Neuropeptides: relatively large transmitter molecules composed of amino acids. (NIH) They aren't made at the presynaptic terminal, they're made in the cell body and also they tend to be released actually away from the synapse and have more of a neuromodulatory and slower effect. (Campbell, BSP210)

Small Molecule Neurotransmitters: much smaller than Neuropeptides. (NIH) Synthesized by enzymes at the presynaptic terminal. (Campbell, BSP210) Also referred to as ‘monoamines.’

Acetylcholine (ACH): functions as a fast transmitter when it works with one receptor and as a “modulator” with a different receptor. Involved with nerves that control muscle movements and heart rhythm. (LeDoux, 59) Synthesized in the "presynaptic terminal." Has the ability to bind both with “ionotropic receptors” and “metabotropic receptors.” (Kolb, 49-50) Released by “motor neurons.” Shown by British pharmacologist Henry Dale to be released by the "vagus nerve" in the "heart" and to be responsible for slowing the "heart rate." The original chemical found by Dale and Otto Loewi in their discovery of “synaptic transmission,” for which they won the Nobel Prize in 1936. (Kandel, 91, 431) In cardiac muscle it is an “inhibitory neurotransmitter” substance. In skeletal muscle it is an “excitatory neurotransmitter” substance. (Also) essential for “learning.” (Doidge, 43) Primary roles: learning, “memory,” muscle contraction. (Hockenbury, 48) In the “peripheral nervous system”, acetylcholine converts “action potentials” in motor neurons into muscular action. Activity in the neurons that release ACH correlates with increasing “arousal” levels. (Koch, 329) Disruption of acetylcholine in the neocortex is believed to play a role in “Alzheimer’s disease.” (LeDoux, 59) In this disease, there is a loss of acetylcholine from the synapses. One of the ways physicians have tried to combat this disease is by prescribing drugs that block an “enzyme” called ‘acetylcholinesterase,’ which breaks down acetylcholine. (Norman Study, 31Mar11) ‘Nerve gas’ works by disrupting acetylcholine transmission at muscles, especially muscles required for normal breathing. (Ledoux, 59)

Amino Acids: the building blocks of protein. Organic chemicals (meaning) they contain carbon-hydrogen bonds. All amino acids have the same basic structure. (Cleveland Clinic)

GABA (Gamma Aminobutyric Acid): The most common inhibitory neurotransmitter in the brain. Manufactured almost exclusively in the brain and spinal cord. (The Brain-Leslie Iversen, 75) Reduces the likelihood of an action potential being generated in the “postsynaptic cell.” (LeDoux, 53) Without GABA inhibition, neurons would send out action potentials continuously under the influence of glutamate, and would eventually literally fire themselves to death. (LeDoux, 56) The brain’s ‘natural tranquilizer.’ Prevents “anxiety” messages from being transmitted from neuron to neuron. When the brain doesn’t produce enough GABA, "depression," anxiety, and "panic attacks" can set in. (Norman Study, 31Mar11) Having normal levels of GABA is crucial to stopping “anxiety.” GABA is the primary target for most of our anti-anxiety medicines. Moving the body triggers the release of GABA - one of the benefits of exercise. (Ratey, 92)

Glutamate: the most common excitatory neurotransmitter. (Kandel4, 69) Plays a critical role in the process of “long term potentiation.” (Goldberg, 112) Actually has two roles in body function. In addition to serving as a neurotransmitter in the brain, it also plays a major part in basic life-sustaining metabolic processes that go on continuously throughout the body. (LeDoux, 53) Increasing the amount of glutamate a sensory neuron sends to a motor neuron, strengthens the “synaptic potential” elicited in the motor cell, thus making it easier for that neuron to fire and cause a movement response. This process is referred to as ‘sensitization’ and 'short-term sensitization' results in "short-term memory." (Kandel, 222) “Cyclic AMP” and “protein kinase A” are also critical to this process. (Kandel, 228)

NMDA Receptor: part of the family of glutamate receptors. Requires two molecules of glutamate and two molecules of glycine because it has an additional voltage channel that is blocked. (Campbell, BSP210)

Glycine: a compound the body uses to create protein. Glycine is one of the amino acids essential to the body’s synthesis of the antioxidant ‘glutathione.’ Cells produce glutathione in order to fight ‘free radicals’ that can otherwise cause oxidative stress and damage cells, proteins, and “DNA.” (WebMD) Receptors for glycine are also ligand-gated channels, their general structure mirroring that of GABA receptors. (NIH)

Biogenic Amines: a category of small molecule neurotransmitters. In terms of synthesis, packaging, release, and degradation, the amine neurotransmitters fall somewhere between the properties of the other small-molecule neurotransmitters and those of the neuropeptides. (NIH) Also referred to as ‘amines’ and ‘monoamines.’

