Activation of capsaicin receptors on the sciatic nerve induces FOS expression in the spinal dorsal horn of adult rats

By using immunohistochemical staining for FOS protein in the spinal cord, the role of capsaicin receptors on the sciatic nerve was investigated. After topical application of capsaicin (1%) to the sciatic nerve, FOS-like immunoreactive (FOS-LI) neurons were observed, chiefly in the superficial laminae of the lumbar dorsal horn. Topical application of capsazepine (5%) or lidocaine (2%) to the sciatic nerve for 15 min before the application of capsaicin reduced the number of FOS-LI neurons in the superficial dorsal horn (by 83.2 +/- 1.7 and 32.4 +/- 1.2%, respectively). One week after pretreatment of the sciatic nerve with colchicine, the number of FOS-LI neurons induced by capsaicin was greatly decreased (by 74.6 +/- 1.7%). Given that FOS protein expression after peripheral noxious stimulation is found in a location similar to that in the present study, our results indicate that the capsaicin receptor on the sciatic nerve is involved in the transmission of noxious information.     

A fluorescence-immunohistochemical study on phosphorylation of ERK1/2, p38, and STAT3 in rat dorsal root ganglia following noxious stimulation of hind paw sensory neurons

A fluorescence-immunohistochemical investigation was performed in lumbar dorsal root ganglia (DRGs) neurons of the rat with regard to ERK1/2-, p38- and STAT3-phosphorylation in response to nociceptor activation in the rat. The stimuli applied were perineural capsaicin treatment of the sciatic nerve, mustard oil application to the hind paw and heat or cold stimulation of the hind paw. The time points of investigations were 15 min/30 min after perineural capsaicin, 30 min/2 h/4 h for mustard oil, 10 min/4 h for cold and 30 min/2 h/8 h for the heat stimulus. All four stimuli lead to a time-dependent, significant 2-3 fold increase in the number of small and medium size DRG cells displaying cytoplasmic staining for p-ERK1/2, but to no activation of satellite cells. Phosphorylated p38 immunoreactivity was increased in the cytoplasma of DRG cells at 2 h after the mustard oil treatment of the hind paw and 30 min after the perineural capsaicin application to the sciatic nerve axons, but not following heat or cold stimuli to the hind paws. Phospho-STAT3 staining was characteristically observed as nuclear and cytoplasmic staining. It was found increased after the perineural capsaicin application to the sciatic nerve axons, however, no marked increase was found with the other 3 noxious stimuli. The present results show that sensory neurons respond with a selective long-lasting increase in p-ERK1/2 in small and medium-size DRG cells, when their axons or axon terminals are stimulated by capsaicin, mustard oil, noxious heat or noxious cold.     

Effect of capsaicin upon afferent and efferent mechanisms of nociception and temperature regulation in birds

(1) Acute capsaicin effects on nociception in the conscious chicken were tested by close arterial injection. The threshold dose to elicit nocifensive and autonomic responses was 50 micrograms, i.e., two to three orders of magnitude higher than in mammals but four times lower than in pigeons. (2) Foot withdrawal from hot water remained unchanged after capsaicin was injected either intravenously in the chicken at a cumulative dose of 600 mg/kg or perineurally at a dose of 100 micrograms into the sciatic nerve of pigeons. (3) Temperature regulation and body temperature in the chicken were not affected by subcutaneous injection of capsaicin, but intravenous infusion at rates of 2-5 or 10-13 mg X min-1 X kg-1 transiently lowered body temperature by 1.5 degrees C and stimulated panting and sometimes vasodilatation of the comb. Repeated capsaicin infusion produced temporary tachyphylaxia but no permanent desensitization. (4) A cumulative dose of 1 g/kg body weight capsaicin reduced the relationship between breathing frequency and respiratory evaporative heat loss in the duck. This deficit was compensated by more pronounced panting and, thus, did not indicate any impairment of temperature regulation. (5) Injection of capsaicin into the sciatic nerve depleted substance P in the dorsal horn of rats. Similar treatment in pigeons caused an increase of substance P immunoreactivity in the dorsal horn. (6) The effects of high capsaicin doses in birds indicate only low susceptibility of afferent neural mechanisms. Some of the effects may be due to a capsaicin action upon efferent neural mechanisms.     

