Inhibition of tetrodotoxin (TTX)-resistant and TTX-sensitive neuronal Na(+) channels by the secretolytic ambroxol.
Ambroxol has a long history for the treatment of airway diseases because of its beneficial effects on surfactant synthesis and mucus-modifying properties. Some findings, however, point to an additional effect on neuronal signal transduction: ambroxol can suppress reflexes such as the cough or the corneal reflex. The airways and the cornea are innervated by C-fibers, which express voltage-gated Na(+) channels with and without sensitivity to tetrodotoxin (TTX). In this study, we performed voltage-clamp experiments to investigate whether ambroxol affects these channel types. In rat dorsal root ganglia, TTX-resistant Na(+) currents were suppressed in a concentration-dependent manner with IC(50) values of 35.2 and 22.5 microM for tonic and phasic block, respectively. Depolarizing prepulses increased the potency of ambroxol, and steady-state inhibition curves were shifted to more negative values. The inhibition was not frequency-dependent. TTX-sensitive currents were inhibited with lower potency (approximately 50% inhibition with 100 microM). Recombinant rat brain IIA channels in Chinese hamster ovary cells were blocked with IC(50) values of 111.5 and 57.6 microM for tonic and phasic block, respectively; in contrast to TTX-resistant channels the block was frequency-dependent. Thus, ambroxol indeed blocks neuronal voltage-gated Na(+) channels, and TTX-resistant channels in sensory neurons were more sensitive than TTX-sensitive. Compared with known local anesthetics (e.g., lidocaine or benzocaine), the potency for Na(+) channel block was relatively high. A recent clinical trial has further confirmed that ambroxol relieved pain and was beneficial in patients who suffered from sore throat.
Department CNS Research, Boehringer Ingelheim Pharma KG, Ingelheim, Germany. weiser@ing.boehringer-ingelheim.com
Thomas
T
Weiser
Nicola
N
Wilson
D000551:Ambroxol / Q000494:pharmacology*; D000779:Anesthetics, Local / Q000494:pharmacology; D000818:Animals; D004594:Electrophysiology; D005100:Expectorants / Q000494:pharmacology*; D005727:Ganglia, Spinal / Q000187:drug effects / Q000378:metabolism; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D026941:Sodium Channel Blockers; D015222:Sodium Channels / Q000502:physiology; D013779:Tetrodotoxin / Q000494:pharmacology*; D014162:Transfection
D000779:Anesthetics, Local; D005100:Expectorants; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D000551:Ambroxol; D013779:Tetrodotoxin