Rats With (Part) Human Brains

Long-time Slashdot reader mspohr shares a report from the Boston Globe's health-news site STAT:
The scientist flicked on a laser, filling the rat's brain with blue light. The rodent, true to its past two weeks of training, scampered across its glass box to a tiny spout, where it was duly rewarded with a drink of water. From the outside, this would appear to be a pretty run-of-the-mill neuroscience experiment, except for the fact that the neurons directing the rat to its thirst-quenching reward didn't contain any rat DNA. Instead, they came from a human "mini-brain" — a ball of human tissue called an organoid — that researchers at Stanford University School of Medicine had grown in a lab and implanted in the rodent's cortex months before.

The experiment — part of a study published Wednesday in Nature — is the first describing human neurons influencing another species' behavior. The study also showed that signals could go the other way; tendrils of human neurons mingled with the rodent brain cells and fired in response to air rustling the rats' whiskers.

The advance opens the door to using such human-rodent chimeras to better understand how the human brain develops and what goes wrong in neurological and psychiatric conditions such as schizophrenia, autism, and epilepsy. When the Stanford scientists implanted organoids grown from the cells of patients with a severe genetic brain disorder, they could watch the neurons develop abnormally with unprecedented clarity.

"This paper really pushes the envelope," said neuroscientist Tomasz Nowakowski, of the University of California, San Francisco, who uses brain organoids in his research on neurodevelopmental disorders but was not involved in the new work. "The field is desperate for more experimental models. And what's really important about this study is it demonstrates that brain organoids can complete their maturation trajectory when transplanted. So it really expands our toolkit for asking more nuanced questions about how genetic mutations lead to behavioral disorders."

It's an example of how stem cells have revolutionized brain research. By "doing their experiments in very young rats whose cortexes are not yet saturated with synapses," the article points out, the researchers "found that the human neurons easily integrated into the animals' rapidly expanding circuitry, which provided them with the stimulation they needed to push past previous developmental barriers."

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