Compound action potential inhibition produced by various antidepressants in the frog sciatic nerve

Astragaloside depresses compound action potential in sciatic nerve of frogs involved in L-type Ca2+-channel dependent mechanism

The sciatic nerve is the largest sensorimotor nerve within the peripheral nervous system (PNS), possessing the ability to produce endogenous neurotrophins. Compound nerve action potentials (CNAPs) are regarded as a physiological/pathological indicator to identify nerve activity in signal transduction of the PNS. Astragaloside (AST), a small-molecule saponin purified from Astragalus membranaceus, is widely used to treat chronic disease. Nonetheless, the regulatory effects of AST on the sciatic nerve remain unknown. Therefore, the present investigation was undertaken to study the effect of AST on CNAPs of frog sciatic nerves. Here, AST depressed the conduction velocity and amplitude of CNAPs. Importantly, the AST-induced responses could be blocked by a Ca2+-free medium, or by applying all Ca2+ channel antagonists (CdCl2/LaCl3) or L-type Ca2+ channel blockers (nifedipine/diltiazem), but not the T-type and P-type Ca2+ channel antagonist (NiCl2). Altogether, these findings suggested that AST may attenuate the CNAPs of frog sciatic nerves in vitro via the L-type Ca2+-channel dependent mechanisms.

Inhibition of Fast Nerve Conduction Produced by Analgesics and Analgesic Adjuvants-Possible Involvement in Pain Alleviation

Nociceptive information is transmitted from the periphery to the cerebral cortex mainly by action potential (AP) conduction in nerve fibers and chemical transmission at synapses. Although this nociceptive transmission is largely inhibited at synapses by analgesics and their adjuvants, it is possible that the antinociceptive drugs inhibit nerve AP conduction, contributing to their antinociceptive effects. Many of the drugs are reported to inhibit the nerve conduction of AP and voltage-gated Na⁺ and K⁺ channels involved in its production. Compound action potential (CAP) is a useful measure to know whether drugs act on nerve AP conduction. Clinically-used analgesics and analgesic adjuvants (opioids, non-steroidal anti-inflammatory drugs, ₂-adrenoceptor agonists, antiepileptics, antidepressants and local anesthetics) were found to inhibit fast-conducting CAPs recorded from the frog sciatic nerve by using the air-gap method. Similar actions were produced by antinociceptive plant-derived chemicals. Their inhibitory actions depended on the concentrations and chemical structures of the drugs. This review article will mention the inhibitory actions of the antinociceptive compounds on CAPs in frog and mammalian peripheral (particularly, sciatic) nerves and on voltage-gated Na⁺ and K⁺ channels involved in AP production. Nerve AP conduction inhibition produced by analgesics and analgesic adjuvants is suggested to contribute to at least a part of their antinociceptive effects.

Inhibition by general anesthetic propofol of compound action potentials in the frog sciatic nerve and its chemical structure

Although the intravenous general anesthetic propofol (2,6-diisopropylphenol) has an ability to inhibit nerve conduction, this has not been fully examined. Various agents inhibit compound action potentials (CAPs) in a manner dependent on their chemical structures. To determine propofol's chemical structure that is important in nerve conduction inhibition, we examined the effects of propofol and its related compounds on fast-conducting CAPs recorded from the frog sciatic nerve by using the air-gap method. Propofol concentration-dependently reduced the peak amplitude of the CAP with a half-maximal inhibitory concentration (IC₅₀) value of 0.14 mM. A similar inhibition was produced by other phenols, 4-sec-butylphenol and 4-amylphenol (IC₅₀ values: 0.33 and 0.20 mM, respectively). IC₅₀ values for these and more phenols (4-isopropylphenol, 4-tert-butylphenol, and 4-ter-amylphenol; data published previously) were correlated with the logarithm of their octanol-water partition coefficients. A phenol having ketone group (raspberry ketone) and alcohols (3-phenyl-1-propanol and 2-phenylethylalcohol) inhibited CAPs less effectively than the above-mentioned phenols. The local anesthetic (LA) benzocaine reduced CAP peak amplitudes with an IC₅₀ of 0.80 mM, a value larger than that of propofol. When compared with other LAs, propofol activity was close to those of ropivacaine, levobupivacaine, and pramoxine, while benzocaine activity was similar to those of cocaine and lidocaine. It is concluded that propofol inhibits nerve conduction, possibly owing to isopropyl and hydroxyl groups bound to the benzene ring of propofol and to its lipophilicity; propofol's efficacy is comparable to those of some LAs. These results could serve to develop propofol-related agents exhibiting analgesia when applied topically.

Inhibition by non-steroidal anti-inflammatory drugs of compound action potentials in frog sciatic nerve fibers

AIMS

Although antinociception produced by non-steroidal anti-inflammatory drugs (NSAIDs) is partly attributed to nerve conduction inhibition, this has not been thoroughly examined yet. The aim of the present study was to reveal quantitatively how various types of NSAIDs affect compound action potentials (CAPs), a measure of nerve conduction.

MAIN METHODS

CAPs were recorded from the frog sciatic nerve by using the air-gap method.

KEY FINDINGS

Soaking the sciatic nerve with acetic acid-based NSAIDs (diclofenac and aceclofenac) reduced the peak amplitude of CAP in a concentration-dependent manner; their IC₅₀ values were 0.94 and 0.47 mM, respectively. Other acetic acid-based NSAIDs (indomethacin, acemetacin and etodolac) also inhibited CAPs [the extent of inhibition: some 40% (1 mM), 40% (0.5 mM) and 15% (1 mM), respectively], except for sulindac and felbinac at 1 mM that had no effects on CAP peak amplitudes. A similar inhibition was produced by fenamic acid-based NSAIDs [tolfenamic acid (IC₅₀ = 0.29 mM), meclofenamic acid (0.19 mM), flufenamic acid (0.22 mM) and mefenamic acid] which are similar in chemical structure to diclofenac and aceclofenac; their derivatives (2,6-dichlorodiphenylamine and N-phenylanthranilic acid) also inhibited. On the other hand, salicylic acid-based (aspirin), propionic acid-based (ketoprofen, naproxen, ibuprofen, loxoprofen and flurbiprofen) and enolic acid-based (meloxicam and piroxicam) NSAIDs had no effects on CAP peak amplitudes.

SIGNIFICANCE

At least a part of antinociception produced by NSAIDs used as a dermatological drug to alleviate pain may be attributed to their inhibitory effects on nerve conduction, which depend on the chemical structures of NSAIDs.