Control of excitatory synaptic transmission by capsaicin is unaltered in TRPV1 vanilloid receptor knockout mice

Several studies have shown that capsaicin could effectively regulate excitatory synaptic transmission in the central nervous system, but the assumption that this effect is mediated by TRPV1 vanilloid receptors (TRPV1Rs) has not been tested directly. To provide direct evidence, we compared the effect of capsaicin on excitatory synapses in wild type mice and TRPV1R knockouts. Using whole-cell patch-clamp techniques, excitatory postsynaptic currents (EPSCs) were recorded in granule cells of the dentate gyrus. First, we investigated the effect of capsaicin on EPSCs evoked by focal stimulation of fibers in the stratum moleculare. Bath application of 10 microM capsaicin reduced the amplitude of evoked EPSCs both in wild type and TRPV1R knockout animals to a similar extent. Treatment of the slices with the TRPV1R antagonist capsazepine (10 microM) alone, or together with the agonist capsaicin, also caused a decrease in the EPSC amplitude both in wild type and TRPV1R knockout animals. Both drugs appeared to affect the efficacy of excitatory synapses at presynaptic sites, since a significant increase was observed in paired-pulse ratio of EPSC amplitude after drug treatment. Next we examined the effect of capsaicin on spontaneously occurring EPSCs. This prototypic vanilloid ligand increased the frequency of events without changing their amplitude in wild type mice. Similar enhancement in the frequency without altering the amplitude of spontaneous EPSCs was observed in TRPV1R knockout mice. These data strongly argue against the hypothesis that capsaicin modulates excitatory synaptic transmission by activating TRPV1Rs, at least in the hippocampal network.