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The Journal of Neurophysiology Vol. 83 No. 2 February 2000, pp. 926-940
Copyright ©2000 by the American Physiological Society
Department of Biomedical Engineering and Center for Hearing Sciences, Johns Hopkins University, Baltimore, Maryland 21205
Davis, Kevin A. and
Eric D. Young.
Pharmacological Evidence of Inhibitory and Disinhibitory Neuronal
Circuits in Dorsal Cochlear Nucleus. J. Neurophysiol. 83: 926-940, 2000. The dorsal cochlear
nucleus (DCN) is rich in both glycine and GABA inhibitory
neurotransmitter systems, and the response properties of its principal
cells (pyramidal and giant cells) are strongly shaped by inhibitory
inputs. For example, DCN principal cells often display highly
nonmonotonic (so-called type IV) input-output functions in response to
best-frequency (BF) tones. In this study, the inhibitory inputs onto
the principal cell types and onto response types of known inhibitory
interneurons were compared before and during iontophoretic application
of the glycine- and GABAA-receptor antagonists, strychnine
and bicuculline. Strychnine eliminates the central (on-BF) inhibitory
area in type IV units, resulting in monotonic BF rate-level curves.
Unexpectedly, bicuculline primarily enhances inhibition in
principal-cell types; for example, type IV units are inhibited at lower
sound levels in the presence of bicuculline. Principal cell types with
weaker inhibitory inputs (type IV-T and type III units) are more
strongly inhibited in the presence of bicuculline and usually are
converted into type IV units. This enhancement of on-BF inhibition by
bicuculline suggests a disinhibitory process involving
GABAA action on a non-GABAAergic inhibitory
pathway. This latter pathway is probably glycinergic and involves type
II units (deep-layer vertical cells) and/or complex-spiking units
(superficial cartwheel cells) because both of these unit types are
disinhibited by bicuculline. One intrinsic GABAA source
could be the superficial stellate cells in DCN because bicuculline
partly blocks the inhibition evoked by somatosensory-stimulated activation of the superficial granule-cell circuitry in DCN. Taken together, the results suggest that glycinergic circuits mediate directly the inhibition of DCN principal cells, but that
GABAAergic circuits modulate the strength of the inhibition.
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