL-RANK axis with OPG inhibited tumor-induced osteoclastogenesis and decreased bone cancer pain.

Constitutive spinal cyclooxygenase-2 participates in the initiation of tissue injury-induced hyperalgesia

Inhibitors of the isozyme cyclooxygenase-2 (COX-2) represent an important advance in pain management, although where and when these inhibitors can exert their antihyperalgesic actions are not completely understood. Here we show that unlike many peripheral tissues in which COX-2 is only expressed in physiologically significant levels after tissue injury, in the normal rat lumbar spinal cord, the majority of neurons and radial glia constitutively express high levels of COX-2 protein. Immediately after peripheral tissue injury and before any measurable upregulation of COX-2 protein in peripheral tissue or spinal cord, inhibition of constitutively expressed spinal COX-2 reduced injury-induced activation of primary afferent neurons, activation of spinal neurons, and the mechanical and thermal hyperalgesia that normally occurs after peripheral tissue injury. The present data demonstrate that constitutively expressed spinal COX-2 plays an important role in the initial hyperalgesia that follows peripheral tissue injury. These results suggest that blocking constitutive spinal COX-2 before tissue injury may reduce the initial peripheral and central sensitization that occurs after tissue injury.

Bone cancer pain: from mechanism to model to therapy

Although bone cancer pain can be severe and is relatively common, very little is known about the basic mechanisms that generate and maintain this debilitating pain. To begin to define the mechanisms that give rise to bone cancer pain, a mouse model was developed using the intramedullary injection and containment of osteolytic sarcoma cells in the mouse femur. These tumor cells induced bone destruction as well as ongoing and movement-evoked pain behaviors similar to that found in patients with bone cancer pain. In addition, there was a significant reorganization of the spinal cord that received sensory input from the cancerous bone, and this reorganization was significantly different from that observed in mouse models of chronic neuropathic or inflammatory pain. To determine whether this mouse model of bone cancer could be used to define the basic mechanisms giving rise to bone cancer pain, we targeted excessive osteoclast activity using osteoprotegerin, a secreted decoy receptor that inhibits osteoclast activity. Osteoprotegerin blocked excessive tumor-induced, osteoclast-mediated bone destruction, and significantly reduced ongoing and movement-evoked pain, and the neurochemical reorganization of the spinal cord. These data suggest that this model can provide insight into the mechanisms that generate bone cancer pain and provide a platform for developing and testing novel analgesics to block bone cancer pain.

Spinal neurons involved in the generation of at-level pain following spinal injury in the rat

Using a conjugate of substance P and the ribosome-inactivating protein saporin, neurons expressing the neurokinin-1 receptor in lamina I of the spinal cord were targeted to determine their role in the expression of a spontaneous pain behavior following intraspinal injections of quisqualic acid in the rat. Treatment was carried out at the time of injury in order to prevent the onset of the behavior, and following onset in order to evaluate the potential clinical utility of this intervention. Treatment at the time of injury resulted in significant decreases in onset-time and severity of pain behavior, while treatment at the time of onset led to a significant reduction of the spontaneous self-directed behavior. The results suggest that the substrate for at-level pain following spinal cord injury includes a population of spinal neurons expressing the neurokinin-1 receptor in the superficial laminae of the spinal cord.

Radiation Treatment Decreases Bone Cancer Pain, Osteolysis and Tumor Size

Radiotherapy is the cornerstone of palliative treatment for primary bone cancer in animals and metastatic bone cancer in humans. However, the mechanism(s) responsible for pain relief after irradiation is unknown. To identify the mechanism through which radiation treatment decreases bone cancer pain, the effect of radiation on mice with painful bone cancer was studied. Analysis of the effects of a 20-Gy treatment on localized sites of painful bone cancers was performed through assessments of animal behavior, radiographs and histological analysis. The findings indicated that radiation treatment reduced bone pain and supported reduced cancer burden and reduced osteolysis as mechanisms through which radiation reduces bone cancer pain.

Mechanisms that generate and maintain bone cancer pain

Although bone cancer pain can be severe and is relatively common, as it frequently arises from metastases from breast, prostate, and lung tumours, very little is known about the basic mechanisms that generate and maintain this chronic pain. To begin to define the mechanisms that give rise to bone cancer pain, we have developed mouse and rat models using the intramedullary injection and containment of tumour cells into the femur. These tumour cells induced bone remodelling as well as ongoing and movement evoked pain behaviours similar to that found in patients with bone cancer pain. In addition there is a significant reorganization of the spinal cord that received sensory input from the cancerous bone and this reorganization generated a neurochemical signature of bone cancer pain that is both dramatic and significantly different from that observed in mouse and rat models of chronic neuropathic or inflammatory pain. These models have provided insight into the mechanisms that drive cancer pain and have begun to allow the development of mechanism-based therapies. Together these advances should reduce tumour-induced pain and suffering and significantly improve the quality of life of cancer patients.

A mechanism-based understanding of bone cancer pain

Although bone cancer pain can be severe and is relatively common, as it frequently arises from metastases from breast, prostate and lung tumours, relatively little is known about the basic mechanisms that generate and maintain this chronic pain. To begin to define the mechanisms that give rise to bone cancer pain, we developed a mouse model using the intramedullary injection and containment of osteolytic sarcoma cells into the mouse femur. These tumour cells induced bone destruction as well as ongoing and movement evoked pain behaviours similar to that found in patients with bone cancer pain. In addition, there was a significant neurochemical reorganization of sensory neurons that innervate the tumour bearing bone as well as in the spinal cord segments that received sensory input from the cancerous bone. This reorganization generated a neurochemical signature of bone cancer pain that was different from that observed in mouse models of chronic neuropathic or inflammatory pain. These data suggest that there is an inflammatory, neuropathic and tumorigenic component to bone cancer pain. Therefore defining when and how these different components drive bone cancer pain may allow the development of more selective analgesic agents to treat this chronic pain state.

