Actions of Analgesics on Nerve Conduction

Actions of Analgesics on Nerve Conduction: Comparison

Please note this is a comparison between Version 1 by Eiichi Kumamoto and Version 6 by Peter Tang.

The aAction potential (AP) conduction in nerve fibers plays a crucialn important role in transmitting nociceptive information from the periphery to the cerebral cortex. N It is possible that analgesics depress nerve AP conduction inhibition possibly , resultsing in analgesia. It is well-known that many tinociception. Many of analgesics are known to suppress nerve AP conduction and also voltage-dependent sodiumsensitive Na⁺ and potassiumK⁺ channels that are involved in producing APs. The cAP conduction. Compound action potential (CAP) recorded from a bundle of nerve fibers is a guide for knowing if analgesics affecthas been used to know whether nerve AP conduction. This entry mentions the inhibitory is affected by analgesics. This review article will introduce the effects of clinically -used analgesics, analgesic adjuvants, and plant-derived analgesics on fast-conducting CAPs and voltage-dependent sodiumsensitive Na⁺ and potassiumK⁺ channels. The efficacies of their effects were compared among the compounds, and it was revealed that some of the compounds have similar efficacies in suppressing CAPsinvolved in AP production. It is suggested that analgesics-induced ninhibition of nerve AP conduction inhibition may contributes to at least a part of their analgesicntinociceptive effects.

antinociception
analgesic
analgesic adjuvant
plant-derived compound
nerve conduction
sciatic nerve
compound action potential
sodium channel
potassium channel
Na+ channel
K+ channel

1. Introduction

Nociceptive ignal of painful stimuli apformation from the periphery to the cerebral cortex is mainly transmitted by action potential (AP) conduction in nerve fibers and chemical transmission at synapses (see [1,2] for reviews). Nociceptive pain is usualied to the skin is mainly conveyed by primary-afferent thin myelinated Aδ-fibers and unmyelinatedly acute and relieved by narcotic analgesics such as opioids and antipyretic analgesics including non-steroidal anti-inflammatory drugs (NSAIDs). On the other hand, pain may persist or recur for longer than three months, which is called chronic pain. One type of chronic pain, neuropathic pain, which occurs as a result of damage to the peripheral or central nervous system (PNS and CNS, respectively), is characterized by a hyper-excitability of neurons near the injured neuronal tissues (see [3] C-fibers to the spinal cord and brain stor review). This type of pain is often resistant to analgesics such as opioids and NSAIDs and is thus treated by using analgesic adjuvants such as α₂-adrem;noceptor agonists, the inantiepileptics, antidepressants and local anesthetics (see [4,5,6,7,8,9,10,11] for reviews). Although the mation is then transmin target of analgesics and analgesic adjuvants, except for NSAIDs and local anesthetics, is generally synapses (see [12,13,14] for reviews), itt is possibled to the brain by t that all of their drugs inhibit nerve AP conduction, partly contributing to their inhibitory effects on pain.

The AP conduction ofis mediated by voltage-gated Na⁺ and K⁺ channels location potentiaed in nerve fibers. Thus, a depolarizing stimulus given to a nerve fiber point activates Na⁺ channels (APexpress) in nerved in membranes of the fiber, allowing Na⁺ entry to the cytoplasm, and ccaused by the gradient of the electrochemical tpotential of Na⁺, leading to a self-regeneransmission at neuron-to-neuron junctiontive production of AP. This in turn results in an outward current (membrane depolarization) in a fiber point adjacent to the point to produce opening of other Na⁺ channels ^[1][2][3][4]and so forth. ASucute nocih a production of AP subsides by a subsequent inactivation of Na⁺ channeptive pain ls and activation of K⁺ chausnnels (see [15,16] for reviews).

