Most of the data from studies on synapses in situ or on synaptosomes have been on cholinergic transmission. There is a vast family of chemical synapses that utilize biogenic amines (see Chap. 12) as neurotransmitter substances. Morphologically, catecholaminergic synapses are similar but possess, in addition to clear vesicles, slightly larger dense-core or granular vesicles of variable dimension (Figs. 1-8 and 1-10). These vesicles were identified first as synaptic vesicles by Grillo and Palay (see Bloom [16]), who segregated classes of granular vesicles based on vesicle and core size, but no relationship was made between granular vesicles and transmitter substances. About the same time, EM autoradiographic techniques were being employed and, using tritiated norepinephrine, Wolfe and co-workers [17] labeled granular vesicles within axonal terminals. Catecholaminergic vesicles generally are classified on a size basis, and not all have dense cores. Another, still unclassified, category of synapses may be the so-called silent synapses observed in CNS tissue both in vitro and in vivo. These synapses are morphologically identical to functional synapses but are physiologically dormant.