Re: What causes certain areas in the brain to respond to change in heomostasis?

Dear Mari,
  You have asked a difficult question.  They mechanisms by which brain 
cells detect and respond to changes in the homeostasis of circulating 
molecules are still not entirely understood.  Certain brain regions like 
the hypothalamus lack a blood brain barrier, so that cells there can 
monitor changes in blood constituents.  For example, there is now good 
evidence that non-neural glial cells in the hypothalamus possess 
high-capacity glucose transporter proteins that allow them to respond to 
changes in blood glucose (see Young, JK, Baker, JH, & Montes, MI "The brain 
response to 2-deoxy glucose is blocked by a glial drug" in the journal 
Pharmacology, Biochem. & Behavior vol. 67, pp. 233-239, 2000; also Leloup, 
C, et al., "Glucose transporter 2 (GLUT2): expression in specific brain 
nuclei" Brain Research vol. 638:221-236, 1994).  Similarly, glial cells may 
participate in the reaction of a respiratory center in the medulla to 
changes in blood pH and CO2 (see Erlichman JS, et al., "Ventilatory effects 
of glial dysfunction in a rat brain stem chemoreceptor region" in J. of 
Applied Physiology vol. 85, p. 1599-1604, 1998).  Cells in the parathyroid 
respond to changes in blood calcium because they possess a newly discovered 
cell membrane protein that binds calcium and regulates the release of 
hormone from these cells (see Brown EM, "Cloning ..of an extracellular Ca+ 
sensing receptor from bovine parathyroid" Nature vol. 366, p. 575, 1993). 
You can read abstracts of these papers by going to the NIH PubMed site 
(www.pubmed.gov) and typing in the names of these authors.

John Young, Dept. Anatomy, Howard Univ. College of Medicine