BASAL EXPRESSION AND INDUCTION OF GLUTAMATE-DECARBOXYLASE AND GABA INEXCITATORY GRANULE CELLS OF THE RAT AND MONKEY HIPPOCAMPAL DENTATE GYRUS

The excitatory, glutamatergic granule cells of the hippocampal dentate gyrus are presumed to play central roles in normal learning and memor y, and in the genesis of spontaneous seizure discharges that originate within the temporal lobe. In localizing the two GABA-producing forms of glutamate decarboxylase (GAD65 and GAD67) in the normal hippocampus as a prelude to experimental epilepsy studies, we unexpectedly discov ered that, in addition to its presence in hippocampal nonprincipal cel ls, GAD67-like immunoreactivity (LI) was present in the excitatory axo ns (the messy fibers) of normal dentate granule cells of rats, mice, a nd the monkey Macaca nemestrina. Using improved immunocytochemical met hods, we were also able to detect GABA-LI in normal granule cell, soma ta and processes. Conversely, GAD65-LI was undetectable in normal gran ule cells. Perforant pathway stimulation for 24 hours, which evoked po pulation spikes and epileptiform discharges in both dentate granule ce lls and hippocampal pyramidal neurons, induced GAD65-, GAD67-, and GAB A-LI only in granule cells. Despite prolonged excitation, normally GAD - and GABA-negative dentate hilar neurons and hippocampal pyramidal ce lls remained immunonegative. Induced granule cell GAD65-, GAD67-, and GABA-LI remained elevated above control immunoreactivity for at least 4 days after the end of stimulation. Pre-embedding immunocytochemical electron microscopy confirmed that GAD67- and GABA-LI were induced sel ectively within granule cells; granule cell layer glia and endothelial cells were GAD- and GABA-immunonegative. In situ hybridization after stimulation revealed a similarly selective induction of GAD65 and GAD6 7 mRNA in dentate granule cells. Neurochemical analysis of the microdi ssected dentate gyrus and area CA1 determined whether changes in GAD- and GABA-LI reflect changes in the concentrations of chemically identi fied GAD and GABA. Stimulation for 24 hours increased GAD67 and GABA c oncentrations sixfold in the dentate gyrus, and decreased the concentr ations of the GABA precursors glutamate-and glutamine. No significant change in GAD65 concentration was detected in the microdissected denta te gyrus despite the induction of GAD65-LI. The concentrations of GAD6 5, GAD67, GABA, glutamate and glutamine in area CA1 were not significa ntly different from control concentrations. These results indicate tha t dentate granule cells normally contain two ''fast-acting'' amino aci d neurotransmitters, one excitatory and one inhibitory, and may theref ore produce both excitatory and inhibitory effects. Although the physi ological role of granule cell GABA is unknown, the discovery of both b asal and activity-dependent GAD and GABA expression in glutamatergic d entate granule cells may have fundamental implications for physiologic al plasticity presumed to underlie normal learning and memory. Further more, the induction of granule cell GAD and GABA. by afferent excitati on may constitute a mechanism by which epileptic seizures trigger comp ensatory interictal network inhibition or GABA-mediated neurotrophic e ffects. (C) 1996 Wiley-Liss, Inc.