'Functional' neuroanatomical tract tracing: analysis of changes in gene expression of brain circuits of interest.

Neuroanatomical tracing when considered as an isolated method produces relatively straightforward answers. Although single-, double- or even triple-tracing paradigms produce valuable data on the organization of brain circuits, the final outcome often is too simplistic since it is not possible to elucidate the activity of these circuits. In this regard, emerging technologies contribute with additional information about the status of neuronal circuits. The laser-guided capture microdissection microscope (LCM) allows the accurate dissection of small brain areas under the microscope that could be further analyzed for gene expression or proteomics. In order to elucidate the gene expression of a given circuit of interest, we have developed a combination of methods comprising (i) fluorescent non-radioactive in situ hybridization for the detection of vGLUT2 mRNA expression combined with retrograde tracing with Fluoro-Gold (FG; analysis performed under the confocal microscope) and (ii) laser-guided capture microdissection of brain areas containing neurons retrogradely labeled with FG followed by the measurement of changes in mRNA levels encoding for vGLUT2 by real-time PCR. Our goal was to detect changes in gene expression of the thalamostriatal pathway in unilaterally 6-OHDA lesioned rats. Taking advantage of this procedure, we found a three-fold increase in vGLUT2 mRNA expression within thalamic neurons projecting to the dopamine-depleted striatum when compared with the activity of the thalamic neurons innervating the control striatum.