IMAGING OF RADIOCARBON LABELED TRACER MOLECULES IN NEURAL TISSUE USING ACCELERATOR MASS-SPECTROMETRY

AUTORADIOGRAPHY is widely and successfully used to image the distribut ion of radiolabelled tracer molecules in biological samples. The metho d is, however, limited in resolution and sensitivity, especially for C -14. Here we describe a new method for imaging C-14-labelled tracers i n sections of biological tissue. A highly focused beam of gallium ions bombards the tissue, which is eroded (sputtered) into constituent ato ms, molecules and secondary ions. The C-14 ions are detected in the se condary beam by the most sensitive method available, namely accelerato r mass spectrometry(1). The specimen is scanned pixel by pixel (1 x 2 mu m), generating an image in a manner analogous to scanning electron microscopy. The method can thus be regarded as a specialized form of s canning secondary ion mass spectrometry (SIMS), referred to here as SI AMS (ref. 2). We have used SIAMS to localize the neurotransmitter gamm a-aminobutyric acid (GABA) in thin sections of cerebral cortex, and sh ow that it can generate C-14 images that are much improved on C-14 aut oradiography. A scan takes 10-20 min and reveals individual axons, neu rons and glial cells at high sensitivity. In principle, the resolution could be increased by up to tenfold, and the method could be extended to some other nuclides.