Having such control over the brain could one day provide a non-invasive treatment for conditions such as Alzheimer’s disease, depression and epilepsy. In the nearer term, quantum dots could be used to treat blindness by reactivating damaged retinal cells.
“Many brain disorders are caused by imbalanced neural activity,” says Lih Lin at the University of Washington, Seattle. “Manipulation of specific neurons could permit the restoration of normal activity levels.”
Methods to stimulate the brain artificially already exist, though each has its drawbacks. Deep brain stimulation is used in Parkinson’s disease to trigger brain cell activity and prevent the abnormal signalling that causes debilitating tremors, but placing the electrodes required is highly invasive. Transcranial magnetic stimulation can stimulate brain cells from
outside the head, but is not highly targeted and so affects large areas of the brain at once. Researchers in optogenetics can control genetically modified brain cells using light but because of these modifications, the technique is not yet deemed safe to use in humans.
Lin’s team has now come up with an alternative using quantum dots â€“ light-sensitive, semiconducting particles just a few nanometres in diameter.
First, they cultivated prostate cancer cells on a film covered with quantum dots. The cell membranes of the cancer cells were positioned next to the dots. The team then shone light onto the nanoparticles.
Energy from the light excites electrons within the quantum dot which causes the surrounding area to become negatively charged (see diagram). This caused some of the cancer cells’ ion channels, which are mediated by a voltage, to open, allowing ions to rush in or out of the cells.
In nerve cells, opening ion channels is a crucial step in generating action potentials â€“ the signals by which the cells communicate in the brain. If the voltage change is large enough, an action potential is generated.
When Lin’s team repeated their experiment with nerve cells, they found that stimulating the quantum dots caused ion channels to open and the nerve cell to fire.
In humans, quantum dots would need to be delivered to brain tissue. Lin claims this shouldn’t be a problem. “A significant advantage is that their surface can be modified with various molecules,” she says. These molecules could be attached to the quantum dots in order to target specific brain cells and could be administered intravenously.
A key hurdle would be delivering the light source to the brain. For this reason, Lin reckons the first use for the technique would be in reactivating damaged cells in the retina, which naturally absorb light. Co-author Fred Reike, who specialises in retinal disease, says that quantum dots have great potential in this area because they directly affect ion channels, which play a key part in the signalling pathways of vision.
“Quantum dots have a great future for biomedical applications,” agrees Kevin Critchley at the University of Leeds, UK, but adds that there are limitations such as potential toxicity issues.
“Based on what we have observed, we are optimistic about the potential of this technology in helping us [answer] biological questions, and eventually diagnose and treat human diseases,” Lin says.