First tadpole model of autism surfaces at conference
Researchers have recreated one of the most popular rodent models of autism in a decidedly slimier animal: the tadpole. The unpublished study was presented Sunday at the 2013 Society for Neuroscience annual meeting in San Diego.
For many years, scientists have known that pregnant women who take valproic acid (VPA), a common epilepsy drug, have an increased risk of having a child with autism. Mouse and rat models of VPA exposure show several features of autism, including impaired social interactions, repetitive behaviors and abnormal brain waves.
These rodents are by far the most popular environmental models of autism, but little is known about how, exactly, VPA harms the developing brain.
“In the rodent models, you have all of these really complex behaviors you’re trying to decipher,” says Eric James, a graduate student in Carlos Aizenman’s laboratory at Brown University in Providence, Rhode Island, who presented the work. The tadpole, in contrast, has more straightforward behaviors and well-defined neural circuits.
“It’s a really simple organism with a simple brain that we can study from the network level all the way down to a single cell,” James says.
Neuroscientists have used Xenopus laevis, the African clawed frog, for many decades because the frogs lay lots of eggs — about 30 to 100 at once. Plus, the eggs develop outside of the body, which is useful for studying prenatal exposures.
“The embryo’s just floating around in the dish, so I can easily manipulate it,” James says.
The same is true of zebrafish, which have also been used to model autism, but tadpole brains and neurons are much larger, making it easier to record from them. “The [tadpole] brain is about one-third of a fingernail,” he says.
The Xenopus model is particularly useful for studying the visual system because its visual circuits have been thoroughly mapped.
James and his colleagues exposed 7-day-old Xenopus embryos to VPA and continued the exposure for another seven to ten days.
The VPA-exposed tadpoles show several abnormal behaviors compared with controls, all of which indicate that they are abnormally sensitive to sensory stimuli and their brain circuits are overly excitable. They have many seizures, for example, which are easily spotted when the animals’ tails curl into a C-shape.
This is intriguing because an imbalance in excitatory signals in sensory brain regions has been proposed as a unifying theory of autism.
In one measure of their differences, a hammer taps against the tadpoles’ culture dish. Normal tadpoles initially startle, but then habituate to the incessant beats. But the VPA-exposed tadpoles never get used to it.
Another task measures ‘avoidance behavior’ in the tadpoles by projecting a moving black dot underneath the culture dish and measuring how close the dot gets before the tadpole tries to escape from it. “The VPA-exposed animals escape much sooner than the others,” James says.
The researchers even looked at what might be considered social behaviors of the tadpoles by measuring how closely they gather together in the dish. Whereas normal tadpoles tend to form small groups, the VPA-exposed animals scatter themselves across their small pond.
All of these behaviors make sense, James says, based on the patterns he found in their brain circuits. By recording from single neurons in the tectum, the region that processes visual information, he found that VPA-exposed tadpoles show an increase in the frequency and duration of spontaneous excitatory signals at the synapse, or junction between neurons. They also have increased network connectivity across the tectum.
The neurons of the exposed cells also have many more branches than controls, which could explain the excess connectivity, he says.
The excess branches may be the result of a lack of synaptic pruning during early development, James says. And this pruning problem, in turn, may stem from an over-production of certain proteins at the synapse.
He plans to use morpholinos — synthetic molecules that curb the expression of specific genes — to knock down certain synaptic proteins and potentially rescue some of the tadpoles’ abnormalities.
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