Abstract
The axons forming the corpus callosum sustain the interhemispheric communication across homotopic cortical areas. We have studied how neurons throughout the columnar extension of the retrosplenial cortex integrate the contralateral input from callosal projecting neurons in cortical slices. Our results show that pyramidal neurons in layers 2/3 and the large, thick-tufted pyramidal neurons in layer 5B showed larger excitatory callosal responses than layer 5A and layer 5B thin-tufted pyramidal neurons, while layer 6 remained silent to this input. Feed-forward inhibitory currents generated by fast spiking, parvalbumin expressing interneurons recruited by callosal axons mimicked the response size distribution of excitatory responses across pyramidal subtypes, being larger in those of superficial layers and in the layer 5B thick-tufted pyramidal cells. Overall, the combination of the excitatory and inhibitory currents evoked by callosal input had a strong and opposed effect in different layers of the cortex; while layer 2/3 pyramidal neurons were powerfully inhibited, the thick-tufted but not thin-tufted pyramidal neurons in layer 5 were strongly recruited. We believe that these results will help to understand the functional role of callosal connections in physiology and disease.
Highlights
The corpus callosum is one of the major fiber bundles of the brain of placental mammals, including humans
Our results show that pyramidal neurons in layers 2/3 and the large, thick-tufted pyramidal neurons in layer 5B showed larger excitatory callosal responses than layer 5A and layer 5B thin-tufted pyramidal neurons, while layer 6 remained silent to this input
Local field potentials and intracellular recordings indicated that electrical stimulation with a bipolar electrode placed on the superficial layers of the agranular retrosplenial cortex (RSC) evoked synaptic responses in the homotopic contralateral cortex (Fig. 1) that were blocked by the application of the AMPA receptor antagonist CNQX 40 lM (Fig. 1d); this allowed us to study the physiology and connectivity of callosal synapses in cortical circuits
Summary
The corpus callosum is one of the major fiber bundles of the brain of placental mammals, including humans. Despite the early postnatal brain contains some callosal fibers immunoreactive to gabaergic markers (Kimura and Baughman 1997), in the young and mature brain, callosal projecting neurons (CPNs) are mainly, if not entirely, pyramidal neurons 2007; Ramos et al 2008), and their synapses on contralateral circuits are excitatory (Kumar and Huguenard 2001, 2003). Contralateral targets of callosal fibers include different subtypes of pyramidal cells (Vogt and Gorman 1982; Kawaguchi 1992; Kumar and Huguenard 2001, 2003) and inhibitory interneurons
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