Presynaptic modulation by neuromedin U of sensory synaptic transmission in rat spinal dorsal horn neurones

Neuromedin U (NMU) is a brain–gut peptide first isolated from the spinal cord. Recent studies on NMU and its receptors have suggested a role of NMU in sensory transmission. Here we report on the localization of NMU in sensory neurones, and the actions of NMU in the substantia gelatinosa (SG) and the deep layer of the dorsal horn (laminae III–V) in adult rat spinal cord slices using the patch-clamp technique. An immunohistochemical study revealed that NMU peptide was present in most of the dorsal root ganglion neurones. In the spinal cord, NMU-immunoreactive neurones were located in the deep layer (laminae III–V), but not in the SG. However, NMU-positive axon terminals were observed in the SG as well as the deep layer. Bath-applied NMU (10 μ m ) increased the frequency, but not amplitude, of miniature excitatory postsynaptic currents (mEPSCs) in the SG and deep layer neurones by 146 ± 14% ( P < 0.01, n = 17) and 174 ± 21% ( P < 0.01, n = 6), respectively, without inducing any postsynaptic membrane currents recorded in tetrodotoxin. On the other hand, NMU did not affect miniature inhibitory postsynaptic currents recorded in tetrodotoxin. These findings, taken together, suggest that NMU acts on the presynaptic terminals of the primary afferent fibres working as an autocrine/paracrine neuromodulator to increase mEPSC frequency of the SG and deep layer neurones. This may account for the spinal mechanisms of the NMU-induced hyperalgesia reported previously.     

Hypocretin-2 (orexin-B) modulation of superficial dorsal horn activity in rat

The hypothalamic peptides hypocretin-1 (orexin A) and hypocretin-2 (Hcrt-2; orexin B) are important in modulating behaviours demanding arousal, including sleep and appetite. Fibres containing hypocretin project from the hypothalamus to the superficial dorsal horn (SDH) of the spinal cord (laminae I and II); however, the effects produced by hypocretins on SDH neurones are unknown. To study the action of Hcrt-2 on individual SDH neurones, tight-seal, whole-cell recordings were made with biocytin-filled electrodes from rat lumbar spinal cord slices. In 19 of 63 neurones, Hcrt-2 (30 n m to 1 μ m ) evoked an inward (excitatory) current accompanied by an increase in baseline noise. The inward current and noise were unaffected by TTX but were blocked by the P_2X purinergic receptor antagonist suramin (300–500 μ m ). Hcrt-2 (30 n m to 1 μ m ) increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in the majority of neurones. The sIPSC increase was blocked by strychnine (1 μ m ) and by TTX (1 μ m ), suggesting that the increased sIPSC frequency was glycine and action potential dependent. Hcrt-2 increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in a few neurones but had no effect on dorsal root-evoked EPSCs in these or in other neurones. Neurones located in outer lamina II, particularly radial and vertical cells, were most likely to respond to Hcrt-2. We conclude that Hcrt-2 has excitatory effects on certain SDH neurones, some of which exert inhibitory influences on other cells of the region, consistent with the perspective that hypocretin has a role in orchestrating reactions related to arousal, including nociception, pain and temperature sense.     

Mechanisms mediating the brain stem control of somatosensory transmission in the dorsal horn of the cat's spinal cord: an intracellular analysis

Summary The effect of brainstem stimulation was studied on neurones recorded intracellularly in the superficial and deeper laminae of the lumbosacral dorsal horn of the spinal cord in anaesthetised cats. Stimulation in the nucleus locus coeruleus (LC) produced a hyperpolarisation in 4/13 multireceptive neurones and produced a biphasic action consisting of a hyperpolarisation which was followed by a depolarisation in 3/13 neurones. These actions were produced irrespective of whether the multireceptive neurone was located in the superficial or deeper laminae of the dorsal horn. Stimulation failed to produce postsynaptic potentials in the remaining 6/13 multireceptive neurones. The amplitude of hyperpolarisation was increased by the passage of depolarising pulses through the recording microelectrode and decreased by hyperpolarising pulses. Stimulation in other brainstem areas such as, the lateral (FTL), paralemniscal (FTP) and central (FTC) divisions of the tegmental field and the nuclei raphe magnus (NRM) and reticularis magnocellularis (RMc) also hyperpolarised neurones in the dorsal horn. The polarity of hyperpolarisation evoked from some brainstem areas (FTP, FTC, RMc) could be reversed to depolarisation by the passive diffusion of ions from the recording microelectrode containing 3M-KCl. Brainstem (LC, NRM, FTP, FTL) stimulation generated long lasting (700 ms) hyperpolarisation on 4/4 selectively nocireceptive neurones of lamina I. There was, however, no effect on the activity of 5/5 neurones recorded in laminae I/II which in addition to receiving excitatory cutaneous inputs were inhibited by heat stimuli. Stimulation in LC also produced dorsal root potentials (DRPs) and reduced the amplitude of simultaneously recorded excitatory postsynaptic potentials (EPSPs) generated by the activation of primary afferent fibres in 3 multireceptive neurones. It is concluded that inhibition of nociceptive transmission in the spinal cord from LC and other brainstem areas may involve both pre- and postsynaptic mechanisms.     

