Projection neurons in the superficial layers of the superior colliculus in the rat: a topographic and quantitative morphometric analysis.

The present study deals with qualitative and quantitative investigations in the superior colliculus of the rat. Tracer studies were correlated with Nissl staining to calculate the quantitative ratio between projection neurons and interneurons in the upper three layers of the superior colliculus. In order to reveal the projections from the superior colliculus, the first group of rats received injections of the tracer FluoroGold into the nucleus lateralis posterior thalami, the lateral geniculate body, the nucleus parabigeminalis, and the predorsal bundle. Commissural connections between the superior colliculus were traced in a second group of animals, which received Biocytin and FluoroGold injections in the upper layers of the right superior colliculus and small deposits of the carbocyanine tracer DiI in the deeper layers of the left superior colliculus. Additionally, double-labelling with FluoroGold tracing and the histochemical detection of NADPH-diaphorase activity was carried out to distinguish between projection neurons and interneurons. These experiments showed that 66% of the neurons within the superficial layers of the superior colliculus were represented by ascending projection neurons, whereas only 2-3% could be identified as descending neurons. Ascending neurons were scattered throughout the three laminae and descending neurons were localized in a cluster-like pattern. Approximately 2-3% of the neurons in the superficial layers were found to be commissural and interlayer neurons which were represented by an identical cell type, since both transcommissural and interlayer processes were originated from their somata. The somata of these commissural-interlayer neurons were all located in the mediorostral part of the superior colliculus and contained NADPH-diaphorase activity. The axon terminals of the interlayer-commissural neurons formed net-like structures which surrounded neuronal somata within the ipsilateral deep layers and within the contralateral upper layers of the superior colliculus, respectively.