In a bundle by tissue injury or damage is a physiological mechanismof nerve fibers exposed to insulator such as oil, sucrose or air, AP conduction in each fiber produces AP current flowing through nerve bundle surface having high resistance that serves to protect a person against injury, which is usually acan be measured as a potential difference, i.e., compound action potential (CAP), by using two electrodes put on the nerve. Sciatic nerve trunk dissected from frogs is a useful preparation to easily and stably record voltage-gated Na⁺-channel bleviated by antipyretiocker tetrodotoxin (TTX)-sensitive and fast-conducting (possibly myelinated Aα-fiber mediated) CAPs by using the nerve trunk exposed to air (air-gap method). A voltage-gated delayed-rectifier K⁺-channel analgesics including non-inhibitor tetraethylammonium increased the half-peak duration of the CAP with no change in its peak amplitude, indicating an involvement of K⁺ channels (stee [17] for review). Althoidal anti-inflammatory drugs (NSAIDs) and narcotic anugh not only fast-conducting but also slow-conducting (C-fiber mediated) CAPs were recorded from the frog sciatic nerve, the latter had much smaller peak amplitude and conduction velocity than the former [18].

2. Actions of Analgesics on Nerve Conduction

2.1. Opioids

Opioids are wellgesics such as -known to inhibit glutamatergic excitatory transmission by activating opioids. O receptors in the other hand, chronic painCNS including the central terminals of primary-afferent fibers, resulting in antinociception ([28,29,30]; see [31,32] for reviewhich may last for a long time, such as three months). Not only central but also peripheral terminal opioid receptors in primary-afferent neurons are thought to be involved in antinociception ([33,34,35,36]; see [37] for more, or occur repeatedly, is a debilitating disease aview). Opioids also exhibit a local anesthetic effect in the PNS. Although it has been reported in decerebrated cats that the perineural administration of an opioid morphine has no effect on CAPs in the superficial radial nerve [38], AP conducompanied by spontaneous pain, etc. and is often restion in peripheral nerve fibers is generally blocked by opioids. For example, opioids such as fentanyl and sufentanil decreased the peak amplitudes of CAPs recorded from peripheral nerve fibers [39] and inhibistant ed peripheral nerve AP conduction [40]. A morphine-induced CAP inhibitio analgesics such as NSAIDs andn in mammalian peripheral nerve fibers was antagonized by a non-specific opioid-receptor antagonist naloxone, indicating an involvement of opioids receptors [41]. NConsisteuropathic pain, which is one type of chronic paint with this observation, binding and immunohistochemical studies have shown the localization of opioid receptors in mammalian peripheral nerve fibers [42,43,44]. It has been, also results from a direct injury given demonstrated that a frog sciatic nerve CAP inhibition produced by opioids is sensitive to naloxone [45]. On the co the peripheral ntrary, there are reports showing that opioids decrease CAP peak amplitudes [39] and supprervousss nerve AP conduction [40] in a manner resysteistant to naloxone.

