GABAergic inhibition in nucleus magnocellularis: Implications for phase locking in the avian auditory brainstem

In the avian auditory brainstem, nucleus magnocellularis (NM) functions to relay phase-locked signals to nucleus laminaris for binaural coincidence de tection. Although many studies have revealed that NM neurons exhibit intrin sic physiological and anatomical specializations for this purpose, the role of inhibition has not been fully explored. The present study characterizes the organization of GABAergic feedback to NM. Anterograde and retrograde l abeling methods showed that NM receives a prominent projection from the ips ilateral superior olivary nucleus (SON). The functional features of this pr ojection were explored in a brain slice preparation. Stimulating fibers fro m the SON evoked long-lasting, depolarizing responses in NM neurons that we re blockable by bicuculline, a GABA(A) receptor antagonist. The slow time c ourse of these responses allowed them to undergo temporal summation during repetitive stimulation. The summed GABAergic response was capable of blocki ng spikes generated in NM neurons by suprathreshold current injection. This inhibitory effect was attributable to a large reduction in input resistanc e caused by a combination of the opening of a GABAergic Cl- conductance and the recruitment of a low-voltage activated K+ conductance. This large redu ction of input resistance increased the amount of current necessary to driv e NM neurons to threshold. The results lead us to propose that GABAergic in hibition enhances phase-locking fidelity of NM neurons, which is essential to binaural coincidence detection in nucleus laminaris.