Although many things have gone wrong in the autistic brain, scientists have recently been focusing on one of the most glaring: a surplus of connections or synapses.

Neuroscientists reported that, at least in lab mice, a drug that restores the healthy “synaptic pruning” that normally occurs during brain development also reverses autistic-like behaviours such as avoiding social interaction.

“We were able to treat mice after the disease had appeared,” neurobiologist David Sulzer of Columbia University Medical Centre, who led the study published in the journal Neuron, said. That suggests the disease could one day be treated in teenagers and adults “though there is a lot of work to be done”, he said.

A synapse is where one neuron communicates with another, forming functional circuits.

With too many synapses, a “brain region that should be talking only to a select number of other regions is receiving irrelevant information from many others”, Ralph-Axel Müller of San Diego State University said. He has done pioneering work in overconnectivity and was not involved in the Neuron study, which he deemed “extremely exciting”.

For the new study, Columbia’s Guomei Tang painstakingly counted synapses in a key region of the cortex of 26 children with autism who had died from other causes and compared that to 22 healthy brains also donated to science.

In the autistic brains, synaptic density was more than 50 per cent higher than that in healthy brains and sometimes two-thirds greater.

It is not clear if too many synapses are the main reason for autism, but many genes linked to autism play a role in synapse pruning. And the discovery that synapse pruning reversed autistic behaviour in the lab mice suggests overconnectivity may be key.

Sulzer’s team used rapamycin, an immunosuppressant drug that prevents organ rejection and is sold by Pfizer as Rapamune. But even if the findings are confirmed − and Sulzer notes that treatments that work in lab animals often fail in people − it is unlikely that rapamycin would be used in people with autism: Its wide-scale immune-suppressing effects would likely cause serious side effects.