The effects of hypothermia, induced during asphyxia, on survival and on learning and maintenance of a conditioned avoidance response was studied in 4-week-old male rats of the Holtzman albino strain.
All animals were asphyxiated in the same gas mixture, consisting of 4% 02, 5 % CO2 and 91 % N2. The mean time of last gasp in this gas mixture, determined in 50 normothermic controls, was 7.40 minutes. No normothermic animal recovered when left at room temperature to recover unaided after the final gasp.
One hundred rats were asphyxiated for 8.02 minutes. During the last 5 minutes of the exposure to the gas mixture the animals were cooled in ice water. After this treatment 100 % of 100 hypothermic rats recovered spontaneously.
Fifty of the hypothermic rats learned a conditioned avoidance response 24 hours before asphyxiation and were tested again for performance and latency 24 hours after asphyxia. In these, accuracy in performance and latency of the conditioned response were maintained.
The remaining 50 hypothermic rats were subjected to the same learning test 24 hours after asphyxiation of the hypothermic rats learned a conditioned avoidance response 24 hours before asphyxiation and were tested again for performance and latency 24 hours after asphyxia. In these, accuracy in performance and latency of the conditioned response. In these the average time to learn and the latency of the conditioned response were slightly prolonged, although the behavior pattern during the learning and the accuracy of performance did not differ from normal controls. However, these effects were seen only in the animals who cooled minimally during the experiment (lowest temperatures 36° to 32°C); the latency and response times of those which cooled more effectively (lowest temperatures 31°to 25°C) were indistinguishable from controls. Six days after asphyxiation all hypothermic animals maintained a normal performance of the conditioned response, suggesting that the observed impairment in the learning animals was of a transient nature.
The present results were discussed in relation to the problem of neonatal asphyxia.