In a feat of miniaturization, researchers have engineered a fluorescence microscope that is three inches long, weighs less than an ounce, and sits perched on a rat's head. Fritjof Helmchen, Winfried Denk, and colleagues at Bell Laboratories in Murray Hill, New Jersey, report in tomorrow's Neuron that their camera can image cortical neurons with subcellular resolution in freely moving animals.
Previous research has used similar technology to visualize blood vessels, neuronal dendrites-even amyloid deposition-in anesthetized mice that were mounted onto a fixed two-or multiphoton microscope (Svoboda et al., 1999; Kimchi et al., 2001). This work pushes the technological envelope by fitting all the necessary technology into a tiny device that awake, behaving rats can carry around while moving about the cage.
Dubbed fiberscope, the device is tethered to a pulsed laser by an optical fiber that carries both the excitation light to the brain and the output signal back to a computer housing control panels and imaging software. Attached surgically to the rat's head, the fiberscope has a tiny, motor-driven objective that focuses the incoming light into the neocortex and a fiber tip that scans the imaged area by a resonance vibration mechanism.
The fiberscope can image blood vessels, capillaries, blood cell movements, and dendrites, though its depth penetration and resolution cannot yet match that of the fixed two-photon microscope, the authors report. The image remained stable while the rats rested and chewed, and fairly stable while they turned and walked steadily. Sudden jerks of the head, however, disturbed the image for a while before it stabilized again.
The present prototype fiberscope should be improved, the scientists write, by making it even smaller so a mouse can carry it. Moreover, the technology currently is limited by the need to label individual neurons with micropipettes and then having to search for those dye-filled cells. It will soon be possible, however, to label populations of cells with genetically encoded functional probes expressing green fluorescent protein, the authors write.
For now, immediate applications for this technology lie in the area of studying dendritic integration of synaptic inputs, and how dendritic activity changes during sleep, wakefulness, and other behavioral states. In the future, Alzheimer's researchers might want to image plaque or tangle formation, or dendritic activity in animal models of this disease.—Gabrielle Strobel
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- Svoboda K, Helmchen F, Denk W, Tank DW. Spread of dendritic excitation in layer 2/3 pyramidal neurons in rat barrel cortex in vivo. Nat Neurosci. 1999 Jan;2(1):65-73. PubMed.
- Kimchi EY, Kajdasz S, Bacskai BJ, Hyman BT. Analysis of cerebral amyloid angiopathy in a transgenic mouse model of Alzheimer disease using in vivo multiphoton microscopy. J Neuropathol Exp Neurol. 2001 Mar;60(3):274-9. PubMed.
- Helmchen F, Fee MS, Tank DW, Denk W. A miniature head-mounted two-photon microscope. high-resolution brain imaging in freely moving animals. Neuron. 2001 Sep 27;31(6):903-12. PubMed.