There Is a Unique Hormone That Grows Memories [Reading Science]

The Moment the Signal for "Growth" Becomes Memory in the Hippocampus

Where are memories formed? We often think of memory as a matter of connections between neurons, that is, synapses. The prevailing explanation is that repeated learning strengthens these connections, resulting in memory retention. However, for a long time, there has been no clear answer as to what signals trigger the very moment when those connections begin to change.

The answer recently proposed by a Korean research team is an unexpected substance: growth hormone, which is known to regulate height and the growth of body tissues. For the first time, it has been identified that growth hormone is rapidly produced in the hippocampus at the moment of learning, helping memory storage cells to mature.

The research team led by Bong-Kyun Kang, head of the Center for Cognition and Sociality at the Institute for Basic Science (IBS), published their findings in the international journal Science Advances. The study demonstrates that growth hormone is essential for the maturation of memory storage cells (engram cells) in the hippocampus. The title of the paper is "Growth hormone is required for hippocampal engram cell maturation."

Maturation of Memory Storage Cells by Growth Hormone (GH). When growth hormone (GH) increases in memory storage cells (engram cells) immediately after learning, presynaptic strengthening occurs in neurons responsible for memory, enhancing their ability to send signals between neurons. Along with this, the number of synaptic spines increases, enabling the neurons to function as memory storage cells (engram cells). Conversely, expressing a mutant that inhibits the action of growth hormone (DN-GH) prevents these synaptic changes, resulting in no conversion to memory storage cells and no memory recall. The lower part of the figure shows that neural signal transmission is enhanced in memory storage cells only when growth hormone is active. Provided by IBS

When Is Memory Determined? The Crucial Hours Right After Learning

Memories are stored in a subset of neurons that are activated during a specific experience. These cells subsequently undergo structural and functional changes, maturing into "memory storage cells," and through this process, memories become retrievable. The problem has been that it was unclear when this maturation begins and what regulates it.

The researchers focused on proteins newly synthesized during memory formation. Proteins are substances that directly drive changes in the structure and function of neurons, and the timing of their production can determine the success or failure of memory formation.

In experiments with mice, the researchers inhibited protein synthesis at different times relative to the learning event. They found that when protein synthesis was blocked after learning was complete, memory recall was maintained. However, when protein synthesis was inhibited during the initial stage, including the moment learning occurred, memory formation was almost completely prevented.

The differences were also clear in memory storage cells. Under normal conditions, synaptic changes that strengthen the connections between neurons occurred after learning. However, when protein synthesis was blocked in the early stage, these changes did not occur at all. The fate of memory was thus determined within a very brief time window immediately after learning.

Photo to aid understanding of the article. Provided by Pixabay

The "Height Hormone" Creates Memories

The researchers discovered that among the proteins synthesized during this early stage, growth hormone rapidly increased in the hippocampus immediately after learning. Notably, growth hormone appeared selectively in memory storage cells that were activated during learning, rather than in random neurons.

To directly verify the role of growth hormone, the team expressed a mutant that blocks growth hormone signaling in hippocampal neurons. As with inhibiting protein synthesis, this prevented synaptic strengthening in memory storage cells and reduced behaviors associated with recalling learned information.

Conversely, when growth hormone was administered externally in conditions where memory formation was suppressed, some of the structural and electrophysiological properties of memory storage cells were restored, and memory recall behaviors reappeared. This proves that growth hormone is not merely an incidental signal in memory formation, but a key factor that directly regulates the maturation of memory storage cells.

The Framework of Memory Research Has Expanded

The significance of this study is not simply in identifying a new molecule. Until now, memory research has focused on neurotransmitters and synaptic changes, while hormones have been treated mainly as regulators of bodily functions.

Bong-Kyun Kang explained, "This study shows that growth hormone, known as a regulator of physical growth, can directly control the functional maturation of memory storage cells," adding, "It provides an opportunity to broaden the neuroscientific framework for understanding memory formation."

Going forward, the researchers plan to investigate the signaling pathways through which growth hormone induces synaptic changes within cells, as well as to identify other regulatory factors that work together during the early stages of memory formation. Memory is no longer just a "story of synapses." As the signal for growth is converted into memory, the complex links are being revealed one by one.