RELATIONAL FEATURES OF ACETYLCHOLINE, NORADRENALINE, SEROTONIN AND GABA AXON TERMINALS IN THE STRATUM-RADIATUM OF ADULT-RAT HIPPOCAMPUS (CA1)

In a well-defined sector of adult rat hippocampus (CA1, stratum radiat um), the ultrastructural features of acetylcholine (ACh), noradrenalin e (NA), serotonin (5-HT) and GABA axon terminals (varicosities) were c ompared by electron microscopy after immunostaining with antibodies ag ainst choline acetyltransferase, NA, 5-HT and glutamic acid decarboxyl ase. Approximately 100 sectional profiles of each type were analyzed f or size, presence of a synaptic membrane specialization (synaptic inci dence) and composition of the microenvironment. An equivalent number o f immunonegative varicosity profiles selected at random from the same micrographs were similarly examined. ACh, NA and 5-HT varicosity profi les were of comparable size, and significantly smaller than GABA profi les. They exhibited a low frequency of junctional specialization, amou nting to 7%, 15% and 21%, respectively, when extrapolated to the whole volume of these terminals. In contrast, GABA varicosities appeared en tirely synaptic. The ACh, NA and 5-HT varicosities also differed from their GABA counterparts in being juxtaposed to a greater number of unl abeled axonal varicosities and a lower number of dendritic branches. I n addition, the microenvironment of immunostained terminals showed a m uch lower number of dendritic spines than that of immunonegative varic osities. This latter finding was viewed as another indication that pre dominantly asynaptic varicosities do not maintain particular relations hips with their immediate surround. It was also concluded that volume transmission represents a major mode of transmission for ACh, NA and 5 -HT in adult rat hippocampus, thus contributing to the properties and functions assigned to these transmitters in this part of brain. (C) 19 95 Wiley-Liss, Inc.