Proactive behavioral effects of theta-driving septal stimulation on conditioned suppression and punishment in the rat

In two experiments, a treatment phase of septal stimulation preceded the acquisition of free operant lever pressing on a random-interval 64-s reinforcement schedule. Male Sprague-Dawley rats, chronically implanted with a bilateral septal stimulating electrode and a unilateral bipolar hippocampal recording electrode, received (a) low-frequency (7.7-Hz) stimulation, which drove the hippocampal theta rhythm, or (b) random-pulse stimulation (average frequency = 7.7 Hz), which produced only nonregular waveforms in the hippocampus. Control animals were implanted but not stimulated. After 12 days of lever-press acquisition, animals were presented while lever pressing with an auditory signal associated with a particular schedule of shock delivery: In Experiment 1, shocks occurred despite the subject's response strategy; in Experiment 2, shocks were delivered only if the rat pressed the lever. In both experiments, lever pressing was suppressed by the auditory stimulus. Theta-driving but not random-pulse septal stimulation proactively increased behavioral tolerance to the effects of electric shock. These results reinforce the idea that proactive behavioral effects of septal stimulation are a consequence of the production of the hippocampal theta rhythm.