GABAergic Inhibition Sharpens the Frequency Tuning and Enhances Phase Locking in Chicken Nucleus Magnocellularis Neurons

GABAergic modulation of activity in avian cochlear nucleus neurons has been studied extensively in vitro. However, how this modulation actually influences processing in vivo is not known. We investigated responses of chicken nucleus magnocellularis (NM) neurons to sound while pharmacologically manipulating the inhibitory input from the superior olivary nucleus (SON). SON receives excitatory inputs from nucleus angularis (NA) and nucleus laminaris (NL), and provides GABAergic inputs to NM, NA, NL, and putatively to the contralateral SON. Results from single-unit extracellular recordings from 2 to 4 weeks posthatch chickens show that firing rates of auditory nerve fibers increased monotonically with sound intensity, while that of NM neurons saturated or even decreased at moderate or loud sound levels. Blocking GABAergic input with local application of TTX into the SON induced an increase in firing rate of ipsilateral NM, while that of the contralateral NM decreased at high sound levels. Moreover, local application of bicuculline to NM also increased the firing rate of NM neurons at high sound levels, reduced phase locking, and broadened the frequency-tuning properties of NM neurons. Following application of DNQX, clear evidence of inhibition was observed. Furthermore, the inhibition was tuned to a broader frequency range than the excitatory response areas. We conclude that GABAergic inhibition from SON has at least three physiological influences on the activity of NM neurons: it regulates the firing activity of NM units in a sound-level-dependent manner; it improves phase selectivity; and it sharpens frequency tuning of NM neuronal responses.