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CONTROL OF BRAIN NEUROTRANSMITTER SYNTHESIS BY PRECURSOR AVAILABILITY AND NUTRITIONAL STATE

CONTROL OF BRAIN NEUROTRANSMITTER SYNTHESIS BY PRECURSOR AVAILABILITY AND NUTRITIONAL STATE

  1. NeuroChi
    The body synthesizes two families of compounds that
    it uses to carry signals from one cell to another the
    hormones and the neurotransmitters. Hormones are
    distributed ubiquitously by the circulation, but only
    cells containing specific receptors can "'decode" them.
    In addition, hormones include a considerable variety
    of chemicals, covering a wide range of polarity and
    molecular weight. Neurotransmitters, in contrast, are
    distributed only to cells that are anatomically close
    to their neurons of origin. They are relatively homogeneous:
    all of the presently accepted group are low
    molecular weight, water-soluble compounds that are
    charged at physiological pH and are closely related
    to amino acids or to such other dietary constituents
    as choline. The studies that are summarized in this
    brief review provide evidence for another major difference
    between most neurotransmitters and all hormones,
    i.e. their different dependencies on precursor
    availability. The rates at which serotonin, acetylcholine,
    and perhaps other neurotransmitters are synthesized
    in the brain depend in part on "open-loop" control
    systems: synthesis increases or decreases as a consequence
    of diet-induced changes in the plasma concentrations
    of their precursors. The production of
    some hormones can be modified by imposing grossly
    unphysiologic limitations on the availability of their
    precursors (for example, the virtual absence of dietary
    iodine or lack of ultraviolet light exposure will limit
    production of thyroxine or vitamin D 3 respectively).
    To our knowledge, however, no evidence exists that
    normal variations in precursor levels ever control the
    rates at which glands produce or secrete steroids, peptides,
    or other hormones. In contrast, the simple
    choice between eating an apple or a chicken leg may
    determine how much serotonin the brain will make
    during the postprandial period, and a week of luncheon
    omelettes would probably elevate acetylcholine
    levels in the basal ganglia.

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