Endothelin and the tumorigenic component of bone cancer pain

Tumors including sarcomas and breast, prostate, and lung carcinomas frequently grow in or metastasize to the skeleton where they can induce significant bone remodeling and cancer pain. To define products that are released from tumors that are involved in the generation and maintenance of bone cancer pain, we focus here on endothelin-1 (ET-1) and endothelin receptors as several tumors including human prostate and breast have been shown to express high levels of ETs and the application of ETs to peripheral nerves can induce pain. Here we show that in a murine osteolytic 2472 sarcoma model of bone cancer pain, the 2472 sarcoma cells express high levels of ET-1, but express low or undetectable levels of endothelin A (ETAR) or B (ETBR) receptors whereas a subpopulation of sensory neurons express the ETAR and non-myelinating Schwann cells express the ETBR. Acute (10 mg/kg, i.p.) or chronic (10 mg/kg/day, p.o.) administration of the ETAR selective antagonist ABT-627 significantly attenuated ongoing and movement-evoked bone cancer pain and chronic administration of ABT-627 reduced several neurochemical indices of peripheral and central sensitization without influencing tumor growth or bone destruction. In contrast, acute treatment (30 mg/kg, i.p.) with the ETBR selective antagonist, A-192621 increased several measures of ongoing and movement evoked pain. As tumor expression and release of ET-1 has been shown to be regulated by the local environment, location specific expression and release of ET-1 by tumor cells may provide insight into the mechanisms that underlie the heterogeneity of bone cancer pain that is frequently observed in humans with multiple skeletal metastases.

Activation of lamina I spinal cord neurons that express the substance P receptor in visceral nociception and hyperalgesia

Spinal lamina I neurons expressing the substance P receptor (SPR) have been shown to play a role in the transmission of somatic inflammatory and neuropathic pain. To evaluate their involvement in visceral nociception in both the noninflamed and inflamed colon, we examined the expression and ligand-induced internalization of the SPR in the rat spinal cord after distention of the noninflamed colon and in rats with inflammation induced by intracolonic instillation of zymosan (3 hours). In the noninflamed animal, acute noxious but not non-noxious colorectal distention induced SPR internalization in lamina I neurons at the thoracolumbar (T13) and lumbosacral (S1) spinal levels, whereas SPR internalization was not detected in lamina I neurons at spinal lumbar segment L4. Although zymosan-induced colorectal inflammation alone did not induce SPR internalization in lamina I neurons, there was an increased number of SPR-expressing lamina I neurons showing SPR internalization in segments T12 through S2 of the spinal cord after colorectal distention. These results show that acute noxious visceral stimuli induce activation of spinal lamina I neurons expressing the SPR and, that after visceral inflammation, there is a marked increase in both the number and rostrocaudal extent of lamina I SPR neurons activated in response to both normally non-noxious and noxious distention of the colon.

Bone cancer pain

BACKGROUND

Bone cancer pain is very common, and patients with this type of pain may be difficult to treat. Development of an experimental model for studying this condition is critical to advancing an understanding of the mechanisms that cause pain in patients with malignant disease.

METHODS

A murine model of bone cancer was studied. Combined analysis of the extent of tumor-induced bone destruction, pain, and neurochemical characterization of the peripheral and central nervous systems was performed to investigate bone cancer pain. Disease-induced bone destruction was assessed by radiographs and histomorphometry. Pain was assessed by spontaneous and elicited behaviors, and neurochemical analysis involved immunohistochemical detection of hyperalgesic peptides and neurochemical markers.

RESULTS

Mice with distal femoral sarcomas exhibited behavioral and neurochemical measures of pain. The pain condition created by malignant bone disease was distinct neurochemically from inflammatory and neuropathic pain states. Experimental evidence indicated that both disease-induced osteolysis and tumors themselves contributed to the generation of pain and that peripheral and central sensitization of the nervous system was present.

CONCLUSIONS

Malignant bone disease creates a unique pain state that involves sensitization of the nervous system. Major contributors to the pain state within the bone tissue are osteoclastic bone resorption and the malignant disease itself.

Increased neurogenesis and brain-derived neurotrophic factor in neurokinin-1 receptor gene knockout mice

It has previously been shown that chronic treatment with antidepressant drugs increases neurogenesis and levels of brain-derived neurotrophic factor in the hippocampus. These changes have been correlated with changes in learning and long-term potentiation and may contribute to the therapeutic efficacy of antidepressant drug treatment. Recently, antagonists at the neurokinin-1 receptor, the preferred receptor for the neuropeptide substance P, have been shown to have antidepressant activity. Mice with disruption of the neurokinin-1 receptor gene are remarkably similar both behaviourally and neurochemically to mice maintained chronically on antidepressant drugs. We demonstrate here that there is a significant elevation of neurogenesis but not cell survival in the hippocampus of neurokinin-1 receptor knockout mice. Neurogenesis can be increased in wild-type but not neurokinin-1 receptor knockout mice by chronic treatment with antidepressant drugs which preferentially target noradrenergic and serotonergic pathways. Hippocampal levels of brain-derived neurotrophic factor are also two-fold higher in neurokinin-1 receptor knockout mice, whereas cortical levels are similar. Finally, we examined hippocampus-dependent learning and memory but found no clear enhancement in neurokinin-1 receptor knockout mice. These data argue against a simple correlation between increased levels of neurogenesis or brain-derived neurotrophic factor and mnemonic processes in the absence of increased cell survival. They support the hypothesis that increased neurogenesis, perhaps accompanied by higher levels of brain-derived neurotrophic factor, may contribute to the efficacy of antidepressant drug therapy.

