Effects of lactate on glucose-sensing neurons in the solitary tract nucleus

For nervous tissue, lactate is a valuable energy substrate that can be extracted from glucose by astrocytes and released for neuronal use. Therefore, we hypothesized that the glucose-sensing neurons that signal the glycemic changes involved in the control of body energy homeostasis may be responsive to extracellular lactate as well. To test this hypothesis, neuronal activity was recorded extracellularly in the solitary tract nucleus of anesthetized rats in order to compare the effects of microelectrophoretic applications of glucose and lactate and of moderate hyperglycemia and to assess the possible effects of lactate on the response to glucose. About 90% of the investigated neurons behaved in a similar manner after local ejections of glucose and lactate. Among them, most neurons activated by glucose were also activated by lactate and all neurons depressed by glucose were also depressed by lactate. This result suggests that the response to these two compounds is mediated by a common mechanism related to their utilization as oxidizible substrates. In half of the tested neurons, the response to glucose was eliminated or significantly reduced after repeated lactate ejections. This inhibitory effect is a likely result of a modification in glucose metabolism induced by a high extracellular lactate level. Most glycemia-sensitive neurons responded similarly to moderate hyperglycemia and to local lactate ejection, suggesting that high brain lactate levels might interfere with the brain mechanisms that mediate glucoprivic eating.