Neurotransmitter - Identifying Neurotransmitters

Identifying Neurotransmitters

The chemical identity of neurotransmitters is often difficult to determine experimentally. For example, it is easy using an electron microscope to recognize vesicles on the presynaptic side of a synapse, but it may not be easy to determine directly what chemical is packed into them. The difficulties led to many historical controversies over whether a given chemical was or was not clearly established as a transmitter. In an effort to give some structure to the arguments, neurochemists worked out a set of experimentally tractable rules. According to the prevailing beliefs of the 1960s, a chemical can be classified as a neurotransmitter if it meets the following conditions:

• There are precursors and/or synthesis enzymes located in the presynaptic side of the synapse.
• The chemical is present in the presynaptic element.
• It is available in sufficient quantity in the presynaptic neuron to affect the postsynaptic neuron.
• There are postsynaptic receptors and the chemical is able to bind to them.
• A biochemical mechanism for inactivation is present.

Modern advances in pharmacology, genetics, and chemical neuroanatomy have greatly reduced the importance of these rules. A series of experiments that may have taken several years in the 1960s can now be done, with much better precision, in a few months. Thus, it is unusual nowadays for the identification of a chemical as a neurotransmitter to remain controversial for very long periods of time.