March 11 (HealthDay News) -- Scientists believe they may have found the biological trigger to a particular type of migraine headache.

In the March 12 issue of Neuron, an Italian university study on mice found that increased levels of the neurotransmitter glutamate in the brain appear to kick off a cortical spreading depression (CSD), a phenomenon that causes migraine sufferers to experience a visual disturbance known as migraine aura. The mice in the study had a gene mutation that has been shown to make people more susceptible to familial hemiplegic migraine (FHM), a subtype of severe migraine accompanied by the aura.

Brain imaging has previously shown that strong neuronal depolarization creeps across the cerebral cortex during CSD, initially increasing electrical signals in the brain, then suppressing neural activity for an extended period of time. Researchers have suspected CSD may flip on certain switches that start a migraine headache.

The researchers from the University of Padova, led by senior author Daniela Pietrobon, found that calcium influx and subsequent glutamate release at cortical pyramidal cell synapses were greater in mice with the FHM mutation. When the release of glutamate -- the major excitatory neurotransmitter in the brain -- was decreased to normal levels, the mice did not experience aura-inducing CSD.

"Our findings provide direct evidence that enhanced glutamate release may explain the facilitation of CSD in the FHM mouse model," Pietrobon, who works in the university's biomedical sciences department, said in a news release issued by the journal's publisher.

SOURCE: Cell Press, news release, March 11, 2009