Nociceptor activation and pain

This paper reviews advances in our knowledge on the physiological properties of human nociceptors and their capacity to signal pain. Conventional microneurography was used in combination with intraneural microstimulation in subjects who estimated the magnitude of pain from nociceptor stimulation. The experimental evidence favours the notion that C polymodal nociceptors can provide a peripheral neuronal basis for determination of heat pain threshold and also an essential peripheral code for suprathreshold magnitude judgments of heat pain. Furthermore, sensitized C polymodal nociceptors can contribute to hyperalgesia after a mild heat injury to hairy skin. Temporal summation is documented for dull, delayed C fibre pain, which is different in quality and less accurately projected than the fast, sharp pain from high-threshold A delta nociceptors. A segmental organization is shown for projected and referred pain from deep structures. Examples are given of central inhibition of pain by a prostaglandin synthetase inhibitor, and by physical manoeuvres such as vibration and cooling. Recent reports on microneurographic findings after nerve injury indicate that the technique may be useful for future studies on pathophysiological pain mechanisms.     

Basic mechanisms of pain associated with deep tissues.

There are important differences in pain arising from deep tissues in comparison to cutaneous pain. These differences can be partially explained by the unique organization of nociceptive systems activated by stimulation of muscle, joint, or viscera. Recent evidence also indicates that stimulation of deep tissues can produce long-lasting changes in central nervous system excitability and, therefore, may play a prominent role in persistent or chronic pain conditions. These findings have important implications for the treatment of chronic deep tissue pain conditions.     

Neuronal application of capsaicin modulates somatic pressor reflexes

Static contraction of skeletal muscle elicits a reflex increase in cardiovascular function. Likewise, noxious stimuli activate somatic nociceptors eliciting a reflex increase in cardiovascular function. On the basis of recent work involving spinothalamic cells in the dorsal horn, we hypothesized that the dorsal horn cells involved in the aforementioned reflexes would be sensitized by applying capsaicin (Cap) to a peripheral nerve. If correct, then Cap would enhance the cardiovascular increases that occur when these reflexes are evoked. Cats were anesthetized, and the popliteal fossa was exposed. Static contraction was induced by electrical stimulation of the tibial nerve at an intensity that did not directly activate small-diameter muscle afferent fibers, whereas nociceptors were stimulated by high-intensity stimulation (after muscle paralysis) of either the saphenous nerve (cutaneous nociceptors) or a muscular branch of the tibial nerve (muscle nociceptors). The reflex cardiovascular responses to these perturbations (contraction or nociceptor stimulation) were determined before and after direct application of Cap (3%) onto the common peroneal nerve, using a separate group of cats for each reflex. Compared with control, application of Cap attenuated the peak change in mean arterial pressure (MAP) evoked by static contraction (DeltaMAP in mmHg: 38 +/- 10 before and 24 +/- 8 after ipsilateral Cap; 47 +/- 10 before and 33 +/- 10 after contralateral Cap). On the other hand, Cap increased the peak change in MAP evoked by stimulation of the saphenous nerve from 57 +/- 8 to 77 +/- 9 mmHg, as well as the peak change in MAP elicited by activation of muscle nociceptors (36 +/- 9 vs. 56 +/- 14 mmHg). These results show that the reflex cardiovascular increases evoked by static muscle contraction and noxious input are differentially affected by Cap application to the common peroneal nerve. We hypothesize that a Cap-induced alteration in dorsal horn processing is the locus for this divergent effect on these reflexes.     

Neural and psychosocial contributions to sex differences in knee osteoarthritic pain

People with osteoarthritis (OA) can have significant pain that interferes with function and quality of life. Women with knee OA have greater pain and greater reductions in function and quality of life than men. In many cases, OA pain is directly related to sensitization and activation of nociceptors in the injured joint and correlates with the degree of joint effusion and synovial thickening. In some patients, however, the pain does not match the degree of injury and continues after removal of the nociceptors with a total joint replacement. Growth of new nociceptors, activation of nociceptors in the subchondral bone exposed after cartilage degradation, and nociceptors innervating synovium sensitized by inflammatory mediators could all augment the peripheral input to the central nervous system and result in pain. Enhanced central excitability and reduced central inhibition could lead to prolonged and enhanced pain that does not directly match the degree of injury. Psychosocial variables can influence pain and contribute to pain variability. This review explores the neural and psychosocial factors that contribute to knee OA pain with an emphasis on differences between the sexes and gaps in knowledge.     

Partial regeneration of the above-elbow amputated rat forelimb. I. Innate responses.