2.1.1. Tramadol

The compound (PNS1RS,2RS)-2-[(dimethylamino)methyl]-1-(3-methoxyphenyl) cyclohexand damage caused in theol hydrochloride (tramadol) is an orally-active opioid which is clinically used as an analgesic in the CNS [46]. cenAlthough tral nervous smadol is metabolized to various compounds such as mono-O-desmethysltem (CNS),ramadol (M1) via N- and O-demethylatiton in is chaanimals and humans [47], M1 is a therapeuticterized by an ally active drug as a central analgesic [46]. Onex of cessive rise in tllular mechanisms for the antinociceptive effect of tramadol is the activation of μ-opioid receptors [48,49]. In agreement with this ide excitability of neurons in the vicinity of injured or damaged neuronal tissuesa, M1 has the highest affinity for cloned μ-opioid receptors among the metabolites of tramadol. M1 is reported to inhibit glutamatergic excitatory transmission in spinal lamina II (substantia gelatinosa) ^[5].neurons Twhis type of pain is alleviated by using analgesicch play a crucial role in regulating nociceptive transmission to the spinal dorsal horn from the periphery, resulting in a decrease in the excitability of the neurons [50,51,52]. In addjuvantsition to such as central activity, tramadol is known to have a local anesthetics, ant effect following its intradermal injection in patients ([53,54,55]; see [56] for reviepw). Consileptics, antidepressants, and α₂-stent with this result, in vivo studies have shown a spinal somatosensory evoked potential inhibition produced by a drenoceptirect application of tramadol to the rat sciatic nerve [57]. Tramadol r agonisteduced the peak amplitude of CAPs ^{[6][7][8][9][10][11][12][13][14][15]}.recorded from Although analgesics and analgee frog sciatic nerve in a concentration-dependent manner in a range of 0.2 to 5 mM [19]. A similar CAP inhibitory ac adjuvants generally depress excittion of tramadol has been reported by other investigators in the frog [58] and ratory synaptic tr sciatic nerve [59,60]. Ananlysmissionis based on the Hill equation demonstrated ^[16][17][18],that half-many oximal inhibitory concentration (IC₅₀) value for their drugs can possiramadol to reduce frog sciatic nerve CAP amplitudes is 2.3 mM; this IC₅₀ value was smaller bly suppressabout three-fold than that (6.6 mM) reported previously for the frog sciatic nerve [58]. Rat sciatic nerve CAPs conwere inhibited by tramadol (37% peak amplitude reduction, whic at 4 mM) less effectively than frog sciatic nerve’s ones [59]. This in part contributes to their inhibitorhibitory action of tramadol in the frog sciatic nerve was not affected by the pretreatment of the sciatic nerve with naloxone (0.01 mM); a μ-opioid receptor agonist (D-Ala², N-Me-Phe⁴, Gly⁵-ol)enkephalin (DAMGO; 1 μM) had no effects on pain. Plants and their con on frog sciatic nerve CAPs. Furthermore, CAPs were affected by much smaller extents by M1 (see below) that is similar in chemical structure to tramadol while exhibiting a higher affinity for μ-opioid receptors than tramadol [61]. These resultituents are s indicate that the tramadol-induced CAP inhibition is not mediated by opioid receptors [19]. Consistent with this result, a sed as folk remedies to pinal somatosensory evoked potential inhibition following the application of tramadol on rat sciatic nerves in vivo was resistant to naloxone [57]. Although tramadol inhibits noradrelieve pain as a drugnaline (NA) and serotonin (5-hydroxytryptamine; 5-HT) reuptake at concentrations similar to those that activate μ-opioid receptors [62,63], a wcombith few side enation of inhibitors of the reuptake of NA and 5-HT (desipramine and fluoxetine, respectively; each 10 μM; see Section 3.3) did not affect frog sciatic nerve ^[19][20][21].

CAPs, conduction indicating no involvement of NA and 5-HT reuptake inhibition in the CAP inhibition [19]. The CAP isnhibition produced by the activaramadol is possibly due to an inhibition of voltage-dgated Na⁺ and K⁺ channepls involvendent sodium and potd in the production of AP. Tramadol concentration-dependently reduced the peak amplitude of TTX-sensitive Na⁺ channel currents recorded from dorsium chanal root ganglion (DRG) neuroblastoma hybridoma cell line ND7/23 cells with an IC₅₀ value of 0.194 mM [64] and from HEK293 cells expressed in ing rat Nav1.2 channels with an IC₅₀ value of 0.103 mM [65]. These values were smaller than that (2.3 mM) for frog sciatic nerve fibers. Thu CAP inhibition. It has been demonstrated that tramadol suppresses the current amplitude of delayed rectifier K⁺-channels (Kv3.1a type) expressed in NG 108-15 cells with an IC₅₀ of 0.025 mM, a value much lesstim than 2.3 mM [66]. Such IC₅₀ values that ifor tramadol to inhibit CAPs, Na⁺ andu K⁺ channes membrane ls were higher than its clinically relevant concentration of about 2 μM in serum [52,67]. Unlike tramadepolarization, a, M1 (1-2 mM) did not affect frog sciatic nerve CAPs. This was confirmed in the frog sciatic nerve whose CAPs were inhibited by tramadol (1 mM; [19]). CAP peak amplied to a certain point on a nerve fiber, opens a sodium channel,tudes were reduced by 9% by M1 at 5 mM. Consistent with such smaller effects of M1, APs conducting on rat primary-afferent fibers were not blocked when the effect of M1 (1 mM) on dorsal root-evoked excitatory postsynaptic currents was examined by applying the patch-clamp technique to lamina II neurons in spinal cord slices [51]. It is interesting to note that tramadol has -OCH₃ boulnd to ting in an influx of sodium ion into he benzene ring while M1 has -OH and thus that the methyl group is present in tramadol but not M1. This result indicates that the difference in chemical structure between tramadol and M1 is responsible for the distinction in CAP inhibition (see Figure 5a in [19] for a comparison of the chelmical structures of the two compounds).