Capsaicin and nociception

The antinociceptive effect of capsaicin to noxious chemical stimuli has been invariably verified. As to thermal or mechanical nociception, however, routine pharmacological methods resulted in conflicting findings. Therefore, using new techniques the nociceptive thresholds of different stimuli were determined on the hindpaw of the rat. After systemic (400 mg/kg s.c.), perineural (1% on the sciatic nerve) and local (5 micrograms into the hindpaw) application of capsaicin the threshold for noxious heat (47.4 +/- 0.08) was shifted upwards by 3.3 degrees C, 4.1 degrees C and 2.9 degrees C, respectively. The changes in mechanonociceptive threshold evoked by pin prick (186 +/- 9 mN force) were more variable. The response to percutaneous xylene application was abolished or markedly inhibited. After systemic application the responsiveness to noxious heat recovered faster than the effect of xylene. C-polymodal nociceptors and some A-delta mechanoheat-sensitive nociceptors isolated from the saphenous nerve of the rat were activated by capsaicin in nanogram doses given close arterially. Five micrograms capsaicin excited few slowly adapting A mechanoreceptors after a long latency, but not A-delta mechanonociceptors or other cutaneous receptors. Proportion of C-polymodal nociceptors was decreased, that of the C-mechanoreceptors was increased after systemic treatment. The role of polymodal-type nociceptors, interaction of other nociceptors, as well as secondary dynamic changes are stressed to explain the antinociceptive effect of capsaicin.     

Changes in fibre populations of the rat hairy skin following selective chemodenervation by capsaicin

Perineural application of capsaicin results in a selective and permanent reduction in the sensitivity to noxious chemical and heat stimuli and elimination of the neurogenic inflammatory response. The present quantitative immunohistochemical study has been undertaken to reveal the populations of cutaneous afferent nerves that are affected by perineural capsaicin treatment. Areas of intact and chemodenervated skin were determined with the aid of the vascular labelling technique. In sections taken from intact skin areas, staining with antibodies against protein gene product 9.5 revealed a rich epidermal innervation. Fibres immunoreactive for growth-associated protein 43 were also abundant; nerve fibres immunoreactive for substance P and calcitonin gene-related peptide were less numerous. Somatostatin- and RT97-immunoreactive fibres were seen only in the subepidermal layer. In sections taken from skin areas supplied by the sciatic nerve treated with capsaicin 3 days previously, the number of epidermal nerve fibres immunoreactive to protein gene product 9.5, growth-associated protein 43, substance P and calcitonin gene-related peptide was reduced by 90%, 95%, 97% and 66%, respectively. These changes persisted for at least 42 days. The findings reveal that the majority of epidermal axons are capsaicin-sensitive and comprise a chemically heterogeneous population. Reductions in cutaneous fibre populations following perineural capsaicin treatment may result from both the degeneration of sensory axons and the depletion of neuron-specific macromolecules. In addition, most cutaneous nociceptive axons may not use the major sensory neuropeptides substance P and calcitonin gene-related peptide as afferent neurotransmitters.     

Peripheral nerve lesion-induced uptake and transport of choleragenoid by capsaicin-sensitive c-fibre spinal ganglion neurons

