Septal neurons containing glutamic acid decarboxylase immunoreactivity project to the hippocampal region in the rat brain.

Injections of the fluorescent dyes Fast Blue or Granular Blue into either the hippocampus (volume approximately 50 nl) or the entorhinal area (100-150 nl) resulted in labeling by retrograde axonal transport of cells in the diagonal band of Broca (dbB) and the medial septum (MS). A large number (approximately 30%) of these cells contained glutamic acid decarboxylase (GAD)-like immunoreactivity, as determined by combined retrograde fluorescent tracing and GAD-immunohistochemistry. Not all GAD positive cells in the dbB and MS were labeled by fluorochromes in a single experiment. The GAD-stained and fluorochrome-containing cells were present at all rostro-caudal levels of the septum and appeared not to belong to any single morphological class of cells. Double staining experiments showed that the GAD-positive cells did not contain acetylcholinesterase reaction product. These findings provide evidence that a significant portion of the septo-hippocampal projection may utilize gamma-aminobutyric acid as a neurotransmitter.