The objectives of this study were to understand the role of ventromedial hypothalamic (VMH) glutamatergic neurotransmission in response to hypoglycemia and to elucidate the effects of recurrent hypoglycemia (RH) on this neurotransmitter. We measured 1) changes in interstitial VMH glutamate levels using microdialysis and biosensors, 2) identified the receptors that mediate glutamate’s stimulatory effects on the counterregulatory responses, 3) quantified glutamate metabolic enzyme levels in the VMH, 4) examined astrocytic glutamate reuptake mechanisms and 5) used 1H-[13C]-NMR spectroscopy to evaluate the effects of RH on neuronal glutamate metabolism. We demonstrated that glutamate acts through kainic acid receptors in the VMH to augment the counterregulatory responses. Biosensors showed the normal transient rise in glutamate levels in response to hypoglycemia is absent in RH animals. More importantly, RH reduced extracellular glutamate concentrations and this was due partly to decreased glutaminase expression. Decreased glutamate was also associated with reduced astrocytic glutamate transport in the VMH. NMR analysis revealed a decrease in 13C4-glutamate but unaltered 13C4-glutamine concentrations in the VMH of RH animals. Our data suggests glutamate release is important for proper activation of the counterregulatory response to hypoglycemia and impairment of glutamate metabolic and re-synthetic pathways with RH may contribute to counterregulatory failure.
This article contains Supplementary Data online at http://diabetes.diabetesjournals.org/lookup/suppl/doi:10.2337/db16-1589/-/DC1.
- Received December 25, 2016.
- Accepted April 10, 2017.
- © 2017 by the American Diabetes Association.