In the present experiments the effect of systemic capsaicin treatment on the retrograde labelling of sensory ganglion cells was studied following the injection of choleratoxin B subunit-horseradish peroxidase conjugate (CTX-HRP) into intact and chronically transected peripheral nerves. In the control rats CTX-HRP injected into intact sciatic nerves labelled medium and large neurons with a mean cross-sectional area of 1,041 +/- 39 gm2. However, after injection of the conjugate into chronically transected sciatic nerves of the control rats, many small cells were also labelled, shifting the mean cross-sectional area of the labelled cells to 632 +/- 118 microm2. Capsaicin pretreatment per se induced a moderate but significant decrease in the mean cross-sectional area of the labelled neurons (879 +/- 79 microm2). More importantly, systemic pretreatment with capsaicin prevented the peripheral nerve lesion-induced labelling of small cells. Thus, the mean cross-sectional areas of labelled neurons relating to the intact and transected sciatic nerves, respectively, did not differ significantly. These findings provide direct evidence for a phenotypic switch of capsaicin-sensitive nociceptive neurons after peripheral nerve injury, and suggest that lesion-induced morphological changes in the spinal cord may be related to specific alterations in the chemistry of C-fibre afferent neurons rather than to a sprouting response of A-fibre afferents.     

Effects of capsaicin on inflammation and on the substance P content of nervous tissues in rats with adjuvant arthritis

Capsaicin (20-80 mg/kg, s.c.) reduced the inflammatory response to inoculation with Mycobacterium butyricum in the rat. The effect was apparent within 24 h, was partial, persisted for well over 20 days, and occurred irrespective of whether capsaicin was administered before or after the onset of inflammation, or at the time when the pathology reached peak. Capsaicin also attenuated the increase in substance P content in sciatic nerve, saphenous nerve, dorsal root ganglia, dorsal roots, and dorsal spinal cord (L4, L5) which occurs in rats with adjuvant arthritis. The data are consistent with a possible role of substance P in the peripheral manifestations of adjuvant arthritis.     

Endothelin as a putative sensory neuropeptide in the guinea-pig: different properties in comparison with calcitonin gene-related peptide

Both endothelin-(ET) and calcitonin gene-related peptide- (CGRP) like immunoreactivity (-LI) were present in a variety of organs and neuronal tissue of the guinea-pig as determined by radioimmunoassay (RIA). Neuronal tissues like dorsal root ganglia (DRG) contained by far the highest levels of both ET- (65 +/- 11 pmol/g) and CGRP-LI (34 +/- 5 pmol/g). The tissue levels of ET-LI remained unchanged after 6-hydroxydopamine and capsaicin-pretreatment, while CGRP-LI was markedly reduced after capsaicin. Chromatographic characterization revealed that the main portion of ET-LI in the DRG, right atrium and lung corresponded to synthetic ET-1. Immunohistochemical studies showed the presence of ET-LI in a few neurons of intact DRG and many neurons in DRG cell-cultures, partly co-existing with CGRP-LI. In the neuronal cells of DRG cultures the ratio between the ET- and CGRP-LI was 1:27 compared to 2:1 in intact DRG. 24 h after ligation of the sciatic or vagal nerves no accumulation of ET-LI was observed above the ligation, while CGRP-LI was increased 4-5-fold. Transection (10 days) of the sciatic nerve caused a 85-95% depletion of CGRP-LI in the distal skin, gastrocnemius muscle and trunk below the transection site, while in the proximal portion of the nerve CGRP-LI increased. No effects on ET-LI in these tissues were observed after sciatic nerve transfection. In release experiments on DRG cell cultures. Langendorff heart preparations or perfused guinea-pig lungs, potassium (60 mM), capsaicin or antidromic nerve stimulation evoked a clear-cut increase in the supernatant levels of CGRP-LI, suggesting release, while no effect on the ET-LI concentration was observed in the effluent. Furthermore, anoxia failed to influence the outflow of ET-LI from the heart and lung. It is concluded that ET-1-LI is present in high levels in spinal ganglia and ET-LI occurs in afferent cell-bodies, but in comparison with CGRP, ET shows remarkable inertness upon various experimental conditions including no evidence for axonal transport, loss after denervation or release. The neuronal ET-LI seems to increase under culture conditions, however. The possible function for the high content of ET-LI in the intact guinea-pig peripheral nervous system remains to be elucidated and may mainly be related to a non-neuronal pool considering the relatively low content of ET-LI compared to CGRP in cultured DRG cells.     

