Inhibitory synaptic mechanisms underlying functional diversity in auditory cortex

In the auditory system, acoustic information conveyed by hair cells and auditory nerves is broken up into different components in central neural circuits, which can then be processed and encoded separately. This process relies on the generation of a variety of response features different from auditory nerve activity. Inhibition in the central circuits proves crucial for the creation and refinement of these functional response properties. Previous understanding of inhibitory mechanisms for auditory information processing has been limited by the methodology of deriving inhibition indirectly from spike and membrane potential responses. Recent application of in vivo whole cell voltage-clamp recordings (iVCRs) to auditory cortical neurons directly reveals the spectral and temporal properties of synaptic inhibition evoked by auditory stimuli. These findings provide new insights into how cortical inhibition shapes spike responses of excitatory neurons through its specific interaction with their excitatory synaptic input. This review highlights our current understanding of cortical in-hibitory mechanisms underlying several fundamental functional properties of auditory cortical neurons. In particular, we propose that the variation in spectro-temporal pattern of cortical inhibition in relation to excitation contributes to the functional diversity of auditory cortex. In the central auditory system, a variety of response features that do not resemble auditory nerve activity are found. For example, although the auditory nerve fire spikes continuously during sound duration, some central auditory neurons only respond transiently to the onset or the offset of sound stimuli (Fig. 1). The firing rate of the auditory nerve increases monotonically as sound intensity increases, whereas that of some central auditory neurons reaches a peak and then declines with further intensity increments. These diverse functional properties may set a foundation for parallel processing of different components of acoustic information. It is believed that the generation of many of these novel response properties depends on inhibitory circuits. However , the detailed underlying mechanisms remain largely unclear, as it has been difficult to directly reveal synap-tic inhibition in previous studies. Recently, in vivo whole cell recording techniques, especially, those of iVCR, have been successfully applied to auditory cortical neurons. These studies provided new insights into the inhibitory synaptic circuitry basis for the generation and refinement of these functional response properties, even though many of them are originated in subcortical nuclei. Cortical responses are know to be strongly influenced by synaptic inhibition, which plays important roles in defining frequency–intensity receptive fields (RFs) and shaping sound-evoked responses of individual cortical neurons …