FIGURE 13.

Changes in hippocampal extracellular glucose levels during place and response training for food reward. A. Changes throughout place and response training. Glucose levels increased after about 5 min into training in all groups, likely responding to rises in blood glucose levels after feeding. At about 15 min into training, glucose levels began to differ based on condition, with those rats engaged in learning the place version of the maze attaining significantly higher levels than those of response-trained or control rats. B. Expanded view of glucose changes during the first minutes of training. Note that glucose levels decreased significantly in the hippocampus in those rats trained on the place task but not in those rats trained on the response task or in controls. These findings suggest that hippocampal processing was engaged by place training in a manner that depleted available extracellular glucose, similar to the results obtained previously during spontaneous alternation testing (compare Fig. 8). In addition, note that the decrease in glucose levels was slightly delayed here relative to the increase in lactate shown in Figure 12. Thus, it appears that the lactate rise anticipates the decrease in glucose, possibly by responses to activation by neuroglia transmitters of receptors on astrocytes.