Protein Synthesis and Rapid Axonal Transport During Regrowth of Dorsal Root Axons

Damage to the sciatic nerve produces significant changes in the relative synthesis rates of some proteins in dorsal root ganglia and in the amounts of some fast axonally transported proteins in both the sciatic nerve and dorsal roots. We have now analyzed protein synthesis and axonal transport after cutting the other branch of dorsal root ganglia neurons, the dorsal roots. Two to three weeks after cutting the dorsal roots, [35S]methionine was used to label proteins in the dorsal root ganglia in vitro. Proteins synthesized in the dorsal root ganglia and transported along the sciatic nerve were analyzed on two-dimensional gels. All of the proteins previously observed to change after sciatic nerve damage were included in this study. No significant changes in proteins synthesized in dorsal root ganglia or rapidly transported along the sciatic nerve were detected. Axon regrowth from cut dorsal roots was observed by light and electron microscopy. Either the response to dorsal root damage is too small to be detected by our methods or changes in protein synthesis and fast axonal transport are not necessary for axon regrowth. When such changes do occur they may still aid in regrowth or be necessary for later stages in regeneration.     

Sympathetic activation by chemical stimulation of white adipose tissues in rats

Injection of leptin into white adipose tissue (WAT) increases sympathetic outflow. The present study was designed to determine the effects of capsaicin and other chemicals in WAT on the sympathetic outflow and blood pressure and the roles of WAT afferents and hypothalamic paraventricular nucleus (PVN) in the adipose afferent reflex (AAR). The AAR was induced by injection of capsaicin, bradykinin, adenosine, adenosine triphosphate (ATP), or leptin into inguinal WAT (iWAT) or retroperitoneal WAT (rWAT) in anesthetized rats. The iWAT injection of capsaicin increased the renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) but not the heart rate. Bradykinin, adenosine, or leptin but not ATP in the iWAT caused similar effects to capsaicin on the RSNA and MAP. Intravenous, intramuscular, or intradermal injection of capsaicin had no significant effects on the RSNA and MAP. The effects of capsaicin in rWAT were similar to that in iWAT on the RSNA and MAP. Furthermore, injection of capsaicin into the iWAT increased the WAT afferent nerve activities, WAT efferent nerve activity, and brown adipose tissue efferent nerve activity. The iWAT denervation or chemical lesion of the PVN neurons with kainic acid abolished the AAR induced by the iWAT injection of capsaicin. These results indicate that the stimulation of iWAT afferents with capsaicin, bradykinin, adenosine, or leptin reflexly increases the RSNA and blood pressure. The iWAT afferents and the PVN are involved in the AAR induced by capsaicin in the iWAT.     

Tellurium-Induced Alterations in 3-Hydroxy-3-Methylglutaryl-CoA Reductase Gene Expression and Enzyme Activity: Differential Effects in Sciatic Nerve and Liver Suggest Tissue-Specific Regulation of Cholesterol Synthesis

The demyelination of peripheral nerves that results from exposure of developing rats to tellurium is due to inhibition of squalene epoxidase, a step in cholesterol biosynthesis. In sciatic nerve, cholesterol synthesis is greatly depressed, whereas in liver, some compensatory mechanism maintains normal levels of cholesterol synthesis. This tissue specificity was further explored by examining, in various tissues, gene expression and enzyme activity of 3-hydroxy-3-methylglutaryl-CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis. Exposure to tellurium resulted in pronounced increases in both message levels and enzyme activity in liver, the expected result consequent to up-regulation of this enzyme in response to decreasing levels of intracellular sterols. In contrast to liver, levels of mRNA and enzyme activity in sciatic nerve were both decreased during the tellurium-induced demyelinating period. The temporal pattern of changes in 3-hydroxy-3-methylglutaryl-CoA reductase message levels in sciatic nerve seen following exposure to tellurium was similar to the down-regulation seen for mRNA specific for PNS myelin proteins. Possible mechanisms for differential control of cholesterol biosynthesis in sciatic nerve and liver are discussed.     

