LOCALIZATION OF PARVALBUMIN, CALRETININ, AND CALBINDIN D-28K IN IDENTIFIED EXTRAOCULAR MOTONEURONS AND INTERNUCLEAR NEURONS OF THE CAT

Calcium-binding proteins have: been shown to be excellent markers of s pecific neuronal populations. We aimed to characterize the expression of calcium-binding proteins in identified populations of the cat extra ocular motor nuclei by means of immunohistochemistry against parvalbum in, calretinin, and calbindin D-28k. Abducens, medial rectus, and troc hlear motoneurons were retrogradely labeled with horseradish peroxidas e from their corresponding muscles. Oculomotor and abducens internucle ar neurons were retrogradely labeled after horseradish peroxidase inje ction into either the abducens or the oculomotor nucleus, respectively . Parvalbumin staining produced the highest density of immunoreactive terminals in all extraocular motor nuclei and was distributed uniforml y. Around 15-20% of the motoneurons were moderately stained with antib ody against parvalbumin, but their axons were heavily stained, indicat ing an intracellular segregation of parvalbumin. Colchicine administra tion increased the number of parvalbumin-immunoreactive motoneurons to approximately 85%. Except for a few calbindin-immunoreactive trochlea r motoneurons (1%), parvalbumin was the only marker of extraocular mot oneurons. Oculomotor internuclear neurons identified from the abducens nucleus constituted a nonuniform population, because low percentages of the three types of immunostaining were observed, calbindin being th e most abundant (28.5%). Other interneurons located within the boundar ies of the oculomotor nucleus were mainly calbindin-immunoreactive. Th e medial longitudinal fascicle contained numerous parvalbumin-and calr etinin-immunoreactive but few calbindin-immunoreactive axons. The majo rity of abducens internuclear neurons projecting to the oculomotor nuc leus (80.7%) contained calretinin. Moreover, the distribution of calre tinin-immunoreactive terminals in the oculomotor nucleus overlapped th at of the medial rectus motoneurons and matched the anterogradely labe led terminal field of the abducens internuclear neurons. Parvalbumin i mmunostained 42% of the abducens internuclear neurons. Colocalization of parvalbumin and calretinin was demonstrated in adjacent semithin se ctions, although single-labeled neurons were also observed. Therefore, calretinin is proven to be a good marker of abducens internuclear neu rons. From all of these data, it is concluded that parvalbumin, calret inin, and calbindin D-28k selectively delineate certain neuronal popul ations in the oculomotor system and constitute valuable tools for furt her analysis of oculomotor function under normal and experimental cond itions. (C) 1998 Wiley-Liss, Inc.