The organization of the striatal output system: a single-cell juxtacellular labeling study in the rat

The aim of the present study was to provide a detailed account of the axona l branching pattern of striatal projection neurons in the rat. Seventy-seve n striatofugal neurons were singly labeled following juxtacellular injectio n of biotin dextran amine. Their axons were entirely reconstructed along th e sagittal plane with the help of a light microscope equipped with a camera lucida. The major findings of this study can be summarized as follows, (1) the striatofugal system originates from medium-sized spiny neurons that pr oject only to globus pallidus (GP, type I, 36.4%), to both GP and substanti a nigra pars reticulata (SNr, type II, 26%), or to globus pallidus, entoped uncular nucleus (EP) and SNr (type III, 37.6%); (2) the striatofugal system displays a high degree of axonal collateralization; about two-thirds of it s axons arborize into two or three striatal target structures; (3) virtuall y all striatofugal axons send collaterals to the GP and none project exclus ively to the EP and,or SNr; (4) the three types of striatal projection neur ons share similar somatodendritic morphology and have no preferential distr ibution in the dorsal striatum. These data, together with those of previous investigations, indicate that the striatofugal system can no longer be con sidered to be a simple dual (direct/indirect) projection system. Instead, i t stands out as a complex and widely distributed neuronal network whose ele ments are endowed with a highly patterned set of axon collaterals, which al lows them to control in an exquisitely precise manner the flow of informati on along the main axis of the basal ganglia. (C) 2000 Elsevier Science Irel and Ltd. All rights reserved.