The discovery of neurotransmitters
Henry Dale discovered acetylcholine in 1914. He found the substance had a similar effect on the body to stimulating the parasympathetic nervous system. He speculated that acetylcholine was a neurotransmitter – a substance released from nerve endings when they were stimulated, which affected organs in the body.
The idea of neurotransmission was not new, but was unproven until Loewi’s experiments in 1921. He connected a frog’s heart, with its nerve trunk intact, to a small glass container filled with Ringer solution. The heart was stimulated electrically. he then monitored the number and strength of the heart beats, and noting how they depended on the stimulation of particular nerve fibres. Afterwards, Loewi transferred the fluid which had been pumped out of the heart into a second heart which he had prepared in the same way. He showed that this fluid alone could change the activity of the heart. The nerve fibres had released substances into the fluid which affected the heart.
The substance responsible for neurotransmission was unidentified. After many years of research and modelling, Loewi found that the transmitter substances in the heart behaved chemically like adrenaline and acetylcholine. In 1929 Dale showed that acetylcholine could be purified from mammalian organs. Dale’s team confirmed Loewi’s findings, showing with experiments on cats, other mammals and frogs that these substances were responsible for neurotransmission throughout the body.