A study led by Jacques Wadiche, Ph.D., and Linda Overstreet-Wadiche, Ph.D., professors in the Department of Neurobiology, was published in Nature Neuroscience, titled "Glutamate concentration tunes AMPA receptor function through conductance-state occupancy." Co-authors include Gokulakrishna Banumurthy, Reagan L. Pennock, Luke T. Coddington, Xiaohui Yan, and Gabrielle N. Smith.
AMPA receptors are largely responsible for fast signaling between neurons. Each receptor can open to different levels, called conducting states, depending on the amount of glutamate that reaches it. This was well-established from past work, but it was unclear how this occurs at synapses where glutamate levels can rapidly change. This study shows that glutamate levels at a synapse can change how AMPA receptors function in real time, including how much calcium they let into the cell. The same receptor can act very differently depending on how much glutamate is around.
To better understand the significance of this research, the communications team sat down with Jacques Wadiche, Ph.D., to discuss his findings and their implications for the broader field.
Q: How does your current study build on previous research in this area?
AMPA receptors are the workhorse of our brains. These receptors mediate the majority of connections between cells in the brain. Because of that role, there has been much work over four decades on both their structure and function. AMPA receptors are activated by glutamate; past work showed that individual receptors can open to different degrees (called conducting states) depending on the number of glutamate molecules that bind to them. That work was done on isolated AMPA receptor proteins under steady-state glutamate concentrations. In brain circuits, however, AMPA receptors at synapses are exposed to glutamate transients that spike into the millimolar range and decrease to almost zero within milliseconds. There had been little work on understanding the relevance of multiple conducting states for synaptic function.
We show that different physiological levels of glutamate at synapses cause AMPA receptors to open to different conducting states. We also found that the different conducting states alter several fundamental properties of AMPA receptors, including the amount of calcium entering the cell. Calcium is important because it acts as a signal inside neurons, one that can trigger longer-lasting changes in neuronal function.
Q: Is your project interdisciplinary? If so, which fields or areas of expertise are involved, and how do they contribute to the research?
The project combines synaptic electrophysiology, two-photon imaging of calcium, glutamate uncaging, ion channel biophysics, genetics, recombinant protein expression, and computational modeling. The physiological experiments allowed us to study AMPARs in an intact circuit, while the more reduced experiments let us isolate the receptors in a much more controlled manner. The modeling helped us make sense of the experimental results and test whether a single mechanism could account for all of them.
Q: How does UAB's collaborative research environment support your project?
The work was truly helped by the collaborative environment between my lab and that of my long-time collaborator, Linda Overstreet-Wadiche, whose lab is physically contiguous with mine. Our labs bring complementary expertise and work closely together, so ideas, approaches, and people move easily between the two groups. That was especially important for this project because it allowed us to pursue unexpected results from several different experimental directions.
Q: What do you hope the biggest impact of this project will be?
Many studies understandably focus on a particular disease or behavior. We are interested in the fundamental properties of synapses themselves that determine how neurons communicate. Our work on AMPA receptor mechanisms is focused by design. These receptors are part of the basic machinery of synapses, so understanding them matters for essentially all brain function.
Our hope is that this changes how neuroscientists think about communication between neurons. We tend to think the properties of a synapse are determined by the proteins present and their chemical modifications, but our study shows that the exact same receptors can behave very differently depending on how much neurotransmitter reaches them. That means synaptic communication is more flexible than we assume, and that changes on either side of a synapse can alter the message being transmitted in ways never previously imagined.
There is a practical consequence as well. For decades, properties such as current-voltage rectification, polyamine block, and calcium permeability have been used as a ‘fingerprint’ to infer which subunits a receptor contains. We find that the same ‘fingerprint’ shifts with glutamate concentration alone with no change in subunit composition. Some results that have been read as changes in receptor composition, including certain types of learning and memory, may instead reflect changes in how much glutamate arrives. I would put that as a cautionary note rather than a conclusion, since working out which findings are affected will require a combination of multiple approaches.
Q: For people who aren't experts in your field, what's the one key takeaway you'd like them to understand about your work?
The main message is that synapses are extremely flexible. The same neurotransmitter can produce different effects depending on how much is released and reaches the receptor. That means the way neurons communicate depends on the conditions under which the receptors are activated, as much as which proteins are present or modified.
Wadiche’s findings point to a synapse far more adaptable than the standard model suggests: the same receptors, exposed to different amounts of neurotransmitter, can send meaningfully different messages. That flexibility could help explain how the brain fine-tunes its circuits, and it raises questions about experiments that have used receptor properties to infer receptor identity. The work also reflects the close, long-running collaboration between Wadiche's lab and fellow Neurobiology researcher, Linda Overstreet-Wadiche, Ph.D.