Scientists Discover ?Volume Control? for Connections between Brain Regions
29 Aug, 2007 12:59 pm
Nerve axons, the hair-like processes that connect brain regions, have been likened to telephone wires in that they faithfully relay information without altering it. However, a recent study shows that, contrary to previous thinking, the information being relayed may be increased or decreased by neurotransmitters (the brain?s own chemicals) or drugs such as nicotine. In other words, axons possess a ?volume control.? This finding may help us understand higher brain functions such as perception and cognition, and help devise treatments for diseases that disrupt connections among brain regions.
In the course of our studies on the auditory thalamo-cortical slice, we made a completely unexpected discovery about how brain cells work.
For years, scientists have known that information travels between brain cells along hair-like extensions called axons. Axons connect a brain cell’s main body, which receives inputs from other brain cells, to the synapse that relays its output. Information processing, it was thought, occurs only in the brain cell’s main body and synapse, but not in the intervening axon. Axons were thought to be like telephone wires that reliably convey signals from one phone to another. But, in our study we found that if you stimulate the axon, the signal can be altered, like giving the telephone wires a volume knob. Our experiment was made possible by having the auditory thalamo-cortical slice, which maintains intact the axons connecting the auditory thalamus and auditory cortex. Using the drug nicotine, we stimulated an axon to determine how it would effect a signal that a brain cell in the auditory thalamus was sending to the auditory cortex. Without applying nicotine, about 35 percent of the messages reached the cortex. But when nicotine was applied to the axon, the success rate nearly doubled to about 70 percent. This finding implies that there are proteins (receptors) on the axon that, in the live brain, must respond to the brain’s own neurotransmitters (chemicals that mediate communication between neurons). We confirmed this implication in the live mouse brain, which also shows that the finding didn’t only apply to brain slices in a dish.
The findings were published online August 19 in the journal Nature Neuroscience.
Our findings may help scientists develop treatments for psychiatric disorders such as depression and schizophrenia in which it is thought that different parts of the brain do not communicate properly with each other. Until now, scientists have thought that in the brain’s cortex – where most cognitive processes occur – information was processed only by the cell body, and not by its axon. The result of our study suggests that we must consider the axons as sites of information processing – and of potential problems when things go wrong.
Our research is funded by the National Institutes of Health (NIDA and NIDCD).
Kawai H., et al, Nicotinic control of axon excitability regulates thalamocortical transmission, Nature Neuroscience (19 Aug 2007)