The metabolic upper critical temperature of the human thermoneutral zone.

INTRODUCTION: The thermoneutral zone (TNZ) defines the range of ambient temperatures at which resting metabolic rate is at a minimum without sensible dry heat loss; the body not needing to defend its core temperature. The TNZ has been defined in a number of species yet surprisingly, in humans only its lower limit has been well characterised; indeed, it is not yet clear if there is an upper limit at which metabolic rate increases. AIM: To evaluate the evidence for a metabolic upper critical temperature to the thermoneutral zone in humans. METHODS: We synthesised current evidence about an upper limit to the human TNZ, highlighting the contradictions in the literature, and then discussed likely explanations for those contradictions. RESULTS: The data from relevant studies differ in terms of whether they indicate that the TNZ has an upper limit, and this was related to the fundamental type of heat exposure protocol employed. Those studies showing evidence for an upper limit associated that limit with a wide range of temperatures. CONCLUSIONS: We offer suggestions for future studies that should clarify the presence/absence of an upper limit to the TNZ and if present, where it lies.

Aspirin (acetylsalicylic acid) effects on behavioral thermoregulation with microwave radiation.

Aspirin is a widely used over-the-counter drug in our society which has wide therapeutic value, yet not all of the behavioral side effects have been studied. Different doses of aspirin solutions were administered (ip) prior to fixed-interval 2-min. schedules of microwave reinforcement in rats tested in a cold environment. Four Sprague-Dawley rats were conditioned to regulate their thermal environment with 5-sec. exposures of MW reinforcement. Friedman's nonparametric test showed significant differences among aspirin and saline-control doses. Post hoc sign tests showed that a moderate dose of aspirin increased operant behavior reinforced by MW radiation, yet lower and higher doses decreased and then increased the rate of responding which resulted in an inverted U-shaped trend. Possible multiple effects of aspirin in terms of its thermoregulatory as well as its pain-tolerance properties, and implications for hypothalamic "set point" are discussed.