Selective labeling by propidium iodide injected into the lateral cerebral ventricle of the rat

Injections of propidium iodide (PI) into the lateral cerebral ventricle of the rat resulted in a bilateral labeling in the septohippocampal nuclei, substantia nigra (SN), ventral tegmental area (VTA), retrorubral nuclei (rr), dorsal and median raphe nuclei, regions within and dorsal to the medial lemniscus of the caudal midbrain, and Purkinje cells in the cerebellum. No labeled neurons were seen in other areas of the brain. The data suggest that PI appears to exhibit selective labeling, and the mechanism underlying the selective labeling is discussed. Combined with Faglu histofluorescence, it was found that all PI-labeled cells in SN-VTA-rr were catecholamine (CA) neurons. After a transection of the medial forebrain bundle immediately before the PI injection, an accumulation of PI was only seen in the distal segments of severed nigrostriatal CA fibers. This provides a strong evidence that PI labeling of CA cells in SN-VTA-rr is due to axonal uptake and retrograde transport.