Certain Nerve Cells Involved in Seizures, Cognitive Deficits in Mice with Dravet Syndrome

Specific nerve cells are critically involved in the development of epileptic seizures, hyperactivity, autistic-like behaviors, and cognitive declines in Dravet syndrome, according to a mouse study from Japan.

The research, “Impairments in social novelty recognition and spatial memory in mice with conditional deletion of Scn1a in parvalbumin-expressing cells,” was published in the journal Neurobiology of Disease.

Dravet syndrome is a rare and severe type of epilepsy characterized by prolonged seizures in the first year of a child’s life. The high number of seizures can cause problems such as cognitive decline, hyperactivity, autistic-like behaviors, impaired spatial learning and memory, and movement and balance problems.

Mutations in one of the two copies of the SCN1A gene are behind the development of Dravet syndrome in 70-80% of patients. These mutations lead to the production of a nonfunctional cell membrane protein, called Nav1.1, that works as a channel to sodium ions — essential for the normal function of certain nerve cells in the brain.

SCN1A mutations in Dravet patients alter the sensitivity of nerve cells to temperature, making warm weather, fevers, or sunlight all triggers for seizures.

Nav1.1 was previously found at high levels in nerve cells with parvalbumin protein in the brains of young mice. Parvalbumin is involved in calcium signaling, which is also key for nerve cell function.

Specifically deleting one copy of the SCN1A gene in parvalbumin-producing nerve cells was shown to cause epileptic seizures and lowered sociability in mice, suggesting these cells could be directly involved in the development of Dravet syndrome.

But how these cells affect the behavior and psychiatric symptoms of Dravet syndrome patients is still poorly understood.

Now, Japanese researchers evaluated and compared the effects of eliminating one SCN1A gene copy in parvalbumin-producing nerve cells or in nerve cells that produce somatostatin — a hormone that controls other hormones in the body — in mice.

A battery of behavioral tests was conducted in male mice (to avoid potential variability due to hormonal fluctuations in the females) after the peak period of epileptic seizures, at the age of 8 weeks.

Researchers found that mice lacking the SCN1A gene in parvalbumin-producing nerve cells had significant behavior and social deficits, along with impaired spatial memory.

These animals showed hyperactive and submissive behavior, defects in olfactory-mediated behavior (taking longer to find buried food), and a lack of social novelty preference — normal mice will investigate a new mouse more so than a familiar one — when compared to healthy mice.

But no noticeable behavior or social abnormalities were observed in mice lacking SCN1A in somatostatin-producing nerve cells.

The results suggest that impaired function of Nav1.1 in parvalbumin-producing nerve cells “is critically involved in the pathogenesis [development] of epileptic seizures, hyperactivity, autistic traits, and cognitive decline in Dravet syndrome,” researchers wrote.

They also noted that because these symptoms were milder than those of mice lacking SCN1A in all cells, other cells, besides those producing parvalbumin, may contribute to Dravet-like symptoms.

“Further studies are required to establish more details of the mechanisms underlying epilepsy and psychiatric conditions in Dravet syndrome,” they added.

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