Tumor necrosis factor α sensitizes spinal cord TRPV1 receptors to the endogenous agonist N-oleoyldopamine

Modulation of synaptic transmission in the spinal cord dorsal horn is thought to be involved in the development and maintenance of different pathological pain states. The proinflamatory cytokine, tumor necrosis factor α (TNFα), is an established pain modulator in both the peripheral and the central nervous system. Up-regulation of TNFα and its receptors (TNFR) in dorsal root ganglion (DRG) cells and in the spinal cord has been shown to play an important role in neuropathic and inflammatory pain conditions. Transient receptor potential vanilloid 1 (TRPV1) receptors are known as molecular integrators of nociceptive stimuli in the periphery, but their role on the spinal endings of nociceptive DRG neurons is unclear. The endogenous TRPV1 receptor agonist N-oleoyldopamine (OLDA) was shown previously to activate spinal TRPV1 receptors. In our experiments the possible influence of TNFα on presynaptic spinal cord TRPV1 receptor function was investigated. Using the patch-clamp technique, miniature excitatory postsynaptic currents (mEPSCs) were recorded in superficial dorsal horn neurons in acute slices after incubation with 60 nM TNFα. A population of dorsal horn neurons with capsaicin sensitive primary afferent input recorded after the TNFα pretreatment had a basal mEPSC frequency of 1.35 ± 0.20 Hz (n = 13), which was significantly higher when compared to a similar population of neurons in control slices (0.76 ± 0.08 Hz; n = 53; P < 0.01). In control slices application of a low concentration of OLDA (0.2 uM) did not evoke any change in mEPSC frequency. After incubation with TNFα, OLDA (0.2 uM) application to slices induced a significant increase in mEPSC frequency (155.5 ± 17.5%; P < 0.001; n = 10). Our results indicate that TNFα may have a significant impact on nociceptive signaling at the spinal cord level that could be mediated by increased responsiveness of presynaptic TRPV1 receptors to endogenous agonists. This could be of major importance, especially during pathological conditions, when increased levels of TNFα and TNFR are present in the spinal cord.     

Capsazepine, a novel capsaicin antagonist, selectively antagonises the effects of capsaicin in the mouse spinal cord in vitro.

The mouse hemisected spinal cord with attached dorsal roots and spinal ganglia in vitro preparation was used to investigate the effects of the capsaicin antagonist, capsazepine (2-[2-(4-chlorophenyl)ethylamino-thiocarbonyl]-7,8-dihydroxy-2,3,4 ,5- tetrahydro-1H-2-benzazepine). The spinal cord and the ganglia were separated by a perspex gap, allowing application of drugs separately to each compartment. Intracellular recordings were made from 37 cells in laminae II-VI of 12 to 20-day-old mice. Brief applications (30 s) of capsaicin (0.8 microM) excited dorsal horn neurones by activating small diameter primary afferent fibres. The response to capsaicin administered to the spinal cord or to the spinal ganglia was antagonised by the capsaicin antagonist, capsazepine (1.5 microM), administered to the same site. Excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of the dorsal root were not affected by capsazepine. Capsazepine itself (5 microM) did not affect the membrane potential of the dorsal horn cells. Capsazepine did not depress the depolarization evoked by substance P. When capsazepine was applied to the spinal cord and capsaicin to the dorsal root ganglion the capsaicin effect was not antagonised. These data suggest that capsaicin-induced depolarization of spinal dorsal horn neurones was mediated via activation of a specific receptor on primary afferent neurones.     

