OCTOPAMINE EFFECTS MIMIC STATE-DEPENDENT CHANGES IN A PROPRIOCEPTIVE FEEDBACK-SYSTEM

The modulatory actions of the biogenic amine octopamine on the femur t ibia (FT) control loop in the stick insect Carausius morosus were exam ined. The response properties of the FT control loop were determined u nder open loop conditions. Mechanical stimulation of the femoral chord otonal organ (fCO) was the input and tibial movement and motoneuronal activity were measured as the output of the system. Following octopami ne injection into the hemolymph of intact, inactive animals, two conse cutive phases occurred at the behavioral level. Octopamine caused init ially an activation of the animal. During this first phase (3.5-12 min duration) the response properties of the FT control loop were similar to those found in animals that were activated by tactile stimuli unde r normal conditions. Afterward, animals became inactive. During this s econd phase (15-20 min duration), the gain of the control loop was zer o and no resistance reflex in the FT joint was generated in response t o fCO stimulation. However, active movements of the tibia could still be elicited. As we could show in restrained animals, where DL-octopami ne was applied topically onto the undesheated mesothoracic ganglion, t he complete suppression of the resistance reflex on the motoneuronal l evel was dose dependent starting at concentrations of 5 x 10(-3) M oct opamine. We could show that octopamine specifically suppressed the pat hways involved in the resistance reflex, while feedback loop responses to fCO stimuli typical for active animals could still be elicited. Ou r results indicate that an increase in the octopamine concentration mi micks activation of the animal: Properties being characteristic for th e control of the FT joint in the inactive animal are inhibited by octo pamine, while properties of the FT control loop typical for the active animal appear to be facilitated following octopamine injection. The r esults clearly demonstrate that different pathways in the neuronal net work underlying the FT control loop are involved in the responses of t he control loop to fCO stimuli in the inactive and active behavioral s tates of the stick insect.