Mice are a useful genetic model for the molecular investigation of learning and memory. For the study of cognitive function of mice, Pavlovian fear conditioning experiments were performed in our laboratory. A single auditory fear conditioning trial was sufficient for the establishment of emotional long-term memory. The presentation of the conditioned auditory stimulus triggered significant changes in the defensive behavior of mice. The objective of our study was to investigate whether heart rate (HR) effects can be used as specific index of fear in freely moving mice. The method used for HR measurements determined the HR change: bradycardia was described for restrained rats, whereas tachycardia was reported for freely moving rats. Our results demonstrated that HR was a useful autonomic parameter indicating associative learning following a one-trial auditory fear conditioning of inbred mice [2].

ECG transmitters (Data Sciences TA10EA-F20) were implanted intraperitoneally [1] during a 15-25 min surgery of deeply anesthetized mice of a weight of 18-23 g. A suture tab on the transmitter body allowed to fix it to the abdominal muscles. Two recording electrodes extending from the transmitter body were guided through the abdominal muscles and placed subcutaneously. One was directed toward the right front leg, the other toward the left hind leg. Special care was taken to place the recording electrodes in such a way that no pressure was exerted onto the skin. In the few cases of wound infection, an antibiotic treatment was provided via the drinking water. After surgery, the body weight was controlled on a daily basis to monitor recovery. Behavioral experiments were performed 14-21 d after surgery, when the mice had regained pre-surgery weight.

HR was measured in the home cage of freely moving mice implanted with an ECG transmitter. For ECG recordings the home cage of a mouse was placed onto the receiver board (Data Sciences RLA1020) which was connected to an analog ECG adaptor (Data Sciences Option R08). ECG signals were recorded digitally with the help of an analog-digital converter (ADInstruments MacLab 4s) and stored on a Macintosh computer. An oscilloscope served as additional signal monitor. Depending on the position of the recording electrodes, at least the R and S waves of the ECG signal could be detected. HR was calculated offline from the R-R intervals of the ECG complex (ADInstruments MacLab Chart 3.5/s). The HR change and HR variability served as autonomic parameters [2]. Precise information on all steps from ECG surgery to equipment and costs of ECG telemetry will be provided. Advantages and disadvantages of the method will be discussed.

References

1. Kramer, K.; van Acker, S.A.B.E.; Voss, H.P.; Grimbergen, J.A.; van der Vijgh, W.J.F.; Bast, A. (1993). Use of telemetry to record electrocardiogram and heart rate in freely moving mice. Journal of Pharmacological and Toxicological Methods, 30, 209-215.
2. Stiedl, O.; Spiess, J. (1997). Effect of tone-dependent fear conditioning on heart rate and behavior of C57BL/6N mice. Behavioral Neuroscience, 111, 703-711.