Sensors

Protein-based optical sensors have been developed for membrane voltage, intracellular calcium concentration, neurotransmitter release, and for a number of second messengers, for example cAMP. Sensors are based on a green fluorescent protein (GFP) chromophore. In one guise fusion proteins are created between GFP and another protein which is able to report some biochemical change. For example, a GFP–Shaker potassium channel fusion protein is able to signal voltage-dependent conformational changes in the channel as alterations in GFP fluorescence. Light emission from GFP derivatives is modulated either by:

- Protonation/deprotonation which is in turn affected by the conformational state of the protein that controls access of the chromophore to the solvent;

- Variations in the electrical properties of neighboring chromophores brought about by changes in their proximity or orientation to each other

To detect rapid events it is necessary to sample at a sufficiently high rate. Thus, to detect individual action potentials lasting one millisecond it is necessary to sample with a frequency greater than 1 kHz if all are to be captured. Short events produce fewer photons and so are harder to see.  Increasing the expression of the sensor is often not the solution because the presence of sensor disturbs the variable under study. For example, the voltage  sensors needed to  detect action potentials change the  capacitance of the  cell membrane in which they are expressed, and  this suppresses synaptic potentials thereby altering the behavior of the neurons under study.