Depressed Metabolism

This is the sixth entry in a series about resuscitation of non-hibernating rodents from circulatory arrest at ultraprofound hypothermic and high subzero temperatures. After successfully reanimating rats from deep body temperatures of 0 – 2 degrees C and subsequent respiratory and cardiac arrest, Andjus allowed the survivors to live for many months afterward in order to observe any long-term effects of hypothermia. What he noticed, beyond temporary weight loss and a couple of rats with impaired temperature regulation, was that animals that had been cooled did not appear to suffer any gross or debilitating effects. Although food intake and sexual behavior were initially diminished, the rats regained healthy appetites within a few days and went on to produce normal offspring within 3 months of cooling.

Andjus, having pioneered a method of resuscitation of rats from ultraprofound hypothermia, also had occasion to take the first look at the effects of hypothermia on learning and memory. In his brief 1955 Nature publication, “Effects of Hypothermia on Behaviour,” Andjus first compared the ability of cooled (0-1 degrees C) vs. untreated (control) rats to learn a serial problem-solving task. Next, he compared two groups of rats cooled to different temperature ranges (0-1 degrees C and 13.4 – 18.5 degrees C) to controls in a classical maze-learning paradigm. Rats were trained on the maze, cooled, tested for retention, and finally trained on a serial problem-solving task.

The results showed a significant impairment in problem-solving ability in rats cooled to 0-1 degrees C compared to controls, but not in rats cooled to 13.4 – 18.5 degrees C. However, the effect was only temporary, as demonstrated by the fact that impairment decreased as the interval between cooling and testing increased. And though memory retention was also affected by hypothermia, Andjus stated that “the differences among experimental and control groups were very small, and in no instance were they statistically significant,” indicating that even severe hypothermia does not produce permanent long-term physical or behavioral changes.

These initial results were supported in another experiment by N. Mrosovsky in 1963, who reported that severe hypothermia did not affect the response of rats to a conditioned avoidance task when cooling was begun only 15 minutes after animals were trained to criterion. In this task, rats were placed in an apparatus with electrified wire flooring such that either side of the cage floor was capable of shocking the animal. To facilitate one-session avoidance learning, the rats were first taught that they could escape from shock by undergoing 20 shock trials at varied time intervals (30, 60, 90, and 120 seconds) in random order. Then they were conditioned to avoid the shock (conditioned response) by responding immediately to a light (conditioned stimulus) that came on inside the dark experimental room 8 seconds before the shock. The light stayed on until the rat crossed the dividing line between the two sides of the apparatus. When they reached the criterion of six successive avoidance responses, experimental animals were returned to their home cages for 15 minutes before cooling was initiated and rewarming was carried out under a bench lamp. Control animals remained in their home cages until retesting.

Both experimental (cooled) and control (untreated) groups were retrained in the avoidance task 13 days after hypothermia. On Day 14, after three successive avoidance responses, training was continued, but the shock came on in the opposite side of the box at the same time as the light (both were on for 8 seconds). The rat was successful in this “reversal procedure” if it stayed on its side of the apparatus while the light was on six consecutive times.

He reported no significant differences in initial learning, citing a median number of trials to criterion of 9.5 for cooled animals and 11.0 for controls on Day 13 retesting. The median number of shocks received was also similar (3 vs. 2) in both groups. There were also no significant differences in reaching criterion on Day 14 re-testing, nor in the reversal procedure.

Mrosovsky wisely points out in his interpretation of these results that

It must not however be assumed from the lack of evidence that hypothermia readily disrupts retention that behavior is unaltered. In the work of Andjus et al. (1956) and that of Sudak and Essman (1961), while retention was not changed, the ability on problem solving and habit reversal were decreased, even several weeks after the cooling.

He goes on to mention that the “motivating conditions” of those experiments are different from his own, which may explain differences in results, but also says that it may be possible that initial learning is more likely to be altered than retention after hypothermia. According to this hypothesis, he classes hypothermia along with anesthetics in  the category of agents having mild retroactive effects on learning and memory (i.e., those affecting memories consolidated immediately before the interfering event).