Aversion induced by electrical stimulation of the mesencephalic locomotor region in the intact and freely moving rat.

Electrical stimulation of the so-called "mesencephalic locomotor region" (MLR) in the acute mesencephalic and restrained rat is known to induce locomotion. In the intact and freely moving rat, electrical stimulation of an area coextensive with MLR is reported to elicit an apparently aversive type of behavioral response. Indeed, the description of this behavioral response is very similar to the description of the prototypical escape reaction elicited by electrical stimulation of the periaqueductal gray (PAG), a structure of the so-called "brain aversive system." In this study, we investigated if, as is the case for PAG stimulations, these MLR electrical stimulations are also aversive in nature. To that end, MLR-stimulated rats were subjected to the switch-off test, in which the stimulated rat learns to interrupt the stimulation by pressing a bar. It was found that electrical stimulation of MLR sites, positive for apparently aversive behavior, supports the learning of the switch-off behavior, which demonstrates the aversive nature of such stimulations. Furthermore, the switch-off latency (time elapsed between the onset of the stimulation and its offset by a press of the bar) was shown to decrease when the intensity of the electrical stimulation is increased or when the interpulse interval is decreased, which suggests that the generated aversive state is graded by the strength of the stimulation. It is, hence, proposed to extend the "brain aversive system" to the MLR.