Reliability of individual differences in initial sensitivity and acute tolerance to nitrous oxide hypothermia.

On average, the hypothermia exhibited by rats receiving 60% nitrous oxide (N2O) eventually abates despite the continued inhalation of the drug (i.e., acute tolerance develops). However, large individual differences occur in both the magnitude of hypothermia achieved and the degree of acute tolerance that develops. To determine whether the degree of temperature loss and subsequent recovery during N2O administration are reliable characteristics of an individual, we measured intraperitoneal temperature via telemetry in 77 Long-Evans rats that each received 60% N2O for 5 h during two sessions separated by 14 days. Good intersession reliability (Pearson's r) was observed for simple change and adjusted change scores for both initial N2O temperature sensitivity (.61 < or = r < or = .62), and acute tolerance development (.46 < or = r < or = .52). In a separate experiment, three groups of rats were selected based on their individual body temperature patterns during an initial N2O administration: (1) insensitive to N2O hypothermia (n = 8); (2) marked hypothermia followed by acute tolerance development (n = 6); and (3) marked hypothermia followed by little acute tolerance development (n = 6). When retested 10 days later, each group exhibited a body temperature profile similar to that observed during the initial N2O exposure. Thus, the temperature profile observed during a rat's initial exposure to 60% N2O reflects a reproducible response for that animal.