Instability of dentate gyrus field potentials in awake and anesthetized rats

Adult male Long-Evans rats were prepared with stimulating and recording electrodes in the perforant path and dentate gyrus, respectively. Urethane-anesthetized acute (ACU) preparations and chronically-implanted freely-moving (CHR) animals received moderate-intensity (50-75% of maximum) stimulation pulses every 15-20 s for 4-5 hr in order to assess the stability of evoked field potentials. Significant increases in both population spike amplitude (+6.3%/hr) and EPSP slope (+2.5%/hr) were seen over the course of testing in the ACU group as a whole, while the CHR group showed significant decreases in EPSP slope (-3.3%/hr) but not spike (-1.2%/hr). Thus, both preparations were unstable, though the group mean drift differed in direction. Field potential drift was also affected by body temperature and stimulation intensity; drift was significantly greater when temperature was not controlled, and responses to moderate-intensity stimulation tended to be less stable than responses to high-intensity pulses. Our results indicate that a drift of 4-6% per hour in individual subjects is common; in unheated acute preparations, drift can equal or exceed 20% per hour (3/7 cases). These findings show that response instability can pose significant problems for electrophysiological investigations of neural plasticity.