Anaerobic glycolysis is crucial for the maintenance of neural activity in guinea pig hippocampal slices

Abstract

To investigate the functional significance of anaerobic and aerobic glycolysis on neural activity and levels of high energy phosphates, we tested the effects of glucose, mannose, fructose lactate, and pyruvate on the maintenance of neural activity and on the levels of ATP and creatine-P (CrP) in hippocampal slices. For an index of neural activity, population spikes (PS) evoked in the granule cell layer were monitored. Immediately after deprivation of glucose, the PS amplitude was gradually reduced and extinguished within 30 mm. Replacement of glucose with either lactate or pyruvate resulted in a decay and loss of PS with a similar time-course as observed during glucose deprivation. However, after the complete loss of neural activity for 10-20 min the PS reappeared and recovered to normal levels. The replacement of glucose with either mannose or fructose resulted in a transient decrease of the PS to 80-70% of the original amplitude in 20 mm, followed by recovery. The time-course of the decrease of PS in the mannose-containing medium was slower than that in the medium containing fructose and the time-course of recovery was faster. ATP and CrP were reduced to 90 and 70% of original level in each slice after glucose deprivation for 30 and 100 mm, respectively. In media containing either lactate, pyruvate, mannose, or fructose, the level of ATP and CrP was maintained at the original level. The anaerobic metabolic rate of glucose, mannose and fructose, determined by the rate of lactate production during complete anoxia, was consistent with the order of the decay and recovery of the PS in mannose and fructose-containing medium. The mode of the transient decay or loss of PS with no apparent reduction in the levels of ATP and CrP in each slice, in the presence of either mannose, fructose or lactate, indicates that anaerobic glycolysis is crucial for the maintenance of PS. The results obtained in this experiment are not in accordance with the reports by Schurr and Fowler in which they showed that lactate can support neural activity, although they did not measure the levels of ATP and CrP in slices. The present experiment disclosed that this discrepancy was due to the difference of slice preparation; namely rapidly prepared slice with shorter period anoxia (1 mm after removal of hippocampal tissue block) gives the results mentioned in the present study whereas slowly prepared slices by vibratome or chopper method with longer period of ischemia (5-20 mm), did not show transient loss of PS after application of lactate. Thus present experiment indicates that glycolytic process is essential for maintaining neural activity for physiological state of slices, if it is admitted to say that rapidly prepared slices is more physiological because they are exposed on shorter period of ischemia, and that more careful attention should be paid for the interpretation of the results of slice experiment according to the method of slice preparation.