The generation of cortical slow potentials in the rat anaesthetised with urethane and their modification by nicotine

Abstract

This paper reports the generation of cortical slow potentials (SP), similar to the human contingent negative variation, in rats anaesthetised with urethane. Rats previously implanted with silver/silver chloride electrodes for recording slow potentials from the frontal cortex were anaesthetised with urethane, and given extensive associative conditioning in which a 100 msec, 70 dB tone (S 1) preceded electric tail shock (S 2). The S 1-S 2 interval was initially set at 0.5 sec, and increased over a number of training sessions to 3 sec. Negative slow potential responses developed during the interval between S 1 and S 2. Once the slow potential response was fully developed, it persisted without habituation during subsequent sessions. Withholding tail shock (S 2) resulted in a rapid extinction of the slow potential response to S 1 alone. In further experiments, rats ( n = 8) that generated steady slow potentials under urethane, were injected with saline ( n = 4) or nicotine (0.4 mg/kg, s.c.; n = 4), and their responses measured 0–10, 15–25 and 30–40 min after administration. While saline did not affect the magnitude of the slow potential responses compared with pre-saline control values, nicotine (0.4 mg/kg) caused significant decreases in the magnitude of slow potentials during all the measurement intervals. These results demonstrate that cortical slow potentials can be generated in rats anaesthetised with urethane, and that these slow potentials are sensitive to drugs such as nicotine. The slow potential responses measured in the rat anaesthetised with urethane thus have similar characteristics to those generated in the conscious rat. Such a preparation may therefore prove useful in carrying out experiments on slow potentials that were previously difficult or impossible in conscious animals.