2.1.2. Other Opioids

In order to reveal whether such according to th structure-activity relationship is seen for other opioids, the effects of morphine, codeine and ethylmorphine on frog sciatic nerve CAPs were examined. Morphine concentration and -dependently reduced CAP peak amplitude; this extent at 5 mM was 15%. Codeine, which has -OCH₃ in place otentiaf -OH in morphine, at 5 mM reduced CAP peak amplitude by 30%. Moreover, CAPs were more effectively inhibited by ethylmorphine, where -OH of morphine is replaced by -OCH₂CH₃; amplitude gradient acreduction at 5 mM was 61%. IC₅₀ value for ethylmoss the cell membranerphine in reducing CAP peak amplitudes was 4.6 mM. CAP inhibitions produced by morphine (10 mM), codeine (5 mM) and ethylmorphine (2 mM) were resistant to naloxone (0.01 mM). Naloxone at a high concentration such as 1 mM by itself reduced by 9% CAP peak amplitudes, but did not affect morphine (10 mM) activity [20]. These results is leads to AP ndicate no involvement of opioid receptors in the CAP inhibition production in a sed by opioids, as reported previously in mammalian peripheral nerves [39,40,68]. A sequencel of-renewing manner, which in tur the opioid-induced CAP peak amplitude reduction was ethylmorphine > codeine > morphine. Thus, CAP amplitude reduction increased in extent with an increase in the number of -CH₂ (see Figure 7A in [20] for a compauses an outward rison of the chemical structures of the three opioids). This result is consistent with the above-mentioned observation that tramadol having -OCH₃ in the benzene ring inhibits CAPs more effecutively than M1 which is different from tramadol only in terrent, i.e., membrane depolarization, to open other sms of the presence of -OH in the ring. Interestingly, this result was obtained in spite of the fact that the chemical structures of morphine, codeine and ethylmorphine are quite distinct from those of tramadol and M1 (see [69] fodr revium chanew). Since the increase in -CH₂ numbels at the points next to it. Such an AP pror is thought to enhance lipophilicity of opioids, lipophilic opioid-channel interaction is suggested to play a pivotal role in nerve AP conduction is block, as shown for local anesthetics [70,71]. Thisubs ided by subsequent sodium channel inactivation and potassium channel opea is supported by the observation that the potency in CAP inhibition in the rat sciatic nerve was in the order of isopropylcocaine (where the methyl ester group of cocaine is replaced with an isopropyl ester group) > cocaethylene (where its methyl ester group is replaced with an ethyl ester group) > cocaine [72]). It is interesting ^[22][23].

Isolation Methods for Testing Analgesic Action on Nerve Fibers

Tto note that the sequence of the affinity of opioidstudy for μ-opioid receptors is morphine > codeine > ethylmorphine [73], the order of AP in mammals is complicwhich is reversed to one for CAP inhibition. If the opioid-induced inhibition of CAPs is mediated by the neμ-opioid receptors, CAP inhibition sequence will be expected to dissect out individualbe morphine > codeine > ethylmorphine. However, this sequence is not seen, a result being consistent with the idea that the opioids-induced frog sciatic nerves to isolate t CAP inhibition is not mediated by opioid receptors. The sam from periphere sequence as that in the frog sciatic nerve has been reported in the rat phrenic nerve [68], although stimulation. For this reason, neuroscience relies on nerve extrthere is a quantitative difference between the two studies. When compared at 5 mM, codeine-induced reduction (about 30%) in frog sciatic nerve CAP peak amplitude was much smaller than that (about 70%) in the rat phrenic nerve, while so a large distinction was not seen in morphine action from relat(about 10%). Frog sciatic nerve CAPs were less sensitive to morphine than those in rabbit and guinea-pig vagus nerves in such that vagus nerve CAP peak amplitudes were reduced by 20–32% at 0.5 mM [41]. APs recorded ivntracely simple animals with conllularly from rat DRG neurons having Aα/β myelinated primary-afferent fibers also exhibited the sequence of ethylmorphine > codeine ≥ morphine (IC₅₀ = 0.70, 2.5 and 2.9 mM, resperved AP mechanisms, sctively) in AP peak amplitude reduction; this inhibition was resistant to naloxone (0.01 mM) [74]. Althoucgh as insects, reptiles, squidsmany drugs including narcotics, antiepileptics, local anesthetics, alcohols and barbiturates block AP conduction in peripheral nerve fibers, suggesting a nonspecific interaction of the drugs with membrane bilayers [75], the chemical structure-specific CAP inhibitior frogn produced by opioids indicates that opioids act on proteins such as voltage-gated Na⁺ and K⁺ channels (see [76] for rexample, refeview). Morphine is reported to suppress peak Na⁺ currents toand steady-state ^[23]).

