Projections from the pretectal complex to the thalamic lateral dorsal nucleus of the cat

The subcortical projections to the lateral dorsal nucleus (LD) of the cat thalamus were studied with retrograde transport techniques. Deposits of horseradish peroxidase (HRP) or fluorescent tracers were placed unilaterally in LD of adult cats, using electrophoretic or pressure injection techniques. Following post-injection survival periods of 1, 2 or 3 days, HRP retrogradely labeled cells were identified in sections reacted with benzidine dihydrochloride; fluorescent labeled cells were identified by fluorescent microscopy. Injections in LD result in retrogradely labeled neurons in all nuclei of the pretectal complex, including the nucleus of the optic tract (NTO), the posterior pretectal nucleus (NPP), the anterior pretectal nucleus (NPA), the pretectal olivary nucleus (NOL), and the medial pretectal nucleus (NPM). Small electrophoretic injections of HRP were used to investigate a possible topographic organization of the pretectal projections. Results from a variety of injection sites indicate only a subtle rostral-caudal gradient. That is, small injection sites in rostral LD result in retrograde labeling of neuron somata in the rostral parts of NTO, NPA and NPP, and throughout NPM. Injections in caudal LD result in labeled cells more caudally situated in NTO, NPA, NPP, and throughout NPM. Injections in the pulvinar (Pul) also result in retrogradely labeled cells in the pretectal complex, particularly NTO, NPP, and NOL. Experiments with injections of distinguishable fluorescent tracers in LD and Pul reveal that many more cells project to Pul than to LD. These experiments also reveal that while neurons that project to LD are intermingled with neurons that project to Pul, the two projections originate from separate subpopulations of cells. These results are discussed in regard to phylogenetic comparison of pretectal projections and subcortical pathways of sensory input to the limbic system.