Identification of projecting neurons in rat neostriatal slices.

Spiny neurons projecting to substantia nigra (SN) were identified by combining retrograde transport of tracers in vivo with intracellular staining in vitro. First, neostriatal projection neurons were retrogradely labeled by the injection of fluorescent tracers into the ipsilateral SN. Then, slices were taken from the neostriata containing the retrogradely labeled cells. Cells were impaled for intracellular recording and stained by intracellular dye injection. All cells were of the spiny type and 75% contained both dyes, the intracellularly injected dye as well as the retrogradely transported tracer. Thus, most intracellular recordings in neostriatal slices are obtained from spiny neurons projecting to SN.