By the early 1900s, scientists had a reasonably clear idea of the anatomy of the nervous system. They knew that individual nerve cells--neurons--formed the basis of that system. They also knew that nerve messages traveled in the form of minute electrical currents along the length of a neuron and then passed from the axon of one cell to the dendrites of a nearby cell.

One major problem remained, however. What was the mechanism by which the nerve message travels across the narrow gap--the synapse--between two adjacent neurons? The British neurologist Thomas R. Elliott (1877-1961) suggested an answer to that question as early as 1903. He proposed the idea that the nerve message is carried from one cell to another by means of a chemical compound. Elliott thought that adrenaline might be this chemical messenger, or neurotransmitter, as it is known today.

Nearly two decades passed before evidence relating to...