Paracrine-like excitation of low-threshold mechanoceptive C-fibers innervating rat hairy skin is mediated by substance P via NK-1 receptors

A Human Sensory Pathway Connecting the Foot to Ipsilateral Face That Partially Bypasses the Spinal Cord

Human sensory transmission from limbs to brain crosses and ascends through the spinal cord. Yet, descriptions exist of ipsilateral sensory transmission as well as transmission after spinal cord transection. To elucidate a novel ipsilateral cutaneous pathway, we measured facial perfusion following painfully-cold water foot immersion in 10 complete spinal cord-injured patients, 10 healthy humans before and after lower thigh capsaicin C-fiber cutaneous conduction blockade, and 10 warm-immersed healthy participants. As in healthy volunteers, ipsilateral facial perfusion in spinal cord injured patients increased significantly. Capsaicin resulted in contralateral increase in perfusion, but only following cold immersion and not in 2 spinal cord-injured patients who underwent capsaicin administration. Supported by skin biopsy results from a healthy participant, we speculate that the pathway involves peripheral C-fiber cross-talk, partially bypassing the cord. This might also explain referred itch and jogger's migraine and it is possible that it may be amenable to training spinal-injured patients to recognize lower limb sensory stimuli.

Peripheral ionotropic glutamate receptors contribute to Fos expression increase in the spinal cord through antidromic electrical stimulation of sensory nerves

Previous studies have shown that peripheral ionotropic glutamate receptors are involved in the increase in sensitivity of a cutaneous branch of spinal dorsal ramus (CBDR) through antidromic electrical stimulation (ADES) of another CBDR in the adjacent segment. CBDR in the thoracic segments run parallel to each other and no synaptic contact at the periphery is reported. The present study investigated whether the increased sensitivity of peripheral sensory nerves via ADES of a CBDR induced Fos expression changes in the adjacent segments of the spinal cord. Fos expression increased in the T8 - T12 segments of the spinal cord evoked by ADES of the T10 CBDR in rats. The increased Fos expression in the T11 and T12, but not T8 - T10 spinal cord segments, was significantly blocked by local application of either N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine maleate (MK-801) or non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) into the receptive field of T11 CBDR. The results suggest that endogenous glutamate released by ADES of sensory nerve may bind to peripheral ionotropic glutamate receptors and activate adjacent sensory nerve endings to increase the sensitivity of the spinal cord. These data reveal the potential mechanisms of neuron activation in the spinal cord evoked by peripheral sensitization.

Electrical signal propagated across acupoints along Foot Taiyang Bladder Meridian in rats

OBJECTIVE

To investigate the electrical signals propagated along Foot Taiyang Bladder Meridian (BL) in a rat model.

METHODS

The experiments were performed on Dark-Agouti (DA), DA.1U and Sprague Dawley (SD) rats. The antidromic electrical stimulation was applied on the nerve innervating "Pishu" (BL 20) to mimic the acupoint electro-acupuncture (EA). The activities recording from adjacent nerve innervating acupoint "Danshu" (BL 19) or "Weishu" (BL 21) were recorded as indics for acupoint, including the mechanical threshold and discharge rate.

RESULTS

After mimic EA on BL 20, C and Aδ units from adjacent BL 19 or BL 21 were sensitized including the decrease in mechanical threshold and increase in discharge rates in DA, DA.1U and SD rats, especially in DA rats. The average discharge rate increased from 2.40±0.26 to 6.06±0.55 and from 1.92±0.42 to 6.17±1.10 impulse/min (P<0.01), and the mechanical threshold decreased from 0.52±0.12 to 0.24±0.05 and from 0.27±0.02 to 0.16±0.01 mmol/L (P<0.01) in C (n=15) and Aδ (n=18) units in DA rats. The net change in discharge rates from C units were 152.5%, 144.7% and 42.4% in DA, DA.1U and SD rats, respectively, among which DA rat's was the highest (P<0.05). In Aδ units, the net change in DA rats were also the highest (221.5%, 139.2% and 49.2% in DA, DA.1U and SD rats).

CONCLUSIONS

These results showed that mimic acupoint EA activated adjacent acupoints along BL in three rat strains, which might be related to propagated sensation along meridians (PSM). In addition, DA rats were more sensitive and might be a good model animal for PSM research.

