13 June 2005: Scientists FRET over neurotransmitter measurement

Molecular biologists have developed a biological nanosensor that can accurately measure levels of the neurotransmitter glutamate in live nerve cells.

Glutamate is a prevalent neurotransmitter in the mammalian brain and regulates a wide range of behaviours, including learning, memory, consciousness and mood. Despite its ubiquity, however, scientists have lacked a method for accurately measuring levels of glutamate in living tissue, which has hampered their understanding of how glutamate levels change over time.

Researchers at the Carnegie Institution, Stanford, US, and Stanford University addressed the problem with fluorescence resonance energy transfer (FRET). This involves attaching two different coloured fluorescent proteins to a type of protein known as a periplasmic binding protein, which binds to various substrate proteins. When a specific periplasmic binding protein binds to a substrate protein it undergoes a structural change, which alters the relative intensity of the light given off by the two attached fluorescent proteins. Thus, the presence of the bound protein is revealed by a change in the colour of the fluorescent light.

Adapting the FRET technology to detect glutamate, the team, led by Sakiko Okumoto at the Carnegie Institution, linked cyan and yellow fluorescent proteins to a bacterial periplasmic binding protein called ybeJ, which previous research had shown binds to glutamate. The researchers expressed this protein construct on the surface of nerve cells extracted from a region of the rat brain known as the hippocampus, an area linked to memory storage.

The researchers found that when they exposed these nerve cells to glutamate, the light emitted by the protein constructs would change colour, with the degree of colour change dependent on the concentration of glutamate. When the researchers triggered glutamate release by the nerve cells - by stimulating them electrically or chemically - the light also changed colour.

’Understanding when and how glutamate is produced, secreted, reabsorbed and metabolised in individual brain cells, in real time, will help researchers better understand disease processes and construct new drugs,’ predicts Okumoto. Jon Evans