Effects of anxiolytic drugs on rewarding and aversive behaviors induced by intracranial stimulation.

In considering the characteristics of the action of anxiolytic drugs and their mechanism in the brain, it may be necessary not only to study the behavioral pharmacology but also to perform brain site research. In the present study, the action of anxiolytic drugs was examined with respect to various behaviors that were induced by stimulating the brain areas related to emotions such as reward (pleasure) or aversion in rats. First, the low rate of response in lateral hypothalamic self-stimulation behavior was induced by schedules of low current brain stimulation, variable interval (VI) and differential reinforcement of low rate (DRL). Anxiolytic drugs such as benzodiazepines facilitated these low-rate responses. The drug susceptibility was highest in the low current stimulation, lower in the VI stimulation, and lowest in the DRL stimulation schedules. Furthermore, it was found by the auto-titration method in intracranial self-stimulation behavior that anxiolytic drugs decreased the threshold of stimulation reward. Second, it was recognized using the decremental lever pressing (DLP) paradigm that anxiolytic drugs increased the threshold of aversive stimulation of mesencephalic dorsal central gray (DCG), and this increasing effect of the drug was antagonized by GABA receptor blockers such as biccuculline. Finally, it was examined whether or not the conflict situation is established by combining brain stimulation reward and aversion, such as foot-shock or DCG stimulation. As a result, the conflict behavior was established by combining not only the brain stimulation reward and foot-shock aversion, but also the brain stimulation reward and DCG stimulation aversion. Further anxiolytic drugs exhibited anti-conflict action in both situations. The susceptibility of anxiolytic drugs was higher with respect to the conflict behavior induced by intracranial reward and aversion than to that induced by the conventional method based on milk reward and foot-shock aversion. These results suggest that behavioral methods using brain stimulation can examine the mechanisms of direct drug action at the brain stimulation site. Indeed, in the present brain stimulation behavioral study, anxiolytic drugs such as benzodiazepines increased the stimulation threshold in lateral hypothalamic self-stimulation and inhibited the DCG aversive stimulation, thus resulting in an anticonflict action of the drugs.