Synapses of identified neurons in the neostriatum.

Studies of the basal ganglia and particularly the neostriatum have described a complex array of neuron types, synapses and putative transmitters. One approach to the study of such an area is to examine identified neurons and thus establish the neural circuits that underlie function. Striatal neurons have been identified under the light microscope by one or more of the following methods: (1) structure, based on Golgi impregnation or the intracellular injection of horseradish peroxidase (HRP); (2) projection area, by the retrograde transport of HRP or the tracing of HRP-injected or Golgi-impregnated axons; (3) chemistry, by immunocytochemistry, histochemistry or autoradiography, to reveal the presence of a selective uptake system for a putative transmitter. Examination of identified neurons in the electron microscope allows the characterization of their afferent synapses (by immunocytochemistry or anterograde degeneration) and their local synaptic output. The afferent and efferent synapses of five classes of identified striatal neurons are discussed: (1) those neurons described in Golgi preparations as medium-size and densely spiny; (2) a large type of striatonigral neuron; (3) GABAergic interneurons; (4) cholinergic neurons; (5) somatostatin-immunoreactive neurons. It is concluded that medium-size densely spiny neurons provide the basic framework of the neural circuits of the neostriatum.