Inward and outward currents in isolated dendrites of Crustacea coxal receptors.

Segments from the nonspiking peripheral dendrites of a crustacean coxal receptor (T fiber) were studied using the voltage clamp technique. The peripheral endings of the T fiber are sensitive to stretch applied to a specialized receptor muscle by rotation of the coxa. The intraganglionary portion of the T fiber is presynaptic to the motor neurons innervating the coxal muscle. Depolarizing commands activated three separate fast channels: (i) a transient inward sodium current, INa, which is blocked by tetrodotoxin (TTX); (ii) a transient outward current, Io1 , having the same voltage-dependent characteristics as INa; and (iii) a second, longer-lasting, outward current, Io2 . Both INa and Io1 were inactivated when segments were clamped at voltages more positive than -50 mV, whereas Io2 could be activated at voltages more positive than -50 mV. Io1 and Io2 were blocked by 4-aminopyridine (4-AP) and by tetraethylammonium (TEA), although Io2 shows a greater sensitivity to TEA than Io1 . It is suggested that Io1 may be a factor in determining the nonspiking behavior of the dendrites and that Io2 may limit the stretch-induced depolarization in the dendrite to a value more negative than that at which the maximum rate of transmitter release occurs. In addition to the three fast currents, the presence of a slow inward and slow outward current could also be demonstrated. The effects of the slow currents were longer in segments cut from the proximal part of the dendrites.