Although a number of recent studies describe the facilitation of limb regeneration by electrical and other forms of stimulation, little is known of innate regenerative capacity in the mammalian limb. The present report describes spontaneous regenerative responses following subtotal forelimb amputation in the young white rat. In one group of animals the forelimb was amputated through the lower humerus and the skin sutured closed. In a second group, adjacent muscle tissue still attached to bone at its origin(s) was interposed between the cut surface of the humerus and the skin. Among animals of the first group (skin closure only) bone growth and limb regenerative responses were generally not observed. Animals of the second group displayed significant elaborations of cartilage and bone at the limb terminus. The appearance and subsequent modification of these tissues suggest that some capacity for limb regeneration exists innately in the young rat and can be more readily evoked than has been recognized heretofore. It is concluded that extant and forthcoming reports of electrically stimulated skeletal tissue growth, repair and regeneration among eutherial mammals should be examined to determine whether reported responses to stimulation represent advances beyond what might be expected from innate replacement processes alone.     

Cardiac,skeletal muscle and serum irisin responses to with or without water exercise in young and old male rats: Cardiac muscle produces more irisin than skeletal muscle

Irisin converts white adipose tissue (WAT) into brown adipose tissue (BAT), as regulated by energy expenditure. The relationship between irisin concentrations after exercise in rats compared humans after exercise remains controversial. We therefore: (1) measured irisin expression in cardiac and skeletal muscle, liver, kidney, peripheral nerve sheath and skin tissues, as also serum irisin level in 10 week-old rats without exercise, and (2) measured tissue supernatant irisin levels in cardiac and skeletal muscle, and in response to exercise in young and old rats to establishing which tissues produced most irisin. Young (12 months) and old rats (24 months) with or without 10 min exercise (water floating) and healthy 10 week-old Sprague-Dawley rats without exercise were used. Irisin was absent from sections of skeletal muscle of unexercised rats, the only part being stained being the perimysium. In contrast, cardiac muscle tissue, peripheral myelin sheath, liver, kidneys, and skin dermis and hypodermis were strongly immunoreactivity. No irisin was seen in skeletal muscle of unexercised young and old rats, but a slight amount was detected after exercise. Strong immunoreactivity occurred in cardiac muscle of young and old rats with or without exercise, notably in pericardial connective tissue. Serum irisin increased after exercise, being higher in younger than older rats. Irisin in tissue supernatants (cardiac and skeletal muscle) was high with or without exercise. High supernatant irisin could come from connective tissues around skeletal muscle, especially nerve sheaths located within it. Skeletal muscle is probably not a main irisin source.     

How does muscle stiffness affect the internal deformations within the soft tissue layers of the buttocks under constant loading?

Mechanical loading of soft tissues covering bony prominences can cause skeletal muscle damage, ultimately resulting in a severe pressure ulcer termed deep tissue injury (DTI). Deformation plays an important role in the aetiology of DTI. Therefore, it is essential to minimise internal muscle deformations in subjects at risk of DTI. As an example, spinal cord-injured (SCI) individuals exhibit structural changes leading to a decrease in muscle thickness and stiffness, which subsequently increase the tissue deformations. In the present study, an animal-specific finite element model, where the geometry and boundary conditions were derived from magnetic resonance images, was developed. It was used to investigate the internal deformations in the muscle, fat and skin layers of the porcine buttocks during loading. The model indicated the presence of large deformations in both the muscle and the fat layers, with maximum shear strains up to 0.65 in muscle tissue and 0.63 in fat. Furthermore, a sensitivity analysis showed that the tissue deformations depend considerably on the relative stiffness values of the different tissues. For example, a change in muscle stiffness had a large effect on the muscle deformations. A 50% decrease in stiffness caused an increase in maximum shear strain from 0.65 to 0.99, whereas a 50% increase in stiffness resulted in a decrease in maximum shear strain from 0.65 to 0.49. These results indicate the importance of restoring tissue properties after SCI, with the use of, for example, electrical stimulation, to prevent the development of DTI.     

刺激皮肤感受野对大鼠多觉型伤害性感受器诱发放电的影响

在大鼠尾部,重复压力刺激皮肤感受野,当间隔时间小于2min时,多觉型伤害性感受器的单位放电数随间隔时间的缩短而减少。压力与辐射热交叉刺激同一感受野,随后刺激的放电数也显著减少。皮下注射致痛剂引起持续性放电的背景上,分别向感受野施加按压、辐射热或电针刺激,随着放电增多后出现一个放电减少的过程。刺激支配尾部的交感神经则使减少的放电显著增多。结果表明,多觉型伤害性感受器受到刺激兴奋后有个感受性降低的过程。本文讨论了这一过程在按摩、针灸缓解痛机制中的可能作用。     

Enolase isozymes in Coho salmon.

