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 post synaptic potentials (EPSPs) produced by these connections changed in amplit ude with the phase of the wingbeat cycle. The largest changes occurred arou nd the phase where elevator motoneurons passed through their minimum membra ne potential. This phase-dependent modulation was neither due to flight-rel ated oscillations in motoneuron membrane potential nor to changes in motone uron input resistance. This indicates that modulation of EPSP amplitude is mediated by presynaptic mechanisms that affect the efficacy of afferent syn aptic input. Primary afferent depolarizations (PADs) were recorded in the t erminal arborizations of tegula afferents, presynaptic to elevator motoneur ons in the same hemiganglion. PADs were attributed to presynaptic inhibitor y input because they reduced the input resistance of the afferents and were sensitive to the gamma-aminobutyric acid antagonist picrotoxin. PADs occur red either spontaneously or were elicited by spike activity in the tegula a fferents. In summary, afferent signaling in the locust flight system appear s to be under presynaptic control, a candidate mechanism of which is presyn aptic inhibition.