Features of contralaterally evoked inhibition in the inferior colliculus

Cells in the central nucleus of the inferior colliculus (ICc) receive a lar ge number of convergent inputs that are not only excitatory but inhibitory as well. While the excitatory responses of ICc cells have been studied exte nsively, less attention has been paid to the effects that inhibitory inputs have on auditory processing in the ICc. The purpose of this study was to e xamine the role of contralaterally evoked inhibition in single ICc cells in awake Mexican free-tailed bats. To study the contralaterally evoked inhibi tion, we created background activity by the iontophoretic application of th e excitatory neurotransmitters glutamate and aspartate and visualized the i nhibition as a gap in the carpet of background activity. We found that 85% of ICc cells exhibit a contralaterally evoked excitation followed by a peri od of inhibition. The inhibition acts primarily through GABAA receptors sin ce the application of bicuculline eliminated or greatly reduced the inhibit ion in all cells. The inhibition has two parts: an early part which is coin cident with the tone stimulus and a later persistent component which outlas ts the tone stimulus by tells of milliseconds. The persistent inhibition ty pically is level-dependent, increasing in duration with increasing sound le vel. The persistent inhibition is also sensitive to the duration of the sti mulus, with short (5 ms) tones being less effective than longer (> 20 ms) t ones in generating persistent inhibition. While the early inhibition has cl ear roles in the shaping of excitatory response properties to a stimulus, t he later persistent component of the inhibition is more enigmatic. The fact that the persistent inhibition lasts well beyond the duration of excitator y inputs to the ICc cell implies that the persistent inhibition may be impo rtant for the temporal segregation of the responses to multiple sound sourc es. (C) 2000 Elsevier Science B.V. All rights reserved.