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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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