Ilgar Mamedov

In complex systems like the brain understanding of connectivity is of paramount importance, as the relationship between parts is often more important than the parts themselves. The aim of my research line is to develop an efficient neuronal tracer that can allow a substantially more specific and comprehensive investigation of the neuronal networks using 1H or 19F MR and Optical imaging techniques. This noninvasive technique is invaluable for longitudinal studies, such as those of plasticity and reorganization, or of neurodegenerative processes. The application of MRI-visible paramagnetic tracers to reveal in vivo connectivity can also provide important subject-specific information for multi-site, multi-electrode intracortical recordings combined with behavioral and physiology experiments widely used in the neuroscience.

Development of multimodal imaging probes for neuroanatomical connectivity and plasticity studies in vivo through of ¹H or ¹⁹F MR and optical imaging.

Introduction and scientific aims

In complex systems like the brain understanding of connectivity is of paramount importance, as the relationship between parts is often more important than the parts themselves. Traditionally, neuronal activity has been studied at different spatio-temporal scales by using methods like intracortical recordings, optical imaging and high-resolution functional magnetic resonance imaging (fMRI). The former techniques can provide no information on large-scale network interactions. Various methods of MRI, on the other hand, including diffusion tensor imaging, measurements of statistical dependence between regions in the resting-state, and graph-theoretical approaches are immensely useful but they too suffer from lack of specificity.

The aim of this research line is to develop an efficient neuronal tracer that can allow a substantially more specific and comprehensive investigation of the neuronal networks using ¹H or ¹⁹F MR and Optical imaging techniques. This noninvasive technique is invaluable for longitudinal studies, such as those of plasticity and reorganization, or of neurodegenerative processes. The application of MRI-visible paramagnetic tracers to reveal in vivo connectivity can also provide important subject-specific information for multi-site, multi-electrode intracortical recordings combined with behavioral and physiology experiments widely used in the neuroscience.

Methods

We developed and applied different classes of neuroanatomical tracers which can be visualized by ¹H or ¹⁹F MRI methods as well as with optical imaging techniques. For the initial studies we conjugated MRI and optical reporters with macromolecules that are extensively applied in neuroanatomical research such as dextran amines or cholera toxin subunit B. We performed in cellulo studies on mouse N18 neuroblastoma cells, which have been incubated with the synthesized tracers under physiological conditions. For our in vivo experiments we directly injected the tracer into the primary motor cortex of the rats (Sprague Dawley). After injection, the animals were scanned at different time points (4d, 6d, 9d and 14d post-injection) to study neuronal transport.

Results and Initial Conclusions

We performed fluorescence microscopy to investigate the cellular uptake and transport properties of the developed tracer. The results demonstrated that our molecules were well internalized well and localized mainly in the cell body of the neuronal cells. Less significant amounts of tracer were also observed also in the neuronal processes and their terminals; they could be the result of the anterograde intracellular transport. On the basis of the cell studies we conclude, that the newly developed tracers were efficiently internalized into neuronal cells.

Our in vivo investigations clearly demonstrated that the internalized molecules reduce T₁ relaxation times of the brain tissue in the regions of interest. Voxel-wise statistical analysis revealed signal enhancement in several well-known subcortical targets of the primary motor cortex, including the dorsolateral thalamus area (Po) and caudate putamen (CPu). Moreover in the image one can observe a cluster of significant voxels that, most likely, correspond to the pyramidal tract as the trace clearly originated from the injection site in M1 and follows toward the CPu region.

Preliminary in vivo experiments on the Dextran based paramagnetic tracers demonstrated the high potential of the method. However, in the future, a broad spectrum of existing neuroanatomical tracers will be used depending on the desired application for given project in neuroscience. Further fine-tuning of the target molecule, injection technique and MRI methods will help to improve in vivo visualization of the tracer.

We are aware that sensitivity of the MR method will never approach that of optical imaging, but it will remain one of the few methods able to achieve the whole brain imaging of living subjects. The high impact of MR-active tracers cannot be overestimated. These molecules will facilitate a variety of new experimental studies on different issues associated with longitudinal research on brain plasticity.

In the context of this and other projects in my group I have been having interaction and collaboration with Oxana Eschenko (Neuromodulation), Henry Evrard (Neuroanotomy), Gisela Hagberg (MRI), Joern Engelmann (Cell Biology), Aneta Keliris (Chemistry)  and Anthony Power (Biochemistry).

References

[1] Mamedov, I., J. Engelmann, O. Eschenko, M. Beyerlein, N.K. Logothetis 'Dual-functional probes towards in vivo studies of brain connectivity and plasticity, ChemComm., 2012, DOI:10.1039/C1CC15991G.

[2] C.W-H. Wu, O. Vasalatiy, N. Liu, H. Wu, S. Cheal, D.-Y. Chen, A.P. Koretsky, G.L. Griffiths, R.B.H. Tootell, L.G. Ungerleider, Neuron, 2011, 70, 229-243;

Figure 1.

Injection of the GdAAZTA-Dextran into the primary motor cortex of the rat. Tracing ability and accumulation in the area of interest (CPu - Caudate putamen, Po- Dorsolateral thalamus).