The effects of GABAergic inhibition on monaural response properties of neurons in the mustache bat's inferior colliculus.

The effects of GABAergic inhibition on response properties of neurons in the inferior colliculus were investigated. The experimental animals were mustache bats and responses were monitored from neurons in the hypertrophied 60 kHz isofrequency contour of the inferior colliculus. The features we report on here are: 1) the maximum discharge rates evoked by tone bursts at each unit's best frequency; 2) the forms of the rate-level functions; 3) the discharge patterns evoked by best frequency tone bursts; and 4) the changes in these response features that were observed when GABAergic inhibition was blocked with bicuculline, an antagonist specific for GABAA receptors. There were three main findings. The first is that bicuculline caused the discharge rate to increase in the majority of neurons. The maximum firing rates in more than half of the units increased by at least 100%, and in 15% of the cells the maximum spike-count increased 400% or more. Of particular interest were the 13 cells that were nearly unresponsive to any tone burst, but responded vigorously to the same stimuli after application of bicuculline. The second main finding is that the increased discharge rates were due either to a change from a phasic to a tonic discharge pattern, or to a change in overall excitability with no change in discharge pattern. The third main finding was that bicuculline changed the shape of the rate-level functions in almost half of the cells studied. The general trend was that units whose pre-drug rate-level functions were upper-threshold were most likely to be changed, followed by regular nonmonotonic and non-saturated monotonic. Units with saturated monotonic functions were the least likely to be affected by bicuculline. These results lead us to suggest that GABAergic inhibition acts on collicular cells in two principal ways. The first way is to modify the effects of the excitatory innervation and thereby shape the response features of collicular neurons. The formation of rate-level functions is but one illustration of the shaping action of GABAergic inhibition. Other features that are shaped by GABAergic inhibition include discharge patterns, thresholds, latencies and tuning curves. The second way is to provide a regulated suppression of evoked activity. We propose that the suppression is situation dependent and may act to enhance the operating range of collicular neurons in situations of particular importance to the animal, such as during periods of selective attention and perhaps in other situations as well.