Sensory modalities conveyed in the hindlimb somatic afferent input to nucleus tractus solitarius.

To determine the somatic sensory modalities conveyed by hindlimb somatic afferent inputs, the discharge of neurons in the nucleus tractus solitarius was recorded in anesthetized rats after electrical stimulation of either the contralateral sciatic nerve or L(6) spinal nerve, which innervates the hindlimb. The discharge of seven of eight cells was increased (P < 0.05) by capsaicin injected into the arterial supply of the hindlimb. Discharge was unaltered in 19 neurons tested for sensitivity to nonnoxious (40 degrees C) and noxious (47 degrees C) heating of the hindlimb skin. In contrast, lightly stroking the skin elicited discharge in 2 of 14 cells, whereas noxious pinching increased activity in 4 other cells. Rhythmic (1- to 3-s) muscle contraction (MC) increased (P < 0.05) discharge in >60% of neurons tested (11 of 18). Static (10- to 30-s) MC significantly (P < 0.05) increased discharge in four cells, two of which were also responsive to rhythmic MC. Rhythmic and sustained muscle stretch increased discharge (P < 0.05) in three of eight neurons tested. These data indicate that nucleus tractus solitarius neurons receive input from low- and high-threshold cutaneous mechanoreceptors, respond to capsaicin delivered into the hindlimb arterial supply, lack thermal sensitivity, and respond to activation of mechanosensitive as well as metabosensitive endings in skeletal muscle.     

Ischemia reperfusion injury of the skeletal muscle after selective deafferentation

The present study analyzes the effect of selective deafferentation on the reperfusion injury of the skeletal muscle when nociceptive sensory fibers of the left sciatic nerve are selectively damaged by capsaicin pretreatment in a rat model following tourniquet ischemia (ISC) applied for 30 min, 1 h, and 2 h on the left hind limb. The isometric tetanic contractile force of the extensor digitorum longus (EDL) muscle was measured after 1 h, and 1, 3, or 7 days of reperfusion. Contractile force of the damaged muscle was compared to the intact contralateral muscle. In another group, ISC was used without capsaicin pre-treatment. After 30 min of ISC, there was no difference between deafferented and non-pretreated groups. Following 1 h ISC, with the exception of 1 h reperfusion, the non-pretreated group produced stronger contractions than the deafferented group. After 2 h ISC, the contractile force of the deafferented muscle was significantly stronger compared to the non-deafferented muscle force at all reperfusion times. In conclusions, it was found that the absence of peptidergic sensory fibers after long-lasting (2 h) ischemia is beneficial in reperfusion injury, whereas the absence of vasodilator peptides has unfavorable effects if tissue damage is milder (after 1 h ischemia).     

Botulinum toxin's axonal transport from periphery to the spinal cord

Axonal transport of enzymatically active botulinum toxin A (BTX-A) from periphery to the CNS has been described in facial and trigeminal nerve, leading to cleavage of synaptosomal-associated protein 25 (SNAP-25) in central nuclei. Aim of present study was to examine the existence of axonal transport of peripherally applied BTX-A to spinal cord via sciatic nerve. We employed BTX-A-cleaved SNAP-25 immunohistochemistry of lumbar spinal cord after intramuscular and subcutaneous hind limb injections, and intraneural BTX-A sciatic nerve injections. Truncated SNAP-25 in ipsilateral spinal cord ventral horns and dorsal horns appeared after single peripheral BTX-A administrations, even at low intramuscular dose applied (5 U/kg). Cleaved SNAP-25 appearance in the spinal cord after BTX-A injection into the sciatic nerve was prevented by proximal intrasciatic injection of colchicine (5 mM, 2 μl). Cleaved SNAP-25 in ventral horn, using choline-acetyltransferase (ChAT) double labeling, was localized within cholinergic neurons. These results extend the recent findings on BTX-A retrograde axonal transport in facial and trigeminal nerve. Appearance of truncated SNAP-25 in spinal cord following low-dose peripheral BTX-A suggest that the axonal transport of BTX-A occurs commonly following peripheral application.     

