Dorsal periaqueductal gray punishment, septal lesions and the mode of action of minor tranquilizers

Abstract

In order to study the role of the septohippocampal system and the dorsal periaqueductal gray (DPAG) substance in punished behavior and in the action of minor tranquilizers, two groups of rats were trained to lever press on a continuous reinforcement schedule of food presentation. In one group, every response was subsequently punished by foot-shock delivery; in the other, by brief electrical stimulation of the DPAG of the mesencephalon. In both groups reponse rates were reduced to less than 10% of pre-punishment rates, but not completely supressed. Response rates did not significantly differ between the two groups, either before or after the introduction of punishment. Septal lesions significantly increased responding in the animals punished by foot-shock, but did not affect responding suppressed by DPAG stimulation. Injection of chlordiazepoxide (5 g/kg, IP) significantly increased punished responding in both groups of rats, before as well as after the septal lesion. Before the septal lesion was made, responding suppressed by foot-shock was significantly more released by chlordiazepoxide than responding punished by DPAG stimulation. These results suggest that in punishment tests using foot-shock, both a behavioral inhibitory system, including the septo-hippocampal structures and an aversive or punishment system, including the DPAG substance, act together to produce response suppression. Both these systems would be depressed by minor tranquilizers in order to cause their anti-punishment and perhaps their anti-anxiety action as well.