PROJECTION STATUS OF CALBINDIN-IMMUNOREACTIVE AND PARVALBUMIN-IMMUNOREACTIVE NEURONS IN THE SUPERFICIAL LAYERS OF THE RATS SUPERIOR COLLICULUS

Immunocytochemistry and retrograde labeling were used to define the th alamic projections of calbindin- and parvalbumin-containing cells in s uperficial layers of the rat's superior colliculus (SC). Quantitative analysis revealed that 90.8 +/- 2.2% (mean +/- standard deviation) of the calbindin-immunoreactive neurons in the stratum griseum superficia le (SGS) projected to the dorsal lateral geniculate nucleus (LGNd) and that 91.3 +/- 4.3% of calbindin-immunoreactive neurons in the stratum opticum (SO) projected to the lateral posterior nucleus (LP). In cont rast, only 17.3 +/- 2.5% of parvalbumin-immunoreactive neurons in the SGS were found to project to the LGNd and 16.5 +/- 3.1% of the parvalb umin-immunoreactive SO cells were retrogradely labeled after LP inject ions. Few of the parvalbumin-immunoreactive neurons in either the SGS (7.2 +/- 2.5%) or the SO (9.2 +/- 2.5%) were GABA positive. The retrog rade-labeling results suggest that parvalbumin-immunoreactive neurons in the rat's SO and SGS may either be primarily interneurons or have d escending projections, while calbindin-containing cells are primarily thalamic projection neurons. These results are consistent with data fr om other rodents, but almost exactly the opposite of data that have be en reported for the cat for these same populations of SC projection ne urons. Such interspecies differences raise questions regarding the fun ctional importance of expressing one calcium-binding protein versus an other in a specific neuronal population.