Neurotransmitters - There and Back Again

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Neurotransmitters - There and Back Again The idea that electrical information could be transferred from one neuron to the next by means of chemical signaling was the subject of intense debate through the first half of the twentieth century. In a key experiment in 1921, Austrian scientist, Otto Loewi established that a message could be transmitted between cells at a junction called a synapse using a chemical substance. Today these chemical substances are called neurotransmitters. Acetylcholine The First Identified Neurotransmitter In his experiment, he used two frog hearts to demonstrate that a chemical substance was involved in passing signals from neurons. Loewi placed the first frog heart, still connected to the vagus nerve, in a chamber containing saline solution. This chamber was connected to a second chamber that contained the second frog heart. Fluid from the first chamber was allowed to flow into the second Loewi chamber. Electrical stimulation of the vagus nerve caused the first heart to slow down. Loewi observed that after a delay, the second heart also slowed down. From this experiment, Loewi hypothesized that electrical stimulation of the vagus nerve released a chemical into the fluid of the first chamber that flowed into the second chamber. He called this chemical “Vagusstoff”. Otto Loewi shared the 1936 Nobel Prize with his friend Henry Dale for demonstrating chemical neurotransmission. Examine the Synapse Pictured Below Imagine what characteristics a substance must have in order to successfully transmit a message across the synaptic cleft. Record your thoughts. What criteria must be met in order for a substance to be considered a neurotransmitter? 1. The substance must be present within the presynaptic cell. Elaborate biochemical pathways are frequently used to produce neurotransmitters. Showing that the enzymes and precursors required to synthesize the substance are present in the presynaptic cell provides evidence that the substance may be a neurotransmitter. However, neurotransmitters such as glutamate or glycine are also needed for protein synthesis and other metabolic reactions. As a result their presence alone is not sufficient to establish them as neurotransmitters. 2. The substance must be released in response to presynaptic depolarization and the release must be Ca2+ dependent. 3. When the substance is released, specific receptors for the substance must be present on the postsynaptic neuron and cause a biological effect. 4. After the substance is released, it must be inactivated either through a reuptake mechanism or by an enzyme that stops the action of the substance. 5. If the substance is applied on the postsynaptic membrane, it should have the same effect as when it is released by the neuron. .

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