Kv7 channels as targets for paracetamol

Paracetamol (acetaminophen, APAP) has been employed in the treatment for various forms of pain for over 60 years, and is possibly the most frequently used drug in the world. Nevertheless, its mechanisms of action remain elusive, as currently available experimental data do not support straightforward conclusions. In the recent past, two new ideas regarding the mechanism of action of APAP have emerged: 1) it is not APAP itself, but rather its metabolites which mediate analgesia; and 2) APAPs targets may be ion channels controlling the functions of neurons involved in pain sensation. Neurons in the pain tracts of mammals pick up noxious signals from the environment and translate them into action potentials, which are then conveyed to brain areas that are instrumental in the perception of pain. These neurons rely on the coordinated gating of several types of ion channels that contribute to all these signaling events. Previously, APAP metabolites were reported to act on transient receptor potential (TRP) channels. Other ion channels that are essential for pain sensation are voltage-gated potassium channels, and member of the Kv7 family are known targets for analgesics. Several molecules have been shown to exert equivalent actions on both TRP and Kv7 channels, amongst these H2S, an active metabolite of analgesic drugs in clinical development. So far evidence for any effect of APAP or its metabolites on voltage-gated potassium channels has been lacking. A proof of concept study demonstrated that one APAP metabolite, N-acetyl-p-benzoquinone imine (NAPQI), supported the gating of Kv7 channels, which in turn impeded action potentials propagation in neurons of the pain tract. Future experiments to be undertaken within the framework of this research project will help expose how NAPQI may affect Kv7 channels at the molecular level and whether these mechanisms contribute to the analgesic actions of APAP. In addition, effects of NAPQI will be compared with those of H2S. Thereby, this project will not only help to resolve the enigma of APAPs mechanism of action, but will also provide information regarding molecular actions of novel analgesics currently in development.

Paracetamol has been employed in the treatment for various forms of pain for over 60 years and is possibly the most frequently used drug in the world. Nevertheless, its mechanism of action remains largely elusive, as currently available experimental data do not support straightforward conclusions. In the recent past, two new ideas regarding the mechanism of action of APAP have emerged: 1) it is not APAP itself, but rather its metabolites which mediate analgesia; and 2) APAP's targets may be ion channels controlling the functions of neurons involved in pain sensation. Neurons in the pain tracts of mammals pick up noxious signals from the environment and translate them into action potentials, which are then conveyed to the brain areas that are instrumental in the perception of pain. To this end, neurons rely on the coordinated gating of several types of ion channels that contribute to all these signaling events. Previously, APAP metabolites were reported to act on transient receptor potential (TRP) channels. Other ion channels that are essential for pain sensation are voltage-gated potassium channels, amongst which are the Kv7 channels. Several molecules have been shown to exert equivalent actions on both TRP and Kv7 channels, but until this project positive evidence for any effect of paracetamol or its metabolites on voltage-gated potassium channels has been lacking. It was shown in the present project that paracetamol reduces mechanical nocifensive behavior in inflamed tissue. This effect relied, in part, on Kv7 channels. Previously it was shown that paracetamol itself did not affect Kv7 channels in neurons, but its toxic metabolite N-acetyl-p-benzoqinone imine. The present project revealed that currents through Kv7.2, Kv7.4, and Lv7.5 subunits were enhanced by NAPQI. The activation of hetermonerization partner Kv7.3 was left-shifted by NAPQI. However, currents through the cardiac isoform Kv7.1 were reduced in response to NAPQI. NAPQI covalently modifies a number of cysteine residues within the channel protein, but only modification of a stretch of three cysteine residues within the S2-S3 linker region is functionally important. Calcium-mediated inhibition of Kv7 channels, induced by inflammatory mediators is prevented by NAPQI in neurons. In addition, NAPQI reduced the sensitivity of Kv7 channels towards PIP2.