neurotransmitter: one of many different body chemicals released by brain and nerve cells, that carry messages from cell to cell across neuronal junctions.

Dictionary of Sexology Project: Main Index

There are five major neurotransmitters:

Acetylcholine:

Dopamine:

Serotonin:

  • Inhibits virtually all activities. Important for sleep onset, mood, eating behavior.

Norepinephrine:

  • Affects arousal, wakefulness, learning, memory, mood.

Endorphins and Enkephalins:

  • Inhibits transmission of pain messages.

What defines a substance as a neurotransmitter?

  1. The substance must be present within the presynaptic neuron. A chemical cannot be secreted from a presynaptic neuron unless it is present there! Because elaborate biochemical pathways are requird to produced neurotransmitters, showing that the enzymes and precursors required to synthesize the substance are present in presynaptic neurons provides some evidence that the substance is used as a transmitter. (However, since the transmitters glutamate, glycine, and aspartate are also needed for protein synthesis and other metabolic reations in all neurons, their presence is not sufficient evidence to establish them as neurotransmitters.)
  2. The substance must be released in response to presynaptic depolarization, which must occur dependant on Ca2+. Another essential criterion for identifying a neurotransmitter is to demonstrate that it is released from the presynaptic neuron in response to presynaptic electrical activity, and that this release requires Ca2+ influx into the presynaptic terminal.
  3. Specific receptors for the substance must be present on the postsynaptic cell. A neurotransmitter cannot act on its target unless specific receptors for the transmitter are present in the postsynaptic membrane. One way to demonstrate receptors is to show that application of exogenous transmitter mimics the postsynaptic effect of presynaptic stimulation.

Neuroscience, Sinaur Associates (QP355.2.N487 1997)

Neurotransmitters can be loosely defined as natural chemical signals which effect the behavior of neurons. For a more rigid definition, see the above criteria. Neurotransmitters can be released at specific synapses, to facilitate neuron-neuron communication, or they can be released by broadcast transmission, in which they are released over a wider area, and have a more modulatory role. Different neurotransmitters can have different effects in different regions of the nervous system. Neurotransmitters are often divided into classical neurotransmitters and nonclassical neurotransmitters. These distinctions are biologically arbitrary, being based only on when the transmitters were discovered. Thus, I will be ignoring that distinction. What follows is a list of known transmitters, their structural type, and their common functions.

Catecholamines
Composed of a catechol nucleus and an amine group. Generally broadcast transmitters, and can have hormonal functions.

Dopamine(DA): Acts as a precursor for norepineprhine in the peripheral nervous system, and has an important functional role in the kidney.

Epinephrine(E): Sympathetic transmitter in frogs.

Norepinephrine(NE): Sympathetic transmitter in mammals.


Indolamines Chemically similar to catecholamines, also broadcast transmitters.

Serotonin(5HT or 5-hydroxytryptamine): Neurotransmitter which acts to increase aggresiveness in invertebrates. Has the opposite sort of effect in vertebrates. Has been implicated to have roles in sleep and modulation of circadian rhythm.


Amino Acids
These are specific transmitters (not broadcast). Absorbed and broken down by glial cells. They sometimes act to modulate the ongoing function of a neuron.

GABA: GABA is the principal inhibitory transmitter in the brain.

(D or L)-glutamate: Glutamate is the principal excitatory transmitter in the brain.

glycine: An inhibitory transmitter.

aspartate: An excitatory transmitter.


Cholines
Acetylcholine: The neuromuscular transmitter of the peripheral nervous system. (It's the chemical that triggers muscle contraction.) In the central nervous system it has been associated with learning and Alzheimer's

Peptides
Neurotransmitters with widespread, occasionally dramatic action. They are usually co-released with classic transmitters. Inactivation occurs through diffusion or enzymatic action (not reabsorption). Typically, peptides have long effects (on the order of minutes). Or they can act on the genome via secondary messengers.

Endorphins


Gases
Gases are unique as neurotransmitters in that they can diffuse anywhere, even through cell membranes. They are broken down quickly, and can be thougt of as having a 'sphere of influence' centered at their point of synthesis.

Carbon monoxide(CO)

Nitrous oxide(NO)



Neurotransmitter receptors
A quick note on neurotransmitter receptors. They come in two general types:
ionotropic: Have a fast action, resuting in the opening of ion channels, affecting membrane voltage.

metabotropic: Have a slower action, and trigger a cascade of chemical actions. Ion channels can open as a result, and there can also be metabolic effects which are more long term.

The NMDA receptor is one of the more interesting receptors. It is responsive to glutamate, but requires activation of both the pre-synaptic and post-synaptic cells to activate. When glutamate binds to the receptor, the channel opens, but is blocked by Mg2+ ions which are too big to fit through. If the post-synaptic is then depolarized, the Mg+2 is forced out and the cell allows Ca2+ and Na+ to enter. Since Ca2+ is known to have chemical effects in neurons, this has been proposed as a mechanism for hebbian learning.


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