Differences in the kinetics of ionic currents in the somatic membrane of identified T- and N-mechanosensory neurons of medical leech.

The ionic currents in the membrane of the body of single T- and N-mechanosensory neurons were studied. Transient inward NA+-current followed by outward K+-current was recorded in the T-cells. The current was activated at membrane potential of -40 mV, reaching its maximum at -10 mV for 2 ms, whereby the reversal potential was about 25 mV. This current was inhibited by tetrodotoxin (TTX, 3 X 10(-6) M) and was eliminated in a Na+-free medium. In an Na+-free medium no inward current was registered upon increasing the Ca2+-concentration and after substituting Ca2+ with 10 mM Ba2+. In the N-cells the inward current was activated at membrane potential of -20 mV, reaching its maximum at -5 mV for 3-4 ms, while the reversal potential was about 45 mV. A time- and potential-dependent residual inward current was observed after application of TTX (3 X 10(-6) M) or in a Na+-free medium. This current increases upon rising the concentration of Ca2+ or when Ca2+ was replaced by 10 mM Ba2+. This current was blocked by methoxyverapamil (D-600, 5 X 10(-5) M). The results obtained manifest differences in the kinetics of the Na+-channels in the T- and N-neurons. In the membranes of the T-cells the Ca2+-channels are either poorly represented or totally lacking. Ca2+-conductivity was established in the N-cells and probably in their membranes exist Ca2+-channels which participate in their excitatory processes.