Biogenic amines (Monoamines)

The biogenic amines comprise (1) the catecholamines norepinephrine, epinephrine, dopamine, and
(2) serotonin (5-Hydroxytryptamine). They are chemical messengers used by neurons as well as by endocrine
and other cells. A catecholamine is an organic compound that contains a catechol nucleus and an amino group.
The monoamines are synthesized within each neuron from the amino acid tyrosine in the presence of
appropriate enzymes. The sequence of enzymatic steps from tyrosine are dopamine to norepinephrine to
epinephrine. Serotonin is synthesized within the neuron from the amino acid tryptophan.
The monamine transmitters are released into the synaptic cleft. The recovery of the released transmitter
occurs by uptake into the presynaptic ending where it is recycled for future use. Neurons releasing
norepinephrine and epinephrine are adrenergic, those releasing doparnine, dopaminergic, and those releasing
serotonin, sertonergic. The activity of these monoamine transmitters is limited by their reuptake by transporters
into the presynaptic ending, where they are recycled into vesicles for future release.
Monoamines may have different specific effects on the postsynaptic cell depending on the type or subtype of
receptor protein. Several varieties of subtypes of receptors are present for each monoamine. These receptors
have different properties and are differentially distributed in the nervous system with the result that they have a
variety of effects and even opposite responses. Thus, it is not possible to state that a specific neurotransmitter
is either excitatory or inhibitory in its action; it can be both depending on the postsynaptic receptor type.
The three biogenic pathways in the brainstem are (1) the dopaminergic pathways (system) which originate in
the midbrain; (2) the noradrenergic pathways (system) which originate in two nuclear groups called the locus
coeruleus (LC) and lateral tegmental nucleus; and (3) the serotonergic pathways (system) which originate in the
raphe nuclei.
[link to www1.indstate.edu]
Serotonin (5-hydroxytryptamine, 5-HT)

Serotonin is present in the cell bodies of neurons located primarily in the raphe nuclei of the brainstem. It is
also found in mast cells (associated with nociception), platelets, and enterochromaffin cells of the gut. Axons
from the raphe nuclei are distributed diffusely throughout the brain and spinal cord. Neurons of the rostral
(midbrain) raphe nuclei have axons that join the median forebrain bundle and terminate in the diencephalon,
striatum, cerebral cortex, and the ependyma lining the ventricles. Those in the middle nuclei (Pons) have
axons that terminate in the cerebellum and reticular formation, and those from the caudal nuclei (medulla) have
axons that project to the spinal nucleus of the trigeminal nerve and to the gray matter of the spinal cord.
Following its release from mast cells and other damaged cells, serotonin is the agent that activates and
sensitizes the nociceptors of the primary "pain" fibers (A-delta and C fibers). In the nervous system, serotonin
is involved with second messenger systems as a modulator. The projections to the spinal trigminal nucleus and
dorsal hom of the spinal cord act to inhibit the pain fibers, and those to the ventral hom act to activate by
facilitation. In addition, serotonin has roles in a complex of physiologic activities including changes in blood
pressure, body temperatures, the sleep-wake cycle, certain psychological and psychotic states, and responses
to certain drugs.
The drug fluoxetine (Prozac) is widely used to help people cope with a range of behavioral symptoms
including degrees of mild to severe depression, deficiency in the ability to experience pleasure, fear of rejection,
and lack of self confidence. It acts by blocking the serotonin transporter in the axon terminal and the reuptake
of serotonin from the synaptic cleft. The result is an increase in the level and duration of the action of serotonin,
which is translated in a few weeks into the therapeutic effect.