The mature electrical properties of identified neurones in grasshopper embryos

Abstract

The mature electrical properties of 5 identified neurons in embryos of the grasshopper S. nitens were examined. The 5 cells arise from 2 different precursor cells: the median neuroblast whose first 3 progeny are called DUM [dorsal unpaired median] 3,4,5; DUM 4,5; and DUM 5; and mid-line precursor 3, which divides once to produce the H cell and the H cell sibling. Electrical coupling was investigated by dual intracellular penetrations. Action potentials were elicited by intracellular stimulation of cell bodies and by extracellular stimulation of axons. The ionic basis of action potentials was investigated by changing the ionic environment and by applying various blocking agents. Most of the mature electrical properties of all 5 cells appear by day 13 of embryonic development. They change little through hatching on day 20. The recorded resting potential for all 5 cells varies from -55 to -60 mV and the recorded input resistance varies from 200-450 M.OMEGA.. All 5 cells show delayed rectification, much of which is blocked by tetraethylammonium (TEA). Their resistance increases as they are hyperpolarized. All 5 neurons generate mature action potentials and in several cases show cell-specific electrical properties by day 13. One cell-specific difference is observed in normal saline. Four of the cells have axons, median neurites and somata which generate action potentials in normal saline; one of the cells (the H cell sibling) has an inexcitable soma and generates action potentials only in its axon. Another cell-specific difference in the soma action potentials of DUM 3,4,5; DUM 4,5; DUM 5; and the H cell is observed when outward current is blocked by TEA. In 3 of the cells, TEA causes the short-duration action potential (2-4 ms) to be converted into a long-duration action potential (100-1000 ms) in which there is an initial spike (Na+-dependent) followed by a long plateau (Ca2+-dependent). In the other cell, DUM 5, at resting potential the addition of TEA only causes a shoulder (Ca2+-dependent) on the falling phase of the action potential. DUM 5 and DUM 4,5 thus have different electrical properties, even though they differ only by a single cell division from the precursor cell. In all 4 neurons which normally generate soma action potentials (DUM 3,4,5; DUM 5; and the H cell), the inward current is carried by both Na+ and Ca2+. On day 13, either inward current alone can generate the overshooting action potential; in contrast, by day 18, neither inward current alone can generate an overshooting response. The inward current of the axon action potential in all 5 cells is carried predominantly by Na+. The progeny of 2 different embryonic precursor cells (median neuroblast and mid-line precursor 3) show a broad spectrum of electrical properties. The mature phenotype of electrical excitability is not a property shared in common by all the progeny of a single embryonic precursor cell in the grasshopper. Conversely, progeny from different precursor cells can share the same mature phenotype of electrical excitability.