Activation of GIRK channels in substantia gelatinosa neurones of the adult rat spinal cord: a possible involvement of somatostatin

Recent studies have suggested that spinal G-protein-coupled, inwardly rectifying K⁺ (GIRK) channels play an important role in thermal nociception and the analgesic actions of morphine and other agents. In this study, we show that spinal GIRK channels are activated by an endogenous neurotransmitter using whole-cell patch-clamp recordings from substantia gelatinosa (SG) neurones in adult rat spinal cord slices. Although repetitive stimuli applied to the dorsal root did not induce any slow responses, ones focally applied to the spinal dorsal horn produced slow inhibitory postsynaptic currents (IPSCs) at a holding potential of −50 mV in about 30% of the SG neurones recorded. The amplitude and duration of slow IPSCs increased with the number of stimuli and decreased with removal of Ca²⁺ from the external Krebs solution. Slow IPSCs were associated with an increase in membrane conductance; their polarity was reversed at a potential close to the equilibrium potential for K⁺, calculated from the Nernst equation. Slow IPSCs were blocked by addition of GDP-β-S into the patch-pipette solution, reduced in amplitude in the presence of Ba²⁺, and significantly suppressed in the presence of an antagonist of GIRK channels, tertiapin-Q. Somatostatin produced an outward current in a subpopulation of SG neurones and the slow IPSC was occluded during the somatostatin-induced outward current. Moreover, slow IPSCs were significantly inhibited by the somatostatin receptor antagonist cyclo-somatostatin. These results suggest that endogenously released somatostatin may induce slow IPSCs through the activation of GIRK channels in SG neurones; this slow synaptic transmission might play an important role in spinal antinociception.     

Medullary raphé lesions do not reduce descending inhibition of dorsal horn neurones of the cat

In anaesthetized cats spinal cold block was used to examine the effects of medullary raphé lesions on the responses of dorsal horn neurones to impulses in unmyelinated primary afferents. Lesions failed to affect these responses, casting doubt on the importance of this region and its serotonergic spinal projections to the control of nociceptive transmission at the spinal level.     

Inhibition of nociceptive responses of dorsal horn neurones in the cat spinal cord by ohmefentanyl, a preferring mu opioid receptor agonist [corrected

An action of ohmefentanyl (OMF), a novel preferring mu opioid receptor agonist, on nociceptive response of the dorsal horn neurone, has been studied in the spinal cat. OMF (0.5-2 micrograms/kg, i.v.) produced a potent naloxone-reversible inhibition of firing of dorsal horn neurones in response to noxious heating of glabrous hindpaw or to impulses in C primary afferents. When ejected into the substantia gelatinosa, OMF produced a relatively selective inhibition of C response, but not A response to excitation of myelinated afferent fibres, of the dorsal horn neurone. An interaction between ohmefentanyl and mu opioid receptors is discussed.     

保留后根的新型脊髓旁矢状面切片技术及其在脊髓突触传递研究中的应用

目的:对保留脊神经后根的脊髓切片技术进行改良,增加所保留的脊神经后根纤维投射到脊髓后角浅层的完整性,以提高实验效率。方法:选用4～5周的SD大鼠,应用振动切片机对其脊髓腰骶膨大分别进行横断面或矢状面切片,在全细胞模式记录下,给予后根刺激观察两者中诱发出兴奋性突触后电流(EPSCs)的成功率,并对其进行比较;调整后根刺激参数分别刺激Aβ、Aδ和C纤维以诱发EPSCs,并对不同纤维诱发的EPSCs进行鉴别。结果:在保留后根的横断面和矢状面切片上诱发出EPSCs的成功率分别是38.43±9.97%和86.36±5.32%,具有显著的统计学差异(P<0.0001);与非保留后根的脊髓切片上诱发出的EPSCs相比,应用保留后根的脊髓横断面切片和矢状面切片所诱发的EPSCs均可通过刺激强度和潜伏期的差异,对不同纤维诱发的EPSCs进行有效的区分。结论:保留后根的脊髓矢状面切片刺激后根反应率显著高于横断面切片,且可对不同纤维诱发的EPSCs进行有效区分。因此,保留后根的脊髓矢状面切片比横断面切片更完整的保留了后根到脊髓后角浅层的投射,可提高实验效率,是研究脊髓中枢突触传递及其可塑性的可靠离体模型。     

