Based on previous species-specific studies, the intracallosal population could be classified as type 1, defined by a dense NADPH-d histochemical reaction (Yan et al. 1996); no type 2 neurons, characterized by low-level NADPH-d activity (Yan et al. 1996), were observed in the cc. Their dendritic trees and the morphology of the perikaryon enabled classification of NADPH-d+/NOSIP neurons into five groups: bipolar (fusiform, #HKI-272 mouse keyword# rectangular), round, polygonal (quadrangular), and pyramidal (triangular-pyriform). Our data therefore indicate that in the rat cc, as in the monkey

(Rockland and Nayyar 2012), there exists a wide neuronal heterogeneity that is actually based only on morphological criteria. The heterogeneity of NO-producing neurons in the cerebral cortex is based on different criteria. NADPH-d+/NOSIP neurons belong to one of two classes, type 1 or type 2 (Yan et al. 1996), based on their content in NO-producing Inhibitors,research,lifescience,medical enzymes. Moreover, nNOS–type 1 neurons display fast-spiking activity, they account for 0.5–2% of the cortical GABAergic population, and in these neurons nNOS is associated with somatostatin and neuropeptide Y (for a review see

Tricoire and Vitalis 2012). It Inhibitors,research,lifescience,medical cannot therefore be excluded that the NADPH-d/nNOS-type 1 neurons found in the rat cc are characterized by chemical heterogeneity. Further double-labeling studies are in progress in our laboratory to test this hypothesis. However, chemical heterogeneity has been Inhibitors,research,lifescience,medical observed in cc neurons, especially in the early stages of postnatal life; some intracallosal neurons contain calretinin, calbindin, GABA, and MAP2 (DeDiego et al. 1994; Riederer et al. 2004). Intracallosal neurons have a wide dendritic field with many dendrites extending into white matter. In the Inhibitors,research,lifescience,medical best cases, they could be followed up to layer VI of the overlying cerebral cortex; they may thus receive synaptic inputs from different sources. Collaterals of cortical afferent and efferent systems could terminate on these dendrites, a

hypothesis that is supported by previous studies. An anterograde tracer injected into different cortical areas anterogradely labeled synaptic terminals establishing synapses Florfenicol on white matter interstitial neurons (Clarke et al. 1993; Shering and Lowenstein 1994). Moreover, both thalamocortical and claustrocortical afferents, which form a dense plexus in layer VI (Zhang and Deschênes 1998; Arnold et al. 2001; Oda et al. 2004), could contact the dendrites of intracallosal neurons, which could thus receive a synaptic input also from neurons located in layer VI—whose axon is confined to the same layer—and/or from collaterals of corticothalamic axons (Briggs 2010). As intracallosal neurons are fully embedded in callosal fibers, another source of influence could be the callosal fibers themselves.

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