OCTOPAMINE MODULATES THE RESPONSES AND PRESYNAPTIC INHIBITION OF PROPRIOCEPTIVE SENSORY NEURONS IN THE LOCUST SCHISTOCERCA-GREGARIA

A multineuronal proprioceptor, the femoral chordotonal organ (feCO), m onitors the position and movements of the tibia of an insect leg. Supe rfusing the locust metathoracic feCO with the neuromodulator octopamin e, or the octopamine agonist synephrine, affects the position (tonic) component of the organ's response, but not the movement (phasic) compo nent. Both octopamine and synephrine act with the same threshold (10(- 6) mol l(-1)). Individual sensory neurones that respond tonically at f lexed tibial angles show increased tonic spike activity following appl ication of octopamine, but those that respond at extended angles do no t. Tonic spiking of phase-tonic flexion-sensitive neurones is enhanced but their phasic spiking is unaffected. Bath application of octopamin e to the feCO increases the tonic component of presynaptic inhibition recorded in the sensory terminals, but not the phasic component. This inhibition should at least partially counteract the increased sensory spiking and reduce its effect on postsynaptic targets such as motor ne urones. Furthermore, some phasic sensory neurones whose spiking is not affected by octopamine nevertheless show enhanced tonic synaptic inpu ts. The chordotonal organ is not known to be under direct efferent con trol, but its output is modified by octopamine acting on its sensory n eurones to alter their responsiveness to mechanical stimuli and by pre synaptic inhibition acting on their central branches. The effects of t his neuromodulator acting peripherally on sensory neurones are therefo re further complicated by indirect interactions between the sensory ne urones within the central nervous system. Increases of sensory neurone spiking caused by neuromodulators may not necessarily lead to paralle l increases in the responses of postsynaptic target neurones.