Unlike prototypic GP-TI cells, none of the GP-TA neurons (n = 5) gave rise to a descending projection axon that targeted downstream BG nuclei. Instead, all reconstructed GP-TA neurons emitted local axon collaterals (although somewhat restricted; see below) and at least one ascending projection axon collateral that targeted striatum (Figure 4). Reconstructing the full axonal arborizations of every labeled GP-TA neuron was beyond the scope of this study, but we visually Dolutegravir ic50 confirmed the extrinsic axonal projections of another nine GP-TA neurons (revealed with Ni-DAB). All but one of these neurons gave rise to only ascending axonal projections that

entered and ramified in striatum; one unusual neuron innervated striatum and EPN. GP-TA neurons thus challenge the widely-held assumption that all GPe neurons innervate STN (Baufreton et al., 2009, Bevan et al., 1998, Smith et al., 1998 and Wichmann and DeLong, 1996). The specific striatal innervation of the two fully-reconstructed GP-TA neurons (cells #6 and #7) was massive; the main axon split to form up to five ascending collaterals that established dense clusters of boutons over large striatal territories (Figures 4A and 4B). Remarkably, each GP-TA neuron gave rise to thousands of axonal boutons in striatum (9,085 and 13,789 boutons for cells #6 and #7). This extensive striatal innervation meant that total axon lengths of GP-TA neurons

(126.2 and 164.2 mm for cells #6 and #7) were considerably longer than those of TCL GP-TI neurons (37.8 and 59.4 mm for cells #1 and #2). Electrophysiological and molecular diversity in GPe is thus mirrored Akt inhibitor drugs by a profound structural diversity. While GP-TI neurons innervate STN and other downstream BG nuclei, GP-TA neurons do not conform to this prototypic connectivity but instead provide a massive innervation of striatum. Our discovery

of a novel GPe cell type that only projects to striatum raises the issue of which types of striatal neuron are innervated. We first tested whether identified GP-TA neurons target the three major classes of aspiny interneuron (two GABAergic, one cholinergic) by revealing immunoreactivity for PV, nitric oxide synthase (NOS), or ChAT (Tepper and Bolam, 2004), respectively, with a light-brown DAB precipitate. The axons of single neurobiotin-labeled GP-TA neurons (n = 3) were revealed with a black Ni-DAB precipitate. Axonal boutons were found in close apposition to the somata and proximal dendrites of all three classes of interneuron, some of which were targeted by clusters of apposed boutons arranged in a “basket-like” manner (Figures 5A–C). Such specialized arrangements are indicative of synaptic contacts established by GPe cells (Bevan et al., 1998 and Sadek et al., 2007). This analysis thus suggests that different classes of striatal interneurons are targeted by GP-TA neurons.