Dopamine: a neurotransmitter that plays a role in coordinating movement, in "attention" and learning, and in behaviors that are reinforcing. (Kolb, 153) Primary roles include movement, “attention,” and “learning.” Influences attention and the ability to concentrate. (Norman Study, 31Mar11) Along with its role in motor commands, it also serves as the main messenger in the "reward systems," guiding a person toward food, drink, mates, and all things useful for survival. Can trigger gambling, overeating, and drug "addiction" behaviors that result from a reward system gone awry. (Eagleman, 156) Dopamine release… has a role in feelings of reward and pleasure. (Kolb, 433) A pleasure-inducing brain chemical. Secreted in the brain of a man when a woman whom he finds attractive looks directly at him. Not secreted when she looks elsewhere. (Goleman, 9) First discovered in the 1950’s by the pharmacologist Arvid Carlsson. (Kandel4,199) Associated disorders include “Parkinson’s disease” (not enough dopamine), “schizophrenia” (too much dopamine), and… “addiction.” (Hockenbury, 48)

Epinephrine: chemical messenger that acts as a neurotransmitter in the “central nervous system,” and as a “hormone” to mobilize the body for fight or flight during times of "stress." (Kolb, 150) Derived from the “amino acid”  ‘tyrosine.’ In response to stress, increases heart rate, raises "blood pressure," and increases levels of blood sugar (“glucose.”) Another function is to increase fatty acids in the blood stream so that your body can use them for fuel. (Norman Study, 31Mar11) Editor's note - also referred to as ‘adrenaline’ because a drug company used 'adrenaline' as a trade name.

Histamine: high concentrations of histamine are found in neurons in the “hypothalamus” that send widespread projections to almost all regions of the brain and spinal cord. The central histamine projections mediate arousal and attention. This partly explains why antihistamines that cross the blood-brain barrier, such as Benadryl®, act as sedatives. Histamine also is released from mast cells in response to allergic reactions or tissue damage. (NIH)

Norepinephrine: found in the brain and in the “parasympathetic” division of the “autonomic nervous system.” (Kolb, 150) The first neurotransmitter scientists studied to try to understand mood. It often amplifies signals that influence attention, "perception," "motivation," and "arousal." (Ratey, 37)  Because of its role in arousal, it can help break an anxiety cycle. Exercise increases norepinephrine both in the moment and over the long term. (Ratey, 107) Primary roles include physical arousal, learning, memory. Associated disorders include depression and stress. (Hockenbury, 48) Attention circuits (pathways) are jointly regulated by norepinephrine and dopamine, which are so similar on a molecular level that they can plug into each others’ receptors. These are the neurotransmitters targeted by “ADHD” medications. (Ratey, 149) Prepares (the body) for action in threatening situations. Also helps regulate blood pressure and maintain the ability to concentrate. Overproduction can lead to irritability, anxiety, and insomnia. (Norman Study, 31Mar11) Higher than normal levels of norepinephrine in the central nervous system have been associated with “mania.” whereas lower than normal levels have been associated with depression. (Kolb, 48) Also referred to as ‘noradrenaline.’

Serotonin: a modulatory “monoamine” neurotransmitter in the brain that has been implicated in the regulation of “mood states”, including depression, anxiety, food intake, and impulsive violence. (Kandel, 448) Acts to enhance the release of glutamate from the presynaptic terminals of sensory neurons. (Kandel, 224) At the synapse between neurons, it activates “CPEB” which is needed for growth of new synaptic “terminals.” (Kandel, 272) Primary role: emotional states. Associated disorders: depression. (Hockenbury, 48) Associated with tranquility, calm, and emotional well-being. Low levels may increase the risk for depression or suicide. (Norman Study, 31Mar11) Also referred to as the ‘happiness neurotransmitter.’

Mediating Neurotransmitters: released by a neuron at the synapse and acts directly on the target cell, either exciting the target cell or inhibiting it. Examples include glutamate and GABA. (Kandel4, 69)

Modulatory Neurotransmitters: fine-tune the action of excitatory and inhibitory neurotransmitters. Examples include dopamine and serotonin. (Kandel4, 69)

Trace Amines: a group of compounds derived from “ammonia” by substituting organic "radicals" for the "hydrogens." Found throughout the brain at low levels, compared to the more abundant neurotransmitters such as dopamine and serotonin. "Amphetamines" and "LSD" bind to receptors that normally bind trace amines. (Lewis, 155)