Sensitization and hippocampal type 2 theta in the rat

Previous research has consistently demonstrated that there are two types of theta which can be recorded from the hippocampus of a variety of species. Type 1 theta occurs during voluntary movements such as walking, running and rearing while type 2 theta occurs during alert immobility. Large amplitude irregular activity (LIA) occurs during more automatic behaviors such as grooming and chewing as well as during non-alert immobility. Type 2 theta can be readily elicited in the unrestrained guinea pig and rabbit. While spontaneous type 2 theta rarely occurs in the rat, short trains of type 2 theta have been observed during conditioning. The purpose of the present study was to examine the relationship between type 2 theta and inescapable shock when the temporal characteristics of the conditioning stimulus (CS) were varied. Three conditioning paradigms (delay, trace, random) were utilized. Type 2 theta production increased over time as a function of the number of shocks and was not due to conditioning. There were no differences in the three groups in the occurrence of type 2 theta during CS presentations or during the inter-stimulus intervals. Previously neutral sensory stimulation also produced type 2 theta after conditioning in all groups. The inescapable shocks placed the rat in a high state of arousal which subsequently sensitized the animal to produce type 2 theta. The data was used to support the hypothesis that a state of arousal is a necessary precursor for the production of type 2 theta.