What is the Meaning of the ATP Surge During Sleep?

833 Critical Topics Forum—Wong-Riley synthesis accounts for only 1.3-2% of total energy consumption , 8,9 and phospholipid turnover consumes only ~5%. 9 By all accounts, the bulk of energy consumed by neurons is for the active transport of ions against their concentration and electrical gradients in relationship to neuronal activity, i.e., the more frequently a neuron's membranes are depolarized by excitatory input, the greater its energy demand for repolarization. Much of this energy is consumed by dendrites, the major receptive sites for excitatory synapses. 10 If anabolic activities are not likely to be the major reason for increased ATP synthesis (and this can be further tested with protein synthesis inhibitors), then what, if any, function(s) during sleep would require such a surge? In rats, both NREM and REM occur in both the light and dark cycles, and bouts of wakefulness exist in the light cycle. 11 If the surge of ATP is related to NREM activity, as Dworak et al. suggested, then shouldn't there be a similar " surge " around the 7th-8th hour of the dark cycle, when there is much NREM activity (see Figures 1 and 7 11)? If brain metabolic activities during wakefulness and REM are greater than those during NREM, 12,13 then the relatively low ATP levels (compared to the " surge ") should reflect greater ATP usage during the waking period that consumes the energy generated. In neurons, energy is not generated unless energy is used. 14 So, the ATP " surge " is not likely to be increased ATP synthesis, as the need and usage are reduced during this time. That leaves a decrease in ATP degradation with an accumulation of unused ATP as another plausible explanation for the " surge. " This is consistent with authors' findings of a delay or prevention of " surge " when the animals were sleep-deprived, i.e., when their energy consumption was increased. A high level of ATP is inhibitory to cytochrome c oxidase, 15 a terminal enzyme of the mitochondrial electron transport chain. Such inhibition limits further energy generation, with a possible benefit of preventing excessive accumulation of reactive oxygen species , a byproduct of energy metabolism. Dworak et al.'s findings are indeed intriguing and provocative. However, the rationale for the ATP " surge, " at least for this reader, deserves further probing. 1 reported a surprising finding , that there was a " surge in ATP …