Different tumors in bone each give rise to a distinct pattern of skeletal destruction, bone cancer-related pain behaviors and neurochemical changes in the central nervous system

Pain is the most common presenting symptom in patients with bone cancer and bone cancer pain can be both debilitating and difficult to control fully. To begin to understand the mechanisms involved in the generation and maintenance of bone cancer pain, we implanted 3 well-described murine tumor cell lines, 2472 sarcoma, B16 melanoma and C26 colon adenocarcinoma into the femur of immunocompromised C3H-SCID mice. Although each of the tumor cell lines proliferated and completely filled the intramedullary space of the femur within 3 weeks, the location and extent of bone destruction, the type and severity of the pain behaviors and the neurochemical reorganization of the spinal cord was unique to each tumor cell line injected. These data suggest that bone cancer pain is not caused by a single factor such as increased pressure induced by intramedullary tumor growth, but rather that multiple factors are involved in generating and maintaining bone cancer pain.

Proteomic analysis of the reactive phenotype of astrocytes following endothelin-1 exposure

Reactive gliosis is an invariant feature of the pathology of central nervous system (CNS) injury and a major determinant of neuronal survival and regeneration. To begin to understand the alterations in astrocyte protein expression that drive glial changes that occur following injury, we used an established model system (endothelin-1 stimulation of hypertrophy) and proteomic analysis to define a discrete set of differentially expressed proteins and post-translational modifications that occur as the astrocytes change from a quiescent to a reactive state. This orchestrated set of changes included proteins involved in cytoskeletal reorganization (caldesmon, calponin, alpha B-crystallin, stathmin, collapsing response mediator protein-2), cell adhesion (vinculin, galectin-1), signal transduction (RACK-1) and astrocyte differentiation (glutamine synthetase). Using proteomic analysis to understand what drives astrocyte expression of these functionally divergent molecules may offer insight into the mechanisms by which astrocytes can exhibit both pro-regenerative and anti-regenerative activities following CNS injury.

Endothelin receptor expression in the normal and injured spinal cord: potential involvement in injury-induced ischemia and gliosis

The endothelins (ETs) are a family of peptides that exert their biological effects via two distinct receptors, the endothelin A receptor (ET(A)R) and the endothelin B receptor (ET(B)R). To more clearly define the potential actions of ETs following spinal cord injury, we used immunohistochemistry and confocal microscopy to examine the protein expression of ET(A)R and ET(B)R in the normal and injured rat spinal cord. In the normal spinal cord, ET(A)R immunoreactivity (IR) is expressed by vascular smooth muscle cells and a subpopulation of primary afferent nerve fibers. ET(B)R-IR is expressed primarily by radial glia, a small population of gray and white matter astrocytes, ependymal cells, vascular endothelial cells, and to a lesser extent in smooth muscle cells. Fourteen days following compression injury to the spinal cord, there was a significant upregulation in both the immunoexpression and number of astrocytes expressing the ET(B)R in both gray and white matter and a near disappearance of ET(B)R-IR in ependymal cells and ET(A)R-IR in primary afferent fibers. Conversely, the vascular expression of ET(A)R and ET(B)R did not appear to change. As spinal cord injury has been shown to induce an immediate increase in plasma ET levels and a sustained increase in tissue ET levels, ETs would be expected to induce an initial marked vasoconstriction via activation of vascular ET(A)R/ET(B)R and then days later a glial hypertrophy via activation of the ET(B)R expressed by astrocytes. Strategies aimed at blocking vascular ET(A)R/ET(B)R and astrocyte ET(B)Rs following spinal cord injury may reduce the resulting ischemia and astrogliosis and in doing so increase neuronal survival, regeneration, and function.

Neurobiology of substance P and the NK1 receptor

Substance P belongs to a group of neurokinins (NKs), small peptides that are broadly distributed in the central nervous system (CNS) and peripheral nervous system (PNS). The biological effects of substance P in the CNS, namely regulation of affective behavior and emesis in the brain and nociception in the spinal cord, are mediated by its binding to the NK1 receptor. The substance P-NK1 (SP-NK1) receptor system is the most extensively studied NK pathway, and in contrast to receptors for other neurotransmitters, such as glutamate, which have high expression throughout the CNS, only a minority of neurons (5% to 7%) in certain CNS areas express the NK1 receptor. The NK1 receptor is distributed in the plasma membrane of cell bodies and dendrites of unstimulated neurons, but upon substance P binding, the NK1 receptor undergoes rapid internalization, followed by rapid recycling to the plasma membrane. Release of substance P is induced by stressful stimuli, and the magnitude of its release is proportional to the intensity and frequency of stimulation. More potent and more frequent stimuli allow diffusion of substance P farther from the site of release, allowing activation of an approximately 3- to 5-times greater number of NK1 receptor-expressing neurons. Recent studies employing pharmacologic or genetic inactivation of NK1 receptors demonstrate the important role of the SP-NK1 receptor system in the regulation of affective behavior and suggest that inhibition of this pathway may be a useful approach to treatment of depression and associated anxiety.

Bone cancer pain

BACKGROUND

Bone cancer pain is very common, and patients with this type of pain may be difficult to treat. Development of an experimental model for studying this condition is critical to advancing an understanding of the mechanisms that cause pain in patients with malignant disease.

METHODS

A murine model of bone cancer was studied. Combined analysis of the extent of tumor-induced bone destruction, pain, and neurochemical characterization of the peripheral and central nervous systems was performed to investigate bone cancer pain. Disease-induced bone destruction was assessed by radiographs and histomorphometry. Pain was assessed by spontaneous and elicited behaviors, and neurochemical analysis involved immunohistochemical detection of hyperalgesic peptides and neurochemical markers.