1. Analysis of the enolase isozymic distribution has been performed in tissues of the Coho salmon, using electrophoretic separation on cellulose acetate strips followed by localization of enzymatic activity. 2. A total of six electrophoretically distinct forms are seen in Coho salmon in patterns that differ both qualitatively and quantitatively from one tissue to another. 3. The isozymes in skeletal muscle and liver are sufficiently similar to one another that a purification procedure previously developed for trout muscle enolase by Cory & Wold (1966) can be used to partially purify enolase from either of the above-mentioned Coho tissues. The main form of enolase in Coho muscle has an isoelectric point of 7.57. 4. Both liver and skeletal muscle enolases can be reversibly denatured in guanidine HCl and subsequently renatured. Liver enolase appeared to renature somewhat faster than muscle enolase under the same conditions. 5. While polyploidy among salmonids may contribute to the complexity of enolase patterns in fish, the differences in isozymic patterns seen from one tissue to another indicate the presence of distinct, nonallelic genes, probably arising through gene duplication.     

Electrical stimulation as a biomimicry tool for regulating muscle cell behavior

There is a growing need to understand muscle cell behaviors and to engineer muscle tissues to replace defective tissues in the body. Despite a long history of the clinical use of electric fields for muscle tissues in vivo, electrical stimulation (ES) has recently gained significant attention as a powerful tool for regulating muscle cell behaviors in vitro. ES aims to mimic the electrical environment of electroactive muscle cells (e.g., cardiac or skeletal muscle cells) by helping to regulate cell-cell and cell-extracellular matrix (ECM) interactions. As a result, it can be used to enhance the alignment and differentiation of skeletal or cardiac muscle cells and to aid in engineering of functional muscle tissues. Additionally, ES can be used to control and monitor force generation and electrophysiological activity of muscle tissues for bio-actuation and drug-screening applications in a simple, high-throughput, and reproducible manner. In this review paper, we briefly describe the importance of ES in regulating muscle cell behaviors in vitro, as well as the major challenges and prospective potential associated with ES in the context of muscle tissue engineering.     

In pursuit of P2X3 antagonists: novel therapeutics for chronic pain and afferent sensitization

Treating pain by inhibiting ATP activation of P2X3-containing receptors heralds an exciting new approach to pain management, and Afferent's program marks the vanguard in a new class of drugs poised to explore this approach to meet the significant unmet needs in pain management. P2X3 receptor subunits are expressed predominately and selectively in so-called C- and Aδ-fiber primary afferent neurons in most tissues and organ systems, including skin, joints, and hollow organs, suggesting a high degree of specificity to the pain sensing system in the human body. P2X3 antagonists block the activation of these fibers by ATP and stand to offer an alternative approach to the management of pain and discomfort. In addition, P2X3 is expressed pre-synaptically at central terminals of C-fiber afferent neurons, where ATP further sensitizes transmission of painful signals. As a result of the selectivity of the expression of P2X3, there is a lower likelihood of adverse effects in the brain, gastrointestinal, or cardiovascular tissues, effects which remain limiting factors for many existing pain therapeutics. In the periphery, ATP (the factor that triggers P2X3 receptor activation) can be released from various cells as a result of tissue inflammation, injury or stress, as well as visceral organ distension, and stimulate these local nociceptors. The P2X3 receptor rationale has aroused a formidable level of investigation producing many reports that clarify the potential role of ATP as a pain mediator, in chronic sensitized states in particular, and has piqued the interest of pharmaceutical companies. P2X receptor-mediated afferent activation has been implicated in inflammatory, visceral, and neuropathic pain states, as well as in airways hyperreactivity, migraine, itch, and cancer pain. It is well appreciated that oftentimes new mechanisms translate poorly from models into clinical efficacy and effectiveness; however, the breadth of activity seen from P2X3 inhibition in models offers a realistic chance that this novel mechanism to inhibit afferent nerve sensitization may find its place in the sun and bring some merciful relief to the torment of persistent discomfort and pain. The development philosophy at Afferent is to conduct proof of concept patient studies and best identify target patient groups that may benefit from this new intervention.     

