Action potentials of embryonic dorsal root ganglion neurones in Xenopus tadpoles.

Abstract

Several classes of action potentials can be distinguished in dorsal root ganglion cells, studied by intracellular recording techniques in X. laevis tadpoles 4.5-51 days old. The ionic basis of the action potential was investigated by changing the ionic environment of the cells and applying various blocking agents. The Ca2+-dependent action potential is a plateau of relatively long duration (mean 8.7 ms). It is unaffected by removal of Na+ but blocked by mM quantities of Co2+. It is present only in small cells. Ca2+/Na+-dependent action potentials. Type I is a spike followed by a plateau or hump of different durations (mean 8.1 ms). The spike is selectively blocked by removal of Na+, leaving the plateau which is in turn blocked by Co2+. It is present in cells of small and intermediate size. Type II is a spike of short duration (mean 2.0 ms) with only an inflection on the falling phase. The spike is blocked by removal of Na+ and no other components can be elicited. The inflection is blocked by Co2+. It is present in cells of all sizes. Type III is similar to type I but is seen only in solutions in which the outward current is blocked. It was observed only very infrequently. Na+-dependent action potentials. Type Ia is a short duration spike (mean 1.1 ms). It is abolished by removal of Na+ or addition of tetrodotoxin (TTX), but largely unaffected by Co2+ or La3+. It is present in cells of all sizes. When the outward current channels are blocked and cells exposed to Na+-free solutions, all cells are capable of producing an action potential in which the inward current is carried by divalent cations. Type Ib is a spike with a smooth, more slowly falling phase. It has the same pharmacological properties as type Ia action potential and is present in cells of small size. Na+-dependent action potentials. Type II is a spike with an inflection on the falling phase (mean duration 3.4 ms). It is prolonged by Co2+ and La3+. Removal of Na+ abolishes the spike but TTX does not block it. It is present in cells of all sizes. The mean resting potential is less than that of cells with Na+-dependent type I action potentials, while the mean input resistance is greater. Tetraethylammonium chloride (TEA) prolongs the different kinds of action potentials. The amount of prolongation varies among cells with a given type of action potential, so that no distinction could be made of the different action potential types based on the effect of TEA. The percent of cells with each kind of action potential varies with the developmental age of the animal. The number of cells with Ca2+ and Ca2+/Na+ action potentials decreases with age, while the number of cells with a Na+ type I action potentials increases. The Na+ type II action potential appears only at later stages. The different classes of action potentials may reflect the developmental stage of the cells and the acquisition of specific functional properties since dorsal root ganglion neurons are likely to proliferate, differentiate and die during the period studied.