Neurofilament Proteins Are Distributed in a Diminishing Proximodistal Gradient Along Rat Sciatic Nerve

Neurofilament (NF) proteins are distributed in a diminishing proximodistal gradient along rat sciatic nerve when compared with total noncollagen or other proteins in nerve. About a twofold decline of NF proteins can be detected by quantitating nerve proteins that have been separated by gel electrophoresis. A similar decrease of immunoreactivity to each NF subunit is seen in distal nerve segments when noncollagen nerve proteins are immunoblotted. Parallel decreases occur in all three NF proteins, thereby maintaining neurofilament subunit stoichiometry along the neuraxis. The same NF gradient can be detected when the NF contents in nerve branches to the gluteus and gastrocnemius muscles are compared with each other and with those in nerve segments taken from the same proximodistal levels of the parent sciatic nerve. The gradient of NF proteins increases during postnatal development and is readily detected by postnatal day 16. During the same period of development, the heavy NF subunit appears for the first time and is rapidly incorporated throughout the sciatic nerve. Hence, the NF gradient becomes manifest during the development and maturation of the adult form of the axonal cytoskeleton. The basis for the proximodistal gradient of NF proteins in peripheral nerve is presently unknown. The extent of the gradient cannot be accounted for on the basis of diminishing numbers of nerve fibers or increasing amounts of other nerve proteins, e.g., collagen, in distal nerve. An alternative interpretation is that the gradient reflects a low level of NF protein turnover during axonal transport.     

Statin treatment reduces oxidative stress-associated apoptosis of sciatic nerve in diabetes mellitus

Abstract

Statins are lipid-lowering drugs that are widely used for treating hyperlipidemia, especially in diabetic patients. The aim of our study was to explore the effects of atorvastatin on oxidative stress and apoptosis in the sciatic nerve due to hyperglycemia. Diabetes was induced by streptozotocin. Atorvastatin was given orally for two weeks beginning from the sixth week. Microscopic examination of sciatic nerve revealed that normal tissue organization was disrupted in streptozotocin induced diabetic rats. Treatment with Atorvastatin reduced the histological damage and protected the morphological integrity of the sciatic nerve in streptozotocin induced diabetes. Increased expressions of transforming growth factor beta-1, endothelial nitric oxide synthase and TUNEL in sciatic nerve from streptozotocin induced diabetes were reduced by Atorvastatin. Atorvastatin could improve the effects of oxidative stress and apoptosis on the sciatic nerve due to diabetes.     

Age-related variations of elements as compared among optic, radial, and sciatic nerves

To elucidate changes of peripheral nerves with aging, the authors studied age-related changes of element contents in the optic, radial, and sciatic nerves by inductively coupled plasma-atomic emission spectrometry. The subjects consisted of seven men and seven women, ranging in age from 61 to 97 yr. The contents of phosphorus and sulfur remained constant through ages 61 to 97 yr in three nerves, the optic, radial, and sciatic nerves. It was found that there were age-related differences in calcium content among the optic, radial, and sciatic nerves: The calcium content of the optic nerve increased progressively with aging; in the radial nerve, it was hardly changed with aging; in contrast, the calcium content of the sciatic nerve decreased gradually with aging. In addition, it was found that in the radial nerve there were moderate correlations between age and zinc or sodium content, whereas significant correlations between age and the content of silicon or iron were found in the sciatic nerve. Furthermore, there was a correlation between the silicon and iron contents in the sciatic nerves.     

