The effect of inhibition on stimulus-specific adaptation in the inferior colliculus.

The inferior colliculus is a center of convergence for inhibitory and excitatory synaptic inputs that may be activated simultaneously by sound stimulation. Stimulus repetition may generate response habituation by changing the efficacy of neuron's synaptic inputs. Specialized IC neurons reduce their response to repetitive tones, but restore their firing when a different and infrequent tone occurs, a phenomenon known as stimulus specific adaptation. Here, using the microiontophoresis technique, we determined the role of GABAA-, GABAB-, and glycinergic receptors in stimulus-specific adaptation (SSA). We found that blockade of postsynaptic GABAB receptors selectively modulated response adaptation to repetitive sounds, whereas blockade of presynaptic GABAB receptors exerted a gain control effect on neuron excitability. Adaptation decreased when postsynaptic GABAB receptors were blocked, but increased if the blockade affected the presynaptic GABAB receptors. A dual, paradoxical effect was elicited by blockade of glycinergic receptors, i.e., both increase and decrease in adaptation. Moreover, simultaneous co-application of GABAA, GABAB, and glycinergic antagonists demonstrated that local GABA- and glycine-mediated inhibition contributes to only about 50% of SSA. Therefore, inhibition via chemical synapses dynamically modulate the strength and dynamics of stimulus-specific adaptation, but does not generate it.