The Role of TRP Channels in Nicotinic Provoked Pain and Irritation from the Oral Cavity and Throat: Translating Animal Data to Humans.

Tobacco smoking-related diseases are estimated to kill more than 8 million people/year and most smokers are willing to stop smoking. The pharmacological approach to aid smoking cessation comprises nicotine replacement therapy (NRT) and inhibitors of the nicotinic acetylcholine receptor, which is activated by nicotine. Common side effects of oral NRT products include hiccoughs, gastrointestinal disturbances and, most notably, irritation, burning and pain in the mouth and throat, which are the most common reasons for premature discontinuation of NRT and termination of cessation efforts. Attempts to reduce the unwanted sensory side effects are warranted, and research discovering the most optimal masking procedures is urgently needed. This requires a firm mechanistic understanding of the neurobiology behind the activation of sensory nerves and their receptors by nicotine. The sensory nerves in the oral cavity and throat express the so-called transient receptor potential (TRP) channels, which are responsible for mediating the nicotine-evoked irritation, burning and pain sensations. Targeting the TRP channels is one way to modulate the unwanted sensory side effects. A variety of natural (Generally Recognized As Safe [GRAS]) compounds interact with the TRP channels, thus making them interesting candidates as safe additives to oral NRT products. The present narrative review will discuss (1) current evidence on how nicotine contributes to irritation, burning and pain in the oral cavity and throat, and (2) options to modulate these unwanted side-effects with the purpose of increasing adherence to NRT. Nicotine provokes irritation, burning and pain in the oral cavity and throat. Managing these side effects will ensure better compliance to oral NRT products and hence increase the success of smoking cessation. A specific class of sensory receptors (TRP channels) are involved in mediating nicotine's sensory side effects, making them to potential treatment targets. Many natural (Generally Recognized As Safe [GRAS]) compounds are potentially beneficial modulators of TRP channels.

Altered Central Sensitization and Pain Modulation in the CNS in Chronic Joint Pain.

Musculoskeletal pain disorders are the second largest contributor to global disability underlining the significance of effective treatments. However, treating chronic musculoskeletal pain, and chronic joint pain (osteoarthritis (OA)) in particular, is challenging as the underlying peripheral and central pain mechanisms are not fully understood, and safe and efficient analgesic drugs are not available. The pain associated with joint pain is highly individual, and features from radiological imaging have not demonstrated robust associations with the pain manifestations. In recent years, a variety of human quantitative pain assessment tools (quantitative sensory testing (QST)) have been developed providing new opportunities for profiling patients and reaching a greater understanding of the mechanisms involved in chronic joint pain. As joint pain is a complex interaction between many different pain mechanisms, available tools are important for patent profiling and providing the basic knowledge for development of new drugs and for developing pain management regimes.

Chronic postoperative pain after primary and revision total knee arthroplasty.

OBJECTIVES: Clinical experience suggests that patients with osteoarthritis (OA) undergoing revision total knee arthroplasty (TKA) experience more chronic complications after surgery compared with patients receiving primary TKA. This study aimed to investigate the difference in pain, mobility, and quality of life (QoL) in patients after revision TKA compared with patients after primary TKA. METHODS: A total of 99 OA patients after revision TKA surgery and 215 patients after primary TKA surgery were investigated in a cross-sectional study using: a pain description of current pain (non-existent, mild, moderate, severe, or unbearable), the pain intensity visual analogue scale, the Knee Society Score, and the Osteoarthritis Research Society International questionnaire. RESULTS: Nineteen percent after primary TKA surgery and 47% after revision TKA surgery experienced severe to unbearable chronic postoperative pain. After revision TKA surgery patients reported higher pain intensities during rest (P=0.039), while walking (P=0.008), and on average over the last 24 hours (P=0.050) compared with the patients after primary TKA surgery. Patients after revision TKA surgery had reduced walking distance (P=0.001), increased use of walking aids (P=0.015), and showed an overall decreased QoL (P<0.001) compared with patients after primary TKA surgery. No significant improvement was found in walking distance (P=0.448) for patients before revision TKA surgery compared with after revision TKA surgery. DISCUSSION: More than twice as many patients have pain after revision surgery compared with patients after primary TKA. Patients after revision TKA surgery have reduced function, poorer QoL, and higher pain intensity compared with patients after primary TKA surgery.

Motor inhibition affects the speed but not accuracy of aimed limb movements in an insect.

