title: Auditory cortical axons contact commissural cells throughout the guinea pig inferior colliculus.
abstract: 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.
authors.affiliation: Department of Anatomy and Neurobiology, Northeast Ohio Medical University, 4209 St. Rt. 44, P.O. Box 95, Rootstown, OH 44272-0095, USA.
authors.forename: Kyle T
authors.initials: KT
authors.lastname: Nakamoto
authors.forename: Colleen S
authors.initials: CS
authors.lastname: Sowick
authors.forename: Brett R
authors.initials: BR
authors.lastname: Schofield
mesh_terms: D000818:Animals; D001303:Auditory Cortex / Q000502:physiology*; D001306:Auditory Pathways / Q000166:cytology; D001369:Axons / Q000378:metabolism / Q000473:pathology*; D001931:Brain Mapping; D005260:Female; D005968:Glutamate Decarboxylase / Q000378:metabolism; D006168:Guinea Pigs; D007245:Inferior Colliculi / Q000473:pathology / Q000502:physiology*; D008297:Male; D008636:Mesencephalon / Q000473:pathology; D008856:Microscopy, Fluorescence; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
chemical_list: D005680:gamma-Aminobutyric Acid; D005968:Glutamate Decarboxylase
keywords: AC; FB; FD; FG; FR; Fast Blue; Fluor0Ruby; FluoroGold; G+; GAD; GAD-immunonegative; GAD-immunopositive; GAD-neg, G−; GB; I-VI; IC; IC central nucleus; IC dorsal cortex; IC lateral cortex; IC rostral cortex; ICc; ICd; IClc; ICrc; auditory cortex; cortical layers; fluorescein dextran; glutamic acid decarboxylase; green RetroBeads; inferior colliculus; ps; pseudosylvian sulcus; rhinal sulcus; rs; white matter; wm