Our brains are made of billions of neurons, which are connected forming complex networks. They communicate between themselves by sending electrical signals, known as action potentials, and chemical signals, known as neurotransmitters, in a process called synaptic transmission. Chemical neurotransmitters are released from one neuron, diffuse to the others and arrive at the targeted cells, generating a signal which excites, inhibits or modulates the cellular activity. The timing and strength of these signals are crucial for the brain to process and interpret sensory information, make decisions, and generate behavior.
Controlling the connections between the neurons would allow us to understand and treat better neurological disorders, rewire or repair the malfunctions of the neural circuits after being damaged, improve our learning capabilities or expand our set of behaviours. There are several approaches to controlling neuronal activity. One possible method is using drugs, that alter the levels of the chemical neurotransmitters in the brain and affect the activity of neurons. Another approach is to use electrical stimulation applied to specific brain regions to activate or inhibit the neurons. A third possibility is using light to control neural activity.
Using photons to control the neuronal activity
Using light to manipulate neuronal activity is a relatively new technique that has been explored in the past. It involves genetically modifying neurons to express light-sensitive proteins, ion channels, pumps or specific enzymes in the target cells. This technique allows researchers to precisely control the activity of concrete populations of neurons with higher precision. There are, however, some limitations. It needs to be delivered very close to the neurons to achieve enough resolution at the level of the synapsis, as light scatters in the brain tissue. Thus, it is often invasive, requiring external interventions. Moreover, the intensity needed to reach the targeted cells can be potentially harmful to them.
To overcome these challenges, a team of ICFO researchers presents in Nature Methods a system that uses photons instead of chemical neurotransmitters as a strategy to control neuronal activity. The ICFO researchers Montserrat Porta, Adriana Carolina González, Neus Sanfeliu-Cerdán, Shadi Karimi, Nawaphat Malaiwong, Aleksandra Pidde, Luis Felipe Morales and Sara González-Bolívar led by Prof. Michael Krieg together with Pablo Fernández and Cedric Hurth, have developed a method to connect two neurons by using luciferases, light-emitting enzymes, and light-sensitive ion channels.
They have developed and tested a system named PhAST -short for Photons as synaptic transmitters- in the roundworm Caenorhabditis elegans, a model organism widely used to study specific biological processes. Resembling how the bioluminescent animals use photons to communicate, PhAST uses the enzymes luciferases to send photons, instead of chemicals, as transmitters between neurons.
Replacing chemical neurotransmitters with photons
