IDENTIFICATION AND CHARACTERIZATION OF RAT ORBICULARIS OCULI MOTONEURONS USING CONFOCAL LASER-SCANNING MICROSCOPY

The eyeblink reflex is one of the most extensively studied behaviors i n mammals. The active downward force that causes lid closure is contro lled by the orbicularis oculi (OO) muscle. To augment our studies on t he neurophysiology and plasticity of the rat eyeblink circuit, here we present the first anatomical paper to focus exclusively on identifyin g and characterizing the OO motoneurons of the rat facial motor nucleu s (FMN). One thousand and twenty-nine cells from four animals were ret rogradely labeled by injecting the OO muscle with HRP and were imaged conventionally. One hundred and one cells from five animals were label ed by injecting the OO muscle with a 3000 mel. wt. fluorescent dextran and were imaged using confocal laser scanning microscopy (CLSM). The latter method resulted in little tissue shrinkage, bright labeling, an d excellent resolution of the soma, dendrites, and axon. Furthermore, it is a histologically simple alternative to HRP for retrograde labeli ng from the neuromuscular junction. Both methods revealed that the OO motoneurons were distributed over the entire length of the FMN, that t hey were concentrated along the dorsal crest of the nucleus, and that they were less numerous in the extreme rostral and caudal regions. As measured using the CLSM method, cell body areas were highly variable, ranging from 317 to 1500 mu m(2), but there was no size gradient along the rostrocaudal extent of the FMN. The neurons exhibited seven prima ry dendrites on average, which gave rise to bifurcating and even trifu rcating secondary dendrites. Using the HRP method, the estimated area of OO motoneurons ranged from 161 to 1381 mu m(2). The combined method s furnished a detailed characterization of the number, spatial distrib ution, and morphology of rat OO motoneurons. Moreover, these methods p rovide a useful way to analyze the circuitry that modulates the rat ey eblink.