We tested this compound in the well-defined circuitry of the rat brain. In the first experiment, GdDOTA-CTB was injected into primary somatosensory cortex (S1) to test for local MR signal enhancement in well-known thalamic targets of S1: the ventral posterolateral thalamic nucleus (VPL), posterior thalamic nuclear group (Po), and reticular thalamic nucleus (Rt) (Koralek et al., 1988, Kaas and Ebner, 1998, Liu and Jones, 1999 and Paxinos, 2004). MRI scans were performed at systematically varied time points to measure the neuronal uptake and transport dynamics of GdDOTA-CTB.

In a second experiment, we validated the above results by comparison with the immunohistochemical staining of CTB in the same animals that previously received GdDOTA-CTB injections and MRI. In addition, we evaluated Selleckchem Antidiabetic Compound Library the extent of possible tissue disruption at the GdDOTA-CTB injection sites using histology. Third, we demonstrated additional MR enhancement in sites expected in/near the injection

site, including the gray matter, the underlying white matter and local intrinsic connections. In addition, we also found patchy enhancement in the caudate/putaman (CPu) in the regions known to have connections with S1 (Gerfen, 1989, Kincaid and Wilson, 1996 and Hoover et al., 2003). The second and third experiments Epigenetics Compound Library in vivo also investigated the direction of transport and whether or not the GdDOTA-CTB traces connections monosynaptically versus multisynaptically. In a fourth experiment, we compared the intraneuronal

transport rate of GdDOTA-CTB with the extracellular diffusion rate of GdDOTA alone, by injecting the two compounds into comparable locations in S1. To confirm that the tract-tracing properties of GdDOTA-CTB are mediated by active uptake and axonal transport mechanisms, we also performed control experiments using Gd-Albumin, a gadolinium-conjugated serum protein. This compound has a molecular weight comparable to that of GdDOTA-CTB, but without any known tract-tracing properties (Nagaraja et al., 2006 and Astary et al., 2010). Fifth, we compared the transport properties of GdDOTA-CTB with those of the MEMRI, in an otherwise-matched experiment. Finally, we investigated not whether GdDOTA-CTB can reveal neuronal tracts in other regions of the brain, by testing it in the olfactory pathway of rats. Following unilateral GdDOTA-CTB injections into S1, we found target-specific enhancement in the main thalamic nuclei known to be connected with S1, namely VPL, Po, and Rt (Figure 1). This presumptive transport was observed when using multiple types of MR sequences: 2D and 3D T1-weighted (T1-W) and 3D T1- inversion recovery (T1-IR) (see Experimental Procedures and Supplemental Information). Depending on the MR sequence used, different brain regions (e.g.