Neurotransmitter

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Chemical structure of D-aspartic acid, a common amino acid neurotransmitter.
Chemical structure of D-aspartic acid, a common amino acid neurotransmitter.

Neurotransmitters are chemicals that are used to relay, amplify and modulate signals between a neuron and another cell.[1] 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 side of the synapse;
  • The chemical is present in the presynaptic element
  • It is available in sufficient quantity in the presynaptic neuron to affect the postsynaptic neuron;
  • There are postsynaptic receptors and the chemical is able to bind to them
  • A biochemical mechanism for inactivation is present.

Contents

[edit] Types of neurotransmitters

There are many different ways to classify neurotransmitters. Dividing them into amino acids, peptides, and monoamines is sufficient for many purposes.

Some more precise divisions are as follows:

The major "workhorse" neurotransmitters of the brain are glutamic acid and GABA.

[edit] Excitatory and inhibitory

Release of excitory neurotransmitters from the presynaptic membrane opens channels in the postsynaptic membrane and leads to an increase in the concentration of sodium ions within the postsynaptic cell and a decrease in that of potassium ions. This leads to a 'depolarisation' of the postsynaptic cell, which is propagated further along the cell membrane by an action potential.

Inhibitory neurotrasmitters encourage the hyperpolarisation of the postsynaptic cell, making it less likely to generate an action potential.

Whether a neurotransmitter acts in an excitatory or inhibitory manner is determined by the reaction of the receptor to its binding. Thus a given chemical can be excitatory at some receptors and inhibitory at others.


[edit] Actions

Some examples of neurotransmitter action:

NB: Neurotransmitters may be specific to particular target organs and have multiple roles around the body. For instance, Acetylcholine is released from both sympathetic and parasympathetic neurons. ACH can be either excitatory to skeletal muscle cells or inhibitory to both smooth muscle and cardiac muscle.

[edit] Neurotransmitter systems

Main article: Neuromodulation

Neurons expressing certain types of neurotransmitters sometimes form distinct systems, where activation of the system affects 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; this fact explains the mode of action of many drugs. Cocaine, for example, blocks the reentering of dopamine back into the presynaptic neuron, leaving these neurotransmitters in the synaptic gap longer. Since the dopamine is in the synapse longer, the neurotransmitter rapidly hit the receptors on the postsynaptic neuron cell, and therefore causing happiness. Excess intake of cocaine can lead to physical addiction. The physical addiction of cocaine is when the neurotransmitters stay in the synapse so long , the the body removes some receptor from the postsynaptic neuron. After the drug affect is done, the person usually fells unhappy, because now the neurotransmitters are less likely to hit the receptor since the body removed many of them during the drug intake. Prozac is a selective serotonin reuptake inhibitor (SSRI), hence potentiating the effect of naturally released serotonin. AMPT prevents the conversion of tyrosine to L-DOPA, the precursor to dopamine; reserpine prevents dopamine storage within vesicles; and deprenyl inhibits monoamine oxidase (MAO)-B and thus increases dopamine levels.

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:

Neurotransmitter systems
System Origin [2] Effects[2]
Noradrenaline system locus coeruleus
  • arousal
  • reward
Lateral tegmental field
Dopamine system dopamine pathways: motor system, reward, cognition, endocrine, nausea
Serotonin system caudal dorsal raphe nucleus Increase (introversion), mood, satiety, body temperature and sleep, while decreasing nociception.
rostral dorsal raphe nucleus
Cholinergic system pontomesencephalotegmental complex
basal optic nucleus of Meynert
medial septal nucleus

[edit] Common neurotransmitters

Category Name Abbreviation Metabotropic Ionotropic
Small: Amino acids Aspartate - -
Neuropeptides N-Acetylaspartylglutamate NAAG Metabotropic glutamate receptors; selective agonist of mGluR3 -
Small: Amino acids Glutamate (glutamic acid) Glu Metabotropic glutamate receptor NMDA receptor, Kainate receptor, AMPA receptor
Small: Amino acids Gamma-aminobutyric acid GABA GABAB receptor GABAA receptor, GABAC receptor
Small: Amino acids Glycine Gly - Glycine receptor
Small: Acetylcholine Acetylcholine Ach Muscarinic acetylcholine receptor Nicotinic acetylcholine receptor
Small: Monoamine (Phe/Tyr) Dopamine DA Dopamine receptor -
Small: Monoamine (Phe/Tyr) Norepinephrine (noradrenaline) NE - -
Small: Monoamine (Phe/Tyr) Epinephrine (adrenaline) Epi - -
Small: Monoamine (Phe/Tyr) Octopamine - -
Small: Monoamine (Phe/Tyr) Tyramine -
Small: Monoamine (Trp) Serotonin (5-hydroxytryptamine) 5-HT Serotonin receptor, all but 5-HT3 5-HT3
Small: Monoamine (Trp) Melatonin Mel Melatonin receptor -
Small: Monoamine (His) Histamine H Histamine receptor -
PP: Gastrins Gastrin - -
PP: Gastrins Cholecystokinin CCK Cholecystokinin receptor -
PP: Neurohypophyseals Vasopressin Vasopressin receptor -
PP: Neurohypophyseals Oxytocin Oxytocin receptor -
PP: Neurohypophyseals Neurophysin I - -
PP: Neurohypophyseals Neurophysin II - -
PP: Neuropeptide Y Neuropeptide Y NY Neuropeptide Y receptor -
PP: Neuropeptide Y Pancreatic polypeptide PP - -
PP: Neuropeptide Y Peptide YY PYY - -
PP: Opioids Corticotropin (adrenocorticotropic hormone) ACTH Corticotropin receptor -
PP: Opioids Dynorphin - -
PP: Opioids Endorphin - -
PP: Opioids Enkephaline - -
PP: Secretins Secretin Secretin receptor -
PP: Secretins Motilin Motilin receptor -
PP: Secretins Glucagon Glucagon receptor -
PP: Secretins Vasoactive intestinal peptide VIP Vasoactive intestinal peptide receptor -
PP: Secretins Growth hormone-releasing factor GRF - -
PP: Somtostatins Somatostatin Somatostatin receptor -
SS: Tachykinins Neurokinin A - -
SS: Tachykinins Neurokinin B - -
SS: Tachykinins Substance P - -
PP: Other Bombesin - -
PP: Other Gastrin releasing peptide GRP - -
Gas Nitric oxide NO - -
Gas Carbon monoxide CO - -
Other Anandamide AEA Cannabinoid receptor -
Other Adenosine triphosphate ATP P2Y12 P2X receptor

[edit] Degradation and elimination

Neurotransmitter must be broken down once it reaches the post-synaptic cell to prevent further excitatory or inhibitory signal transduction. For example, acetylcholine, (ACH) (an excitatory neurotransmitter), is broken down by acetylcholinesterase (AchE). Choline is taken up and recycled by the pre-synaptic neuron to synthesize more ACH. Other neurotransmitters such as dopamine are able to diffuse away from their targeted synaptic junctions and are eliminated from the body via the kidneys, or destroyed in the liver. Each neurotransmitter has very specific degradation pathways at regulatory points, which may be the target of the body's own regulatory system or recreational drugs.

[edit] Sub-Neurotransmitters

In the process of neurotransmission, certain unknown components are existent and active, though their activities are subtle.

These components assist and desist procession of neurotransmission by aiding communication or stimulating the building blocks of the neurotransmitter itself. Examples are:

Mesoendodextrine Mesotetradextrine

[edit] See also

[edit] References

  1. ^ n_08/12571289 at Dorland's Medical Dictionary
  2. ^ 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. 

[edit] External links

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