Piperine is a pungent alkaloid found in many species of the genus Piper. The structure of this molecule somewhat resembles that of capsaicin, another culinary alkaloid, and the best known of the vanilloid receptor-1 (TRPV1) agonists. Recent studies have demonstrated that piperine can modulate gastrointestinal transit and may even have a gastroprotective effect (Szolcsanyi & Bartho 2001). We describe here our efforts to characterise the effects of piperine at human TRPV1 using electrophysiological methods.

Whole-cell patch-clamp recordings were made from HEK 293 cells stably expressing hTRPV1 (Hayes et al. 2000). The extracellular solution consisted of (mM): NaCl, 130; KCl, 5; BaCl₂, 2; MgCl₂, 1; glucose, 30; Hepes-NaOH, 25; pH 7.3. Electrodes were filled with (mM): CsCl, 140; MgCl₂, 4; EGTA, 10; Hepes-CsOH, 10; pH 7.3. Data are expressed as mean ± S.E.M., Student’s t test was used to analyse statistical significance (P < 0.05).

Piperine, activated inward currents in HEK 293 cells expressing TRPV1 with an estimated mean EC₅₀ = 38 ± 2 µM (n = 5-9). Neither piperine nor capsaicin had any effect on parental HEK 293 cells. The current activated by piperine (30 µM) was reversibly antagonised by the TRPV1 inhibitor capsazepine (10 µM), 94 ± 5.1 % block, (n = 3); and the non-competitive TRPV1 blocker ruthenium red (10 µM), 95 ± 4.7 %, (n = 4). Over a -70 to +70 mV voltage ramp, the currents elicited by 30 µM piperine and 1 µM capsaicin yielded indistinguishable current-voltage relationships; these were outwardly rectifying with reversal potentials of 0 ± 0.4 mV and -1 ± 0.8 mV, respectively. A high concentration of piperine (100 µM) displayed 2.86 ± 0.91 fold (n = 6) greater efficacy than 1 µM capsaicin, a concentration that produces ▓ge│90 % maximal response. In addition, different desensitisation profiles were observed with capsaicin 1 µM producing little macroscopic desensitisation during applications lasting 20 s while currents activated by 100 µM piperine desensitised 50 % in 9.9 ± 0.7 s (n = 5). Weak acidification modulates TRPV1 activity. At pH 6.5 a similar (P > 0.05) degree of potentiation (I_pH6.5/I_pH7.3) was observed for responses to 1 µM capsaicin (2.42 ± 0.32, n = 7) and 30 µM piperine (2.56 ± 0.27, n = 4).

These data indicate that two different TRPV1-activating alkaloids can produce distinct response phenotypes at hVR1 and suggest that capsaicin may be regarded as a partial agonist of this conductance. Agonist desensitisation strategies may have further applications in treating pain and gastrointestinal disorders.

F.N. McNamara is in receipt of EU Framework V Postdoctoral Fellowship.