Mechanisms of Dopamine-dependent Plasticity in Auditory Cortex

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One form of plasticity in the auditory cortex can be induced by combining electrical stimulation of dopamine neurons with a pure tone stimulation ; the pure tone will be represented by an enlarged cortical area after repeated stimulation with the combined stimuli. Similar plasticity occurs when pure tone stimulation is combined with stimulation of cholinergic neurons in the basal forebrain. Here we asked how dopamine might induce the plasticity by examining the acute effect of dopamine on cortical response to acoustic stimulations. Using optical imaging with voltage sensitive dyes, we were able to record auditory response in the cortex with millisecond temporal resolution. Examining the tonotopic responses in the cortex allowed us identifying both the primary and the secondary cortices. Such examinations also led to the finding of several new fields in the auditory cortices. Application of dopamine or its agonists to the cortex, however, caused no change in the auditory response. Systemic application of L-DOPA also failed to induce a significant change in the response. We thus set out to study the effect of acetylcholine. Application of Oxotromorine suppressed both the initial excitatory response and the following inhibitory responses to acoustic stimulations. We wondered next if there is any interaction between dopamine and acetylcholine. We found that co-application of dopamine and acetylcholine induced no change in the auditory response, indicating that dopamine completely suppressed all effects of acetylcholine. These results suggest a novel mechanism for dopamine in inducing plasticity in the cortex by interacting with acetylcholine.