When reaching toward a target, human subjects use slower movements to achieve higher accuracy, and this can be accompanied by increased limb impedance (stiffness, viscosity) that stabilizes movements against motor noise and external perturbation. In arthropods, the activity of common inhibitory motor neurons influences limb impedance, so we hypothesized that this might provide a mechanism for speed and accuracy control of aimed movements in insects. We recorded simultaneously from excitatory leg motor neurons and from an identified common inhibitory motor neuron (CI1) in locusts that performed natural aimed scratching movements. We related limb movement kinematics to recorded motor activity and demonstrate that imposed alterations in the activity of CI1 influenced these kinematics. We manipulated the activity of CI1 by injecting depolarizing or hyperpolarizing current or killing the cell using laser photoablation. Naturally higher levels of inhibitory activity accompanied faster movements. Experimentally biasing the firing rate downward, or stopping firing completely, led to slower movements mediated by changes at several joints of the limb. Despite this, we found no effect on overall movement accuracy. We conclude that inhibitory modulation of joint stiffness has effects across most of the working range of the insect limb, with a pronounced effect on the overall velocity of natural movements independent of their accuracy. Passive joint forces that are greatest at extreme joint angles may enhance accuracy and are not affected by motor inhibition.

The effect of topical capsaicin-induced sensitization on heat-evoked cutaneous vasomotor responses.

Brief, localized, cutaneous, non-painful thermal stimuli can evoke a transient vasomotor response, causing increased cutaneous blood flow and elevated skin temperature. The aims of this study were to investigate 1) if cutaneous sensitization by topical application of capsaicin (TRPV1 receptor agonist) can facilitate the size, duration and spatial extent of this vasomotor response and 2) if males and females respond differently. Thermal pulses (43°C for 60 seconds) were applied on left/right volar forearms of 15 age-matched males and females. Skin temperature and cutaneous blood flow were measured 1, 5, 10, 15, and 30 minutes after heat application before and after topical capsaicin (1%, 30 min application) with contralateral arm serving as the control. Recordings were made from the region of interest at distances of 2, 4, 6, 8, and 10 cm from the capsaicin application site. Sensitization significantly enhanced skin temperature for up to 30 min and compared with non-sensitized skin at 10 min. Females showed the strongest response after sensitization, but the response lasted longer and spread more widely in males. The blood flow responses were significantly longer after capsaicin (from 5 to 30 minutes after thermal application). This increased blood flow extended outside the treated area up to 10 min after stimulation. After sensitization, the area under the blood flow response curves showed significantly stronger responses in females, spreading 4 cm outside the stimulation site. Cutaneous sensitizing caused prolonged and spatially expanded vasomotor responses to standardized thermal stimulation with sex specific differences.

The UVB cutaneous inflammatory pain model: a reproducibility study in healthy volunteers.

BACKGROUND: The human ultraviolet-B (UVB) experimental pain model induces cutaneous neurogenic inflammation, involves hyperalgesia, and is widely used as a pharmacological screening pain model. AIM: To estimate the test-retest reliability of the UVB pain model by application of a comprehensive set of vasomotor and quantitative sensory assessment methods and to estimate sample sizes required for parallel or crossover pharmacological screening studies when this model is considered to be applied. METHODS: The upper arms of 15 healthy male volunteers were UVB irradiated with three times the minimal erythema dose. Neurogenic inflammation was assessed by measuring erythema index, superficial blood flow and skin temperature at baseline, 1 day, 2 days and 3 days post irradiation. Sensory changes were assessed by brush stroke, von Frey hairs, pressure algometry, heat-evoked pain, stimulus response function to weight calibrated pin-prick stimulation, and the area of secondary hyperalgesia. The experiment was repeated with a two-week interval. Systematic bias, Coefficient of variation (CV), and intra-class correlation (ICC) were calculated within and between UVB irradiations. The sample sizes for parallel and crossover studies were calculated. RESULTS: Neurogenic inflammation (erythema index) and primary hyperalgesia (pin-prick stimulation) remained significant for 3 days, and were highly reproducible within and between the UVB irradiations resulting of low sample sizes (4-26) in both parallel and crossover studies. CONCLUSION: Based on sample size calculations, it is recommended to use the erythema index to assess neurogenic inflammation, and pin-prick stimulation for primary hyperalgesia for both parallel and crossover pharmacological screening studies.

A formal mathematical framework for physiological observations, experiments and analyses.

Experiments can be complex and produce large volumes of heterogeneous data, which make their execution, analysis, independent replication and meta-analysis difficult. We propose a mathematical model for experimentation and analysis in physiology that addresses these problems. We show that experiments can be composed from time-dependent quantities, and be expressed as purely mathematical equations. Our structure for representing physiological observations can carry information of any type and therefore provides a precise ontology for a wide range of observations. Our framework is concise, allowing entire experiments to be defined unambiguously in a few equations. In order to demonstrate that our approach can be implemented, we show the equations that we have used to run and analyse two non-trivial experiments describing visually stimulated neuronal responses and dynamic clamp of vertebrate neurons. Our ideas could provide a theoretical basis for developing new standards of data acquisition, analysis and communication in neurophysiology.

