Cholinergic and GABAergic neurotransmission in the fascia dentata: electron microscopic immunocytochemical studies in rodents and primates.

This chapter summarizes immunocytochemical studies on the cholinergic and GABAergic innervation of dentate neurons. There are at least three types of neuron that give rise to the GABAergic innervation of dentate granule cells. First, there are the basket cells located in and directly underneath the granular layer. Their axons form a pericellular plexus around the cell bodies and proximal dendrites of the granule cells. Second are the dentate axo-axonic cells. These neurons are located in the innermost portion of the molecular layer and give rise to rows of boutons that impinge on the axon initial segments of the granule cells. Finally, there are GABAergic neurons in the septal region that are known to project to the hippocampus and fascia dentata. All types of GABAergic neurons establish symmetric synapses. Basket cells and axo-axonic cells are major inhibitory components of the fascia dentata. The septohippocampal GABAergic neurons selectively contact other GABAergic cells in the fascia dentata thereby serving disinhibition of the granule cells. The cholinergic fibers arising from the medial septum form a diffuse network in all layers of the fascia dentata. Electron microscopy reveals that both symmetric and asymmetric synapses are established. Cholinergic terminals contact granule cells as well as GABAergic and peptidergic neurons in the hilar region. The above data were obtained in rats. Preliminary studies in monkeys have shown that the types of cholinergic synapse are very similar in the rodent and primate fascia dentata. However, some differences were noted in the types of GABAergic synapse. We have thus observed numerous asymmetric synapses with spines in addition to the well-known symmetric synapses with dendritic shafts, cell bodies and axon initial segments.