RESULTS

Mice with distal femoral sarcomas exhibited behavioral and neurochemical measures of pain. The pain condition created by malignant bone disease was distinct neurochemically from inflammatory and neuropathic pain states. Experimental evidence indicated that both disease-induced osteolysis and tumors themselves contributed to the generation of pain and that peripheral and central sensitization of the nervous system was present.

CONCLUSIONS

Malignant bone disease creates a unique pain state that involves sensitization of the nervous system. Major contributors to the pain state within the bone tissue are osteoclastic bone resorption and the malignant disease itself.

Endothelin B receptors are expressed by astrocytes and regulate astrocyte hypertrophy in the normal and injured CNS

The ability of mammalian central nervous system (CNS) neurons to survive and/or regenerate following injury is influenced by surrounding glial cells. To identify the factors that control glial cell function following CNS injury, we have focused on the endothelin B receptor (ET(B)R), which we show is expressed by the majority of astrocytes that are immunoreactive for glial acid fibrillary protein (GFAP) in both the normal and crushed rabbit optic nerve. Optic nerve crush induces a marked increase in ET(B)R and GFAP immunoreactivity (IR) without inducing a significant increase in the number of GFAP-IR astrocytes, suggesting that the crush-induced astrogliosis is due primarily to astrocyte hypertrophy. To define the role that endothelins play in driving this astrogliosis, artificial cerebrospinal fluid (CSF), ET-1 (an ET(A)R and ET(B)R agonist), or Bosentan (a mixed ET(A)R and ET(B)R antagonist) were infused via osmotic minipumps into noninjured and crushed optic nerves for 14 days. Infusion of ET-1 induced a hypertrophy of ET(B)R/GFAP-IR astrocytes in the normal optic nerve, with no additional hypertrophy in the crushed nerve, whereas infusion of Bosentan induced a significant decrease in the hypertrophy of ET(B)R/GFAP-IR astrocytes in the crushed but not in the normal optic nerve. These data suggest that pharmacological blockade of astrocyte ET(B)R receptors following CNS injury modulates glial scar formation and may provide a more permissive substrate for neuronal survival and regeneration.

Simultaneous reduction in cancer pain, bone destruction, and tumor growth by selective inhibition of cyclooxygenase-2

More than half of all chronic cancer pain arises from metastases to bone, and bone cancer pain is one of the most difficult of all persistent pain states to fully control. Several tumor types including sarcomas and breast, prostate, and lung carcinomas grow in or preferentially metastasize to the skeleton where they proliferate, and induce significant bone remodeling, bone destruction, and cancer pain. Many of these tumors express the isoenzyme cycloxygenase-2 (COX-2), which is involved in the synthesis of prostaglandins. To begin to define the role COX-2 plays in driving bone cancer pain, we used an in vivo model where murine osteolytic 2472 sarcoma cells were injected and confined to the intramedullary space of the femur in male C3HHeJ mice. After tumor implantation, mice develop ongoing and movement-evoked bone cancer pain-related behaviors, extensive tumor-induced bone resorption, infiltration of the marrow space by tumor cells, and stereotypic neurochemical alterations in the spinal cord reflective of a persistent pain state. Thus, after injection of tumor cells, bone destruction is first evident at day 6, and pain-related behaviors are maximal at day 14. A selective COX-2 inhibitor was administered either acutely [NS398; 100 mg/kg, i.p.] on day 14 or chronically in chow [MF. tricyclic; 0.015%, p.o.] from day 6 to day 14 after tumor implantation. Acute administration of a selective COX-2 inhibitor attenuated both ongoing and movement-evoked bone cancer pain, whereas chronic inhibition of COX-2 significantly reduced ongoing and movement-evoked pain behaviors, and reduced tumor burden, osteoclastogenesis, and bone destruction by >50%. The present results suggest that chronic administration of a COX-2 inhibitor blocks prostaglandin synthesis at multiple sites, and may have significant clinical utility in the management of bone cancer and bone cancer pain.

Origins of skeletal pain: sensory and sympathetic innervation of the mouse femur

Although skeletal pain plays a major role in reducing the quality of life in patients suffering from osteoarthritis, Paget's disease, sickle cell anemia and bone cancer, little is known about the mechanisms that generate and maintain this pain. To define the peripheral fibers involved in transmitting and modulating skeletal pain, we used immunohistochemistry with antigen retrieval, confocal microscopy and three-dimensional image reconstruction of the bone to examine the sensory and sympathetic innervation of mineralized bone, bone marrow and periosteum of the normal mouse femur. Thinly myelinated and unmyelinated peptidergic sensory fibers were labeled with antibodies raised against calcitonin gene-related peptide (CGRP) and the unmyelinated, non-peptidergic sensory fibers were labeled with the isolectin B4 (Bandeira simplicifolia). Myelinated sensory fibers were labeled with an antibody raised against 200-kDa neurofilament H (clone RT-97). Sympathetic fibers were labeled with an antibody raised against tyrosine hydroxylase. CGRP, RT-97, and tyrosine hydroxylase immunoreactive fibers, but not isolectin B4 positive fibers, were present throughout the bone marrow, mineralized bone and the periosteum. While the periosteum is the most densely innervated tissue, when the total volume of each tissue is considered, the bone marrow receives the greatest total number of sensory and sympathetic fibers followed by mineralized bone and then periosteum. Understanding the sensory and sympathetic innervation of bone should provide a better understanding of the mechanisms that drive bone pain and aid in developing therapeutic strategies for treating skeletal pain.

