Voltage- and stage-dependent uncoupling of Rohon-Beard neurones during embryonic development of Xenopus tadpoles

1. Electrical coupling of Rohon-Beard neurones in the spinal cord of Xenopus laevis has been studied from the time of closure of the neural tube (stage 20, 22h after fertilization of the egg) to the free swimming tadpole (stage 49, 12d old). Pairs of cells were examined by impaling both simultaneously with single micro-electrodes having current-passing and voltage-recording capabilities.2. At the earliest time studied (stage 20), before Ca(2+) action potentials are detected, cells are electrically coupled. Coupling coefficients as high as 0.6 can be recorded. Tests with small hyperpolarizing and depolarizing current pulses demonstrate that the coupling does not show rectification.3. The coupling is voltage-dependent: depolarization or hyperpolarization of one cell with respect to another, above a threshold, causes relative uncoupling of the cells. The coupling coefficient falls to approximately 10% of its initial value when the difference in potential between the two cell bodies is approximately 75 mV. Cells usually become recoupled at the termination of the current pulse. Other, unidentified cells in the spinal cords of the same embryos show coupling that is not voltage-dependent.4. Voltage-dependent uncoupling and recoupling persist when cells are depolarized by high K(+), and in the presence of 30 mM-Co(2+), suggesting that chemical synapses are not involved. They are also unaffected by addition of Rb(+), Cs(+) or TEA(+) to the extracellular solution, elevated levels of Ca(2+), or replacement of Na(+) with Tris or Cl(-) with isethionate, suggesting that conductance changes in the surface membrane, such as anomalous rectification, are not responsible.5. Lowering the intracellular pH with CO(2)-HCO(3) (-) buffered saline does not abolish electrical coupling but appears to eliminate its voltage dependence.6. Slightly later (e.g. stage 21), cells that do not yet produce Ca(2+) action potentials while coupled will do so when their input resistance is increased by uncoupling them from their neighbours.7. Later still (e.g. stage 23), cells make Ca(2+) action potentials while coupled, and an action potential in one cell can trigger an action potential in other cells to which it is coupled. Ca(2+) action potentials that do not bring other coupled cells to threshold for impulse initiation can transiently reduce the strength of coupling. Repetitive firing of these Ca(2+) action potentials at a low frequency does not cause permanent uncoupling of the cells.8. Rohon-Beard neurones become electrically uncoupled about stage 25 (early tail-bud, 28h old). Coupling disappears around the time of appearance of the Na(+) component of the action potential, although coupling that is voltage-dependent or independent can still be seen between other, unidentified cells. No electrical coupling of Rohon-Beard cells was detected at later stages of development.