May as well combine two interests here.

1: Brain Res. 2001 Jan 5;888(1):117-127.

Acute stress increases permeability of the blood-brain-barrier through
activation of brain mast cells.

Esposito P, Gheorghe D, Kandere K, Pang X, Connolly R, Jacobson S, Theoharides
TC.

Departments of Pharmacology and Experimental Therapeutics, Tufts University
School of Medicine, 136 Harrison Avenue, Boston, MA 02111, USA.

Disruption of the blood-brain-barrier (BBB) is important in the pathophysiology
of various inflammatory conditions of the central nervous system (CNS), such as
multiple sclerosis (MS), in which breakdown of the BBB precedes any clinical or
pathological findings. There is some evidence that relapsing-remitting MS
attacks may be correlated with certain types of acute stressful episodes. Stress
typically activates the hypothalamic-pituitary-adrenal (HPA) axis through the
release of corticotropin releasing hormone (CRH), leading to production of
glucocorticoids that down regulate immune responses. However, acute stress also
has pro-inflammatory effects that appear to be mediated through activation of
mast cells. Here we show that acute stress by immobilization increased
permeability of rat BBB to intravenous 99Technetium gluceptate (99Tc). This
effect was statistically significant in the diencephalon and the cerebellum,
while it was absent in the cerebral cortex where there are not mast cells.
Immobilization stress also induced activation of mast cells in diencephalon, the
site where most mast cells are found in the rat brain. Both BBB permeability and
mast cell activation were inhibited by the 'mast cell stabilizer' disodium
cromoglycate (cromolyn). These results expand the pathophysiology of mast cells
and implicate them in CNS disorders, that may possibly be induced or exacerbated
by stress.

PMID: 11146058 [PubMed - indexed for MEDLINE]

...might be that one of a number of factors leading to PD is that our individual responses to stress result in varied amounts of hormonal release which lead to individualized disruption of the BBB.

Toxins find their way from the gut by way of its similarly compromised barrier into the bloodstream and thence into the brain. Among those toxins are bacterial LPS as well as metals, pesticides, etc.

LPS potentiates the toxicity of mercury, rotenone, etc.

In previously sensitized individuals, LPS activates the microglia (brain immune cells) leading to over-reaction and self harm.

One of the higher concentrations of microglia is in the substantia nigra.

This is not the whole tapestry, but seems reasonable as one of the primary threads.

http://www.fi.edu/brain/stress.htm

The site covers a lot more than stress. Clear, understandable, cool pictures, good design too.

If you barely understand what your neuro is talking about, a half hour here will bring you up to speed. Spend a couple of hours and you will know more than he does.