Spinal neurons that possess the substance P receptor are required for the development of central sensitization

In previous studies, we have shown that loss of spinal neurons that possess the substance P receptor (SPR) attenuated pain and hyperalgesia produced by capsaicin, inflammation, and nerve injury. To determine the role of SPR-expressing neurons in modulating pain and hyperalgesia, responses of superficial and deep lumbar spinal dorsal horn neurons evoked by mechanical and heat stimuli and by capsaicin were made after ablation of SPR-expressing neurons using the selective cytotoxin conjugate substance P-saporin (SP-SAP). Morphological analysis and electrophysiological recordings were made after intrathecal infusion of vehicle, saporin alone, or SP-SAP. SP-SAP, but not vehicle or SAP alone, produced an approximately 62% decrease in SPR-expressing neurons in the dorsal horn. Loss of SPR-expressing neurons diminished the responses of remaining neurons to intraplantar injection of capsaicin. Peak responses to 10 microg of capsaicin were approximately 65% lower in animals pretreated with SP-SAP compared with controls. Additionally, sensitization to mechanical and heat stimuli that normally follows capsaicin was rarely observed. Importantly, responses to mechanical and heat stimuli in the absence of capsaicin were not altered after SP-SAP treatment. In addition, nociceptive neurons did not exhibit windup in the SP-SAP-treated group. These results demonstrate that SPR-expressing neurons located in the dorsal horn are a pivotal component of the spinal circuits involved in triggering central sensitization and hyperalgesia. It appears that this relatively small population of neurons can regulate the physiological properties of other nociceptive neurons and drive central sensitization.

Efficacy of systemic morphine suggests a fundamental difference in the mechanisms that generate bone cancer vs inflammatory pain

Pain is the cancer related event that is most disruptive to the cancer patient's quality of life. Although bone cancer pain is one of the most severe and common of the chronic pains that accompany breast, prostate and lung cancers, relatively little is known about the mechanisms that generate and maintain this pain. Recently, we developed a mouse model of bone cancer pain and 16 days following tumor implantation into the intramedullary space of the femur, significant bone destruction and bone cancer pain-related behaviors were observed. A critical question is how closely this model mirrors human bone cancer pain. In the present study we show that, as in humans, pain-related behaviors are diminished by systemic morphine administration in a dose dependent fashion that is naloxone-reversible. Humans suffering from bone cancer pain generally require significantly higher doses of morphine as compared to individuals with inflammatory pain and in the mouse model, the doses of morphine required to block bone cancer pain-related behaviors were ten times that required to block peak inflammatory pain behaviors of comparable magnitude induced by hindpaw injection of complete Freund's adjuvant (CFA) (1-3mg/kg). As these animals were treated acutely, there was not time for morphine tolerance to develop and the rightward shift in analgesic efficacy observed in bone cancer pain vs. inflammatory pain suggests a fundamental difference in the underlying mechanisms that generate bone cancer vs. inflammatory pain. These results indicate that this model may be useful in defining drug therapies that are targeted for complex bone cancer pain syndromes.

Molecular mechanisms of cancer pain

Pain is the most disruptive influence on the quality of life of cancer patients. Although significant advances are being made in cancer treatment and diagnosis, the basic neurobiology of cancer pain is poorly understood. New insights into these mechanisms are now arising from animal models, and have the potential to fundamentally change the way that cancer pain is controlled.

Noninvasive imaging of peripherally injected Alzheimer's disease type synthetic A beta amyloid in vivo

The pathological hallmark of Alzheimer's disease (AD) is accumulation in the brain of amyloid composed of the 40-mer peptide A beta. Many fundamental questions about the biology of (AD) remain unanswered because there is currently no method of quantifying A beta amyloid in vivo. A noninvasive method of detecting and quantifying A beta amyloid in vivo would have wide application for the premortem diagnosis of AD and the efficient evaluation of candidate therapeutics aimed at inhibiting the formation and growth of A beta amyloid. Taking advantage of the extraordinarily high affinity of A beta for itself, we have synthesized an N'-terminal diethylenetriaminepentaacetic acid (DTPA) derivative of A beta possessing the kinetic activity and specificity for A beta amyloid desired of a probe to be used for noninvasive imaging. DTPA-A beta(3-40) is readily labeled with (111)InOAc(3) to yield a stable probe with exquisite specificity for naturally occurring and synthetic A beta amyloid in vitro. Moreover, (111)In-DTPA-A beta(3-40), administered intravascularly can specifically deposit onto and label previously injected synthetic A beta amyloid and be imaged in vivo with a gamma camera. The present results demonstrate the design, synthesis, and use of an A beta amyloid-specific probe and methods for its use as a noninvasive imaging agent. In vivo imaging of A beta amyloid represents an important step toward the development of biochemically based objective tools for the assessment of progression of AD and efficacy of potential therapeutics.

Apolipoprotein E affects amyloid formation but not amyloid growth in vitro: mechanistic implications for apoE4 enhanced amyloid burden and risk for Alzheimer's disease

The transition from the partially folded soluble Abeta monomer to insoluble Abeta amyloidfibrils is seminal to the formation and growth of amyloid plaques in Alzheimer's disease (AD). A detailed understanding of the role of AD risk factors in these processes is essential to understanding the physiochemical nature of this conformational rearrangement. The apolipoprotein E epsilon4 allele, a risk factor for AD, affects AD pathology by increasing amyloid burden relative to the much more common epsilon3 allele. In the present study, in vitro models were employed to probe the effect of these proteins on kinetically distinct steps in Abeta fibrillogenesis. Formation of Abeta amyloid was faster in the presence of apoE4 than apoE3, while growth of existing plaques was unaffected by either isoform. Further, experiments with Abeta stereoisomers establish that this effect of apoE3 is mediated through interaction with oligomeric fibrillogenic intermediates rather than through specific contacts with monomeric Abeta. Consistent with the altered pathology and enhanced risk for AD associated with inheritance of the epsilon4 allele, we conclude that APOE epsilon4 is a risk factor for AD not due to a pathological gain of function of apoE4 but to a loss of protective function of apoE3.