APK⁺ currents flowing on the sin single myelinated nerve fibers isolated from the frog sciatic nerve, leading to the prolongation of APs [77]. Intracellularface ofly-applied morphine reduced voltage-gated Na⁺ and K⁺ channel curve trent amplitudes in squid giant axons [78]. Bath-applied morphine redunkced TTX-sensitive Na⁺ channel current amplitude in DRG neuronsistingblastoma hybridoma cell line ND7/23 cells with an IC₅₀ value of 0.378 mM [64], although Nav1.2 channy fibeels expressed in HEK293 cells were unaffected by morphine at 1 mM [65]. In support of s can be measured as a compound auch an idea about ion channel inhibition, it has been reported that an opioid meperidine, which is used for AP conduction blockade and thus analgesia, inhibits Na⁺-channels in a manner similar tion po that of lidocaine [79]. Table 1 summarizes IC₅₀ values for frog sciatential (CAP) by immersing the ic nerve fast-conducting CAP inhibitions produced by opioids together with those for rat sciatic nerve in aCAPs and voltage-gated Na⁺ channels.

Table 1. Comparison of IC₅₀ values in inhibiting frog or rat sciatic nerve fast-conducting CAPs and TTX-sensitive Na⁺ channels among opioids.

Opioids	Frog CAP	Rat CAP	TTX-Sensitive	References
	IC₅₀ (mM)	IC₅₀ (mM)	Na⁺ Channel Current
			IC₅₀ (mM)
Tramadol	2.3	37% reduction	0.194	[19,59,64]
		(4 mM)	0.103	[65]
Mono-O-desmethyl -tramadol	9% reduction			[19]
Mono-O-desmethyl -tramadol	(5 mM)
Morphine	15% reduction		0.378	[20,64]
	(5 mM)
Codeine	30% reduction			[20]
	(5 mM)
Ethylmorphine	4.6			[20]

Here, where IC₅₀ values are not available, it is partly shown for comparison how CAP amplitudes are reduced by drugs, where % value indicates the extent of the reduction at the concentration shown in parentheses. In clisolator such as air, oil, or sucrosenical practice, although administration of opioids into the nerve sheath results in pain relief (for instance, see [80]), mand then by putting two electrodes on the nerve. CAPs, y of pain treatments by use of opioids are due to systemic administration of centrally-penetrating opioids, leading to their actions in the PNS and CNS, both of which acontribute to analgesia (see [81] for re sensitive to tetrodotoxin (TTX),view). It is possible that centrally-administrated opioids act on not only the CNS but also the PNS, because opioids are reported to be transported to the periphery from brain by P-glycoprotein [82]. In supporthat blocks vo of an important role of opioids in the PNS, subcutaneous administration of N-methyl-morphine, which did notage-dependen pass through the blood brain barrier, resulted in antinociception in an acetic acid-writhing model in mice [35]. It hasodium channels, and are fast- been reported that a subcutaneously-administrated opioid loperamide, which cannot penetrate into the brain, exhibited an antinociceptive effect in the formalin test in rats [34]. Sucoh anducting (possibly action of opioids in the PNS appeared to be mediated by primaopioid receptors in peripheral terminals of primary-afferent fibers ([33,34,35,36,83]; see [37] for review). In addition, thick myelinatee inhibitory effect of opioids on nerve AP conduction also might contribute to local analgesia following the peripheral perineural administration of opioids (for instance, see [84]) that are expected Aαto lead to a direct action fibers), can be eaof opioids at high doses on peripheral nerves. Since codeine is metabolized to morphine via O-demethylation in humans and anilymals ([85,86]; obseee [81] for reved in the sciaiew), peripherally-administrated codeine might have a similar effect to that of morphine.

