GABAERGIC AND GLYCINERGIC INHIBITION SHARPENS TUNING FOR FREQUENCY MODULATIONS IN THE INFERIOR COLLICULUS OF THE BIG BROWN BAT

Discrimination of amplitude and frequency modulated sounds is an impor tant task of auditory processing. Experiments have shown that tuning o f neurons to sinusoidally frequency- and amplitude-modulated (SFM and SAM, respectively) sounds becomes successively narrower going from low er to higher auditory brain stem nuclei. In the inferior colliculus (I C), many neurons are sharply tuned to the modulation frequency of SFM sounds. The purpose of this study was to determine whether GABAergic o r glycinergic inhibition is involved in shaping the tuning for the mod ulation frequency of SFM sounds in IC neurons of the big brown bat (Ep tesicus fuscus). We recorded the response of 56 single units in the ce ntral nucleus of the IC to SFM stimuli before and during the applicati on of the gamma-aminobutyric acid-A (GABAA) receptor antagonist bicucu lline or the glycine receptor antagonist strychnine. To evaluate tunin g to the modulation frequency, the normalized spike count (normalized according to the maximal response for each condition tested) was plott ed versus the modulation frequency and the upper and lower 50% cutoff points were determined. Bicuculline increased the upper cutoff in 46% of the neurons by greater than or equal to 25%. The lower cutoff decre ased in 48% of the neurons tested. In some neurons (similar to 30%), a sharpening of the tuning by bicuculline was observed. Strychnine indu ced an increase of the upper cutoff in almost half of the neurons. Com pared with bicuculline these changes were smaller. The lower cutoff de creased in 50% of the neurons with strychnine. The synchronization coe fficient (SC) was calculated and compared for three modulation frequen cies (50, 100, and 200 Hz) between predrug and drug condition. For all neurons, synchronization decreased (n = 36) or did not change (n = 26 ) during drug application. This was mainly an effect of the prolonged discharge in response to each cycle. Under predrug conditions, many ne urons exhibited selectivity to the direction of the FM, hence they onl y responded once to each cycle. In a minority of neurons, direction se lectivity was abolished by drug application. The main finding was that neuronal inhibition sharpens tuning to the modulation frequency in th e majority of neurons. In general, changes induced by bicuculline or s trychnine were comparable.