Tooth extraction-induced internalization of the substance P receptor in trigeminal nucleus and spinal cord neurons: imaging the neurochemistry of dental pain

Although pains arising from the craniofacial complex can be severe and debilitating, relatively little is known about the peripheral and central mechanisms that generate and maintain orofacial pain. To better understand the neurons in the trigeminal complex and spinal cord that are activated following nociceptive stimuli to the orofacial complex, we examined substance P (SP) induced internalization of substance P receptors (SPR) in neurons following dental extraction in the rat. Unilateral gingival reflection or surgical extraction of a rat maxillary incisor or molar was performed and tissues harvested at various time points post-extraction. Immunohistochemical analysis of brainstem and cervical spinal cord sections was performed using an anti-SPR antibody and confocal imaging. Both the number and location of neurons showing SPR internalization was dependent on the location and extent of tissue injury. Whereas extraction of the incisor induced internalization of SPR in neurons bilaterally in nucleus caudalis and the spinal cord, extraction of the molar induced strictly unilateral internalization of SPR-expressing neurons in the same brain structures. Minor tissue injury (retraction of the gingiva) activated SPR neurons located in lamina I whereas more extensive and severe tissue injury (incisor or molar extraction) induced extensive SPR internalization in neurons located in both laminae I and III-V. The rostrocaudal extent of the SPR internalization was also correlated with the extent of tissue injury. Thus, following relatively minor tissue injury (gingival reflection) neurons showing SPR internalization were confined to the nucleus caudalis while procedures which cause greater tissue injury (incisor or molar extraction), neurons showing SPR internalization extended from the interpolaris/caudalis transition zone through the C7 spinal level. Defining the population of neurons activated in orofacial pain and whether analgesics modify the activation of these neurons should provide insight into the mechanisms that generate and maintain acute and chronic orofacial pain.

Enhanced responses of spinal dorsal horn neurons to heat and cold stimuli following mild freeze injury to the skin

The effects of a mild freeze injury to the skin on responses of nociceptive dorsal horn neurons to cold and heat stimuli were examined in anesthetized rats. Electrophysiological recordings were obtained from 72 nociceptive spinal neurons located in the superficial and deep dorsal horn. All neurons had receptive fields (RFs) on the glabrous skin of the hindpaw, and neurons were functionally divided into wide dynamic range (WDR) and high-threshold (HT) neurons. Forty-four neurons (61%) were classified as WDR and responded to both innocuous and noxious mechanical stimuli (mean mechanical threshold of 12.8 +/- 1.6 mN). Twenty-eight neurons (39%) were classified as HT and were excited only by noxious mechanical stimuli (mean mechanical threshold of 154.2 +/- 18.3 mN). Neurons were characterized for their sensitivity heat (35 to 51 degrees C) and cold (28 to -12 degrees C) stimuli applied to their RF. Among WDR neurons, 86% were excited by both noxious heat and cold stimuli, while 14% responded only to heat. For HT neurons, 61% responded to heat and cold stimuli, 32% responded only to noxious heat, and 7% responded only to noxious cold. Effects of a mild freeze injury (-15 degrees C applied to the RF for 20 s) on responses to heat and cold stimuli were examined in 30 WDR and 22 HT neurons. Skin freezing was verified as an abrupt increase in skin temperature at the site of injury due to the exothermic reaction associated with crystallization. Freezing produced a decrease in response thresholds to heat and cold stimuli in most WDR and HT neurons. WDR and HT neurons exhibited a mean decrease in response threshold for cold of 9.0 +/- 1.3 degrees C and 10.0 +/- 1.6 degrees C, respectively. Mean response thresholds for heat decreased 4.0 +/- 0.4 degrees C and 4.3 +/- 1.3 degrees C in WDR and HT neurons, respectively. In addition, responses to suprathreshold cold and heat stimuli increased. WDR and HT neurons exhibited an 89% and a 192% increase in response across all cold stimuli, and a 93 and 92% increase in responses evoked across all heat stimuli, respectively. Our results demonstrate that many spinal neurons encode intensity of noxious cold as well as noxious heat over a broad range of stimulus temperatures. Enhanced responses of WDR and HT neurons to cold and heat stimuli after a mild freeze injury is likely to contribute to thermal hyperalgesia following a similar freeze injury in humans.

Osteoprotegerin diminishes advanced bone cancer pain

Bone cancer pain most commonly occurs when tumors originating in breast, prostate, or lung metastasize to long bones, spinal vertebrae, and/or pelvis. Primary and metastatic cancers involving bone account for approximately 400,000 new cancer cases per year in the United States alone, and >70% of patients with advanced breast or prostate cancer have skeletal metastases. Whereas pain resulting from bone cancer can dramatically impact an individual's quality of life, very little is known about the mechanisms that generate and maintain this pain. To begin to define the mechanisms that give rise to advanced bone cancer pain, osteolytic 2472 sarcoma cells or media were injected into the intramedullary space of the femur of C3H/HeJ mice, and the injection hole was sealed using dental amalgam, confining the tumor cells to the bone. Twelve days after injection of 2472 tumor cells, animals showed advanced tumor-induced bone destruction of the injected femur, bone cancer pain, and a stereotypic set of neurochemical changes in the spinal cord dorsal horn that receives sensory inputs from the affected femur. Administration of osteoprotegerin, a naturally secreted decoy receptor that inhibits osteoclast maturation and activity and induces osteoclast apoptosis, or vehicle was begun at 12 days, when significant bone destruction had already occurred, and administration was continued daily until day 21. Ongoing pain behaviors, movement-evoked pain behaviors, and bone destruction were assessed on days 10, 12, 14, 17, and 21. The neurochemistry of the spinal cord was evaluated at days 12 and 21. Results indicated that osteoprotegerin treatment halted further bone destruction, reduced ongoing and movement-evoked pain, and reversed several aspects of the neurochemical reorganization of the spinal cord. Thus, even in advanced stages of bone cancer, ongoing osteoclast activity appears to be involved in the generation and maintenance of ongoing and movement-evoked pain. Blockade of ongoing osteoclast activity appears to have the potential to reduce bone cancer pain in patients with advanced tumor-induced bone destruction.

