The Teen Brain Builds Synapse Hotspots Scientists Never Saw Before

The teenage brain isn’t just trimming connections—it’s secretly building powerful new neural hotspots that may shape the mind for life.

Adolescence is a major turning point not only for social and physical development, but also for how the brain works. During these years, mental abilities such as planning ahead, solving problems, and making complex decisions continue to take shape. Despite the importance of this stage, scientists still do not fully understand how brain circuits are built and refined during this time.

At the center of this process are synapses, the connections that allow neurons to communicate with one another. For many years, researchers believed that the number of synapses rises throughout childhood and then declines during adolescence. This idea led to the theory that excessive “synaptic pruning,” the removal of unused or weaker connections, could contribute to neuropsychiatric conditions. Schizophrenia, which can involve hallucinations, delusions, and disorganized thinking, has often been cited as one possible outcome of this process.

A Discovery That Challenges Long-Held Assumptions

Scientists at Kyushu University have now uncovered evidence that complicates this traditional view. In a study published in Science Advances on January 14, the research team found that adolescence is not only a time of synapse loss. The brain also creates new, tightly packed clusters of synapses in specific parts of neurons during this developmental window.

“We did not set out to study brain disorders,” says Professor Takeshi Imai at Kyushu University’s Faculty of Medical Sciences. “After developing a high-resolution tool for synaptic analysis in 2016, we looked at the mouse cerebral cortex out of curiosity. Beyond seeing the beauty of the neuronal structure, we were surprised to discover a previously unknown high-density hotspot of dendritic spines, the tiny protrusions in dendrites where excitatory synapses are formed.”

Focusing on a Critical Layer of the Cortex

The cerebral cortex contains six layers, each playing a role in forming complex brain networks. Imai’s team focused their attention on Layer 5 neurons, which collect information from multiple sources and send signals outward as the cortex’s final output. Because of this role, these neurons act as a crucial control point for brain processing.

To examine these cells in detail, the researchers used SeeDB2, the tissue clearing agent Imai’s team developed, along with super-resolution microscopy. This approach allowed them to study transparent brain samples and track dendritic spines throughout entire Layer 5 neurons in unprecedented detail.

Synapse Hotspots Appear During Adolescence

The analysis revealed something unexpected. A small section of the neuron’s apical dendrite contained an unusually dense concentration of dendritic spines, forming what the researchers describe as a “hotspot.” Further investigation showed that this hotspot does not exist early in life and instead emerges during adolescence.

By tracking mice across different developmental stages, the team pinpointed when this change occurs. In two-week-old mice, before weaning, dendritic spines were spread fairly evenly across the neuron. Between three and eight weeks of age, a period roughly corresponding to early childhood through adolescence, spine density increased sharply in one specific region of the apical dendrite. Over time, this localized growth produced the dense synapse hotspot.

“These findings suggest that the well-established ‘adolescent synaptic pruning’ hypothesis needs to be reconsidered,” says Imai.

Implications for Schizophrenia Research

The findings may also shed new light on schizophrenia. “While synaptic pruning occurs broadly across dendrites, synapse formation also takes place in specific dendritic compartments during adolescent cortical development. Disruption of this process may be the key factor in at least some types of schizophrenia,” says Ryo Egashira, the study’s first author and a graduate student at Kyushu University’s Graduate School of Medical Sciences, when the research was conducted.

To test this idea, the researchers studied mice with mutations in genes associated with schizophrenia, including Setd1a, Hivep2, and Grin1. Early development appeared normal, with typical spine density up to two or three weeks after birth. During adolescence, however, synapse formation was significantly reduced. As a result, the characteristic hotspot failed to form.

For decades, schizophrenia has been linked mainly to excessive synaptic pruning. This study offers a different perspective, suggesting that problems with building new synapses during adolescence may play a critical role. Still, the researchers caution that their work focused only on mice, and it is not yet known whether the same mechanisms apply to primates or humans.

What Comes Next

“Moving forward, we hope to identify which brain regions are forming these new synaptic connections during adolescence,” says Imai. “That will tell us what circuits are actually being built during this developmental window. Understanding how and when these connections form can advance our knowledge of both brain development and the mechanisms underlying neuropsychiatric disorders.”

Reference: “Dendritic compartment-specific spine formation in layer 5 neurons underlies cortical circuit maturation during adolescence” by Ryo Egashira, Meng-Tsen Ke, Nao Nakagawa-Tamagawa, Satoshi Fujimoto, Shigenori Inagaki, Tsuyoshi Takagi, Tsuyoshi Miyakawa, Yoshiaki Tagawa and Takeshi Imai, 14 January 2026, Science Advances.
DOI: 10.1126/sciadv.adw8458

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