Self-stimulation of an 'aversive' brain structure: the mesencephalic central gray area.

Four experiments were performed in BALB/c mice implanted either in the lateral hypothalamus (LH) or in the dorsal part of the mesencephalic central gray area (CG) or both in LH and CG. In the first 3 experiments the animals were placed in a Y-maze where they could successively trigger and turn off continuous electrical stimulation. Expt. 1 showed that mice stimulated in CG (like animals stimulated in LH) are able to discriminate between the reinforced and the non-reinforced arm of the Y-maze in order to self-administer the stimulation. This behavior was observed whether the experimental situation was a simple spatial discrimination or a more complex light-dark discrimination. During the second experiment, the animals could choose between stimulation of low intensity (in one arm) or high intensity (in the other arm). We observed that, like LH animals, CG implanted mice chose the higher stimulation, despite its strong aversive component. On the other hand, Expt. 3 showed that when the animals could chose between CG stimulation, whose intensity was progressively increased, and LH stimulation whose intensity remained constant (low) they preferred the latter. When the mice were placed in a lever-press box where they could obtain 0.2 s of electrical stimulation (last experiment), LH animals showed vigorous self-stimulation behavior, whereas CG animals showed only a weak response rate. These results show that, in spite of its aversiveness, dorsal CG stimulation has an appetitive component whose magnitude may vary under different experimental conditions.