Tuesday, January 15, 2013

Primer on Neurotransmitters

Synapse diag1
Synapse diag1 (Photo credit: Wikipedia)

The brain contains billions and billions of neurons. These cells communicate with one another by releasing small endogenous chemical messengers, called neurotransmitters, into the synapse, where they are then taken up by specific receptors on neighboring cells. There are many types of neurotransmitters in the brain—what they have in common is that they are produced inside a neuron, released into the synapse, and then cause an excitatory or inhibitory effect on receptor cells, helping to propagate or downgrade action potentials.[i] 
Neurotransmitters are often classified into two types: small-molecule transmitters and neuropeptides. Small-molecule transmitters can be further differentiated into monoamines like dopamine and amino acids like glutamate. The neuropeptide class includes endorphins, insulin, and oxytocin. Typically, small-molecule transmitters are direct actors on neighboring cells. Neuropeptides, on the other hand, are better suited for more subtle modulatory effects.[ii]
Originally, neuroscientists believed that each type of neuron released only a single, unique neurotransmitter. This theory, referred to as Dale’s Law or Dale’s Principle after the observations of English neuroscientist Henry Hallett Dale, was first put forward by Australian neurophysiologist and Nobel Laureate John Eccles. Further examination, however, showed that neurons synthesize and release more than one type of neurotransmitter at their terminals. Eccles later revised Dale’s principle to suggest that specific neurons do not release just a single type of neurotransmitter but rather the same set of transmitter types at their synapses.[iii] Today, most neuroscientists posit that most axonal branches of a neuron release the same neurotransmitter(s)—which explains why different neuron types are still referred to as “dopaminergic” or “serotonergic” cells in scientific publications.[iv]
To date, scientists have identified more than 60 different neurotransmitters in the human brain—and expect to find more in the future. They are learning that neurotransmitters like acetylcholine, dopamine, glutamate, serotonin, norepinephrine, GABA, and others play important roles in human cognition and behavior. And while neurotransmitters are too often discussed as having a single role or function, neuroscientists are finding that they are multi-faceted, complex, and interact with one another in a variety of different ways. For example, dopamine has long been thought of as the neurotransmitter involved with reward processing. But new research suggests that the release of acetylcholine results in the release of dopamine—and, ultimately, both influence reward processing and learning. [v],[vi]


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