Neuroscientists create technique to rapidly switch neurons off and on to study function
La Jolla, CA – Using molecules involved in insect molting, researchers at the Salk Institute for Biological Studies have created a laboratory method that can quickly turn off neurons in the brain and spinal cord of live animals - and can just as rapidly switch them back on.
The so-called "AlstR/AL system," detailed in the July 20 issue of Neuron, represents a significant advance in the study of neuroscience, the scientists say, because it now is possible to precisely control the activity of specific neuronal cell types in order to understand their role in complex networks.
This novel method will help explain how information is processed in the brain; and which specific nerve cells work together to mediate sensation, cognition, and motor control. Understanding the neural circuits that underlie brain function is a crucial step toward developing better devices to repair the diseased brain.
"This technique is well suited for detailed studies of neural circuitry, perception and behavior at a resolution that has never before been possible," says the study's lead author, Edward M. Callaway, Ph.D., a full professor in the Systems Neurobiology Laboratories.
"Other methods used to date to study the role of specific neurons by silencing them often end up either killing the cells of interest or are too slow to adequately understand the purpose of these cells," Callaway says.
Neuroscientists are trying to tease apart the contribution of each of the thousands of kinds of neurons found within the mammalian nervous system. "We want to understand the function of different types of trees that constitute this complex forest," he says.
"In the mammalian nervous system, billions of neurons are located next to each other and each of these has thousands of intertwined processes. From a distance this looks like an impossible tangle, but when you look closer you see that this jungle is actually composed of distinct neuron types" explains Callaway. "Each type of neuron plays a precise role and is connected to other neurons in a unique way. So, we need to understand how these specific circuits work, and how they control behavior. We can do that now by turning off specific types of neurons, all at once, without affecting other nearby neurons."
The study describes how the eight-member Salk team crafted the method and tested it in mice, rats, ferrets and monkeys, focusing on neurons in the brain's cortex and thalamus.
The technique involves two steps. The first is to deliver a gene with the help of a viral vector. The gene is taken up by all nerve cells but is only active in the class of neuronal cells to which it is specifically targeted. It codes for production of the Drosophila allatostatin receptor (AlstR). Allatostatin is named for its role in the regulation of metamorphosis in some insects; Corpora allata, specialized tissue in the head of insects, secrete hormones that regulate the process.
This insect receptor has no innate function in a mammalian brain, and "by itself, it can't do anything," Callaway says.
The second step is to deliver allatostatin (AL), the hormone ligand that sticks on to the allatostatin receptor and activates it. "Activating the receptor completely shuts down activity of the neurons, and since this is an insect peptide, there are no side effects in the animals," Callaway says. It takes just a minute to silence the neurons and they can stay inactivated for hours until the researchers choose to reverse the process.
To do that, they simply rinse off the AL peptide. "As soon as the peptide washes away, the nerve cells recover function, and you can safely repeat the process again and again," he says.
"The researchers demonstrated that the method works to reversibly silence brain neurons in general, and future studies will use it to turn specific types of neurons on and off," Callaway says. In this study, they also proved that the technique can silence neuronal activity in the spinal cords of transgenic animals whose nerve cells have been altered to naturally express AlstR, including individual neurons.
"We are very excited about this technique," Callaway says. "We think it will have a broad impact in our efforts to understand how the brain and spinal cord function."
Researchers who contributed to this study include co-first authors Elaine M. Tan Ph.D., and Yoshiaki Yamaguchi, Ph.D., in the Systems Neurobiology Laboratory, Gregory Horwitz, Ph.D. and Thomas Albright Ph.D. in the Vision Center, Simon Gosgnach, Ph.D., and Martyn Goulding, Ph.D., in the Molecular Neurobiology Laboratories, and Edward S. Lein, Ph.D., a former researcher in the Vision Center and now at the Allen Institute for Brain Science in Seattle.
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.