FEATURES OF IPSILATERALLY EVOKED INHIBITION IN THE DORSAL NUCLEUS OF THE LATERAL LEMNISCUS

The dorsal nucleus of the lateral lemniscus (DNLL) is a binaural nucle us whose neurons are excited by stimulation of the contralateral ear a nd inhibited by stimulation of the ipsilateral ear. Here we report on several features of the ipsilaterally evoked inhibition in 95 DNLL neu rons of the mustache bat. These features include its dependence on int ensity, its tuning and the types of stimuli that are capable of evokin g it. Inhibition was studied by evoking discharges with the iontophore tic application of glutamate, and then evaluating the strength and dur ation of the inhibition of the glutamate evoked background activity pr oduced by stimulation of the ipsilateral ear. Excitatory responses wer e evoked by stimulation of the contralateral ear with best frequency ( BF) tone bursts. Glutamate evoked discharges could be inhibited in all DNLL neurons and the inhibition often persisted for periods ranging f rom 10 to 50 ms beyond the duration of the tone burst that evoked it. The duration of the persistent inhibition increased with stimulus inte nsity. Stimulus duration had little influence on the duration of the p ersistent inhibition. Signals as short as 2 ms suppressed discharges f or as long as 30 ms after the signal had ended. The frequency tuning o f the total period of inhibition and the period of persistent inhibiti on were both closely matched to the tuning evoked by stimulation of th e contralateral ear. Moreover, the effectiveness of complex signals fo r evoking persistent inhibition, such as brief FM sweeps and sinusoida lly amplitude and frequency modulated signals, was comparable to that of tone bursts at the neuron's excitatory BF, so long as the complex s ignal contained frequencies at or around the neuron's excitatory BF. W e also challenged DNLL cells with binaural paradigms. In one experimen t, we presented a relatively long (40 ms) BF tone burst of fixed inten sity to the contralateral ear, which evoked a sustained discharge, and a shorter, 10 ms signal of variable intensity to the ipsilateral ear. As the intensity of the 10 ms ipsilateral signal increased, it genera ted progressively longer periods of persistent inhibition and thus the discharges were suppressed for periods far longer than the 10 ms dura tion of the ipsilateral signal. With interaural time disparities, ipsi lateral signals that led contralateral signals evoked a persistent inh ibition that suppressed the responses to the trailing contralateral si gnals for periods of a least 15 ms. This suggests that an initial bina ural sound that favors the ipsilateral ear should suppress the respons es to trailing sounds that normally would be excitatory if they were p resented alone. We hypothesize a circuit that generates the persistent inhibition and discuss how the results with binaural signals support that hypothesis. (C) 1998 Published by Elsevier Science B.V. All right s reserved.