The Journal of Neurophysiology Vol. 81 No. 2 February 1999, pp. 959-962
Copyright ©1999 by the American Physiological Society

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Phase-Dependent Presynaptic Modulation of Mechanosensory Signals in the Locust Flight System

 ¹Fachbereich Biologie, Universität Kaiserslautern, D-67653 Kaiserslautern, Germany; and  ²Vergleichende Neurobiologie, Universität Ulm, D-89069 Ulm, Germany

Phase-dependent presynaptic modulation of mechanosensory signals in the locust flight system. In the locust flight system, afferents of a wing hinge mechanoreceptor, the hindwing tegula, make monosynaptic excitatory connections with motoneurons of the elevator muscles. During flight motor activity, the excitatory postsynaptic potentials (EPSPs) produced by these connections changed in amplitude with the phase of the wingbeat cycle. The largest changes occurred around the phase where elevator motoneurons passed through their minimum membrane potential. This phase-dependent modulation was neither due to flight-related oscillations in motoneuron membrane potential nor to changes in motoneuron input resistance. This indicates that modulation of EPSP amplitude is mediated by presynaptic mechanisms that affect the efficacy of afferent synaptic input. Primary afferent depolarizations (PADs) were recorded in the terminal arborizations of tegula afferents, presynaptic to elevator motoneurons in the same hemiganglion. PADs were attributed to presynaptic inhibitory input because they reduced the input resistance of the afferents and were sensitive to the -aminobutyric acid antagonist picrotoxin. PADs occurred either spontaneously or were elicited by spike activity in the tegula afferents. In summary, afferent signaling in the locust flight system appears to be under presynaptic control, a candidate mechanism of which is presynaptic inhibition.

0022-3077/99 $5.00 Copyright © 1999 The American Physiological Society