What are the relative contributions of excitation and inhibition to the frequency and amplitude tuning of auditory cortical neurons? Although tuning could arise simply from convergence of tuned excitatory thalamocortical inputs, pharmacological and other evidence suggests that inhibition plays a critical role. Do the inhibitory inputs to cortical neurons show frequency or amplitude tuning? Do excitatory and inhibitory inputs to a neuron show the same tuning, or are inhibitory inputs stronger away from the best frequency? Is the relative timing of excitatory and inhibitory inputs fixed, or could it shift with frequency? These questions can only be addressed by measuring the stimulus dependence of the relative magnitudes and time courses of the excitatory and inhibitory synaptic inputs to a given neuron. To achieve this, we recorded from auditory cortical neurons in the anesthetized rat using the whole-cell voltage clamp technique. Synaptic currents were recorded in response to tones presented at different holding potentials. Synaptic conductance was computed from the time-varying synaptic I-V curve for each stimulus, and decomposed into excitatory and inhibitory components. Different neurons displayed each of the abovementioned roles for inhibition. Neurons displayed either tuned or relatively untuned inhibition, and inhibitory tuning profiles were either similar to or different from the excitatory profile in the same neurons. Excitatory and inhibitory inputs showed either fixed timing across stimuli, or stimulus-dependent time shifts. This heterogeneity of roles for inhibition is consistent with the diversity of inhibitory interneurons and the local circuits they form in the cerebral cortex.
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