Discovery of Neurotransmitters

Back in 1920, an Austrian scientist named Otto Loewi discovered the first neurotransmitter. In his experiment (which came to him in a dream), he used two frog hearts. One heart (heart #1) was still connected to the vagus nerve. Heart #1 was placed in a chamber that was filled with saline. This chamber was connected to a second chamber that contained heart #2. So, fluid from chamber #1 was allowed to flow into chamber #2.

Electrical stimulation of the vagus nerve (which was attached to heart #1) caused heart #1 to slow down. Loewi also observed that after a delay, heart #2 also slowed down.

From this experiment, Loewi hypothesized that electrical stimulation of the vagus nerve released a chemical into the fluid of chamber #1 that flowed into chamber #2. He called this chemical "Vagusstoff". We now know this chemical as the neurotransmitter called acetylcholine.

Neurotransmitter Criteria

1. It must be produced within a neuron.

2. It must be found within a neuron.

3. When a neuron is stimulated (depolarized), a neuron must release the chemical.

4. When a chemical is released, it must act on a post-synaptic receptor and cause a biological effect.

5. After a chemical is released, it must be inactivated. Inactivation can be through a reuptake mechanism or by an enzyme that stops the action of the chemical.

6. If the chemical is applied on the post-synaptic membrane, it should have the same effect as when it is released by a neuron.

Neurotransmitter Types

There are many types of chemicals that act as neurotransmitter substances. Below is a list of some of them.

Small Molecule Neurotransmitter Substances

Acetylcholine (ACh)Dopamine (DA)Norepinephrine (NE)
Serotonin (5-HT)Histamine

Amino Acids

Gamma-aminobutyric acid (GABA)GlycineGlutamate

Neuroactive Peptides - partial list!!

gastrinsubstance Pneurotensinglucagon
oxytocinprolactinthyrotropinangiotensin II
sleep peptidesgalaninneuropeptide Y thyrotropin-releasing hormone
gonadotropnin-releasing hormone growth hormone-releasing hormone luteinizing hormone vasoactive intestinal peptide

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Neurotransmitter Types

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