A new way of delivering drugs to the brain has been developed by scientists at the University of Oxford.
They used the body's own transporters - exosomes - to deliver drugs in an experiment on mice.
The authors say the study, in Nature Biotechnology, could be vital for treating diseases such as Alzheimer's, Parkinson's and Muscular Dystrophy.
The Alzheimer's Society said the study was "exciting" and could lead to more effective treatments.
One of the medical challenges with diseases of the brain is getting any treatment to cross the blood-brain barrier.
The barrier exists to protect the brain, preventing bacteria from crossing over from the blood, while letting oxygen through.
However, this has also produced problems for medicine, as drugs can also be blocked.
In this study the researchers used exosomes to cross that barrier.
Exosomes are like the body's own fleet of incredibly small vans, transporting materials between cells.
The team at Oxford harvested exosomes from mouse dentritic cells, part of the immune system, which naturally produce large numbers of exosomes.
They then fused the exosomes with targeting proteins from the rabies virus, which binds to acetylcholine receptors in brain cells, so the exosome would target the brain.
They filled the exosomes with a piece of genetic code, siRNA, and injected them back into the mice.
The siRNA was delivered to the brain cells and turned off a gene, BACE1, which is involved in Alzheimer's disease.
The authors reported a 60% reduction in the gene's activity.
"These are dramatic and exciting results" said the lead researcher Dr Matthew Wood.
"This is the first time this natural system has been exploited for drug delivery."
The research group believes that the method could modified to treat other conditions and other parts of the body.
Dr Wood said: "We are working on sending exosomes to muscle, but you can envisage targeting any tissue.
"It can also be made specific by changing the drug used."
The researchers are now going to test the treatment on mice with Alzheimer's disease to see if their condition changes.
The team expect to begin trials in human patients within five years.
Dr Susanne Sorensen, head of research at the Alzheimer's Society, said: "In this exciting study, researchers may have overcome a major barrier to the delivery of potential new drugs for many neurological diseases including Alzheimer's.
She said the blood-brain barrier had been an "enormous issue as many potential drugs have not been properly tested because you couldn't get enough of them into the brain."
She added: "If this delivery method proves safe in humans, then we may see more effective drugs being made available for people with Alzheimer's in the future."
Dr Simon Ridley, head of research at Alzheimer's Research UK, said: "This is innovative research, but at such an early stage it's still a long way from becoming a treatment for patients.
"Designing drugs that cross the blood brain barrier is a key goal of research that holds the promise of improving the effectiveness of Alzheimer's treatments in the future."
Exosomes may have other medical applications.
Alexander Seifalian, a professor of nanotechnology and regenerative medicine at University College London, told the BBC: "Experimental evidence indicates that exosomes can prime the immune system to recognize and destroy cancer cells, making them a potential tool as cancer vaccines."
He also said exosomes "could well form the cornerstone of nanoscale drug delivery systems of the future."
He added: "The apparent versatility and established biosafety of exosomes underscores the potential of these biological membrane vesicles to be of tremendous potential in the realm of nanotechnology and regenerative medicine."