Expression and localization of endothelin receptors: implications for the involvement of peripheral glia in nociception

The endothelins (ETs) are peptides that have a diverse array of functions mediated by two receptor subtypes, the endothelin A receptor (ET(A)R) and the endothelin B receptor (ET(B)R). Pharmacological studies have suggested that in peripheral tissues, ET(A)R expression may play a role in signaling acute or neuropathic pain, whereas ET(B)R expression may be involved in the transmission of chronic inflammatory pain. To begin to define the mechanisms by which ET can drive nociceptive signaling, autoradiography and immunohistochemistry were used to examine the distribution of ET(A)R and ET(B)R in dorsal root ganglia (DRG) and peripheral nerve of the rat, rabbit, and monkey. In DRG and peripheral nerve, ET(A)R-immunoreactivity was present in a subset of small-sized peptidergic and nonpeptidergic sensory neurons and their axons and to a lesser extent in a subset of medium-sized sensory neurons. However, ET(B)R-immunoreactivity was not seen in DRG neurons or axons but rather in DRG satellite cells and nonmyelinating ensheathing Schwann cells. Thus, when ETs are released in peripheral tissues, they could act directly on ET(A)R-expressing sensory neurons and on ET(B)R-expressing DRG satellite cells or nonmyelinating Schwann cells. These data indicate that ETs can have direct, nociceptive effects on the peripheral sensory nervous system and that peripheral glia may be directly involved in signaling nociceptive events in peripheral tissues.

The molecular dynamics of pain control

Pain is necessary for survival, but persistent pain can result in anxiety, depression and a reduction in the quality of life. The discriminative and affective qualities of pain are both thought to be regulated in an activity-dependent fashion. Recent studies have identified cells and molecules that regulate pain sensitivity and the parallel pathways that distribute nociceptive information to limbic or sensory areas of the forebrain. Here, we emphasize the cellular and neurobiological consequences of pain, especially those that are involved in the generation and maintenance of chronic pain. These new insights into pain processing will significantly alter our approach to pain control and the development of new analgesics.

Murine models of inflammatory, neuropathic and cancer pain each generates a unique set of neurochemical changes in the spinal cord and sensory neurons

The aim of this investigation was to determine whether murine models of inflammatory, neuropathic and cancer pain are each characterized by a unique set of neurochemical changes in the spinal cord and sensory neurons. All models were generated in C3H/HeJ mice and hyperalgesia and allodynia behaviorally characterized. A variety of neurochemical markers that have been implicated in the generation and maintenance of chronic pain were then examined in spinal cord and primary afferent neurons.Three days after injection of complete Freund's adjuvant into the hindpaw (a model of persistent inflammatory pain) increases in substance P, calcitonin gene-related peptide, protein kinase C gamma, and substance P receptor were observed in the spinal cord. Following sciatic nerve transection or L5 spinal nerve ligation (a model of persistent neuropathic pain) significant decreases in substance P and calcitonin gene-related peptide and increases in galanin and neuropeptide Y were observed in both primary afferent neurons and the spinal cord. In contrast, in a model of cancer pain induced by injection of osteolytic sarcoma cells into the femur, there were no detectable changes in any of these markers in either primary afferent neurons or the spinal cord. However, in this cancer-pain model, changes including massive astrocyte hypertrophy without neuronal loss, increase in the neuronal expression of c-Fos, and increase in the number of dynorphin-immunoreactive neurons were observed in the spinal cord, ipsilateral to the limb with cancer. These results indicate that a unique set of neurochemical changes occur with inflammatory, neuropathic and cancer pain in C3H/HeJ mice and further suggest that cancer induces a unique persistent pain state. Determining whether these neurochemical changes are involved in the generation and maintenance of each type of persistent pain may provide insight into the mechanisms that underlie each of these pain states.

Activation barriers to structural transition determine deposition rates of Alzheimer's disease a beta amyloid

Brain amyloid composed of the approximately 40-amino-acid human beta-amyloid peptide A beta is integral to Alzheimer's disease pathology. To probe the importance of a conformational transition in Abeta during amyloid growth, we synthesized and examined the solution conformation and amyloid deposition activity of A beta congeners designed to have similar solution structures but to vary substantially in their barriers to conformational transition. Although all these peptides adopt similar solution conformations, a covalently restricted Abeta congener designed to have a very high barrier to conformational rearrangement was inactive, while a peptide designed to have a reduced barrier to conformational transition displayed an enhanced deposition rate relative to wild-type A beta. The hyperactive peptide, which is linked to a heritable A beta amyloidosis characterized by massive amyloid deposition at an early age, displayed a reduced activation barrier to deposition consistent with a larger difference in activation entropy than in activation enthalpy relative to wild-type A beta. These results suggest that in Alzheimer's disease, as in the prion diseases, a conformational transition in the depositing peptide is essential for the conversion of soluble monomer to insoluble amyloid, and alterations in the activation barrier to this transition affect amyloidogenicity and directly contribute to human disease.

