The development of electrical properties of identified neurones in grasshopper embryos.

Abstract

The development of the electrical properties of 5 identified neurons in grasshopper embryos between days 10-13 of embryogenesis (hatching occurs on day 20) were examined. DUM [dorsal unpaired median] 3,4,5; DUM 4,5; DUM 5; the H cell; and the H cell sibling are the progeny of 2 different precursor cells. Electrical coupling and electrical excitability were assayed by intracellular recordings. Midway through embryogenesis, on day 10, the 5 cells are highly electrically coupled to each other and are electrically inexcitable. The temporal sequence of the development of electrical excitability and electrical uncoupling is described for DUM 3,4,5; 4,5; and 5. The H cell and H cell sibling undergo the same sequence 1 day later. The 1st non-linear membrane property to appear is delayed rectification which appears on day 11 and can be blocked by tetraethylammonium (TEA). In some cells at about day 11, the addition of TEA to normal saline unmasks a Na+-dependent action potential in the axon. The 1st action potential in normal saline is a Na+-dependent response that appears in the axon at day 11-11.5. The next stage of excitability in normal saline is the appearance about day 11.5 of a Na+-dependent action potential in the median neurite between the soma and the 2 axons. In some cells at about day 11.5, the addition of TEA unmasks an excitable response in the soma. Overshooting action potentials appear in the soma about day 12; the inward current is carried by both Na+ and Ca2+; TEA causes a prolonged shoulder on the falling phase of the action potential. A short time later, TEA causes a long-duration Ca2+ plateau. A progressive decrease in the degree of electrical coupling among the cells occurs between days 10 and 12.5. Complete uncoupling is never observed before day 11, but has always occurred by day 12.5. Two methods were used to demonstrate that electrical coupling does not mask the presence of excitable inward current channels and thus make the cells appear inexcitable. First, the cells were exposed to veratridine. The cells which normally generate excitable Na+ responses are depolarized by it; the younger inexcitable cells are not. Secondly, the cells were electrically isolated by killing the somata of their neighbors. The input resistance increased, yet the extent of excitability remained unchanged. There is variability in the precise temporal relationship of excitability and uncoupling. Pairs of cells from different embryos of the same age can generate the same type of action potentials and yet be coupled in one embryo and uncoupled in another. Electrical excitability and uncoupling appear to be causally unrelated and independent events, occurring at about the same developmental stage.