^^^^^^ WARM GREETINGS FROM ^^^^^^^^^^^^ :-)
Ivan Suzman 49/39/36 [log in to unmask] :-)
Portland, Maine land of lighthouses 52 deg. F :-)
********************************************************************
Adapted from: Human Anatomy and Physiology
Benjamin/Cummings
by Elaine N. Marieb, 1992
Revised by Ivan Suzman.
Dear list members,
I hope the following will assist all of you in understanding how
levodopa reaches the brain.
The blood-brain barrier is a protective anatomical arrangement
that helps the brain's environment to remain stable. No other
organ is so absolutely dependent on a constant internal chemistry.
In other parts of the body, extra-cellular concentrations of
hormones, amino acids (building blocks of protein), and ions
are in a state of flux, particularly after eating or exercising.
If the brain were exposed to such chemical variations,.its
nerve cells would fire uncontrollably. This is in part because
some hormones and amino acids serve as neurotransmitters,
and some ions (particularly potassium) modify the neuronal
firing.
Blood-carried substances within the brain's capillaries are
separated from the tissue of the brain by (1) the structure of the
capillary walls ; (2) a relatively thick basal lamina on the external
surfaces of these capillaries, and (3) the structure of the astrocytes,
which are star-shaped projections from the neurons of the brain
tissues to the capillaries.
The relative impermeability of brain capillaries results in a
virtually complete blood-brain barrier.
The blood-brain barrier is selective. A point of great importance in
Parkinson's Disease, is that essential amino acids like
phenylalanine (which is the important ingredient in Sinemet),
glucose, and some electrolytes diffuse into the brain.
Blood-borne wastes such as urea, as well as large proteins
are prevented from entering the brain tissue. Small non-essential
amino acids and potassium ions are also prevented from
entering the brain.
The blood-brain barrier is ineffective against fats, fatty acids,
oxygen and carbon dioxide. Fat-soluble
molecules such as blood-borne alcohol, nicotine, anesthetics,
and THC (from marijuana) easily pass by diffusion to the
brain. In some areas of the brain, the blood-brain barrier is
either partial or absent. In these areas, blood-born molecules
gain easy access to brain tissue. One such area is the
vomiting sensor of the brain stem.
Another is the hypothalamus which is extremely important in
PD. Hydration (water balance), body temperature and many
metabolic activities are regulated here. The lack of a blood-
brain barrier in these areas allows "quick-testing" of the
composition of the blood.
The blood-brain barrier in infants and fetuses is incomplete.
Biochemical injury to the brain by toxic exposure is more
likely in children than in adults. Traumatic injury to the brain
may cause localized breaches in the blood-brain barrier.
Medical management of the biochemistry of the brain can
be achieved in some cases by infusing drugs through
gateways in the blood-brain barrier. The drug manitol is one
that is used to create these gateways.
October 23, 1999 Portland, Maine :-)
