title: Axon terminals from the nucleus isthmi pars parvocellularis control the ascending retinotectofugal output through direct synaptic contact with tectal ganglion cell dendrites.
abstract: The optic tectum in birds and its homologue the superior colliculus in mammals both send major bilateral, nontopographic projections to the nucleus rotundus and caudal pulvinar, respectively. These projections originate from widefield tectal ganglion cells (TGCs) located in layer 13 in the avian tectum and in the lower superficial layers in the mammalian colliculus. The TGCs characteristically have monostratified arrays of brush-like dendritic terminations and respond mostly to bidimensional motion or looming features. In birds, this TGC-mediated tectofugal output is controlled by feedback signals from the nucleus isthmi pars parvocellularis (Ipc). The Ipc neurons display topographically organized axons that densely ramify in restricted columnar terminal fields overlapping various neural elements that could mediate this tectofugal control, including the retinal terminals and the TGC dendrites themselves. Whether the Ipc axons make synaptic contact with these or other tectal neural elements remains undetermined. We double labeled Ipc axons and their presumptive postsynaptic targets in the tectum of chickens (Gallus gallus) with neural tracers and performed an ultrastructural analysis. We found that the Ipc terminal boutons form glomerulus-like structures in the superficial and intermediate tectal layers, establishing asymmetric synapses with several dendritic profiles. In these glomeruli, at least two of the postsynaptic dendrites originated from TGCs. We also found synaptic contacts between retinal terminals and TGC dendrites. These findings suggest that, in birds, Ipc axons control the ascending tectal outflow of retinal signals through direct synaptic contacts with the TGCs.
authors.affiliation: Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Santiago, Región Metropolitana, Chile.
authors.forename: Cristian
authors.initials: C
authors.lastname: González-Cabrera
authors.affiliation: Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Santiago, Región Metropolitana, Chile.
authors.forename: Florencia
authors.initials: F
authors.lastname: Garrido-Charad
authors.affiliation: Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Santiago, Región Metropolitana, Chile.
authors.forename: Jorge
authors.initials: J
authors.lastname: Mpodozis
authors.affiliation: Medical Research Council Anatomical Neuropharmacology Unit, Department of Pharmacology, University of Oxford, Oxford, OX1 2JA, United Kingdom.
authors.forename: J Paul
authors.initials: JP
authors.lastname: Bolam
authors.affiliation: Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Santiago, Región Metropolitana, Chile.|Facultad de Medicina, Universidad Finis Terrae, Providencia, Santiago, Región Metropolitana, Chile.
authors.forename: Gonzalo J
authors.initials: GJ
authors.lastname: Marín
mesh_terms: D000818:Animals; D002645:Chickens; D002772:Cholera Toxin / Q000378:metabolism; D003712:Dendrites / Q000648:ultrastructure*; D005260:Female; D017950:Ganglia, Sensory / Q000166:cytology*; D008297:Male; D008854:Microscopy, Electron; D008953:Models, Anatomic; D009474:Neurons / Q000502:physiology*; D010835:Phytohemagglutinins; D017729:Presynaptic Terminals / Q000502:physiology* / Q000648:ultrastructure; D013477:Superior Colliculi / Q000166:cytology* / Q000502:physiology; D014795:Visual Pathways / Q000502:physiology*
chemical_list: D010835:Phytohemagglutinins; C100156:leukoagglutinins, plants; D002772:Cholera Toxin
keywords: RRID:AB_10013220; RRID:AB_2336126; RRID:AB_2336654; RRID:nif-0000-23420; attention; optic tectum; stimulus selection; superior colliculus; ultrastructure; vision