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 GABA_A-, GABA_B-, and glycinergic receptors in stimulus-specific adaptation (SSA). We found that blockade of postsynaptic GABA_B receptors selectively modulated response adaptation to repetitive sounds, whereas blockade of presynaptic GABA_B receptors exerted a gain control effect on neuron excitability. Adaptation decreased when postsynaptic GABA_B receptors were blocked, but increased if the blockade affected the presynaptic GABA_B 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 GABA_A, GABA_B, 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.