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.
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This work was supported by the Spanish Ministerio de Economía y Competitividad Grants SAF2016-75803-P and Junta de Castilla y León Grant SA343U14 to MSM. YAA held PhD fellowships from the Mexican Consejo Nacional de Ciencia y Tecnología and Secretaría de Educación Pública. We thank to Dr. Nell B. Cant and Edward Bartlett for their thoughtful comments on a previous version of the manuscript. In addition, we thank Collen Gabel for her assistance in performing strychnine experiments and Otto García-Garibay for his help to generate Figs. 1b–5b.
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Conflict of interest
The authors declare that they have no conflict of interest.
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