2.2. NSAIDs

Antinoc nerve trunk isolatediception produced by NSAIDs is mediated by various mechanisms such as (1) inhibition of the synthesis of prostaglandins from frogs by arachidonic acid by inhibiting the cyclooxygenase enzyme ([87,88]; sexpe [89,90,91] for reviewsing the ), (2) inhibition of acid-sensitive ion channels [92] and transierve trunknt receptor potential (TRP) channels [93,94], (3) acto aivation of several K⁺ channels ([95,96,97,98,99]; see [100,101] for revirews), (know4) substance P depletion [102], (5) an interasction with the air-gdrenergic system [103] apnd (6) an methinvolvement of opioids [104,105] and endocannabinoids (see [106] for review). AThe half-peak duration of the CAP was increasidea about an involvement of mechanisms other than cyclooxygenase inhibition in antinociception is supported by a voltage-dependethe observation that there is a dissociation between anti-inflammation and antinociception produced by NSAIDs [107]. An acet delayed rectifier potassiic acid-based NSAID diclofenac reduced frog sciatic nerve CAP peak amplitudes in a partially reversible manner. Diclofenac activity was concentration-dependent in a range of 0.01–1 mM with an IC₅₀ value of 0.94 m channel M. Another acetic acid-based NSAID aceclofenac (a carboxymethyl ester of diclofenac) also exhibited a similar CAP inhibitor, tetraethylammonium, withy action. CAP peak amplitudes were concentration-dependently reduced by aceclofenac in a range of 0.01–1 mM with an IC₅₀ of 0.47 mM, a valut any alteration in itse smaller than that of diclofenac. Other acetic acid-based NSAIDs had an efficacy smaller than those of diclofenac and aceclofenac. Indomethacin at 1 mM reduced CAP peak amplitude, which ines by 38% and acemetacin (where the -OH group of indomethacin is substituted by -OCH₂COOH) at 0.5 mM did so by 38%. Etodolacated that p at 1 mM reduced CAP peak amplitudes by only 15%, and sulindac and felbinac at 1 mM had no effects on CAP peak amplitudes [21]. A similar frog sciatassium channels are involved in CAP prodic nerve CAP inhibition was produced by fenamic acid-based NSAIDs (tolfenamic acid, meclofenamic acid, mefenamic acid and flufenamic acid) whose chemical structures are similar to those of diclofenac and aceclofenac. Tolfenamic acid concentration-dependently reduced CAP peak amplitudes in a range of 0.01–0.2 mM with an IC₅₀ value of 0.29 mM. The actionvity of meclofenamic acid ^[24].(where Although the frog sciatic nee chloro group bound to the benzene ring of tolfenamic acid is changed in number and position) was concentration-dependent in a range of 0.01–0.5 mM with an IC₅₀ value of 0.19 mM. Moreove exhibits both fast-conducting and slow-conducting (Aδ-fiber and C-fiber mediatedr, mefenamic acid (where the chloro group bound to the benzene ring of tolfenamic acid is replaced by methyl group) concentration-dependently reduced CAP peak amplitudes in a range of 0.01–0.2 mM with the extent of 16% at 0.2 mM. CAP peak amplitudes were concentration-dependently reduced by flufenamic acid (where one out of two methyl groups bound to the benzene ring of mefenamic acid is lacking and another one is replaced by -CF₃) with an ICAPs₅₀ of 0.22 mM, a value the lattecomparable to those of tolfenamic acid and meclofenamic acid [21]. 2,6-Dichlorodiphenylamine CAPsand N-phenylanthrave much smaller nilic acid (which are similar in chemical structure to diclofenac and tolfenamic acid while being not NSAIDs) reduced frog sciatic nerve CAP peak amplitudes a; the former compound lacks the -CH₂COOH group of diclofenac and conduction velocities than the formthe latter one lacks chloro and methyl groups bound to the benzene ring of tolfenamic acid. 2,6-Dichloro- diphenylamine activity was concentration-dependent in a range of 0.001-0.1 mM with the extent of 45% at 0.1 mM; N-phenylanthr onesanilic acid activity was concentration-dependent in a range of ^[25]0.

