Increased white blood cell count in this substance can indicate Gullain-Barré syndrome, and testing this liquid is one half of the diagnostic procedure for encephalitis.
High concentrations of proline in this substance can increase the rate of glutamate release.  This substance is produced by and circulated via the ependymal cells of the choroid plexus. It is typically found in the subarachnoid space, and arachnoid villi are sites of its reabsorption. Increased white blood cell count in this substance can indicate Gullain-Barré syndrome, and testing this liquid is one half of the diagnostic procedure for encephalitis. For 10 points, identify this fluid, the target of a lumbar puncture, a procedure colloquially known as a "spinal tap."
High concentrations of proline in this substance can increase the rate of glutamate release.  
{cerebrospinal fluid} [or CSF]