Isolation of metabolically distinct synaptosomes on Percoll gradients

Synaptosomes were prepared from whole rat brain by six different methods based on gradients of sucrose, Ficoll or Percoll. In these, the synthesis and calcium-specific release of amino acids were assessed by two different procedures. Preparations based on sucrose showed the least calcium-specific release, followed by Ficoll-derived synaptosomes. As previously described, Percoll gave two separate populations of synaptosomes, both very active in terms of release of aspartate, glutamate, and GABA. The data involving release and synthesis were not identical, but did agree in the following: in low-density synaptosomes, haloperidol blocked both the release and synthesis of glutamate, but was without effect in the heavier population. 2-chloroadenosine and 2-oxoglutarate affected both release and synthesis only in the high-density population. Dopamine blocked aspartate release and synthesis only in the high-density population. These results suggest that haloperidol interferes with glutamate release and synthesis via a mechanism which may not involve adenosine, serotonin, or dopamine.