Alzheimer's disease amyloid propagation by a template-dependent dock-lock mechanism

Amyloid plaques composed of the peptide Abeta are an integral part of Alzheimer's disease (AD) pathogenesis. We have modeled the process of amyloid plaque growth by monitoring the deposition of soluble Abeta onto amyloid in AD brain tissue or synthetic amyloid fibrils and show that it is mediated by two distinct kinetic processes. In the first phase, "dock", Abeta addition to the amyloid template is fully reversible (dissociation t(1/2) approximately 10 min), while in the second phase, "lock", the deposited peptide becomes irreversibly associated (dissociation t(1/2) >> 1000 min) with the template in a time-dependent manner. The most recently deposited peptide dissociates first while Abeta previously deposited becomes irreversibly "locked" onto the template. Thus, the transition from monomer to neurotoxic amyloid is mediated by interaction with the template, a mechanism that has also been proposed for the prion diseases. Interestingly, two Abeta peptides bearing primary sequence alterations implicated in heritable Abeta amyloidoses displayed faster lock-phase kinetics than wild-type Abeta. Inhibiting the initial weak docking interaction between depositing Abeta and the template is a viable therapeutic target to prevent the critical conformational transition in the conversion of Abeta((solution)) to Abeta((amyloid)) and thus prevent stable amyloid accumulation. While thermodynamics suggest that inhibiting amyloid assembly would be difficult, the present study illustrates that the protein misfolding diseases are kinetically vulnerable to intervention.

Osteoprotegerin blocks bone cancer-induced skeletal destruction, skeletal pain and pain-related neurochemical reorganization of the spinal cord

Bone cancer pain is common among cancer patients and can have a devastating effect on their quality of life. A chief problem in designing new therapies for bone cancer pain is that it is unclear what mechanisms drive this distinct pain condition. Here we show that osteoprotegerin, a secreted 'decoy' receptor that inhibits osteoclast activity, also blocks behaviors indicative of pain in mice with bone cancer. A substantial part of the actions of osteoprotegerin seems to result from inhibition of tumor-induced bone destruction that in turn inhibits the neurochemical changes in the spinal cord that are thought to be involved in the generation and maintenance of cancer pain. These results demonstrate that excessive tumor-induced bone destruction is involved in the generation of bone cancer pain and that osteoprotegerin may provide an effective treatment for this common human condition.

Neurochemical and cellular reorganization of the spinal cord in a murine model of bone cancer pain

The cancer-related event that is most disruptive to the cancer patient's quality of life is pain. To begin to define the mechanisms that give rise to cancer pain, we examined the neurochemical changes that occur in the spinal cord and associated dorsal root ganglia in a murine model of bone cancer. Twenty-one days after intramedullary injection of osteolytic sarcoma cells into the femur, there was extensive bone destruction and invasion of the tumor into the periosteum, similar to that found in patients with osteolytic bone cancer. In the spinal cord, ipsilateral to the cancerous bone, there was a massive astrocyte hypertrophy without neuronal loss, an expression of dynorphin and c-Fos protein in neurons in the deep laminae of the dorsal horn. Additionally, normally non-noxious palpation of the bone with cancer induced behaviors indicative of pain, the internalization of the substance P receptor, and c-Fos expression in lamina I neurons. The alterations in the neurochemistry of the spinal cord and the sensitization of primary afferents were positively correlated with the extent of bone destruction and the growth of the tumor. This "neurochemical signature" of bone cancer pain appears unique when compared to changes that occur in persistent inflammatory or neuropathic pain states. Understanding the mechanisms by which the cancer cells induce this neurochemical reorganization may provide insight into peripheral factors that drive spinal cord plasticity and in the development of more effective treatments for cancer pain.

Transmission of chronic nociception by spinal neurons expressing the substance P receptor

Substance P receptor (SPR)-expressing spinal neurons were ablated with the selective cytotoxin substance P-saporin. Loss of these neurons resulted in a reduction of thermal hyperalgesia and mechanical allodynia associated with persistent neuropathic and inflammatory pain states. This loss appeared to be permanent. Responses to mildly painful stimuli and morphine analgesia were unaffected by this treatment. These results identify a target for treating persistent pain and suggest that the small population of SPR-expressing neurons in the dorsal horn of the spinal cord plays a pivotal role in the generation and maintenance of chronic neuropathic and inflammatory pain.

Spinal opioid analgesia: how critical is the regulation of substance P signaling?

Although opioids can reduce stimulus-evoked efflux of Substance P (SP) from nociceptive primary afferents, the consequences of this reduction on spinal cord nociceptive processing has not been studied. Rather than assaying SP release, in the present study we examined the effect of opioids on two postsynaptic measures of SP release, Fos expression and neurokinin-1 (NK-1) receptor internalization, in the rat. The functional significance of the latter was first established in in vitro studies that showed that SP-induced Ca(2+) mobilization is highly correlated with the magnitude of SP-induced NK-1 receptor internalization in dorsal horn neurons. Using an in vivo analysis, we found that morphine had little effect on noxious stimulus-evoked internalization of the NK-1 receptor in lamina I neurons. However, internalization was reduced when we coadministered morphine with a dose of an NK-1 receptor antagonist that by itself was without effect. Thus, although opioids may modulate SP release, the residual release is sufficient to exert maximal effects on the target NK-1 receptors. Morphine significantly reduced noxious stimulus-induced Fos expression in lamina I, but the Fos inhibition was less pronounced in neurons that expressed the NK-1 receptor. Taken together, these results suggest that opioid analgesia predominantly involves postsynaptic inhibitory mechanisms and/or presynaptic control of non-SP-containing primary afferent nociceptors.