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100

 Functional Assessment of Urinary Neuro-biogenic Amines—A COMPREHENSIVE GUIDE

ferent vagal nerve. Glutamate may be

primarily oxidized and metabolized in

the intestines, and serves to nourish the

enterocytes of the gut mucosa as gluta-

mine. Amino acid decarboxylation reac-

tions in the gastrointestinal microbiota

are known to generate biogenic amines

and neuroactive molecules which may

not only affect the levels of glutamate

and GABA, but affect gut function.

Animal studies indicate that free glu-

tamate in the gut lumen may activate

the vagus nerve and stimulate the brain.

Glutamate signaling via taste and gut re-

ceptors may affect physiologic functions

such as digestion, thermoregulation and

energy production.

Receptors:

Glutamate signaling may occur

through a variety of Glutamate recep-

tors. There are four types of glutamate

receptors. Combinations of different

glutamate receptor subtypes may be

found in different areas of the brain and

the peripheral nervous system, and may

determine how individual neurons re-

spond to glutamate. Glutamate receptor

function or conformation may be affect-

ed by mutations or single nucleotide

polymorphisms (SNPs) in the DNA that

codes for each receptor subtype.

Ionotropic receptors regulate so-

dium, potassium or calcium (charged

ions) flow into and out of neurons. The

level of ions inside the neuron deter-

mines its action potential and ability

to fire. There are three families of iono-

tropic receptors:

Amino-3-hydroxy-5-methyl-4-

isoxazole propionic acid (AMPA)

control sodium and potassium influx

into neurons

post-synaptic

mediate fast excitatory transmissions

in the brain

AMPA receptors have been found

in the gastrointestinal tract (animal

studies)

GluR1-4 are AMPA receptors

Kainate receptors

control sodium and potassium influx

into neurons

primarily pre-synaptic

may regulate glutamate release

GluR5-7 and GluR KA-1 and -2 are

kainate receptors

N-methyl-D-aspartate (NMDA)

controls calcium influx and sodium/

potassium influx

binding of magnesium ions in the

receptor’s ion channel prevents

the influx of calcium and acts as a

receptor blocker

dysfunctions may contribute to

neuropsychiatric disorders (mood,

schizophrenia)

animal studies indicate that NMDA

receptors may contribute to learning

conditioned fear (fear-potential

startle), which may contribute to

development of post-traumatic

stress disorder (PTSD)

primarily post-synaptic

receptors are concentrated in the

limbic system of the brain

NMDA receptors have been found

in the gastrointestinal tract (animal

studies)

animal studies indicate that exercise

may upregulate NMDA receptor

expression