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Abstract

In order to understand the functional activity in the brain, a detailed knowledge of anatomical connections between different brain regions is of great importance. Unfortunately the requirement for highly invasive technologies precludes most studies in living subjects. In spite of the rapid progress in the field of animal research combined with the neuroimaging technology, currently applied techniques demonstrate the absence of a completive method which can be used for such studies. In contrast to conventional techniques, volume imaging with MRI-visible neuronal tracers provides a complete description of large-scale three-dimensional (3-D) networks. The recent attempts to conjugate the commonly used paramagnetic complex Gd-DOTA with classical neuroanatomical tracers such as cholera toxin subunit B (CTB) or biocytin demonstrated the high potential of such systems to act in noninvasive connectivity studies.1,2 Here we have developed efficient neuronal tracers that can allow a more complete investigation of the neuronal networks using MR and optical imaging techniques. Modified mono- or bis- Gd-AAZTA3,4 complexes acting as a MR reporter and tetramethylrhodamine as optical reporter were conjugated to Dextran (MW 10,000) which is used extensively in neuroanatomical research (Fig.1 up). This enables the investigation of neuroanatomical connectivity in the brain by both MR and optical imaging. Fluorescence microscopy of neuronal cells incubated in vitro with different concentrations of the tracer molecules clearly demonstrated their effective internalization and localization mainly within the cell body of the cells but also transport along the cellular processes. In vivo injection into the motor cortex (M1) of rat demonstrate signal enhancement in several well-known subcortical targets of M1, including the somatosensory cortex (S1) and Caudate putamen (CPu) confirming the high potential of the new contrast agents and their applicability for in vivo connectivity studies (Fig.1 down). Such systems can enable a diversity of new experimental studies on various neuroscientific issues associated with longitudinal research on brain plasticity and neurode- or regeneration.