Electrical interaction between antidromically stimulated frog motoneurones and dorsal root afferents: enhancement by gallamine and TEA

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Abstract
1. Electrical interactions have been studied in the isolated frog spinal cord preparation. It is found that gallamine and tetraethylammonium chloride (TEA) markedly enhance all non‐cholinergic synaptic interactions, including the electrical interaction between motoneurones (VR‐VRP). In addition, in the presence of either of these drugs, a short‐latency interaction is seen to exist between antidromically stimulated motoneurones and dorsal root afferents (early VR‐DRP). The early VR‐DRP is rarely seen in the absence of gallamine or TEA.2. The early VR‐DRP is of the same short latency as the VR‐VRP and fulfils the same criteria for electrical interaction: it increases in amplitude with cooling from 17–10° C, it is not blocked by a wide variety of pharmacological blocking agents, and it is suppressed by both Mg2+ and Ca2+, with no antagonism of action between the two.3. The early VR‐DRP appears as a cluster of unitary events: all‐or‐none spikes conducted out the dorsal root fibres. No initial graded slow potentials are seen. Often there are two peaks in the response.4. The early VR‐DRP is facilitated by a dorsal root volley, with a time course normally intermediate between that of the orthodromic ventral root potential (DR‐VRP) and the dorsal root potential (DR‐DRP). This orthodromic facilitation apparently is achieved by increasing invasion of motoneurone dendritic trees and depolarization of dorsal root afferents toward threshold.5. If the same ventral root is stimulated twice, or adjacent roots stimulated at different intervals, the second early VR‐DRP, like the VR‐VRP, is seen to be occluded for 10–20 msec, then facilitated to supranormal amplitudes. It is concluded that motoneurone dendrites are presynaptic to both interactions.6. Evidence is presented that gallamine and TEA act by increasing the duration of activity both in axon terminals and in antidromically invaded motoneurones. Often second or multiple spikes result. The increased duration of depolarization can increase transmitter release at terminals and increase coupling at electrical junctions.7. Possible morphological correlates for the two electrical interactions are discussed. It is speculated that the motoneurone interaction arises at numerous areas of close apposition between dendrites in dendritic ‘thickets’, and that the early VR‐DRP is mediated by fewer, lower‐resistance, electrical junctions between dendrites and afferent nerve terminals.

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