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Laminar segregation of GABAergic neurons in the avian nucleus isthmi pars magnocellularis: A retrograde tracer and comparative study

Authors

  • Macarena Faunes,

    Corresponding author
    1. Departamento de Biología, Facultad de Ciencias, Universidad de Chile, 7800003, Santiago, Chile
    2. Centro de Investigaciones Médicas, Escuela de Medicina, Pontificia Universidad Católica de Chile, 8330023, Santiago, Chile
    • Departamento de Anatomía Normal, Escuela de Medicina, Pontificia Universidad Católica de Chile, Lira 44, 8330023 Santiago, Santiago de Chile

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  • Sara Fernández,

    1. Departamento de Biología, Facultad de Ciencias, Universidad de Chile, 7800003, Santiago, Chile
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  • Cristián Gutiérrez-Ibáñez,

    1. University Centre for Neuroscience, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada
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  • Andrew N. Iwaniuk,

    1. Department of Neuroscience, Canadian Centre for Behavioural Neuroscience, University of Lethbridge, Lethbridge, Alberta, T1K 3M4, Canada
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  • Douglas R. Wylie,

    1. University Centre for Neuroscience, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada
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  • Jorge Mpodozis,

    1. Departamento de Biología, Facultad de Ciencias, Universidad de Chile, 7800003, Santiago, Chile
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  • Harvey J. Karten,

    1. Department of Neurosciences, School of Medicine, University of California, San Diego, La Jolla, California 92093-0608, United States of America
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  • Gonzalo Marín

    1. Departamento de Biología, Facultad de Ciencias, Universidad de Chile, 7800003, Santiago, Chile
    2. Facultad de Medicina, Universidad Finis Terrae, 7501015, Santiago, Chile
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Abstract

The isthmic complex is part of a visual midbrain circuit thought to be involved in stimulus selection and spatial attention. In birds, this circuit is composed of the nuclei isthmi pars magnocellularis (Imc), pars parvocellularis (Ipc), and pars semilunaris (SLu), all of them reciprocally connected to the ipsilateral optic tectum (TeO). The Imc conveys heterotopic inhibition to the TeO, Ipc, and SLu via widespread γ-aminobutyric acid (GABA)ergic axons that allow global competitive interactions among simultaneous sensory inputs. Anatomical studies in the chick have described a cytoarchitectonically uniform Imc nucleus containing two intermingled cell types: one projecting to the Ipc and SLu and the other to the TeO. Here we report that in passerine species, the Imc is segregated into an internal division displaying larger, sparsely distributed cells, and an external division displaying smaller, more densely packed cells. In vivo and in vitro injections of neural tracers in the TeO and the Ipc of the zebra finch demonstrated that neurons from the external and internal subdivisions project to the Ipc and the TeO, respectively, indicating that each Imc subdivision contains one of the two cell types hodologically defined in the chick. In an extensive survey across avian orders, we found that, in addition to passerines, only species of Piciformes and Rallidae exhibited a segregated Imc, whereas all other groups exhibited a uniform Imc. These results offer a comparative basis to investigate the functional role played by each Imc neural type in the competitive interactions mediated by this nucleus. J. Comp. Neurol. 521:1727–1742, 2013. © 2012 Wiley Periodicals, Inc.

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