Neurotransmitters are broadly classified into two main types: excitatory or inhibitory neurotransmitters. Excitatory neurotransmitters increase the likelihood that the postsynaptic neuron or target cell will generate an action potential (i.e., excitation), whereas, inhibitory neurotransmitters decrease the likelihood that the postsynaptic cell will generate an action potential (i.e., inhibition).
Examples of excitatory neurotransmitters include glutamate, acetylcholine, and dopamine. Glutamate is the most abundant excitatory neurotransmitter in the central nervous system. It is synthesized from alpha-ketoglutarate and upon binding to glutamate receptors, causes depolarization or influx of sodium ions into the cell, causing the membrane potential to become net positive. Acetylcholine is another excitatory neurotransmitter that is important in regulating synaptic transmission in the peripheral nervous system. Acetylcholine is synthesized from choline and acetyl-CoA via the enzyme choline acetyltransferase. Acetylcholine acts on nicotinic acetylcholine receptors and plays an important role in skeletal muscle contraction. It causes local depolarization of muscle fibers at the neuromuscular junction, which is the first step in muscle contraction.
Dopamine is another excitatory neurotransmitter that is synthesized from tyrosine via tyrosine hydroxylase, an enzyme that adds a hydroxyl group to the substrate (e.g., tyrosine). Dopamine is involved in reward processing, motivation, and regulation of mood. At low doses, dopamine may lead to vasodilation in the renal vasculature, and at moderate to higher doses, dopamine may exert cardiovascular effects (e.g., increased heart rate and contractility).
Examples of inhibitory
neurotransmitters include gamma-aminobutyric acid (GABA) and glycine. GABA is the primary
inhibitory neurotransmitter in the central nervous system. It is synthesized from glutamate via an enzyme called glutamate decarboxylase, which is a type of enzyme that removes carboxyl groups from a substrate (e.g., glutamate). In the central nervous system, GABA binds to receptors (e.g., GABA-A and GABA-B), thereby opening chloride ion channels. Chloride ions cause the membrane potential to become net negative, known as hyperpolarization, and inhibit action potential generation. GABA transmission maintains a balance between neuronal depolarization and hyperpolarization.
Glycine is one of many amino acids in the
body and is an important
inhibitory neurotransmitter primarily found in the
spinal cord and
brainstem. Glycine is synthesized from serine via an enzyme called serine hydroxymethyltransferase, which is an enzyme that
transfers hydroxymethyl groups to a substrate (i.e., serine). Like GABA, glycine binds to ligand-gated chloride channels, and the influx of negatively charged chloride ions results in membrane hyperpolarization.