Glutamic acid decarboxylase (GAD) immunocytochemistry of developing rabbit hippocampus

Immunocytochemical techniques were used to examine the synaptic relations of inhibitory interneurons in the developing rabbit hippocampus. Glutamic acid decarboxylase (GAD), the synthesizing enzyme for the inhibitory neurotransmitter GABA, was found in interneurons of immature (8 d old) as well as mature (30 d old) tissue. GAD-immunoreactivity was seen in somata, dendrites, and axon terminals of interneurons at both ages. Electron-microscopic examination revealed that GAD-positive "terminals" in immature tissue were often not associated with the usual synaptic specializations, but were rather in simple apposition to the "postsynaptic" element. In mature tissue, GAD-positive terminals made symmetric contacts primarily with pyramidal cell somata, initial segments, and proximal dendrites. In addition, GAD-positive terminals synapsed onto both GAD-positive and GAD-negative interneuron profiles.