CONTRIBUTION OF GABA-MEDIATED AND GLYCINE-MEDIATED INHIBITION TO THE MONAURAL TEMPORAL RESPONSE PROPERTIES OF NEURONS IN THE INFERIOR COLLICULUS

1. To determine the contribution of inhibition to the generation of th e temporal response patterns of neurons in the inferior colliculus (IC ), the effects of iontophoretically applied gamma-aminobutyric acid (G ABA), glycine, and the GABA(A) and glycine receptor antagonists, bicuc ulline and strychnine were studied on 121 neurons in the IC of urethan -anesthetised guinea pig. 2. The neurons temporal discharge patterns w ere classified into six categories on the basis of their peristimulus time histograms (PSTHs). 1) Onset units fired at the stimulus onset an d could be divided into two subtypes: narrow (1-2 spikes only) or broa d (response lasting up to similar to 30 ms). 2) Pauser units had a pre cisely timed onset peak separated from a lower level of sustained acti vity by either a marked reduction or complete cessation of firing. 3) Chopper units had three or more clearly defined peaks near stimulus on set or evidence of regularly spaced peaks over the duration of the sti mulus. 4) Onset-chopper units had three clearly defined peaks at onset but no sustained firing. 5) On-sustained units had a clearly defined single onset peak followed by a lower level of sustained activity. 6) Sustained units fired throughout the stimulus, but lacked an onset pea k. 3. Iontophoretic application of GABA and glycine produced a dose-de pendent reduction in firing rate in 76% (42/55) and 79% (11/14) of uni ts, respectively. Application of bicuculline or strychnine increased t he discharge rate in 91% (64/70) and 94% (16/17) of neurons, respectiv ely. 4. The effects of bicuculline and strychnine on PSTH class were s tudied in detail on 70 neurons. Changes in discharge rate were accompa nied by changes in PSTH in 49% (34/70) of neurons tested with bicucull ine and 41% (7/17) tested with strychnine. Pauser units were the most affected with 69% changing their PSTH class, but some units in all PST H classes, except the chopper group, exhibited changes in PSTH pattern after application of bicuculline. The majority of units (similar to 5 0%) that changed PSTH pattern in the presence of bicuculline became ch opper units. Units of all PSTH classes could become choppers, but the proportion of units showing this change was dependent on the unit's co ntrol response pattern. All seven units that changed PSTH class with s trychnine also became choppers. Changes in PSTH, including the appeara nce of a chopper pattern, did not depend on either a unit's control di scharge rate or the magnitude of the change in discharge rate induced by the antagonists. 5. Bicuculline and strychnine had no significant e ffect on latency for units in the chopper, onset-chopper, onset, pause r, and on-sustained groups. A few sustained and unclassified units tha t had long predrug latencies did show marked reductions in latency whe n tested with bicuculline. 6. The majority of units did not fire spont aneously, and neither bicuculline or strychnine produced a significant increase in spontaneous rate. 7. In many units, the changes in firing rate did not occur equally over the duration of the response. Firing rates at the onset and in the last quarter of the sustained response w ere compared. Three effects of bicuculline and strychnine were observe d. For 80% of units the largest change in firing rate occurred in the sustained response, while in 14% of units the change was greatest at o nset. The change in onset and sustained firing rate for the remaining 6% of units was equal. The different effects of the antagonists on the onset and sustained components of the response varied with the unit's control PSTH type. This suggests that there are differences in the ti ming and strength of the inhibitory inputs impinging on different neur ons. 8. Histological verification of recording sites was possible for 59/70 units of which 68% were located in the central nucleus of the IC (CNIC). All the reported effects occurred in the CNIC, but similar ch anges also were seen in the dorsal cortex and external cortex of the I C. 9. We conclude that GABAergic and glycinergic inhibition exerts a s trong influence on the temporal firing pattern of collicular units. Af ter the application of either bicuculline or strychnine, the majority of units become choppers, showing that many IC neurons have the capaci ty to fire with a regular discharge pattern. These inhibitory effects may be mediated both by temporal variations in the interaction of exci tatory and inhibitory inputs, or by the influence of these inputs on s pecific membrane conductances.