Classic and Modern Meridian Studies: A Review of Low Hydraulic Resistance Channels along Meridians and Their Relevance for Therapeutic Effects in Traditional Chinese Medicine

Meridian theory is one of the core components of the theory of traditional Chinese medicine (TCM). It gives an integral explanation for how human life works, how a disease forms, and how a therapy acts to treat a disease. If we do not understand the meridians, it is hard to understand the TCM. People in China and abroad had been working hard for 50 years, trying to understand the meridians; then 15 years ago a breakthrough idea appeared when we realized that they are low resistance fluid channels where various chemical and physical transports take place. The channel is called low hydraulic resistance channel (LHRC) and the chemical transport is named volume transmission (VT). This review aims to give a full understanding of the essence of meridian and its works on the therapies of TCM.

Induction of Hyperalgesia in Pigs through Blocking Low Hydraulic Resistance Channels and Reduction of the Resistance through Acupuncture: A Mechanism of Action of Acupuncture

According to the classic theory of Chinese medicine, pain is due to the blockage in meridian channels, and acupuncture was invented to treat pain by “dredging” the channels. To test the theory, a hyperalgesia model was made by injecting hydrogel into low hydraulic resistance channel (LHRC) in 12 anaesthetized minipigs. Tail-flick threshold and ear-flick threshold were measured using a thermal radiation dolorimeter, and relative flick threshold (RFT) was calculated. Hydraulic resistance (HR) was measured with a biological HR measuring instrument on low HR points on LHRC and on control points with higher HR located outside LHRC; readings were recorded before, during, and after acupuncture treatment. RFT decreased after blocking the LRHC and was still significantly decreased 2 days and 4 days afterwards. No significant changes occurred when injecting saline into the same points or injecting gel into points outside the channel. Subsequent acupuncture reduced HR on LRHC along meridians but had no significant effect on sites with higher HR located outside LHRC. One of the mechanisms of action of acupuncture treatment for chronic pain may be that acupuncture affects peripheral tissue by reducing the HR in LHRC along meridians, improving the flow of interstitial fluid and removing algogenic substances and thereby relieving pain.

Mechanical allodynia in human glabrous skin mediated by low-threshold cutaneous mechanoreceptors with unmyelinated fibres

We recently showed that C-tactile fibres (CTs) in human hairy skin (anterior leg) mediate crossover between innocuous touch and noxious touch, i.e. mechanical allodynia. Although there is no evidence for existence of a phenotypically identical class of CTs in human glabrous skin, the 'qualia' of affective stimuli are comparable across skin types. In 42 healthy subjects, muscle pain was induced by infusing hypertonic saline (5 %) into flexor carpi ulnaris muscle. Concurrently, sinusoidal vibration (200 Hz-200 μm) was applied to glabrous skin of little finger. The neural substrate of allodynia was determined by employing conduction blocks of myelinated (ulnar nerve compression) and unmyelinated (low-dose intra-dermal anaesthesia) fibres. In order to compare the expression of allodynia across spinal segments and skin types, vibration was also applied to glabrous skin of index finger and hairy skin of dorsal forearm. In addition, high-precision brushing stimuli were applied at speeds of 1.0 and 3.0 cm s(-1) to digital glabrous skin with absent myelinated fibres. During muscle pain, vibration caused a significant and reproducible increase in pain (allodynia). This effect persisted during blockade of myelinated fibres, but was abolished by inactivation of unmyelinated cutaneous fibres. The vibration-evoked effects were found to be comparable across spinal segments and skin types. Furthermore, brushing produced a near-identical expression of C-fibre-mediated allodynia. Prior to induction and upon cessation of muscle pain, vibration and brushing were reported as non-painful. Based on these results, we postulate that a functional homologue of the CTs (hairy skin) mediates allodynia in human glabrous skin.

