NEUROSYSTEMS Indirect pathway between the primary auditory and visual cortices through layer V pyramidal neurons in V2L in mouse and the effects of bilateral enucleation M. E. Larame´e, 1 T. Kurotani, 2,3 K. S. Rockland, 3,4 G. Bronchti 1 and D. Boire 1,5 1 Groupe de Recherche en Neurosciences, De ´ partement de Chimie-Biologie, Universite ´ du Que ´ bec a ` Trois-Rivie ` res, Trois-Rivie ` res, QC, Canada G9A 5H7 2 Department of Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Nagoya, Japan 3 Lab for Cortical Organization and Systematics, RIKEN Brain Science Institute, Wako-shi, Saitama, Japan 4 RIKEN-MIT Center for Neural Circuit Genetics, MIT Cambridge, MA, USA 5 E ´ cole d’optome ´ trie, Universite ´ de Montre ´ al, Montre ´al, QC, Canada Keywords: blindness, cortico-cortical, heteromodal, morphology, plasticity Abstract Visual cortical areas are activated by auditory stimuli in blind mice. Direct heteromodal cortical connections have been shown between the primary auditory cortex (A1) and primary visual cortex (V1), and between A1 and secondary visual cortex (V2). Auditory afferents to V2 terminate in close proximity to neurons that project to V1, and potentially constitute an effective indirect pathway between A1 and V1. In this study, we injected a retrograde adenoviral vector that expresses enhanced green fluorescent protein under a synapsin promotor in V1 and biotinylated dextran amine as an anterograde tracer in A1 to determine: (i) whether A1 axon terminals establish synaptic contacts onto the lateral part of V2 (V2L) neurons that project to V1; and (ii) if this indirect cortical pathway is altered by a neonatal enucleation in mice. Complete dendritic arbors of layer V pyramidal neurons were reconstructed in 3D, and putative contacts between pre-synaptic auditory inputs and postsynaptic visual neurons were analysed using a laser- scanning confocal microscope. Putative synaptic contacts were classified as high-confidence and low-confidence contacts, and charted onto dendritic trees. As all reconstructed layer V pyramidal neurons received auditory inputs by these criteria, we conclude that V2L acts as an important relay between A1 and V1. Auditory inputs are preferentially located onto lower branch order dendrites in enucleated mice. Also, V2L neurons are subject to morphological reorganizations in both apical and basal dendrites after the loss of vision. The A1–V2L–V1 pathway could be involved in multisensory processing and contribute to the auditory activation of the occipital cortex in the blind rodent. Introduction Visually deprived humans have greater abilities in processing non- visual stimuli than sighted individuals (Lessard et al., 1998; Ro ¨der et al., 1999; Alary et al., 2009; Wan et al., 2010). These capabilities appear to be correlated with the activation of their striate and extrastriate visual areas by somatosensory (Sadato et al., 1996, 2004; Cohen et al., 1997) and auditory (Kujala et al., 1995; Gougoux et al., 2004, 2005; Hertrich et al., 2009) stimuli. This non-visual activation of the occipital cortex has also been observed in visually deprived mice (Chabot et al., 2007; Larsen et al., 2009; Van Brussel et al., 2011), rats (Piche et al., 2007), hamsters (Izraeli et al., 2002), opossums (Kahn & Krubitzer, 2002; Karlen et al., 2006), cats (Yaka et al., 1999, 2000; Sanchez-Vives et al., 2006) and blind mole rats (Heil et al., 1991; Bronchti et al., 2002). These results suggest underlying cross-modal plasticity following the loss of visual inputs in different species. Several hypotheses have been proposed to explain how cross- modal interactions are established in visually deprived individuals (Bavelier & Neville, 2002). Firstly, subcortical pathways could directly convey non-visual activity to the primary visual cortex (V1). Indeed, the inferior colliculus projects to V1 via direct projections to the lateral geniculate nucleus (LGN) in blind rodents (Doron & Wollberg, 1994; Piche ´ et al., 2004; Chabot et al., 2007). The second hypothesis involves connections to V1 and to the secondary visual area (V2) from parietal, frontal and temporal associative areas, where multisensory processing occurs (Macaluso et al., 2000; Calvert, 2001; Driver & Noesselt, 2008). These multisensory cortical areas project to visual, auditory and somatosensory unisensory cortices (Kennedy & Bullier, 1985; Cappe & Barone, 2005). The visual cortex in blind subjects could participate in the processing of a non- visual stimulus by amplifying these connections. Finally, visual deprivation could strengthen direct cortico-cortical connections between unimodal primary sensory cortices. The presence of these Correspondence: Dr D. Boire, 1 Groupe de Recherche en Neurosciences, as above. E-mail: Denis.Boire@uqtr.ca Received 5 October 2010, revised 9 March 2011, accepted 11 April 2011 European Journal of Neuroscience, Vol. 34, pp. 65–78, 2011 doi:10.1111/j.1460-9568.2011.07732.x ª 2011 The Authors. European Journal of Neuroscience ª 2011 Federation of European Neuroscience Societies and Blackwell Publishing Ltd European Journal of Neuroscience