Neurotransmitters are chemicals that are used to relay, amplify and modulate signals between a neuron and another cell. According to the prevailing beliefs of the 1960s, a chemical can be classified as a neurotransmitter if it meets the following conditions:
- There are precursors and/or synthesis enzymes located in the presynaptic neuron;
- The chemical must be present in the presynaptic element
- It is available in sufficient quantity in the presynaptic neuron to affect the postsynaptic neuron;
- There must be postsynaptic receptors and the ability for the chemical to bind to said receptors
- A biochemical mechanism for inactivation must be present.
Types of neurotransmitters
There are many different ways to classify neurotransmitters. Often, dividing them into amino acids, peptides, and monoamines is sufficient for many purposes.
Some more precise divisions are as follows:
- Around 10 "small-molecule neurotransmitters" are known:
- monoamines (epinephrine E, norepinephrine NE, dopamine DA, serotonin 5-HT, and melatonin)
- 3 or 4 amino acids, depending on exact definition used: (primarily glutamic acid, GABA, aspartic acid & glycine)
- Purines, (Adenosine, ATP, GTP and their derivatives)
- Fatty acids are also receiving attention as the potential endogenous cannabinoid.
- Over 50 neuroactive peptides (vasopressin, somatostatin, neurotensin, etc.) have been found, among them hormones such as LH or insulin that have specific local actions in addition to their long-range signalling properties.
- Single ions, such as synaptically-released zinc, are also considered neurotransmitters by some.
- Nitrogen monoxide (NO)
The major "workhorse" neurotransmitters of the brain are glutamic acid (=glutamate) and GABA.
Some examples of neurotransmitter action:
- Acetylcholine - voluntary movement of the muscles
- Norepinephrine - wakefulness or arousal
- Dopamine - voluntary movement and motivation, "wanting", pleasure, associated with addiction and love
- Serotonin - memory, emotions, wakefulness, sleep and temperature regulation
- GABA (gamma aminobutyric acid) - inhibition of motor neurons
- Glycine - spinal reflexes and motor behaviour
- Neuromodulators - sensory transmission-especially pain
It is important to appreciate that it is the receptor that dictates the neurotransmitter's effect
Neurons expressing certain types of neurotransmitters sometimes form distinct systems, where activation of the system causes effects in large volumes of the brain, called volume transmission.
The major neurotransmitter systems are the noradrenaline (norepinephrine) system, the dopamine system, the serotonin system and the cholinergic system.
Drugs targeting the neurotransmitter of such systems affects the whole system, and explains the mode of action of many drugs;
Diseases may affect specific neurotransmitter systems. For example, Parkinson's disease is at least in part related to failure of dopaminergic cells in deep-brain nuclei, for example the substantia nigra. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.
A brief comparison of the major neurotransmitter systems follows:
| System || Origin  || Effects
| Noradrenaline system
|| locus coeruleus
| Lateral tegmental field
| Dopamine system
|| dopamine pathways:
- mesocortical pathway
- mesolimbic pathway
- nigrostriatal pathway
- tuberoinfundibular pathway
| motor system, reward, cognition, endocrine, nausea
| Serotonin system
|| caudal dorsal raphe nucleus
|| Increase (introverson), mood, satiety, body temperature and sleep, while decreasing nociception.
| rostral dorsal raphe nucleus
| Cholinergic system
|| pontomesencephalotegmental complex
- short-term memory
| basal optic nucleus of Meynert
| medial septal nucleus
- ^ a b Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone, page 474 for noradrenaline system, page 476 for dopamine system, page 480 for serotonin system and page 483 for cholinergic system.. ISBN 0-443-07145-4.