Resumo

Hippocampal theta rhythms are oscillations (≈ 4 to 12 Hz) associated with complex behaviors requiring mnemonic processing, such as exploratory locomotor behavior in rats. CA1 region of hippocampus was recorded to investigate the changes in theta activity during contextual fear conditioning paradigm. Microelectrodes array consists of 18 teflon coated tungsten sharp wires (diameter 50 μm separated by 400 μm; impedance 1-3 MΩ at 1 kHz measured in saline). Specific stereotaxic coordinates used to implant in the hippocampus were 3.0 - 4.2 mm caudal to bregma; 1.6 - 3.6 mm lateral to bregma e 2.2 mm depth for layer CA1. The array was implanted in male Wistar rats (n=5) at the right hemisphere, in accordance with NIH guidelines for survival surgery. Rats were anesthetized with Ketamine (100 mg/kg) and Xylazine (5 mg/kg) following induction with Halothane. One week after surgery, multi-unit activities were recorded (Plexon Inc, TX). The experiment consists of two recording section days. In the first day, the rats explore freely the conditioning chamber for 5 minutes (context pre-exposure). 24 hours later, the animals were placed in conditioning chamber for 7 minutes. After 2 minutes they were presented with 5 unsignaled footshocks (2 s duration, 0.70mA, 1 minute apart). Theta analysis was submitted on a 10s windows of exploratory locomotor behavior during context pre-exposure (windows 1) and 10s windows immediately before each footstock (windows 2, 3, 4, 5). Using Welch’s method we calculated the spectral ratios of windows 2 to 1, 3 to 1, 4 to 1 and 5 to 1. We showed that at significance level of 5% the strongest theta activity occurred after aversive experience and this increase is maintained from the first to fifth footshock. No significant difference was observed in others frequencies (alpha, beta, delta or gamma). The results suggest that theta activity could reflect increase of demands on information processing during aversive learning.