Intracortical Pathways Determine Breadth of Subthreshold Frequency Receptive Fields in Primary Auditory Cortex

To examine the basis of frequency receptive fields in auditory cortex (ACx), we have recorded intracellular (whole cell) and extracellular (local field potential, LFP) responses to tones in anesthetized rats. Frequency receptive fields derived from excitatory postsynaptic potentials (EPSPs) and LFPs from the same location resembled each other in terms of characteristic frequency (CF) and breadth of tuning, suggesting that LFPs reflect local synaptic (including subthreshold) activity. Subthreshold EPSP and LFP receptive fields were remarkably broad, often spanning five octaves (the maximum tested) at moderate intensities (40-50 dB above threshold). To identify receptive-field features that are generated intracortically, we microinjected the GABA(A) receptor agonist muscimol (0.2-5.1 mM, 1-5 microl) into ACx. Muscimol dramatically reduced LFP amplitude and reduced receptive-field bandwidth, implicating intracortical contributions to these features but had lesser effects on CF response threshold or onset latency, suggesting minimal loss of thalamocortical input. Reversal of muscimol's inhibition preferentially at the recording site by diffusion from the recording pipette of the GABA(A) receptor antagonist picrotoxin (0.01-100 microM) disinhibited responses to CF stimuli more than responses to spectrally distant, non-CF stimuli. We propose that thalamocortical and intracortical pathways preferentially contribute to responses evoked by CF and non-CF stimuli, respectively, and that intracortical projections linking frequency representations determine the breadth of receptive fields in primary ACx. Broad, subthreshold receptive fields may distinguish ACx from subcortical auditory relay nuclei, promote integrated responses to spectrotemporally complex stimuli, and provide a substrate for plasticity of cortical receptive fields and maps.