The effects of low body temperatures on deposition of tracers in the mammalian brain

Abstract

Different views of the “blood-brain barrier” are described, and the barrier is presented, not as a simple structural or functional barrier similar to a biological membrane, but as a series of structures and mechanisms which determine the rate of deposition of hematogenous substances in the brain. Reported effects of hypothermia and hibernation on this system are reviewed. The deposition of protein, ion and carbohydrate tracers was examined in euthermic and hypothermic rats as well as in euthermic, hypothermic, and hibernating bats, ground squirrels, and hamsters. Artifically hypothermic animals from all mammalian species in the study demonstrated increased uptake of the ion and carbohydrate tracers although not the protein tracer, but hibernating animals with equally low or lower core temperatures showed no increase in cerebral deposition of any of the tracers over that found in euthermic controls. In rats, the increased deposition of rubidium in the hypothermic brain was demonstrated to be independent of the duration of hypothermia, and evidence was gathered which was consistent with the alternate hypothesis that a critical temperature is reached before the change in rate of cerebral deposition of the tracer. Thermistors were implanted in rat brains, and data were gathered to suggest that this critical temperature in the brain is near 18.8 °C. Differential counting and autoradiography demonstrated that the rubidium tracer was fairly equally distributed in the brain. Neonatal rats and poikilothermic frogs did not show increased deposition of tracers in the hypothermic condition, in comparison to euthermic controls. High doses of dexamethasone lessened, but did not eliminate, the increased deposition of rubidium in the brain of hypothermic rats. Large intravenous doses of ouabain moderately increased the deposition of rubidium tracer in the brains of euthermic rats. These findings are discussed in terms of the effect of alterations in the barrier system on hypothermic animals, and the significance of intact barrier systems in hibernating, poikilothermic and neonatal animals.