F01–2 mM with the extent of 23% at 1 mM [21]. With respect-conducting CAPs recorded from the to other types of NSAIDs, salicylic acid-based (aspirin; 1 mM), propionic acid-based (ketoprofen, naproxen, ibuprofen, loxoprofen and flurbiprofen; each 1 mM) and enolic acid-based [meloxicam (0.5 mM) and piroxicam (1 mM)] NSAIDs had no effects on frog sciatic nerve wCAP amplitudes [21]. CAP amplitude re found to be inhibited byductions produced by the NSAIDs would be mediated by an inhibition of TTX-sensitive voltage-gated Na⁺ chantinociceptive dnels that are involved in frog CAP production. In support of this idea, diclofenac decreased the peak amplitudes of TTX-sensitive Na⁺-channel currugents in a rat DRG [108] and mouse trigeminanner l ganglion neurons [109]. A similar diclofepenac-induced Na⁺-channdent on their col inhibition has been reported in rat myoblasts [110] ancd ventratioicular cardiomyocytes [111]. Flufenamic acid as and well as diclofenac decreased Na⁺-channel curremical snt amplitudes in rat hippocampal CA1 neurons [112,113,114]. Altrhouctgh IC₅₀ valures. Among the drugs, there are (0.22 mM) for flufenamic acid in frog sciatic nerve CAP inhibition was similar to that of Na⁺ channelinically us inhibition (0.189 mM) in rat hippocampal CA1 neurons [114], IC₅₀ value (0.94 mM) for d antinociceptive drugs iiclofenac in CAP inhibition was much larger than those (0.00851 and 0.014 mM in rat myoblasts and DRG neurons, respectively) of Na⁺ channcelu inhibition [108,110]. Regarding rank order among NSAIDs, ^[26],the order for CAP inhibition at 0.5 mM wany types of os flufenamic acid > diclofenac > indomethacin >> aspirin = naproxen = ibuprofen [21]; this was in part sioidsmilar to those for Na⁺ such as tramadolannel inhibition in rat cardiomyocytes (diclofenac ^[27][28],> mnany amide-proxen ≥ ibuprofen; [111]) and ealster-type local aneso in rat DRG neurons (diclofenac > flufenamic acid > indomethacin > aspirin; [108]). With respeticsct to TTX-resistant ^[29],Na⁺ chantiepilepticnnels, diclofenac at 0.3 mM reduced peak current amplitudes ^[30],by about 20% intidepress rat trigeminal ganglion neurons [115]; Nav1.8 channel currents ^[31],were inhibitedexmedetomidine (DEX; (+ by flufenamic acid and tolfenamic acid (current amplitude reduction: ca. 30 and 30%, respectively, at 0.1 mM; [116]). TTX-(S)-4-[sensitive Nav1-(2,3-d.7 channel currents were more sensitimethylphenyl)-ethyl]-1H-imidazove to flufenamic acid and tolfenamic acid (reduction: ca. 60 and 70%, respectively, at 0.1 mM) than Nav1.8 ones [116]. Alternative, which is an α₂-ly, chemical irritation-induced adctivity increnoceptor agonistease of cat corneal sensory nerve fibers was suppressed in extent by NSAIDs; ^[32]),this suppression wand diverse kinds of s different in magnitude among distinct types of NSAIDs [109,117]. Na⁺-channel inhibitinociceptive compounds isolated from plantson produced by NSAIDs appeared to be different in extent among preparations. Concentrations required for NSAIDs to have a significant inhibitory effect on frog sciatic nerve CAPs were in general higher than those ^[33].needed to inhibit Na⁺ channels; Tthis entry will dmay be attributed to various reasons including the fact that not only Na⁺ channels but also K⁺ channels aribee involved in determining CAP amplitudes. To my knowledge, the effects of antinocicececlofenac, indomethacin, etodolac, acemetacin, meclofenamic acid and mefenamic acid on voltage-gated Na⁺ channels have not been reported. Table 2 summarizes IC₅₀ value drugs on CAPs evoked s for frog sciatic nerve fast-conducting CAP inhibitions produced by NSAIDs together with those for voltage-gated Na⁺ channels.

Table 2. Comparison of IC₅₀ values in inhibiting frog sciatic nerve fast-conducting CAPs, TTX-sensitive or -resistant Na⁺ channels among NSAIDs.

