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A neurotransmitter’s agonist
is a molecule that has the same effect on the postsynaptic neuron as the neurotransmitter
itself does. An antagonist is a molecule
that blocks the effect that the neurotransmitter normally has on the post-synaptic
neuron.
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It was long thought that a given neuron released
only one kind of neurotransmitter. But today, many experiments show that a single
neuron can produce several different neurotransmitters. |
Neurons that use GABA and glutamate as neurotransmitters
are used by more than 80% of the neurons in the brain and constitute the most
important inhibition and excitation systems, respectively, of the substantia nigra
pars compacta (SNc). |
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This section describes
a few of the best known neurotransmitters that are involved in many functions
in both the central and the peripheral nervous systems. Apart from acetylcholine,
they all belong to the family of amines or amino acids.
Neurotransmitter
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Example
of Disorder Involving It
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Molecular
Structure
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Acetylcholine
is a very widely distributed excitatory neurotransmitter that triggers muscle
contraction and stimulates the excretion of certain hormones. In the central nervous
system, it is involved in wakefulness, attentiveness, anger, aggression, sexuality,
and thirst, among other things. | | Alzheimer’s
disease is associated with a lack of acetylcholine in certain regions of the brain. | |
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Dopamine
is a neurotransmitter involved in controlling movement and posture. It
also modulates mood and plays a central role in positive
reinforcement and dependency. | | The
loss of dopamine in certain parts of the brain causes the muscle rigidity typical
of Parkinson’s disease. | |
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GABA
(gamma-aminobutyric acid) is an inhibitory neurotransmitter that is very
widely distributed in the neurons of the cortex. GABA contributes to motor control,
vision, and many other cortical functions. It
also regulates anxiety. | | Some
drugs that increase the level of GABA in the brain are used to treat epilepsy
and to calm the trembling of people suffering from Huntington’s disease. | |
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Glutamate
is a major excitatory neurotransmitter that is associated with learning
and memory. | | It
is also thought to be associated with Alzheimer’s disease, whose first symptoms
include memory malfunctions. | |
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Norepinephrine
is a neurotransmitter that is important for attentiveness, emotions, sleeping,
dreaming, and learning. Norepinephrine is also released as a hormone into the
blood, where it causes blood vessels to contract and heart rate to increase. | | Norepinephrine
plays a role in mood disorders such as manic depression. | |
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Serotonin
contributes to various functions, such as regulating body temperature,
sleep, mood, appetite, and pain. | | Depression,
suicide, impulsive behaviour, and agressiveness all appear to involve certain
imbalances in serotonin. | | | credit: The Brain McGrill | | |
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