Stimulation of saphenous afferent nerve produces vasodilatation of the vasa nervorum via an axon reflex-like mechanism in the sciatic nerve of anesthetized rats

The role of calcitonin gene-related peptide in peripheral and central pain mechanisms including migraine

Calcitonin gene-related peptide (CGRP) is a 37-amino acid peptide found primarily in the C and Aδ sensory fibers arising from the dorsal root and trigeminal ganglia, as well as the central nervous system. Calcitonin gene-related peptide was found to play important roles in cardiovascular, digestive, and sensory functions. Although the vasodilatory properties of CGRP are well documented, its somatosensory function regarding modulation of neuronal sensitization and of enhanced pain has received considerable attention recently. Growing evidence indicates that CGRP plays a key role in the development of peripheral sensitization and the associated enhanced pain. Calcitonin gene-related peptide is implicated in the development of neurogenic inflammation and it is upregulated in conditions of inflammatory and neuropathic pain. It is most likely that CGRP facilitates nociceptive transmission and contributes to the development and maintenance of a sensitized, hyperresponsive state not only of the primary afferent sensory neurons but also of the second-order pain transmission neurons within the central nervous system, thus contributing to central sensitization as well. The maintenance of a sensitized neuronal condition is believed to be an important factor underlying migraine. Recent successful clinical studies have shown that blocking the function of CGRP can alleviate migraine. However, the mechanisms through which CGRP may contribute to migraine are still not fully understood. We reviewed the role of CGRP in primary afferents, the dorsal root ganglion, and in the trigeminal system as well as its role in peripheral and central sensitization and its potential contribution to pain processing and to migraine.

Neural mechanism of localized changes in skeletal muscle blood flow caused by moxibustion-like thermal stimulation of anesthetized rats

Moxibustion-like thermal stimulation (MTS) was applied to the gastrocnemius muscle to measure local muscle blood flow (MBF) in the stimulated region and the change in the MBF in the region, and its mechanism was examined. In the experiment, we used urethane-anesthetized rats under artificial respiration and observed the change caused by gastrocnemius MTS using a laser Doppler blood-flow meter. MTS applied to the gastrocnemius muscle caused a two-phase response in blood flow that showed a transient decrease followed by an increase without blood pressure change. It is suggested that the increase in response occurs because of an axon reflex that has a reflex arc below the spinal cord, and the decrease in response is caused by direct stimulation of postganglionic muscle sympathetic fibers.

Different patterns of blood flow response in the trapezius muscle following needle stimulation (acupuncture) between healthy subjects and patients with fibromyalgia and work-related trapezius myalgia

Needle stimulation (acupuncture) has recently been shown to increase blood flow in the tibialis anterior muscle and overlying skin in healthy subjects (HS) and patients with fibromyalgia (FM). The aim of the present study was to examine the effect of needle stimulation on local blood flow in the trapezius muscle and overlying skin in HS and two groups of patients suffering from chronic pain in the trapezius muscle, i.e., FM and work-related trapezius myalgia (TM) patients. Two modes of needling, deep muscle stimulation (Deep) and subcutaneous needle insertion (SC), were performed at the upper part of the shoulder and blood flow was monitored for 60 min post-stimulation. Blood flow changes were measured non-invasively by using a new application of photoplethysmography. Increased blood flow in the trapezius muscle and overlying skin was found in all three groups following both Deep and SC. In HS, Deep was superior to SC in increasing skin and muscle blood flow, whereas in FM, SC was as effective as, or even more effective, than Deep. In the severely affected TM patients, no differences were found between the stimuli, and generally, a lesser blood flow response to the stimuli was found. At Deep, the muscle blood flow increase was significantly larger in HS, compared to the two patient groups. Positive correlations were found between muscle blood flow at Deep and pressure pain threshold in the trapezius muscle, neck movement and pain experienced at the stimulation, and negative correlations were found with spontaneous pain-related variables, symptom duration and age, pointing to less favorable results with worsening of symptoms, and to the importance of nociceptor activation in blood flow increase. It was hypothesized that the different patterns of muscle blood flow response to the needling may mirror a state of increased sympathetic activity and a generalized hypersensitivity in the patients. The intensity of stimulation should be taken into consideration when applying local needle stimulation (acupuncture) in order to increase the trapezius muscle blood flow in chronic pain conditions.