In pursuit of P2X3 antagonists: novel therapeutics for chronic pain and afferent sensitization

Treating pain by inhibiting ATP activation of P2X3-containing receptors heralds an exciting new approach to pain management, and Afferent's program marks the vanguard in a new class of drugs poised to explore this approach to meet the significant unmet needs in pain management. P2X3 receptor subunits are expressed predominately and selectively in so-called C- and Aδ-fiber primary afferent neurons in most tissues and organ systems, including skin, joints, and hollow organs, suggesting a high degree of specificity to the pain sensing system in the human body. P2X3 antagonists block the activation of these fibers by ATP and stand to offer an alternative approach to the management of pain and discomfort. In addition, P2X3 is expressed pre-synaptically at central terminals of C-fiber afferent neurons, where ATP further sensitizes transmission of painful signals. As a result of the selectivity of the expression of P2X3, there is a lower likelihood of adverse effects in the brain, gastrointestinal, or cardiovascular tissues, effects which remain limiting factors for many existing pain therapeutics. In the periphery, ATP (the factor that triggers P2X3 receptor activation) can be released from various cells as a result of tissue inflammation, injury or stress, as well as visceral organ distension, and stimulate these local nociceptors. The P2X3 receptor rationale has aroused a formidable level of investigation producing many reports that clarify the potential role of ATP as a pain mediator, in chronic sensitized states in particular, and has piqued the interest of pharmaceutical companies. P2X receptor-mediated afferent activation has been implicated in inflammatory, visceral, and neuropathic pain states, as well as in airways hyperreactivity, migraine, itch, and cancer pain. It is well appreciated that oftentimes new mechanisms translate poorly from models into clinical efficacy and effectiveness; however, the breadth of activity seen from P2X3 inhibition in models offers a realistic chance that this novel mechanism to inhibit afferent nerve sensitization may find its place in the sun and bring some merciful relief to the torment of persistent discomfort and pain. The development philosophy at Afferent is to conduct proof of concept patient studies and best identify target patient groups that may benefit from this new intervention.

     

Localization of the actin-binding protein fesselin in chicken smooth muscle

This report compares cellular localization of fesselin in chicken smooth, skeletal and cardiac muscle tissues using affinity purified polyclonal fesselin antibodies. Western blot analyses revealed large amounts of fesselin in gizzard smooth muscle with lower amounts in skeletal and cardiac muscle. In gizzard, fesselin was detected by immunofluorescence as discrete cytoplasmic structures. Fesselin did not co-localize with talin, vinculin or caveolin indicating that fesselin is not associated with dense plaques or caveolar regions of the cell membrane. Immunoelectron microscopy established localization of fesselin within dense bodies. Since dense bodies function as anchorage points for actin and desmin in smooth muscle cells, fesselin may be involved in establishing cytoskeletal structure in this tissue. In skeletal muscle, fesselin was associated with desmin in regularly spaced bands distributed along the length of muscle fibers suggesting localization to the Z-line. Infrequently, this banding pattern was observed in heart tissue as well. Localization at the Z-line of skeletal and cardiac muscle suggests a role in contraction of these tissues.     

Adipose tissue,the immune system and exercise fatigue: how activated lymphocytes compete for lipids

Adipose depots that contain lymph nodes, and probably intermuscular fat in skeletal and cardiac muscle, are specialized to provision adjacent tissue in a paracrine mode. Perinodal adipocytes respond selectively to various cytokines and incorporate proportionately more polyunsaturated fatty acids. Lipolysis in the adipocytes of node-containing depots can be stimulated via inflammation of the enclosed lymph nodes. Repeated immune stimulation elicits properties characteristic of perinodal adipocytes in those elsewhere in the same depot, and hours later in other node-containing depots, but not in nodeless depots. Such site-specific properties of adipose tissue enable partitioning of dietary and metabolic supplies of fatty acids between competing tissues. Local interactions emancipate the peripheral immune system from competing with other tissues for lipids during immune responses, and may be especially important during periods of high demand, such as strenuous exercise. Biopsies of subcutaneous adipose tissue from sites remote from lymph nodes do not adequately represent the composition of fatty acids available to the immune system in situ, and perhaps that supplied to other tissues. Intermuscular fat in skeletal and cardiac muscle may also indicate paracrine relationships between adipocytes and "end-user" tissues. The concept of paracrine interactions between certain adipocytes and "user" tissue may account for the widespread contiguity between these tissues in vivo.     

Demystifying SP cell purification: viability, yield, and phenotype are defined by isolation parameters

Side population (SP) cells isolated from bone marrow, skeletal muscle, and skin have been shown to engraft in dystrophic muscle. However, there have been questions on the phenotypical heterogeneity, tissue of origin, and relationships among SP cell populations extracted from different tissues. Studies on bone marrow SP cells have followed a consistent protocol for their isolation and results obtained are concordant. In contrast, protocols for the isolation of muscle SP cells vary greatly, and consequently reports on their phenotype, differentiation potential and origin have been inconsistent. To address this controversy, we demonstrate that isolation parameters, such as tissue dissociation, cell counting, Hoechst concentration, and stringency in the selection of SP cells, have an effect on the yield, viability, and homogeneity of SP cells derived from bone marrow, skeletal muscle, and skin. In this paper, we demonstrate that SP cells isolated from the bone marrow are distinct from SP cells extracted from skeletal muscle and skin tissues. This study offers an explanation for the controversy surrounding muscle SP cells, provides a detailed standardized protocol for their isolation, and highlights basic guidelines for reproducible and reliable isolation of SP cells from any tissue.     