In vivo and in vitro effects of peripheral galanin on nociceptive transmission in naive and neuropathic states

Galanin is widely distributed in the nervous system and is consistently upregulated in both dorsal root ganglion and spinal neurones by peripheral nerve injury. This study investigates the peripheral effects of galanin on nociceptive neurones using in vitro and in vivo electrophysiological techniques in naive and neuropathic rats. Using an in vitro skin-nerve preparation recording from single nociceptive fibres, galanin (1 microM) significantly inhibited firing induced by noxious heat in 65% of fibres examined. In the remaining 35% of fibres, galanin (1 microM) induced a facilitation of the responses to noxious heat. To examine the effect of peripheral galanin in vivo, extracellular recordings from convergent dorsal horn neurones were made in anaesthetised naive sham-operated and spinal nerve-ligated (SNL) rats. Injection of galanin (0.1-10 microg) into hindpaw receptive fields inhibited responses to innocuous mechanical, noxious mechanical and noxious heat stimuli in a proportion of neurones in each animal group and facilitated the remaining neurones. However, a higher proportion of neurones (80-90%) was inhibited by peripheral galanin administration in SNL rats compared with naive (45-55%) and sham (70-80%) rats. These results show that galanin can have both excitatory and inhibitory effects on peripheral sensory neurones, perhaps reflecting differential receptor activation, and that the proportion of these receptors may change following peripheral neuropathy.     

Increased nociceptive input rapidly modulates spinal GABAergic transmission through endogenously released glutamate

Stimulation of nociceptive primary afferents elicits pain by promoting glutamatergic transmission in the spinal cord. Little is known about how increased nociceptive input controls GABAergic tone in the spinal dorsal horn. In this study, we determined how increased nociceptive inflow affects GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) of lamina II neurons by using whole cell recordings in rat spinal cord slices. Bath application of capsaicin for 3 min induced a long-lasting inhibition of sIPSCs in 50% of the neurons tested. In the other half of the neurons, capsaicin either increased the frequency of sIPSCs (34.6%) or had no effect on sIPSCs (15.4%). The GABA(A) current elicited by puff application of GABA was not altered by capsaicin. Capsaicin did not inhibit sIPSCs in rats treated with intrathecal pertussis toxin. Also, capsaicin failed to inhibit sIPSCs in the presence of ionotropic glutamate receptor antagonists or in the presence of both LY341495 and CPPG (group II and group III metabotropic glutamate receptor antagonists, respectively). However, when LY341495 or CPPG was used alone, capsaicin still decreased the frequency of sIPSCs in some neurons. Additionally, bradykinin significantly inhibited sIPSCs in a population of lamina II neurons and this inhibitory effect was also abolished by LY341495 and CPPG. Our study provides novel information that stimulation of nociceptive primary afferents rapidly suppresses GABAergic input to many dorsal horn neurons through endogenous glutamate and activation of presynaptic group II and group III metabotropic glutamate receptors. These findings extend our understanding of the microcircuitry of the spinal dorsal horn involved in nociception.     

Antagonism of mu-opioid receptor-mediated inhibitions of nociceptive neurones by U50488H and dynorphin A1-13 in the rat dorsal horn

We have studied the effects of two highly selective kappa-opioid receptor agonists, U50488H and dynorphin A1-13 on the powerful inhibitions of rat dorsal horn nociceptive neurones produced by the potent mu-opiate receptor agonist, Tyr-D-Ala-Gly-Me-Phe-Gly-ol (DAGO). Extracellular single unit recordings were made from 35 convergent neurones which could be excited by impulses in A beta- and C-fibre afferents following transcutaneous electrical stimulation of the ipsilateral hind paw. The mu- and kappa-agonists were applied directly onto the surface of the spinal cord. DAGO (0.19, 0.48 and 1.9 nmol) dose-dependently inhibited C-fibre evoked responses with little effect on A beta-evoked activity. The spinal application of dynorphin A1-13 (6.2 nmol) and U50488H (28 nmol) rapidly reversed the spinal inhibitory effect of DAGO indicating that these kappa-ligands are likely to act as mu-receptor antagonists in the rat dorsal horn.     