Understanding propagated sensation along meridians by volume transmission in peripheral tissue

Propagated sensation along meridians (PSM) is a phenomenon that a sensation moves along meridians during stimulation of an acupoint. PSM has an appearance rate of 1.3% among people and have characteristics of low speed, going toward afflicted sites and being blocked by physical pressure which is difficult to be explained by known neural and blood transmission. Volume transmission (VT) is a widespread mode of intercellular communication in the central nervous system that occurs in the extracellular fluid and in the cerebrospinal fluid. VT signals moves from source to target cells via energy gradients leading to diffusion and convection (flow) which is slow, long distance and much less space filling. VT channel diffuse forming a plexus in the extracellular space with two parameters of volume fraction and tortuosity. Some experiments showed an information transmission between adjacent and distant acupoints along meridians cross spinal segments. This process is a cross-excitation between peripheral nerve terminals which is related to nonsynaptic transmission. Some neurotransmitters or neuropeptides such as glutamate, adenosine triphosphate (ATP) and neuropeptide such as substance P, neurokinin A and calcitonin gene-related peptide relate with the cross-excitation which can be regards as VT signals. Comparing the characteristics of PSM and VT, many similar aspects can be found leading to an assumption that PSM is a process of VT in peripheral tissue along meridians. The reason why VT signals transmit along meridians is that the meridian is rich in interstitial fluid under the condition of low hydraulic resistance which has been proven experimentally. According to Darcy's law which descript the flow of interstitial fluid and conservation equation, interstitial fluid will move toward meridians and flow along meridians that restrict the VT signals within the channel and accelerate the flow according to Fick's diffusion law. During the process, a degranulation of histamine from mast cells happens on the route which can expand capillary and increase the blood perfusion and interstitial fluid which had already been observed. The mechanism of PSM is featured by alternative axon reflex (wired transmission, WT) and VT in peripheral tissue along meridians, sending simultaneously a continuous sensate signal to control nerve system which can be felt like a PSM.

C-tactile fibers contribute to cutaneous allodynia after eccentric exercise

We recently showed that during acute muscle pain, C-tactile (CT) fibers mediate allodynia in healthy human subjects. In this study, we pursued the following questions: Do CTs contribute to allodynia observed in delayed onset muscle soreness (DOMS)? Is CT-mediated allodynia reproducible in a clinical pain state? In 30 healthy subjects, DOMS was induced in anterior compartment muscles of the leg by repeated eccentric contractions. DOMS was confirmed by mapping the emergence of tender points (decreased pressure pain thresholds). Furthermore, we measured pressure pain thresholds in a clinical subject who presented with activity-triggered heel pain but no resting pain. Cutaneous vibration (sinusoidal; 200 Hz-200 μm)--an otherwise innocuous stimulus--was applied to anterolateral leg before exercise, during DOMS, and following recovery from DOMS. The peripheral origin of allodynia was determined by employing conduction blocks of unmyelinated (intradermal anesthesia) and myelinated (nerve compression) fibers. In DOMS state, there was no resting pain, but vibration reproducibly evoked pain (allodynia). The blockade of cutaneous C fibers abolished this effect, whereas it persisted during blockade of myelinated fibers. In the clinical subject, without exposure to eccentric exercise, vibration (and brushing) produced a cognate expression of CT-mediated allodynia. These observations attest to a broader role of CTs in pain processing.

PERSPECTIVE

This is the first study to demonstrate the contribution of CT fibers to mechanical allodynia in exercise-induced as well as pathological pain states. These findings are of clinical significance, given the crippling effect of sensory impairments on the performance of competing athletes and patients with chronic pain and neurological disorders.

Neural acupuncture unit: a new concept for interpreting effects and mechanisms of acupuncture

When an acupuncture needle is inserted into a designated point on the body and mechanical or electrical stimulation is delivered, various neural and neuroactive components are activated. The collection of the activated neural and neuroactive components distributed in the skin, muscle, and connective tissues surrounding the inserted needle is defined as a neural acupuncture unit (NAU). The traditionally defined acupoints represent an anatomical landmark system that indicates local sites where NAUs may contain relatively dense and concentrated neural and neuroactive components, upon which acupuncture stimulation would elicit a more efficient therapeutic response. The NAU-based local mechanisms of biochemical and biophysical reactions play an important role in acupuncture-induced analgesia. Different properties of NAUs are associated with different components of needling sensation. There exist several central pathways to convey NAU-induced acupuncture signals, Electroacupuncture (EA) frequency-specific neurochemical effects are related to different peripheral and central pathways transmitting afferent signals from different frequency of NAU stimulation. More widespread and intense neuroimaging responses of brain regions to acupuncture may be a consequence of more efficient NAU stimulation modes. The introduction of the conception of NAU provides a new theoretical approach to interpreting effects and mechanisms of acupuncture in modern biomedical knowledge framework.

