Auditory cortical axons contact commissural cells throughout the guinea pig inferior colliculus

Oct 22, 2013Hearing research

Hearing brain nerve fibers connect with cross-communication cells across the guinea pig midbrain hearing area

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Abstract

Approximately 23-34% of commissural cells in the left inferior colliculus are contacted by auditory cortical axons.

Auditory cortex projections influence how cells in the inferior colliculi respond to sound.
The direct projection from the auditory cortex to the contralateral inferior colliculus is relatively small, raising questions about its effects.
Contacts between cortical axons and commissural cells were identified through fluorescent tracer injections.
Majority (>86%) of labeled commissural cells in the left inferior colliculus were not GABAergic.
Most of the contacted commissural cells appear to be excitatory, suggesting a significant role in auditory processing.
This pathway may play a key role in how the auditory cortex affects responses in the contralateral inferior colliculus.

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Projections from auditory cortex (AC) affect how cells in both inferior colliculi (IC) respond to acoustic stimuli. The large projection from the AC to the ipsilateral IC is usually credited with the effects in the ipsilateral IC. The circuitry underlying effects in the contralateral IC is less clear. The direct projection from the AC to the contralateral IC is relatively small. An unexplored possibility is that the large ipsilateral cortical projection contacts the substantial number of cells in the ipsilateral IC that project through the commissure to the contralateral IC. Apparent contacts between cortical boutons and commissural cells were identified in the left IC after injection of different fluorescent tracers into the left AC and the right IC. Commissural cells were labeled throughout the left IC, and many (23-34%) appeared to be contacted by cortical axons. In the central nucleus, both disc-shaped and stellate cells were contacted. Antibodies to glutamic acid decarboxylase (GAD) were used to identify GABAergic commissural cells. The majority (>86%) of labeled commissural cells were GAD-immunonegative. Despite low numbers of GAD-immunopositive commissural cells, some of these cells were contacted by cortical boutons. Nonetheless, most cortically contacted commissural cells were GAD-immunonegative (i.e., presumably glutamatergic). We conclude that auditory cortical axons contact primarily excitatory commissural cells in the ipsilateral IC that project to the contralateral IC. These corticocollicular contacts occur in each subdivision of the ipsilateral IC, suggesting involvement of commissural cells throughout the IC. This pathway - from AC to commissural cells in the ipsilateral IC - is a prime candidate for the excitatory effects of activation of the auditory cortex on responses in the contralateral IC. Overall this suggests that the auditory corticofugal pathway is integrated with midbrain commissural connections.

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Auditory cortical axons contact commissural cells throughout the guinea pig inferior colliculus

Abstract

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