Direct inhibition of substantia gelatinosa neurones in the rat spinal cord by activation of dopamine D2-like receptors

Dopaminergic innervation of the spinal cord is largely derived from the brain. To understand the cellular mechanisms of antinociception mediated by descending dopaminergic pathways, we examined the actions of dopamine (DA) on nociceptive transmission by using behavioural studies and whole-cell patch-clamp recordings from substantia gelatinosa (SG) neurones in the spinal cord. Intrathecal administration of DA increased the mechanical nociceptive threshold and this effect was mimicked by a D2-like receptor agonist, quinpirole, but not by a D1-like receptor agonist, SKF 38393. In current-clamp mode of patch-clamp recordings, bath application of DA hyperpolarized the membrane potential of SG neurones and suppressed action potentials evoked by electrical stimulation of a dorsal root. In voltage-clamp mode, DA induced an outward current that was resistant to TTX, was blocked by the addition of Cs⁺ or GDP-β-S in the pipette solution, and was inhibited in the presence of Ba⁺. The DA-induced current reversed its polarity at a potential close to the equilibrium potential of the K⁺ channel calculated from the Nernst equation. The DA-induced outward current was mimicked by quinpirole, but not by SKF 38393. The DA-induced outward current was suppressed by a D2-like receptor antagonist, sulpiride, but not by a D1-like receptor antagonist, SCH 23390. In contrast, DA did not cause any significant change in amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs). These results indicate that DA mainly acts on postsynaptic SG neurones to induce an outward current via G-protein-mediated activation of K⁺ channels through D2-like receptors. This may be a possible mechanism for antinociception by the descending dopaminergic pathway.     

Descending projections from the rostral ventromedial medulla (RVM) to trigeminal and spinal dorsal horns are morphologically and neurochemically distinct

Neurons in the rostral ventromedial medulla (RVM) are thought to modulate nociceptive transmission via projections to spinal and trigeminal dorsal horns. The cellular substrate for this descending modulation has been studied with regard to projections to spinal dorsal horn, but studies of the projections to trigeminal dorsal horn have been less complete. In this study, we combined anterograde tracing from RVM with immunocytochemical detection of the GABAergic synthetic enzyme, GAD67, to determine if the RVM sends inhibitory projections to trigeminal dorsal horn. We also examined the neuronal targets of this projection using immunocytochemical detection of NeuN. Finally, we used electron microscopy to verify cellular targets. We compared projections to both trigeminal and spinal dorsal horns. We found that RVM projections to both trigeminal and spinal dorsal horn were directed to postsynaptic profiles in the dorsal horn, including somata and dendrites, and not to primary afferent terminals. We found that RVM projections to spinal dorsal horn were more likely to contact neuronal somata and were more likely to contain GAD67 than projections from RVM to trigeminal dorsal horn. These findings suggest that RVM neurons send predominantly GABAergic projections to spinal dorsal horn and provide direct input to postsynaptic neurons such as interneurons or ascending projection neurons. The RVM projection to trigeminal dorsal horn is more heavily targeted to dendrites and is only modestly GABAergic in nature. These anatomical features may underlie differences between trigeminal and spinal dorsal horns with regard to the degree of inhibition or facilitation evoked by RVM stimulation.     

Modulation of AMPA excitatory postsynaptic currents in the spinal cord dorsal horn neurons by insulin

Glutamate AMPA receptors are critical for sensory transmission at the spinal cord dorsal horn (DH). Plasma membrane AMPA receptor endocytosis that can be induced by insulin may underlie long term modulation of synaptic transmission. Insulin receptors (IRs) are known to be expressed on spinal cord DH neurons, but their possible role in sensory transmission has not been studied. In this work the effect of insulin application on fast excitatory postsynaptic currents (EPSCs) mediated by AMPA receptors evoked in DH neurons was evaluated. Acute spinal cord slices from 6 to 10 day old mice were used to record EPSCs evoked in visually identified superficial DH neurons by dorsal root primary afferent stimulation. AMPA EPSCs could be evoked in all of the tested neurons. In 75% of the neurons the size of the AMPA EPSCs was reduced to 62.1% and to 68.9% of the control values when 0.5 or 10 μM insulin was applied. There was no significant change in the size of the AMPA EPSCs in the remaining 25% of DH neurons. The membrane permeable protein tyrosine kinase inhibitor, lavendustin A (10 μM), prevented the insulin induced AMPA EPSC depression. Our results suggest a possible role of the insulin pathway in modulation of sensory and nociceptive synaptic transmission in the spinal cord.     