Analysis of the Acid Phosphatases of Rat Sensory Gangli by Isoelectric Focussing on Polyacrylamide Gels

The acid phosphatases of rat spinal and trigeminal ganglia have been separated by isoelectric focussing on polyacrylamide gels into two main classes with pI's of 7.1 and 5.4. A low-pI acid phosphatase is also present in the mouse but not in the guinea pig. Evidence is presented that in the rat the pI 5.4 enzyme represents the nonlysosomal acid phosphatase that has been identified histochemically in one population of sensory neurones. This pI 5.4 acid phosphatase is partly membrane-bound and is selectively depleted by capsaicin administration. In lumbar dorsal root ganglia the enzyme is selectively depleted by sectioning the sciatic nerve and undergoes rapid axonal transport, a greater amount being transported peripherally than centrally. The results are discussed with reference to the possible function of this nonlysosomal acid phosphatase.     

急性神经损伤引起脊髓背角C-纤维诱发电位长时程增强

神经损伤引起神经病性疼痛,表现为持续性痛超敏和痛觉过敏。目前对神经病性疼痛的机制尚缺乏了解。我们以往的工作表明强直电刺激坐骨神经可引起脊髓背角C-纤维诱发电位的长时程增强(long-term potentiation,LTP),该LTP被认为是病理性疼痛的突触模型。本研究的目的在于探讨急性神经损伤是否能在完整动物的脊髓背角诱发出C-纤维诱发电位LTP。在以测试刺激(10～20V,0．5ms)电刺激坐骨神经的同时在脊髓背角用微电极记录C一纤维诱发电位。分别用强直刺激、剪断或夹捏坐骨神经诱导LTP。结果发现：(1)剪断或夹捏坐骨神经都可以诱导脊髓背角C-纤维诱发电位的LTP,该LTP可持续到实验结束(3～9h),在剪断神经前10min用利多卡因局部阻滞坐骨神经则可完全阻断LTP的产生;(2)神经损伤诱导的LTP可被NMDA受体阻断剂AP5所阻断;(3)用单次强直刺激引起LTP后,切断坐骨神经可使LTP的幅度进一步增大,而用多次强直电刺激使LTP饱和后,损伤神经则不能使LTP进一步增大。切断神经引起LTP后,强直电刺激也不能使LTP进一步增大。这些结果表明,急性神经损伤可以诱导脊髓背角C纤维诱发电位LTP,且切断神经能更有效地诱导LTP。该试验进一步支持我们的设想,即脊髓背角C-纤维诱发电位LTP可能在病理性疼痛的形成中起重要作用。     

Inhibitory effect of anandamide on resiniferatoxin-induced sensory neuropeptide release in vivo and neuropathic hyperalgesia in the rat

Anandamide (AEA) is an endogenous cannabinoid ligand acting predominantly on the cannabinoid 1 (CB(1)) receptor, but it is also an agonist on the capsaicin VR(1)/TRPV(1) receptor. In the present study we examined the effects of AEA and the naturally occurring cannabinoid 2 (CB(2)) receptor agonist palmitylethanolamide (PEA) on basal and resiniferatoxin (RTX)-induced release of calcitonin gene-related peptide (CGRP) and somatostatin in vivo. Since these sensory neuropeptides play important role in the development of neuropathic hyperalgesia, the effect of AEA and PEA was also examined on mechanonociceptive threshold changes after partial ligation of the sciatic nerve. Neither AEA nor PEA affected basal plasma peptide concentrations, but both of them inhibited RTX-induced release. The inhibitory effect of AEA was prevented by the CB(1) receptor antagonist SR141716A. AEA abolished and PEA significantly decreased neuropathic mechanical hyperalgesia 7 days after unilateral sciatic nerve ligation, which was antagonized by SR141716A and the CB(2) receptor antagonist SR144528, respectively. Both SR141716A and SR144528 increased hyperalgesia, indicating that endogenous cannabinoids acting on CB(1) and peripheral CB(2)-like receptors play substantial role in neuropathic conditions to diminish hyperalgesia. AEA and PEA exert inhibitory effect on mechanonociceptive hyperalgesia and sensory neuropeptide release in vivo suggesting their potential therapeutical use to treat chronic neuropathic pain.