This is a first draft which has not been edited. If you have questions, or want to help in the writing or editing process, please contact the author: jaronwilliams@mail.weber.edu

Chemical Neurotransmitters

Synaptic activity and cellular communication is critical to proper neuronal functioning. As discussed in the previous chapter, neurons utilize electrical signals across the synapse to communicate with other neurons and cell types. Along with electrical signals, neurons also utilize chemical signaling in the form of chemical neurotransmitters. In this chapter, we will briefly discuss chemical neurotransmitters and the role they play in neuronal function.

Chemical neurotransmitters are substances released by a neuron in synaptic vesicles that have a specific effect on a specific neuron or effector site. Neurotransmitters are similar to hormones in many ways but differ in a few key aspects. While the actions of hormones and neurotransmitters are very similar, they differ in that neurotransmitters typically act on effector sites closer to the site of release. Where hormones can travel long distances via the blood stream to various effector sites. There are 7 categories and more than 100 known neurotransmitters , however in this chapter we will focus on three main categories of chemical neurotransmitters, Biogenic Amines, Amino Acids, and Polypeptides, as well as some additional neurotransmitters such as ATP, ACH, and adenosine.

Biogenic Amines

The most abundant form of neurotransmitter in the body, biogenic amines, includes dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline), histamine, and serotonin. These various transmitters play a critical role in many body functions both in the central and peripheral nervous systems. These can be further categorized into catecholamine transmitters (norepinephrine, epinephrine, and dopamine) and serotonin in its own category. We will discuss each transmitter in brief detail below. Histamine is also referred to as a biogenic amine but its structure differs from catecholamines and serotonin.

• Dopamine: commonly referred to as the pleasure chemical, dopamine is responsible for feelings of pleasure in the body. Dopamine has also been linked to addiction tendencies. Dopamine is primarily synthesized in the substantia nigra (black substance) located in the midbrain. Low levels of dopamine within the substantia nigra have been correlated with symptoms of Parkinson’s disease as dopamine also plays a crucial role in movement control.
• Norepinephrine (noradrenaline): this neurotransmitter is involved in activating the sympathetic nervous system, also known as the fight or flight response. It works to increase alertness, contracts blood vessels, and increases blood flow.
• Epinephrine (adrenaline): epinephrine also plays a role in the activation of the sympathetic nervous system. Epinephrine works more to increase heart rate and increase blood flow during stressful situations.
• Serotonin: this neurotransmitter is heavily involved in mood and digestion regulation. Serotonin has been linked with various mood disorders such as depression and anxiety which is why many people diagnosed with such disorders are prescribed SSRI’s (selective serotonin re-uptake inhibitors).
• Histamine: this biogenic amine plays a large role in the bodies inflammatory response. It’s action includes vasculature and smooth muscle control. Histamine is largely concentrated in the hypothalamus.

Amino Acids

These neurotransmitters serve dual purpose, as neurotransmitters and as universal cellular constituents. The body uses amino acids as building blocks for various proteins throughout the body. However not all amino acids are used as neurotransmitters, the body uses three main amino acids to transmit signals which include glutamate, GABA, and glycine.

• Glutamate:  glutamate is the most common neurotransmitter within the central nervous system, most frequently used at excitatory synapses. Glutamate is involved in memory and learning processes.
• GABA: gamma-aminobutyric acid, commonly known as GABA, also falls into the amino acid neurotransmitter classification. GABA is commonly used by inhibitory neurons and interneurons in the central nervous system. As such, it aides in reducing reactions in the central nervous system and produces what has been described as a calming effect.
• Glycine: glycine is most commonly used by inhibitory interneurons of the spinal cord where it aids in the processing of various sensory inputs including movement, vision, and audition.

Polypeptides 

Named because of their chemical structure, polypeptides consist of a chain of amino acids to form a peptide. These peptides are then used as neurotransmitters throughout the nervous system. The most common polypeptides used in the body are known as endorphins. While there are many different endorphins produced in the body the function of each is nearly identical, as such, we will discuss them in a group.

• Endorphins: These neurotransmitters have been dubbed the “feel good” chemicals of the body. Endorphins are released during a variety of physical activities including exercise, periods of excitement, and sexual activity. Endorphins create a sense of well-being and euphoria as well as aid in pain reduction.

Acetylcholine

Acetylcholine, also known as ACH, does not fit into any of the main three categories directly. While it does have similarities with some categories, it is not considered an amino acid nor is it derived from one. It is referred to as a low-molecular-weight amine transmitter. Found in both the central and peripheral nervous systems, ACH is an excitatory neurotransmitter that helps with learning and memory functions in the central nervous system. In the peripheral nervous system, ACH helps with maintaining proper heart rate and blood pressure.

ATP and Adenosine

ATP is most commonly known as the main source of energy for cellular activities, however, when modified, ATP and its degradation product adenosine can function as important neurotransmitters.

• Adenosine: this neurotransmitter has an inhibitory effect in the central nervous system. Specifically, the stimulatory effect of caffeine depends on the inhibition of adenosine binding to caffeine receptors.
• ATP: when released by damaged tissues, ATP can transmit pain section between sensory neurons. When released in the central axons of the dorsal root ganglion, ATP excites purine receptors on neurons in the dorsal horn of the spinal cord.