Chronic morphine treatment increases the expression of the neural cell adhesion molecule in the dorsal horn of the mouse spinal cord

It is well known that prolonged exposure to morphine results in tolerance to morphine-induced antinociception. In the present study, we found that mice that were tolerant to morphine-induced antinociception exhibited an increase in immunoreactivity for the neural cell adhesion molecule in the dorsal horn of the spinal cord, which was highly overlapped with immunoreactivity for the increased metabotropic glutamate receptor 5 induced by morphine. These findings support the idea that repeated stimulation of μ-opioid receptors increases the expression of neural cell adhesion molecule and metabotropic glutamate receptor 5. This phenomenon leads to the enhanced excitatory synaptic transmission in the dorsal horn of the spinal cord, and in turn suppresses the morphine-induced antinociception.     

Functional GABA(A)-receptor-mediated inhibition in the neonatal dorsal horn

Neonatal nociceptive circuits and dorsal horn cells are characterized by an apparent lack of inhibitory control: receptive fields are large and thresholds low in the first weeks of life. It has been suggested that this may reflect immature GABA(A)-receptor (GABA(A)R) signaling whereby an early developmental shift in transmembrane anion gradient is followed by a longer period of low Cl- extrusion capacity. To investigate whether functional GABA(A)R-mediated inhibition does indeed undergo postnatal regulation at the level of dorsal horn circuits, we applied the selective GABA(A)R antagonist gabazine to the spinal cord in anesthetized rat pups [postnatal day (P) 3 or 21] while recording spike activity in single lumbar dorsal horn cells in vivo. At both ages, blockade of GABA(A)R activity resulted in enlarged hind paw receptive field areas and increased activity evoked by low- and high-intensity cutaneous stimulation, revealing comparable inhibition of dorsal horn cell firing by spinal GABA(A)Rs at P3 and P21. This inhibition did not require descending pathways to the spinal cord because perforated patch-clamp recordings of deep dorsal horn neurons in P3 spinal cord slices also showed an increase in evoked spike activity after application of gabazine. We conclude that spinal GABAergic inhibitory transmission onto single dorsal horn cells "in vivo" is functional at P3 and that low Cl- extrusion capacity does not restrict GABAergic function over the normal range of evoked sensory activity. The excitability of neonatal spinal sensory circuits could reflect immaturity in other intrinsic or descending inhibitory networks rather than weak spinal GABAergic inhibition.     

Inflammation induced increase of fluoride resistant acid phosphatase (FRAP) activity in the spinal dorsal horn in rats

Fluoride-resistant acid phosphatase (FRAP) has been suggested as an enzymatic marker for nociceptive primary afferent terminals in the spinal dorsal horn, however there has not been demonstrated a direct functional relation between FRAP activity and an increased nociceptive transmission. For this purpose, we quantitated FRAP activity in the spinal dorsal horn of the rat in a heat-induced cutaneous inflammatory model. Male Sprague-Dawley rats anaesthetised with thiopental were separated in two groups where the left hindpaw was submerged during 60 s either in water at room temperature (control group) or in water at 60 degrees C (inflammation group) which induce in this group a progressive hindpaw inflammation. After 8 h, the lumbar enlargement of the spinal cord was extracted, cut in slices and 1 mm micropunch fragments were obtained from the right and left dorsal horn. The activity of FRAP was determined using the Gomori colorimetric method and corrected by the protein concentrations. FRAP activity in the left dorsal horn was statistically higher than right dorsal horn in the inflammation group (3.05+/-0.54 versus 1.91+/-0.23 u/g per l; P<0.05). Also, FRAP activity from the left dorsal horn of the control and inflammation groups show a significant increase in the last group (3.05+/-0.54 versus 2.17+/-0.23 u/g per l; P<0.05). This results demonstrate that FRAP is not only an enzymatic marker for neuronal and fibre integrity of nociceptive primary afferents but also it is associated to the nociceptive afferent activation.