Inhibition has little effect on response latencies in the inferior colliculus

The inferior colliculi of all mammals are characterized by a wide range of first-spike response latencies that can greatly exceed the minimum time required for the transmission of input through the lower brainstem. The mechanisms that account for long response latencies of up to 50 ms are unclear, but one hypothesis is that an early inhibition plays a role in shaping latency. To test this hypothesis, response latencies were measured in the inferior colliculi of the pallid and mustached bats before and during the blockade of GABAa and glycine receptors. The effect of blocking inhibition on response latency was compared under stimulus conditions that produced the shortest latency in the predrug condition. Multibarrel "piggyback" electrodes were used to iontophoretically apply bicuculline and strychnine sequentially while recording from single neurons. Predrug latencies ranged from 9 to 26 ms in the pallid bat and from 4 to 17 ms in the mustached bat. Despite large increases in response magnitude and response duration following disinhibition, the blockade of inhibitory receptors had modest effects on response latency. In the pallid bat, blocking GABA receptors produced latency changes that ranged from -3.8 to +0.2 ms, while blocking glycine receptors produced changes from -0.1 to +1.7 ms. Similarly, in the mustached bat, blocking GABA receptors caused changes ranging from -10.3 to +1.4 ms; blocking glycine receptors in the mustached bat caused changes from -3.6 to +1.0 ms. The large change of -10.3 ms was an exception. In both species, the majority of neurons showed changes of <1 ms. We conclude that a fast, early inhibitory input does not appear to play a significant role in shaping the wide range of response latencies present in the inferior colliculi of mustached and pallid bats.