NSAIDs		Frog CAP IC₅₀ (mM)	TTX-Sensitive Na⁺ Channel Current IC₅₀ (mM)	TTX-Resistant Na⁺ Channel Current IC₅₀ (mM)	References
Acetic Acid-Based	Diclofenac	0.94	0.00851, 0.014	ca. 20% reduction (0.3 mM)	[21,108,110,115]
	Aceclofenac	0.47			[21]
	Indomethacin	38% reduction (1 mM)			[21]
	Acemetacin	38% reduction (0.5 mM)			[21]
	Etodolac	15% reduction (1 mM)			[21]
	Sulindac	n.d. (no effect, 1 mM)			[21]
	Felbinac	n.d. (no effect, 1 mM)			[21]
Fenamic Acid-Based	Tolfenamic acid	0.29	ca. 70% reduction (0.1 mM)	ca. 30% reduction (0.1 mM)	[21,116]
	Meclofenamic acid	0.19			[21]
	Mefenamic acid	16% reduction (0.2 mM)			[21]
	Flufenamic acid	0.22	ca. 60% reduction (0.1 mM)	ca. 30% reduction (0.1 mM)	[21,116]
			0.189		[114]
Salicylic Acid-Based	Aspirin	n.d. (no effect, 1 mM)			[21]
Propionic Acid-Based	Ketoprofen	n.d. (no effect, 1 mM)			[21]
	Naproxen	n.d. (no effect, 1 mM)			[21]
	Ibuprofen	n.d. (no effect, 1 mM)			[21]
	Loxoprofen	n.d. (no effect, 1 mM)			[21]
	Flurbiprofen	n.d. (no effect, 1 mM)			[21]
Enolic Acid-Based	Meloxicam	n.d. (no effect, 0.5 mM)			[21]
Enolic Acid-Based	Piroxicam	n.d. (no effect, 1 mM)			[21]

Here, when IC₅₀ values are not available, it is partly shown for comparison how the CAPs and channels are affected by drugs, where % value indicates the extent of the reduction at the concentration shown in parentheses; n.d.: not determined. NSAIDs (diclofen the ac, aceclofenac, tolfenamic acid, meclofenamic acid and flufenamic acid), which are more effective in frog sciatic nerves CAP inhibition compared to the other NSAIDs [21], have twof benzene frogs and argue how nerve APrings that bind a hydrophilic substituent group, both of which rings are linked by -NH- (see Figures 1Aa, 1Ba, 3Aa, 3Ba, 3Da in [21] for the chemical struconductitures of the five NSAIDs), as seen in local anesthetics (see Section 3.4). Mefenamic acid (where one inhibitionsof the two benzene rings has a hydrophobic substituent group; see Figure 3Ca in [21]) approduced by drugs diffeared to be less effective, albeit not examined at a higher concentration due to a less solubility of this drug (see above). CAPs were effectively inhibited by 2,6-dichlorodiphenylamine and N-phenylanthr among tanilic acid that are not NSAIDs while being similar in chemical structure to NSAIDs having two benzene rings (see Figures 4Aa and 4Ba in [21]). CAPs were also depressed by bisphem.nol A that have For cotwo benzene rings that bind a hydrophilic group such as -OH [26]. Much evidence demponstrates tharison, the effects oft the other actions of NSAIDs depend on their chemical structures. For instance, an involvement of NO-cGMP-K⁺ channels in antinociceptivon mediate drugd by NSAIDs was dependent on their chemical structures [98,118]. Nonselective on peripheral nerve CAPs in mammals and vcation channels in the rat exocrine pancreas were suppressed by flufenamic acid and mefenamic acid but not indomethacin, aspirin and ibuprofen [119]. There was a distinction in depressing TRP meltage-deastatin-3 channels between diclofenac and aceclofenac [94]. Although NSAIDs not only supprendent sodium andss but also activate TRP ankyrin-1 channels, this activation also differed in magnitude among NSAIDs [120]. pMoreotassium channels that arver, there was a distinction among NSAIDs in the activities of mitochondrial oxidative phosphorylation or electron transport system that may be involved in prtheir adverse side effects [121]. Althoducing APs will also be mgh the concentrations of NSAIDs tested in the frog sciatic nerve are generally much higher than those for voltage-gated Na⁺-channel intioned, provided that data are availablehibition, such high concentrations are likely when NSAIDs are used at high concentrations in the direct vicinity of nerve fibers. At least a part of analgesia caused by NSAIDs used as a dermatological drug for antinociception may be due to a nerve conduction inhibition through their inhibitory action on voltage-gated Na⁺ channels [122].