Morphological evidence for functional capsaicin receptor expression and calcitonin gene-related peptide exocytosis in isolated peripheral nerve axons of the mouse

Rat sciatic nerve axons express capsaicin, proton and heat sensitivity and respond to stimulation with a Ca2+-dependent and graded calcitonin gene-related peptide (CGRP) release. In this study we demonstrate that similar functions, including capsaicin-induced CGRP release, are to be found in the desheathed sciatic nerve of the mouse. We have morphologically investigated the mechanisms of this axonal release in regions away from the active zones of synapses. Capsaicin receptor 1 (TRPV1) and CGRP immunostaining was performed using electron microscopic visualization. TRPV1 was identified in the axoplasm and inside vesicles--presumably on axonal transport--as well as in considerable quantity in the axonal plasma membrane of unmyelinated nerve fibers. Most of the unmyelinated axons were immunopositive for CGRP and in unstimulated nerves CGRP-containing vesicles almost entirely filled the axoplasm. After capsaicin stimulation (10(-6) M for 5 min), the fibers appeared depleted of CGRP with only few vesicles remaining as well as some residual staining of the axoplasm. In addition a large number of vesicles were fused with the axonal membrane, forming classical exocytotic figures--the omega structures--lined with CGRP immunoreactive product. These results present morphological evidence for the distribution of TRPV1 along unmyelinated axons in peripheral nerve and also provide the first demonstration of vesicular neuropeptide exocytosis along unmyelinated axons in peripheral nerve.

Vasodilatation of muscle microvessels induced by somatic afferent stimulation is mediated by calcitonin gene-related peptide release in the rat

In anesthesized rats, the effects of electrical stimulation (ES) to the saphenous nerve on the microcirculation of the gracilis muscle were assessed through the measurement of two different hemodynamic parameters: (a). the muscle blood flow (MBF) using a laser Doppler flowmeter; and (b). the changes in diameter of the muscle arterioles observed directly using an intravital microscope system. Ipsilateral ES (5 V, 20 Hz, for 30 s) produced increases in MBF and mean arterial pressure (47+/-10% and 18+/-5%) over the baseline, while no significant changes in MBF were observed in the contralateral muscle. Neither selective nor simultaneous alpha- and beta-adrenergic blockade altered the increases in MBF induced by ipsilateral ES. The arteriolar diameter was found to increase by 38.9+/-5% following ipsilateral ES. This response in diameter was abolished after the topical application of a calcitonin gene-related peptide receptor antagonist (CGRP(8-37)). Contralateral ES produced a decrease in arteriolar diameter by 26+/-14%. Thus, ipsilateral nerve ES produced vasodilative responses in the muscle accompanied by increases in MBF independently of the sympathetic activity. Furthermore, CGRP was found directly involved in the reflex neural regulation of the muscle microcirculation, which suggests the participation of an axon reflex mechanism.

Acupuncture mechanisms for clinically relevant long-term effects--reconsideration and a hypothesis

From the author's direct involvement in clinical research, the conclusion has been drawn that clinically relevant long-term pain relieving effects of acupuncture (>6 months) can be seen in a proportion of patients with nociceptive pain. The mechanisms behind such effects are considered in this paper. From the existing experimental data some important conclusions can be drawn: 1. Much of the animal research only represents short-term hypoalgesia probably induced by the mechanisms behind stress-induced analgesia (SIA) and the activation of diffuse noxious inhibitory control (DNIC). 2. Almost all experimental acupuncture research has been performed with electro-acupuncture (EA) even though therapeutic acupuncture is mostly gentle manual acupuncture (MA). 3. Most of the experimental human acupuncture pain threshold (PT) research shows only fast and very short-term hypoalgesia, and, importantly, PT elevation in humans does not predict the clinical outcome. 4. The effects of acupuncture may be divided into two main components--acupuncture analgesia and therapeutic acupuncture. A hypothesis on the mechanisms of therapeutic acupuncture will include: 1. Peripheral events that might improve tissue healing and give rise to local pain relief through axon reflexes, the release of neuropeptides with trophic effects, dichotomising nerve fibres and local endorphins. 2. Spinal mechanisms, for example, gate-control, long-term depression, propriospinal inhibition and the balance between long-term depression and long-term potentiation. 3. Supraspinal mechanisms through the descending pain inhibitory system, DNIC, the sympathetic nervous system and the HPA-axis. Is oxytocin also involved in the long-term effects? 4. Cortical, psychological, "placebo" mechanisms from counselling, reassurance and anxiety reduction.

Calcitonin gene-related peptide produces skeletal muscle vasodilation following antidromic stimulation of unmyelinated afferents in the dorsal root in rats

In anesthetized rats, the contribution of calcitonin gene-related peptide (CGRP) to antidromic vasodilation of skeletal muscle blood flow (MBF) following electrical stimulation of muscle afferent was investigated by measuring biceps femoris MBF using laser Doppler flowmetry. Repetitive antidromic electrical stimulation of unmyelinated C fibers in ipsilateral dorsal roots at the 3rd-5th lumbar segments for 30 s caused an increase in MBF for 3-15 min (mean 4.5 min) without significant change in systemic arterial blood pressure. The increase in skeletal MBF started about 10 s after the onset of stimulation, and peaked at approximately 130% of the control value at about 30 s after the end of the 30 s period of stimulation. The MBF response was totally abolished by topical application of hCGRP (8-37), a CGRP receptor antagonist. It is concluded that antidromic vasodilation in skeletal muscles following stimulation of unmyelinated C afferents in dorsal roots is independent of systemic blood pressure and is mediated essentially by CGRP. It is suggested that this CGRP-related antidromic vasodilation may be important in the clinical improvement of skeletal MBF produced by physical therapy, e.g. acupuncture.