Desensitization properties of AMPA receptors at the cerebellar mossy fiber granule cell synapse.

Native AMPA receptors (AMPARs) exhibit rapid and profound desensitization in the sustained presence of glutamate. Desensitization therefore contributes to short-term depression at synapses in which glutamate accumulates. At synapses that do not exhibit desensitization-dependent depression, AMPARs are thought to be protected against prolonged or repetitive exposure to synaptically released glutamate. At the cerebellar mossy fiber to granule cell (GC) synapse, in which high release probability and glutamate spillover produce a substantial buildup of glutamate concentration in the cleft ([Glut]cleft) during high-frequency transmission, only moderate desensitization of the phasic AMPAR EPSC occurs. To investigate how such currents are produced, we examined the kinetic properties of synaptic AMPARs in GCs using glutamate uncaging. Photolysis of 4-methoxy-7-nitroindolinyl-caged L-glutamate with large illumination spots produced step-like increases in [Glut]cleft that could be used to systematically probe AMPAR kinetics. At low levels of activation, synaptic AMPARs exhibited little desensitization. With larger activations, the desensitization time course became faster, but the level of desensitization was only weakly dependent on receptor occupancy. Indeed, a substantial desensitization-resistant current component remained (17%) in saturating glutamate. Photolysis with small illumination spots produced brief [Glut]cleft waveforms and transient AMPAR activations, similar to the EPSC current components. Paired-pulse uncaging with such spots revealed little desensitization after spillover-like activations and modest depression after activations that mimicked quantal and spillover components together. Our results show that GC AMPARs exhibit a resistance to desensitization at low occupancies and that this property is crucial for sustaining high-frequency transmission at a synapse in which glutamate accumulates.

Rapid vesicular release, quantal variability, and spillover contribute to the precision and reliability of transmission at a glomerular synapse.

The amplitude and shape of EPSC waveforms are thought to be important determinants of information processing and storage in the brain, yet relatively little is known about the origins of EPSC variability or how it affects synaptic signaling. We investigated the stochastic determinants of AMPA receptor-mediated EPSC variability at cerebellar mossy fiber-granule cell (MF-GC) connections by combining multiple-probability fluctuation analysis (MPFA) and deconvolution methods. The properties of MF connections with a single release site and the effects of the rapidly equilibrating competitive antagonist kynurenic acid on EPSCs suggest that receptors are not saturated by glutamate during a quantal event and that quanta sum linearly over a wide range of release probabilities. MPFA revealed an average of five vesicular release sites per MF-GC connection. Our results show that the time course of vesicular release is rapid (decay, tau = 75 micros) and independent of release probability, introducing little jitter in the shape or timing of the quantal component of the EPSC at physiological temperature. Moreover, the peak vesicular release rate per release site after an action potential (AP) (approximately 3 ms(-1)) is substantially higher than previously reported for central synapses. Interaction of amplitude fluctuations arising from quantal release and quantal size with the slower, low variability spillover-mediated current produce substantial variability in EPSC shape. Our simulations of MF-GC transmission suggest that quantal variability and transmitter spillover extend the voltage from which AP threshold can be crossed, improving reliability, and that fast vesicular release allows precise signaling across MF connections with heterogeneous weights.

Modulation of glutamate mobility reveals the mechanism underlying slow-rising AMPAR EPSCs and the diffusion coefficient in the synaptic cleft.

Fast- and slow-rising AMPA receptor-mediated EPSCs occur at central synapses. Fast-rising EPSCs are thought to be mediated by rapid local release of glutamate. However, two controversial mechanisms have been proposed to underlie slow-rising EPSCs: prolonged local release of transmitter via a fusion pore, and spillover of transmitter released rapidly from distant sites. We have investigated the mechanism underlying slow-rising EPSCs and the diffusion coefficient of glutamate in the synaptic cleft (Dglut) at cerebellar mossy fiber-granule cell synapses using a combination of diffusion modeling and patch-clamp recording. Simulations show that modulating Dglut has different effects on the peak amplitudes and time courses of EPSCs mediated by these two mechanisms. Slowing diffusion with the macromolecule dextran slowed slow-rising EPSCs and had little effect on their amplitude, indicating that glutamate spillover underlies these currents. Our results also suggest that under control conditions Dglut is approximately 3-fold lower than in free solution.