SIMULTANEOUS MEASUREMENT BY HPLC OF THE EXCITATORY AMINO-ACID TRANSMITTER CANDIDATES HOMOCYSTEATE AND HOMOCYSTEINE SULFINATE SUPPORTS A PREDOMINANT ASTROCYTIC LOCALIZATION

Primary cultures of mouse cerebral cortex neurons, cerebellar granule cells and cortical astrocytes were maintained in vitro for respectively 8-10, 7-10 and 21-24 days. Following these times, amino acids were extracted from the cells by use of ice-cold 70% (v/v) ethanol and the extracts lyophilised. The lyophilised extracts when resuspended were subjected to reverse-phase high performance liquid chromatographic (HPLC) analysis for detection of free amino acids. Samples of cell culture growth medium and water blanks were treated in a similar manner. Identification of L-homocysteate (HCA) and L-homocysteine sulphinate (HCSA) was undertaken by matching retention times with regard to external standards and by 'spiking' cell extracts with authentic compounds. On this basis, HCA and HCSA were consistently detectable in astrocytes at levels of, respectively, 72.3 +/- 33.7 pmol/mg protein (n=24) and 49.4 +/- 28,7 pmol/mg protein (n=24). However, in neurons, a peak corresponding to HCSA could not be detected above the background noise, while the area of the peak corresponding to HCA was always greater than, but not significantly different from, that of the background noise present in water blanks. HCA and HCSA were not detectable in the serum used for preparation of the cell culture growth medium. Taken together, these findings indicate a predominant localisation of HCA and HCSA in astrocytes which, at least in culture, appear to possess the metabolic machinery necessary for synthesising and storing these amino acids without any neuronal influence.