New Research Challenges Understanding of Synapses in Adolescent Brain Development

Adolescence is a pivotal period not just for social development but also for significant changes in brain architecture. While advanced cognitive functions like planning and decision-making become refined, the intricate mechanisms behind these transformations remain complex and not fully understood.

Traditionally, the prevailing view among neuroscientists has been that synapses—the connections that enable neuronal communication—increase in number during childhood and subsequently decline through a process known as "synaptic pruning." This pruning was believed to contribute to various neuropsychiatric disorders, particularly schizophrenia, which is marked by symptoms like hallucinations and disorganized thought patterns.

New findings from a team at Kyushu University, published on January 14 in Science Advances, present a challenging perspective on this long-held belief. Their research indicates that rather than merely eliminating synaptic connections, the adolescent brain actively develops new clusters of synapses in designated areas of neurons.

Professor Takeshi Imai, part of this investigative team, remarks, "We weren't originally focused on brain disorders. After creating a high-resolution tool for synaptic analysis in 2016, our curiosity led us to inspect the mouse cerebral cortex, where we stumbled upon a previously unnoticed high-density area of dendritic spines—the sites where excitatory synapses form."

The study examined the cerebral cortex's Layer 5, a crucial region that integrates sensory information and functions as a key output center. Using their custom tissue-clearing agent SeeDB2 combined with super-resolution microscopy, the researchers effectively mapped dendritic spines throughout these neurons, unveiling a dense concentration of spines in a particular dendritic area, which they labeled as a "hotspot." Notably, this hotspot was not present in early life but became prominent during adolescence.

The research team meticulously tracked spine distribution across different developmental stages. They discovered that in two-week-old mice, dendritic spines were relatively evenly distributed. However, between three and eight weeks—an age range representing the transition from childhood to adolescence—the density of spines surged in specific regions, ultimately leading to the formation of this dense synaptic hotspot.

"This discovery urges us to rethink the well-established hypothesis of 'adolescent synaptic pruning'," said Imai, reinforcing the need for a paradigm shift in understanding this developmental phase.

The implications of these findings extend to understanding the etiology of certain brain disorders. Ryo Egashira, the first author of the study and a graduate student at Kyushu University, noted, "While synaptic pruning occurs across dendrites, our findings indicate that synapse formation is localized and distinctly situated within specific dendritic areas during adolescent development. Disruption of this localized growth may contribute to some schizophrenia types."

To investigate this hypothesis further, the researchers studied mice with mutations in schizophrenia-associated genes such as Setd1a, Hivep2, and Grin1. Their early development appeared stable, showing normal spine density until about two to three weeks after birth. However, during adolescence, a marked reduction in synapse formation was observed, disrupting the development of these synaptic hotspots.

This evolving perspective suggests that schizophrenia might not solely be a result of synaptic loss but could also involve impaired synapse formation during a critical developmental window. However, the researchers caution that their findings, based on murine models, may not directly translate to higher primates or humans, leaving open questions about the universality of these processes.

Looking ahead, Imai's team aims to identify which specific brain regions are involved in forming these new synapses during adolescence. "Understanding where and when these connections develop will enhance our grasp of brain maturation and could elucidate mechanisms underlying neuropsychiatric disorders," he concluded.

Materials provided by Kyushu University. Note: Content may be edited for style and length.

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