What is so important about glutamate? It makes and breaks connections between neurons, and it turns on other neurons to stimulate them into action. Glutamate neurotransmission is mediated through receptors that allow the passage of sodium or calcium ions into neurons; the receptors were named according to the chemical tools that were historically used to study them. For example, the sub type of glutamate receptors known as N-methyl- d-A spartate (NMDA) allows the entry of calcium ions into neurons. Following the
entry of calcium ions, some truly interesting things begin to happen inside the neuron that leads to the production of what you might call a “memory.” Calcium ions activate a complex cascade of biochemical changes that ultimately involve the genes of the neuron and that may actually change how the neuron behaves for the rest of your life. These biochemical changes may also alter how one neuron communicates with hundreds of other neurons. Think of this neural process as a symphony of musicians playing together for the first time. Initially, everyone is playing his or her own song. Then the conductor arrives and hands out a
musical score; all of the musicians begin to play in a complex pattern of rhythms that conveys information. Like the conductor, calcium ions entering via NMDA channels initiate the process of forming an ensemble of neuronal activity. Your neurons are the musicians, and when they become linked to each other according
to some common pattern of activity, they form an ensemble that plays a particular song, or memory, which can recur only when that particular ensemble of neurons plays the same pattern together. In this analogy, memories can be seen as symphonies of activity in our brains, and just as we enjoy playing the same tunes
over and over again, so we also enjoy replaying pleasant memories. Unfortunately, glutamate’s actions can prime us to play unpleasant or traumatic memories over and over again as well when they are triggered by innocent events in our daily lives. In addition, the entry of calcium ions into neurons may sometimes become excessive as a result of aging, disease, or stroke and may initiate some harmful processes that may contribute to the removal of synapses or even the death of neurons. This information tells us quite a lot about the role of glutamate: when it works correctly, memories can be formed; when it does not work correctly, as when it induces too much calcium to enter the neuron, then death and destruction follow and memory is lost. Thus, maintaining a good balance of function related to the entry of calcium ions is a challenging but critical requirement for neurons, and the amino acid neurotransmitter glutamate plays a critical role in this process.
Glutamate also has a unique function in brain development. When you were very young, the neurons in your brain developed many connections, or synapses, with other neurons to optimize your ability to learn a great deal of information quickly, such as how to move your hands and feet, what your mother’s voice sounds like, or what the color red looks like. But as you grew older (during adolescence), your brain became a bit like an over-wired computer — for it to work better and faster, with less likelihood of failing, it became advantageous for it to remove unnecessary “wires,” or connections. This is where glutamate’s other unique abilities come into play. Your brain uses glutamate to prune synapses that have become unnecessary, which in turn allows the remaining neural circuits to function more efficiently. Later, when you’re an adult, glutamate is critical for allowing your brain to be “plastic,” to mold your responses to the environment so that you increase your chances of survival. Thus, like the Roman god Janus, the neurotransmitter glutamate has two faces: One is important for the early brain development and function in our past; the other is important
for brain pruning and subsequent function in our future. Meanwhile, its staying power can sometimes be a mixed blessing. For example, as mentioned, traumatic memories formed through glutamate’s actions can continue to haunt us long after the event that created those memories has occurred. The best example of this is called post-traumatic stress disorder; the unpleasant memories that characterize this disorder are very difficult to treat because of the amazing efficacy of glutamate to form lasting changes in the brain.
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