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The Journal of Neuroscience, April 15, 2000, 20(8):2954-2963
GABAergic Inhibition in Nucleus Magnocellularis: Implications for Phase Locking in the Avian Auditory Brainstem
Pablo Monsivais, Lichuan Yang, and Edwin W Rubel Virginia Merrill Bloedel Hearing Research Center and Department of Otolaryngology, Head and Neck Surgery, University of Washington, Seattle, Washington 98195
In the avian auditory brainstem, nucleus magnocellularis (NM) functions to relay phase-locked signals to nucleus laminaris for binaural coincidence detection. Although many studies have revealed that NM neurons exhibit intrinsic 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 labeling methods showed that NM receives a prominent projection from the ipsilateral superior olivary nucleus (SON). The functional features of this projection were explored in a brain slice preparation. Stimulating fibers from the SON evoked long-lasting, depolarizing responses in NM neurons that were blockable by bicuculline, a GABAA receptor antagonist. The slow time course of these responses allowed them to undergo temporal summation during repetitive stimulation. The summed GABAergic response was capable of blocking spikes generated in NM neurons by suprathreshold current injection. This inhibitory effect was attributable to a large reduction in input resistance caused by a combination of the opening of a GABAergic Cl
conductance and the recruitment of a low-voltage activated K+ conductance. This large reduction of input resistance increased the amount of current necessary to drive NM neurons to threshold. The results lead us to propose that GABAergic inhibition enhances phase-locking fidelity of NM neurons, which is essential to binaural coincidence detection in nucleus laminaris.
Key words: GABA; nucleus magnocellularis; avian auditory brainstem; phase locking; superior olivary nucleus; K+ conductance
Copyright © 2000 Society for Neuroscience 0270-6474/00/2082954-10$05.00/0
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