Electrotonic coupling between frog spinal motoneurons. An electrophysiological and morphological study

Abstract

Summary In motoneurons of the isolated spinal cord of frogs ( Rana esculenta ) short latency depolarizations (‘VR-EPSPs’), produced by stimulation of different ventral roots, were investigated. During increased extracellular magnesium ion concentration, postsynaptic potentials were reversibly suppressed. In addition, the antidromic spike invasion of the somatodendritic membrane of motoneurons was blocked because of membrane hyperpolarization and the subsequent increment in threshold depolarization. Consequently, the antidromic focal potential diminished in amplitude and the VR-EPSPs disappeared to a similar extent. At the same time, the neuron input resistance increased and the extracellular concentration of potassium ions, [K + ] e , decreased below control levels. However, antidromic spikes again invaded the somatodendritic membrane of motoneurons, despite the maintained action of magnesium ions, whenever the bulk of motoneurons were depolarized by increasing [K + ] e . As a result, the VR-EPSPs reappeared, whereas all chemical PSPs remained blocked. PSPs were also blocked reversibly by application of manganese ions. No effects were seen either in [K + ] e , membrane potential, passive membrane properties, or in antidromic spike invasion of motoneurons, and VR-EPSPs remained unchanged. On the basis of these results, it was concluded that the VR-EPSP is electronically propagated between different motoneurons. Electron microscopic inspection of the membranes of frog motoneuron somata and dendrites, as exposed by freeze-fracturing, revealed numerous membrane specializations of gap junction character. As a typical feature of these gap junctions, regions of densely packed particles surrounded small particle free regions. As a consequence of these findings the term VR-EPSP, as employed by Kubota and Brookhart, should be used with some reservation. If the proposed interpretation of the signal should prove to be correct, the designation ‘ventral root electrotonic junction potential’ (VR-EJP) would, in our opinion, be more appropriate.