Modulation of transient receptor potential (TRP) channels by Chinese herbal extracts

Many transient receptor potential (TRP) channels are activated or blocked by various compounds found in plants. TRPV1 (transient receptor potential vanilloid 1) and transient receptor potential ankyrin 1 (TRPA1) are members of the TRP ion channel family which are implicated as cough receptors. Several plant extracts are used in traditional Chinese medicine (TCM) for the treatment of cough. This report evaluated the effect of four such herbals for their effect on cough receptors. Of the herbals tested, Pipaye (Folium Eriobotryae) extracts, especially the 50% ethanol extract, showed the highest activity for the suppression of TRPV1 activation by its agonist capsaicin. Pipaye is able to elicit Ca-flux through TRPV1 as well as through unknown channels. Thus the inhibition of capsaicin elicited Ca-flux in TRPV1 cells by Pipaye is largely due to desensitization of the receptor. Pipaye extract has a similar, but less pronounced effect, on TRPA1. Also tested were three pure components, ursolic acid (UA), peimine (PM) and peiminine (PN) derived from TCM for their effect on cough receptors. Only UA tested at 100 μM showed potent antagonistic activity towards TRPV1. Both PM and PN had no activity when tested alone, however, both were able to enhance the UA effect. A similar, but less marked, inhibition was also noted for TRPA1.