Lithium has been a cornerstone treatment for bipolar disorder for more than half a century, praised for its ability to steady mood when life feels like it’s swinging wildly between extremes. Yet one question has stubbornly lingered: what is lithium actually doing inside the brain? An article published in Nature by Palmos et al. proposed an intriguing answer…

Lithium Increases Hippocampal Volume

A growing body of brain-imaging research has pointed to a tantalizing clue. People with bipolar disorder who take lithium long-term often show larger hippocampal volumes than patients who are unmedicated or on different treatments.

The hippocampus—a seahorse-shaped structure tucked deep in the brain—is crucial for memory, learning, and mood regulation. If lithium is linked to a “bigger” hippocampus, it may be changing brain tissue in a meaningful way.

But bigger doesn’t necessarily mean better. And it doesn’t explain how the change happens. One possibility is that lithium protects existing cells from damage. Another, more dramatic idea is that lithium encourages the hippocampus to grow new ones.

That brings us to neurogenesis: the process of generating new neurons. For a long time, scientists assumed adults couldn’t make new brain cells. We now know that’s not entirely true.

One region—the dentate gyrus, part of the hippocampus—appears to keep a slow “trickle” of new neurons going even in adulthood. Estimates suggest hundreds of new neurons may be added each day in the human dentate gyrus. The pace can be influenced by experience, stress, genetics, and medication.

How Could Lithium Support Neurogenesis?

Animal studies have hinted for years that lithium might be neurogenic. The missing link has been human evidence: does lithium directly push human hippocampal cells toward becoming new neurons?

The study you shared takes a bold step toward answering that question by moving the investigation into a human-based lab model. Researchers worked with human hippocampal progenitor cells—think of them as “starter” cells that haven’t chosen their final identity yet. Under the right conditions, they can either keep dividing or differentiate, maturing into specialized brain cells like neurons or glia.

The team exposed these cells to lithium over an extended period (about two weeks in culture—long enough to count as “chronic” exposure in a lab setting). They tested a lower dose similar to what’s typically found in patients’ bloodstreams and a higher dose commonly used in laboratory research to reveal biological effects.

Then they looked for telltale molecular “name tags” on the cells. These tags are proteins that mark where a cell is in its developmental journey:

• DCX (doublecortin) flags neuroblasts—young cells on the path to becoming neurons.
• MAP2 marks more mature neurons.
• S100β marks glia, the support cells that help neurons survive and function.

The results were striking: high-dose lithium increased the proportion of cells becoming neuroblasts, neurons, and glia. In other words, lithium didn’t just keep these starter cells alive—it nudged more of them toward becoming the brain’s working components.

Just as importantly, lithium did not increase markers of cell death, suggesting the effect wasn’t simply a stress response or toxicity. And it didn’t clearly boost the most direct marker of rapid cell division compared with untreated cells. The central shift seemed to be about maturation, not just multiplying cell numbers.

To understand what might be driving that shift, the researchers also measured gene activity using RNA sequencing, a technique that reads which genes are switched on or off across the whole genome.

High-dose lithium altered the activity of hundreds of genes, with patterns pointing to neuronal development and the extracellular matrix—the connective “scaffolding” surrounding cells that helps guide how neurons grow, extend branches, and wire into networks.

That scaffolding matters because neurogenesis isn’t just about making new cells. For a new neuron to become useful, it has to develop and connect—sending out dendrite-like branches, forming synapses, and integrating into existing circuits.

The most intriguing piece of the study is how it tries to connect lab findings to real-world brain anatomy. The researchers compared lithium-responsive genes in their cell model to genes identified in large genetic studies of hippocampal structure.

They found that genes turned up by lithium overlapped specifically with genes linked to the size of the molecular layer of the dentate gyrus—a layer known for dense dendritic branching and connectivity, especially relevant for newly formed neurons.

Conclusion

Taken together, the findings paint a coherent picture: lithium may help the hippocampus not only by preserving tissue, but by encouraging the development of new neurons and glia, and by shifting gene programs tied to the architecture of the dentate gyrus.

This doesn’t prove that lithium makes new neurons grow in the living human brain in exactly the same way it does in a dish. A lab model can’t fully recreate the complexity of a real brain—blood supply, immune signalling, and neural circuitry all matter. And the strongest effects appeared at a higher experimental dose.

But the study does something the field has long needed: it provides direct evidence in human hippocampal cells that lithium can push the machinery of neurogenesis forward, while also linking that molecular signature to human brain structure.

For a drug that has spent decades saving lives while keeping some of its secrets, that’s a meaningful step toward understanding what “mood stabilization” might look like at the level of brain growth and repair.