Brains of amphetamine-treated mice (top) show enhanced expression of c-fos relative to saline-treated mice (bottom). Image courtesy of Drs. Christina Liu and Philip Liu, Massachusetts General Hospital, Boston, Massachusetts.
Which super power would molecular neuroscientists want most? X-ray vision is the likely choice. No current methods allow researchers to measure or localize gene expression inside the brains of live animals. Now Liu et al. report a technique that combines in situ hybridization with magnetic resonance imaging (MRI) in a recent article in the Journal of Neuroscience.
Superparamagnetic iron oxide nanoparticles (SPION) are contrast agents used as reporters in MRI. SPIONs have magnetic properties only while inside magnetic fields, and their small size, approximately one thousand times smaller than most neurons, allows them to incorporate into cells.
Upon activation, many regions of the brain show increased expression of the oncogene c-fos, which, together with c-jun, acts as a transcription factor at activator protein-1 promoter elements in DNA. The authors conjugated biotinylated oligodeoxynucleotides complementary to c-fos mRNA to avidin-modified SPIONs to generate the SPION-cfos complex.
The authors injected fluorescently labeled SPION-cfos into the left cerebral ventricles of live mice. SPION and c-fos oligodeoxynucleotides localized to the same brain regions. In adjacent sections of the ipsilateral hippocampus, the authors found prussian blue staining, indicating the presence of iron, and fluorescence, indicating the presence of c-fos oligodeoxynucleotides. Fluorescent label localized to neuron cell bodies and axons, but not glial fibrillary acidic protein-positive cells, suggesting that neurons, but not glia, absorb SPION-cfos.
In highly sensitive T2-weighted MRI, magnetic resonance signal correlated with c-fos expression. Relative to mice injected with either free SPION or SPION conjugated to nontargeting, random oligodeoxynucleotides, mice injected with SPION-cfos showed increased signal in the somatosensory cortex and striatum within seven hours of injection, which returned to baseline within two days. Expression of 
-actin is higher than that of c-fos when measured by standard in situ hybridization in the mouse brain. Similarly, mice injected with SPION-actin showed increased signal in the contralateral somatosensory cortex relative to mice injected with SPION-cfos, suggesting that magnetic resonance signal generated by this new method was qualitatively similar to standard in situ hybridization data.
Amphetamine treatment induces c-fos expression in the nucleus accumbens, striatum and cingulate. Amphetamine treatment did not alter signal in mice injected with SPION conjugated to a random oligodeoxynucleotide sequence. However, mice treated with SPION-cfos and amphetamine showed increased signal particularly in the nucleus accumbens, but also in the striatum, cingulate, and prelimbic and infralimbic cortices, relative to mice treated with SPION-cfos and saline.
Therefore, the authors' patent-pending technique localized gene expression in the brains of live mice. Similar to the current use of MRI techniques to diagnose strokes and lesions, perhaps one day this method will be used to diagnose psychiatric and neurological disorders in humans.