Skeletal muscle tissue engineering

The reconstruction of skeletal muscle tissue either lost by traumatic injury or tumor ablation or functional damage due to myopathies is hampered by the lack of availability of functional substitution of this native tissue. Until now, only few alternatives exist to provide functional restoration of damaged muscle tissues. Loss of muscle mass and their function can surgically managed in part using a variety of muscle transplantation or transposition techniques. These techniques represent a limited degree of success in attempts to restore the normal functioning, however they are not perfect solutions. A new alternative approach to addressing difficult tissue reconstruction is to engineer new tissues. Although those tissue engineering techniques attempting regeneration of human tissues and organs have recently entered into clinical practice, the engineering of skeletal muscle tissue ist still a scientific challenge. This article reviews some of the recent findings resulting from tissue engineering science related to the attempt of creation and regeneration of functional skeletal muscle tissue.     

Stimulation of protein synthesis by both insulin and amino acids is unique to skeletal muscle in neonatal pigs

In neonatal pigs, the feeding-induced stimulation of protein synthesis in skeletal muscle, but not liver, can be reproduced by insulin infusion when essential amino acids and glucose are maintained at fasting levels. In the present study, 7- and 26-day-old pigs were studied during 1) fasting, 2) hyperinsulinemic-euglycemic-euaminoacidemic clamps, 3) euinsulinemic-euglycemic-hyperaminoacidemic clamps, and 4) hyperinsulinemic-euglycemic-hyperaminoacidemic clamps. Amino acids were clamped using a new amino acid mixture enriched in nonessential amino acids. Tissue protein synthesis was measured using a flooding dose of L-[4-(3)H]phenylalanine. In 7-day-old pigs, insulin infusion alone increased protein synthesis in various skeletal muscles (from +35 to +64%), with equivalent contribution of myofibrillar and sarcoplasmic proteins, as well as cardiac muscle (+50%), skin (+34%), and spleen (+26%). Amino acid infusion alone increased protein synthesis in skeletal muscles (from +28 to +50%), also with equivalent contribution of myofibrillar and sarcoplasmic proteins, as well as liver (+27%), pancreas (+28%), and kidney (+10%). An elevation of both insulin and amino acids did not have an additive effect. Similar qualitative results were obtained in 26-day-old pigs, but the magnitude of the stimulation of protein synthesis by insulin and/or amino acids was lower. The results suggest that, in the neonate, the stimulation of protein synthesis by feeding is mediated by either amino acids or insulin in most tissues; however, the feeding-induced stimulation of protein synthesis in skeletal muscle is uniquely regulated by both insulin and amino acids.     

Biomimetic approach to tissue engineering

The overall goal of tissue engineering is to create functional tissue grafts that can regenerate or replace our defective or worn out tissues and organs. Examples of grafts that are now in pre-clinical studies or clinical use include engineered skin, cartilage, bone, blood vessels, skeletal muscle, bladder, trachea, and myocardium. Engineered tissues are also finding applications as platforms for pharmacological and physiological studies in vitro. To fully mobilize the cell's biological potential, a new generation of tissue engineering systems is now being developed to more closely recapitulate the native developmental milieu, and mimic the physiologic mechanisms of transport and signaling. We discuss the interactions between regenerative biology and engineering, in the context of (i) creation of functional tissue grafts for regenerative medicine (where biological input is critical), and (ii) studies of stem cells, development and disease (where engineered tissues can serve as advanced 3D models).     

Electrical injury mechanisms: dynamics of the thermal response

The thermal response of the human upper extremity to large electric currents was examined using an axisymmetric unidimensional model containing bone, skeletal muscle, fat, and skin in coaxial cylindrical geometry. Appropriate thermal and electrical properties were assigned to each tissue, and the tissue response to joule heating was determined by a finite-element numerical technique. We found that when the tissues are electrically in parallel, skeletal muscle sustained the largest temperature rise and then heated adjacent tissues. Thus, when bone is not in series with other tissues, joule heating of bone is unlikely to be responsible for thermal damage to adjacent tissue. In addition, the effect of tissue perfusion on the thermal response was found to be essential for rapid cooling of the centrally located tissues.