Octreotide inhibits capsaicin-induced activation of C and Aδ afferent fibres in rat hairy skin in vivo

1. The present study investigated whether the somatostatin receptor (SSTR) agonist, octreotide, could inhibit the activation of dorsal skin afferent fibres induced by local injection of capsaicin in the rat. 2. Single unit activity from Aδ mechano-heat sensitive (AMH; n = 41) and C mechano-heat sensitive (CMH; n = 30) afferents was recorded after their isolation in thin filaments from the dorsal cutaneous nerve branches. The effect of subcutaneous octreotide injection on the change in discharge rate and mechanical threshold induced by capsaicin was determined. 3. Capsaicin (0.05%) injection into the edge of the receptive field of both AMH and CMH units increased their discharge rate and decreased their mechanical threshold. Pre-injection of octreotide inhibited these responses, and co-application of SSTR antagonist, cyclosomatostatin, reversed the inhibitory effect of octreotide. 4. The present study provides electrophysiological evidence that the signal evoked by the somatostatin receptor inhibits the activation and mechanical sensitization evoked by capsaicin in the terminals in small-diameter sensory neurons.

Spinal or systemic TY005, a peptidic opioid agonist/neurokinin 1 antagonist, attenuates pain with reduced tolerance

BACKGROUND AND PURPOSE

The use of opioids in treating pain is limited due to significant side effects including somnolence, constipation, analgesic tolerance, addiction and respiratory depression. Pre-clinical studies have shown that neurokinin 1 (NK(1) ) receptor antagonists block opioid-induced antinociceptive tolerance and may inhibit opioid-induced rewarding behaviours. Here, we have characterized a bifunctional peptide with both opioid agonist and NK(1) antagonist pharmacophores in a rodent model of neuropathic pain.

EXPERIMENTAL APPROACH

Rats were evaluated for behavioural responses to both tactile and thermal stimuli in either an uninjured, sham- or nerve-injured state. TY005 (Tyr-DAla-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bn(CF(3) )(2) ) was delivered spinally or systemically to assess the antinociceptive effects after acute exposure. Motor skills were evaluated using the rotarod test to determine potential sedative effects. Spinal TY005 was given chronically to sham- or nerve-injured animals to determine the development of tolerance.

KEY RESULTS

Bolus injections of TY005 produced dose-dependent antinociception in non-injured animals and alleviated nerve injury-induced thermal and tactile hypersensitivities (i.e. antihyperalgesia) more effectively than morphine. Sedative effects were not evident from the rotarod test at doses that were antihyperalgesic, nor at doses threefold higher. Repeated administration of TY005 did not lead to the development of antihyperalgesic tolerance or alter sensory thresholds.

CONCLUSIONS AND IMPLICATIONS

Collectively, the data suggest that opioid agonist/NK(1) antagonist bifunctional peptides represent a promising novel approach to the management of chronic pain without the development of tolerance, reducing the need for escalation of doses and unwanted side effects associated with opiates alone.

Somatostatin inhibits activation of dorsal cutaneous primary afferents induced by antidromic stimulation of primary afferents from an adjacent thoracic segment in the rat

To investigate the effect of somatostatin on the cross-excitation between adjacent primary afferent terminals in the rats, we recorded single unit activity from distal cut ends of dorsal cutaneous branches of the T10 and T12 spinal nerves in response to antidromic stimulation of the distal cut end of the T11 dorsal root in the presence and absence of somatostatin and its receptor antagonist applied to the receptive field of the recorded nerve. Afferent fibers were classified based upon their conduction velocity. Mean mechanical thresholds decreased and spontaneous discharge rates increased significantly in C and Adelta but not Abeta fibers of the T10 and T12 spinal nerves in both male and female rats following antidromic electrical stimulation (ADES) of the dorsal root from adjacent spinal segment (DRASS) indicating cross-excitation of thin fiber afferents. The cross-excitation was not significantly different between male and female rats. Microinjection of somatostatin into the receptive field of recorded units inhibited the cross-excitation. This inhibitory effect, in turn, was reversed by the somatostation receptor antagonist cyclo-somatostatin (c-SOM). Application of c-SOM alone followed by ADES of DRASS significantly decreased the mechanical thresholds and increased the discharge rates of C and Adelta fibers, indicating that endogenous release of somatostatin plays a tonic inhibitory role on the cross-excitation between peripheral nerves. These results suggest that somatostatin could inhibit the cross-excitation involved in peripheral hyperalgesia and have a peripheral analgesic effect.