ABSTRACT
Early in the history of neuroanatomy, the medium spiny (MS) neurone, the main neuronal class of the mammalian striatum, was characterized as a medium-sized cell body, with densely spiny dendrites, and local intensely-ramifying axon collaterals (Cajal, 1911). The relatively small cell body and intensive local axonal arborization prompted early researchers to consider these neurones to be interneurones (Vogt and Vogt, 1920). Despite some indications from Golgi studies (Leontovich, 1975; DiFiglia, Pasik and Pasik, 1976), it was not until intracellular neuronal tracers became available that it was established that in fact the main axon of MS neurones, besides giving off local collaterals, left the striatum to provide an intense innervation of other nuclei of the basal ganglia (Chang, Wilson and Kitai, 1981; Kawaguchi, Wilson and Emson, 1990). Through this projection, MS neurones inhibit neurones in target nuclei such as the globus pallidus (Park, Falls and Kitai, 1982). The nature of synaptic transmission at the terminals of local MS axon collaterals has not been described so far. Multiple failed attempts to uncover such transmission at the striatal level have left as an open question the functional impact of these local axon collaterals (Jaeger, Kita and Wilson, 1994). Nonetheless, given that no synaptic specialization of
collaterals within the striatum has taken place, the prevalent conclusion among most researchers is that local synapses between MS neurones are also inhibitory.
