Human neurons are transplanted into the brains of rats

They report today (Oct.
12) in the journal Nature that researchers have successfully transplanted human neurons into the brains of young mice for
the first time.
Human cells form connections with rat neurons and can be used to control rat behavior
.

To experiment with human cells, especially when studying the effects of certain drugs, scientists have developed a model called organoids: tiny structures that grow from stem cells that mimic the human brain or other organs
.
However, they themselves cannot replicate the complexities of
neuronal development in real humans.

To further develop organoids, the scientists transplanted human brain organoids into the brains of few-day-old mice, whose immune systems were suppressed to prevent rejection of human cells
.
By using young rats instead of adult rats, the researchers hope that the cells will fuse together more smoothly and form more connections
in the rat brain.
In fact, they report that human neurons mature successfully, are six times larger than neurons grown in a petri dish, and form connections
to rat neurons through synapses.
Lead author Sergiu Pasca, a neuroscientist at Stanford University, said: "These cells have just entered another stage
of maturity.

To test whether transplanted human neurons affected the behavior of young mice, Pasca and other researchers genetically modified human cells using a technique called optogenetics, which allows cells to respond to
light.
Then, when they fed the rats with water, they used a fiber-optic cable to emit blue light
on human neurons integrated with the rats.
Rats quickly associate light with water: after just two weeks, they start licking the water, expecting to drink when
the light hits.

The study raises ethical questions
about the treatment of animals.

Taimie Bryant, professor of animal law at UCLA, said: "I don't think it makes moral sense
to treat animals as resources that humans can use.
In my opinion, the consciousness of rats is quite remarkable without human manipulation, and the destruction of rat brains symbolizes an attitude towards nature that jeopardizes the prospects
for the continued survival of human and non-human animals on Earth.
Other experts have expressed concern that changes in the rat brain could open the door
to higher cognitive abilities in lab animals.
Julian Savulescu, a bioethicist at the National University of Singapore, told Technology Review: "This increases the possibility that you are creating an enhanced rat that may be more cognitive than the average mouse
.
"

Pasca told Technology Review that the rats in the study did not show any evidence
of advanced cognition or human behavior.
Ethical concerns aside, he believes such work could significantly advance drug development and help solve complex human diseases
.
"Challenging the disease will require a bold approach," he said
.
"We need to build human models that generalize more aspects of the human brain to study these unique human conditions
.
"