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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. Hippocampus 1999; 9:333–339. © 1999 Wiley‐Liss, Inc.