To carry out their search for filopodia, the trio used a sensitive microscopy technique called eMAP.They studied 2,234 synapses between cortical nerve cells of a type called pyramidal neurons, which have thousands of synapses each.Peering through an emap microscope is enough to determine which cellular protrusions are filopodia.But it cannot show which synapses on them are silent.To do that, they needed to test how the filopodia responded to glutamate, the brain's main excitatory neurotransmitter.First, they had to deliver a controlled flow of glutamate to the particular synapse they wanted to test.To this end, they poured a soup of "caged" glutamate over the neuron under examination.This form of the molecule is inert until hit with energy from the intersection of two laser beams.Aiming those at the synapse under study enabled them to uncage the neurotransmitter and see, by measuring the electrical activity in that part of the neuron using an ultrafine electrode, whether the synapse responded.They found that mature pyramidal-neuron protrusions generated electrical activity when exposed to glutamate, as expected.Filopodia did not, confirming the silence of their synapses.Silent synapses are, however, useless unless they can be switched on at the appropriate moment.And the researchers confirmed this is possible.They were able to induce the silent versions on filopodia to turn into mature, active synapses by pairing the simulated release of glutamate with a subsequent surge of electricity inside the neuron.
