A GABAergic Tecto–Tegmento–Tectal Pathway in Pigeons Martin Stacho, 1 * Sara Letzner, 1 Carsten Theiss, 2 Martina Manns, 1 and Onur G€ unt€ urk€ un 1 1 Department of Biopsychology, Faculty of Psychology, Institute of Cogntive Neuroscience, Ruhr-University Bochum, 44801 Bochum, Germany 2 Department of Cytology, Faculty of Medicine, Ruhr-University Bochum, 44801 Bochum, Germany ABSTRACT Previous studies have demonstrated that the optic tecta of the left and right brain halves reciprocally inhibit each other in birds. In mammals, the superior colliculus receives inhibitory g-aminobutyric acid (GABA)ergic input from the basal ganglia via both the ipsilateral and the contralateral substantia nigra pars reticulata (SNr). This contralateral SNr projection is important in intertectal inhibition. Because the basal ganglia are evolutionarily conserved, the tectal projec- tions of the SNr may show a similar pattern in birds. Therefore, the SNr could be a relay station in an indi- rect tecto–tectal pathway constituting the neuronal sub- strate for the tecto–tectal inhibition. To test this hypothesis, we performed bilateral anterograde and ret- rograde tectal tracing combined with GABA immunohis- tochemistry in pigeons. Suprisingly, the SNr has only ipsilateral projections to the optic tectum, and these are non-GABAergic. Inhibitory GABAergic input to the contralateral optic tectum arises instead from a nearby tegmental region that receives input from the ipsilateral optic tectum. Thus, a disynaptic pathway exists that possibly constitutes the anatomical substrate for the inhibitory tecto–tectal interaction. This pathway likely plays an important role in attentional switches between the laterally placed eyes of birds. J. Comp. Neurol. 000:000–000, 2016. V C 2016 Wiley Periodicals, Inc. INDEXING TERMS: substantia nigra; tecto–tectal interaction; nucleus of the lateral ponto-mesencephalic tegmen- tum; retrograde and anterograde tracing; GABA; parvalbumin; RRIDs: AB_211712; AB_258833; AB_2201526; AB_477652; AB_477329; SciRes_000111 The optic tectum (TeO) is a phylogenetically old, paired midbrain structure that is present in all verte- brates. It encapsulates the torus semicircularis, some- times called the tectum acusticum. Because the tectum opticum and torus semicircularis assume separate loca- tions along the longitudinal axis of the brainstem in mammals, the mammalian TeO is called the superior colliculus (SC). The laminated appearance and general connectivity pattern of the TeO are similar across taxa (Grofov a et al., 1978; Grover and Sharma, 1981; North- cutt, 1982; L az ar et al., 1983; Welker et al., 1983; Huerta and Harting, 1984; Luksch, 2003; Hellmann et al., 2004). The vertebrate TeO is an essential relay station for visuomotor transformation and plays a cru- cial role in saccadic eye movements, prey-catching, spatial attention, and stimulus selection (Sparks and Mays, 1990; Ewert et al., 1999, 2001; Luque et al., 2005; Wurtz, 2009; Knudsen, 2011; Mysore and Knud- sen, 2011; Krauzlis et al., 2013). The left and right TeO modulate their activity patterns via the tectal (CT) and the posterior commissures (CP) (Robert and Cu enod, 1969a,b; Niida, 1973; Mascetti and Arriagada, 1981; Rhoades et al., 1981, 1986; Keysers et al., 2000). These commissures have been implicated in interhemi- spheric transfer of visual discriminations in fish (Mark, 1966; Ingle and Campbell, 1977; Hemsley and Savage, 1989), in transfer of habituated stimuli and in lateral- ized visuomotor behavior in birds (Hamassaki and Britto, 1987; G€ unt€ urk€ un and B€ ohringer, 1987), and in transfer of black–white discrimination, recovery from cortical hemianopia, and to some extent transfer of motion and brightness discrimination in mammals Grant sponsor: German Research Foundation (DFG); Grant numbers: SFB864 and MA4485/1 and MA4485.2 (to M.M.). *CORRESPONDENCE TO: Martin Stacho, Ruhr-University Bochum, Insti- tute of Cognitive Neurosciences, Faculty of Psychology, Department of Biopsychology, Universit€ atstraße 150, 44801 Bochum, Germany. E-mail: Martin.Stacho@rub.de Received November 24, 2015; Revised March 10, 2016; Accepted March 10, 2016. DOI 10.1002/cne.23999 Published online Month 00, 2016 in Wiley Online Library (wileyonlinelibrary.com) V C 2016 Wiley Periodicals, Inc. The Journal of Comparative Neurology | Research in Systems Neuroscience 00:00–00 (2016) 1 RESEARCH ARTICLE