Serotoninergic afferents in the monkey neostriatum

Golgi and electron microscopic observations of the neostriatum of macaque monkeys reveal the presence of thin axons of extrinsic origin which produce a profuse arborization of very fine beaded branches. The varicosities contain pleomorphic synaptic vesicles, and may form strongly asymmetric axospinous synapses. Immunocytochemical methods utilizing an unlabeled antibody against serotonin followed by peroxidase-antiperoxidase complex and further histochemical visualization demonstrate that some elements conforming to the above description exhibit specific immunoreactivity. Relatively few of the labeled profiles form synapses and did so with small dendritic spines. In these cases the membrane specializations are strongly asymmetric. The neuropil also contains many unstained elements of similar morphology. The findings provide the morphologic features of serotoninergic axons in the neostriatum. Their properties may be shared by other monoaminergic afferents. It is suggested that serotonin is released as a neurotransmitter from synapsing boutons, and as a neuromodulator from non-synapsing varicosities. This distinction may correlate with the serotonin receptors 1 and 2 which apparently are responsible for excitatory and inhibitory actions, respectively.