Projections of the mesencephalic locomotor region in the rat

Retrograde labeling and electrophysiological techniques were used to determine the organization of efferent projections of the mesencephalic locomotor region (MLR) in the rat. Injections of horseradish peroxidase (HRP) or fluorescent dyes were made into suspected terminal zones of the MLR. After allowing for retrograde transport, the same animals underwent precollicular transection under anesthesia. The MLR was identified physiologically by the induction of controlled locomotion on a treadmill at low current (less than 50 microA) levels. Stimulation sites were marked and the brain processed accordingly. Retrogradely labeled cells in proximity (less than 0.5 mm) to locomotion-inducing stimulation sites were considered to be putative MLR neurons and were plotted according to stereotactic coordinates. These studies revealed the intrinsic organization of MLR neurons projecting to various brainstem sites. This organization may be related to the different types of transmitter systems involved in mediating MLR function.