HUBER, A.S.*; YAGER, D. D.; University of Maryland, College Park: Inhibition Shapes the Responses of an Auditory Interneuron of the Praying Mantis

The mantis auditory interneuron, 501-T3, responds to the ultrasonic echolocation cries of bats. It has a distinctive firing pattern that changes during a bat attack sequence. Behavioral and physiological data suggest that 501-T3 triggers the evasive behavior that protects the insect from capture. 501-T3�s characteristic firing pattern begins with a high rate (400-600 spikes/s) initial, phasic burst of action potentials followed by a pause (8-18 msec) before the response becomes tonic at a lower rate (100-200 spikes/s). We hypothesize that inhibition mediated by glycine or GABA plays a role in determining the firing pattern of 501-T3. We used microelectrodes to pressure inject the antagonists strychnine (glycine) and picrotoxin (GABA) into the neuropil near the neurites of 501-T3 while monitoring the neuron�s response to sound pulses. Strychnine decreases rather than increases the number of action potentials in the response and the rate of firing. This suggests that glycine does not directly inhibit 501-T3, but still acts via a more complex pathway. Picrotoxin doubles the number of action potentials in the auditory response, and shortens or eliminates the pause, which is consistent with a loss of inhibition. After injecting picrotoxin, the phasic firing rate is unchanged, but the tonic firing rate increases (200-350 spikes/s), suggesting the onset of the inhibition occurs near the end of the initial burst. Thus, the timing and strength of GABAergic inhibition shapes the firing pattern of 501-T3 by terminating the initial burst, which creates a pause in firing, and altering the firing rate during the tonic phase.