Three kinds of current in response to substance P in bullfrog DRG neurons

In response to SP applied externally, neurons freshly isolated from bullfrog dorsal root ganglion (DRG) showed three kinds of current (I _SP), i.e. slow, fast and moderately activating I_SPs. All the three kinds of I_SP were inward currents, and were completely blocked by either peptide antagonist of SP receptor spantide or non-peptide antagonist of SP receptor WIN51708. The slow activating I_SP showed slow kinetic features. Replacement of NaCl in external solution by NMDG had no effect on this kind of I_SP, while Ba²⁺ abolished it almost completely, thus the ionic mechanism underlying slow activating I _SP was deduced to be the closure of K⁺ channels. The fast activating I_SP in bullfrog DRG neurons, just as in rat DRG neurons, was proved to be caused by the opening of Na⁺ preferring non-selective cation channel, for it was abolished almost completely by replacement of NaCl in external solution with equimolar NMDG. The moderately activating I_SP was similar to the fast activating I_SP in current configuration, however, its kinetic characteristics lay between those of fast and slow activating I_SPs. Either NMDG or Ba²⁺ suppressed this kind of I_SP partially. Therefore the moderately activating I_SP might be mediated by non-selective cation channel. We used repatch technique to explore the intracellular mechanism underlying the three kinds of I_SP and found that the three kinds of I_SP were caused by the activity of either G-protein coupled channel (slow activating I_SP) or directly opened channel (fast activating I _SP) or both (moderately activating I_SP ).