Fig. 1. Experiment design and data analysis: rectal temperature (Δ) and mixed expired carbon dioxide (▪) as a function of time. The maximum response intensity of nonshivering thermogenesis is defined as the ratio between the maximum and the minimum (baseline) level of mixed expired carbon dioxide. The thermoregulatory threshold temperature is defined as the temperature at which a sustained increase of mixed expired carbon dioxide is detectable. Fent+Nalx = fentanyl plus naloxone; Mep = meperidine; Mep+Atip = meperidine plus atipamezole; Mep+Nalx = meperidine plus naloxone; Sal = saline; Sal+Nalx = saline plus naloxone.