A local route to pain relief

Abstract

Local anaesthetics stop pain, but block all other sensations too. In rats, one molecular delivery vehicle makes an unusual local anaesthetic specific for pain — provided a little spice is added to the mix first. The snag with most local anaesthetics is their lack of specificity. They are lipophilic, so can enter virtually any neuron, where they indiscriminately block sodium channels in the membrane. A way of blocking the activity of specific pain-sensing neurons without affecting other sensory or motor neurons could be used to create a more targeted form of local anaesthesia, and that is the prospect held out by Binshtok et al. in this issue. They report that the lidocaine derivative QX-314 can be targeted to pain-sensing neurons. Normally QX-314 can't cross the cell membrane. But antipain specificity is ensured by allowing it to enter via the TRPV1 channel, a capsaicin receptor found only in pain-sensing neurons. Co-application of QX-314 and capsaicin in rats blocked mechanical and thermal pain, inducing local anaesthesia, without the paralysis seen in 'normal' lidocaine anaesthesia.