Leao, R. | bioRxiv Science

bioRxiv2018

A voltage-dependent depolarization induced by low external glucose in neurons of the nucleus of the tractus solitarius of rats: interaction with KATP channels regulated by external glucose.

The brain demands constant supply of glucose-derived energy for cell metabolism and survival, and brain hypoglycemia may compromise synaptic functions. The brainstem nucleus of the tractus solitarius (NTS) is an integrative center for autonomic counterregulatory responses to hypoglycemia, and recent evidences have demonstrated that NTS neurons can also sense fluctuations in glucose levels in the extracellular milieu. Glucose-sensing neurons rely on glucose metabolism to respond to changes in glucose availability, but the ionic mechanisms underlying the low-glucose sensitivity are not well elucidated in NTS neurons. Therefore, this work aimed to investigate the effect of low extracellular glucose (0.5 mM) on the electrophysiological properties of NTS neurons of rats, by whole-cell patch-clamp. We showed that in NTS neurons maintained in 5 mM extracellular glucose, low glucose induces a depolarization in most neurons, which was correlated with membrane potential, decreased with depolarization. ATP-sensitive potassium (KATP) channels contribute to the hyperpolarized resting membrane potential (RMP) of NTS neurons, and blockage of KATP channels produced depolarization of the membrane and occludes the effect of low glucose, thus acting as modulators of low-glucose sensing in NTS neurons. However, this effect is caused by depolarization, since membrane hyperpolarization after KATP blockage restores the effect of low glucose. Additionally, we demonstrated that the incubation of NTS neurons in high glucose (10 mM) prior to recordings leads to more depolarized RMP in these neurons, what contributes to increase the number of neurons unresponsive to a low-glucose challenge, possibly via depolarization by inhibition of KATP channels following increased amounts of high glucose-derived intracellular ATP. We conclude that NTS neurons depolarize the membrane in response to the application of a low-glucose solution, but this effect is occluded by membrane depolarization triggered by KATP blockage. This suggests a homeostatic regulation of the membrane potential by glucose.

physiology↗

bioRxiv2016

THE CREATION OF A NEGATIVE SLOPE CONDUCTANCE REGION BY THE ACTIVATION OF THE PERSISTENT SODIUM CURRENT PROLONGS NEAR-THRESHOLD SYNAPTIC POTENTIALS

A change of the input resistance (Rin) of the neuron involves a change in the membrane conductances by opening and closing of ion channels. In passive membranes, i.e., membranes with only linear leak conductances, the increase or decrease of these conductances leads to a decrease or increase of the Rin and the membrane time constant ({tau}m). However, the presence of subthreshold voltage dependent currents can produce non-linear effects generating deviations from this relationship, especially the contradictory effect of negative conductances, as produced by the sodium-persistent current (INaP), on the Rin. In this work we aimed to analyze experimentally and theoretically the impact of the negative conductance produced by INaP on Rin. Experiments of whole-cell patch-clamp conducted in CA1 hippocampus pyramidal cells from brain slices showed a paradoxical voltage-dependent decrease of the Rin and the {tau}m in subthreshold membrane potentials close to the firing threshold after the perfusion with TTX, which inhibits INaP. This effect is postulated to be a result of the negative slope conductance in the subthreshold region produced by this conductance. The analysis of the experimental data, together with simulations found that the slope conductance of INaP is negative for subthreshold membrane potentials and its magnitude is voltage dependent in the same range observed for the voltage-dependence of Rin and {tau}m. The injection of an artificial INaP using dynamic-clamp in the presence of TTX restored the Rin and {tau}m to its original values. Additionally the injection of an artificial leak current with a negative conductance in the presence of TTX restored the Rin and {tau}m as the artificial Inap did. On the other hand, the injection of an artificial leak current with a positive conductance in the presence of TTX had no effect on the Rin and {tau}m. We conclude that INaP increases the Rin and {tau}m by the negative slope conductance observed in its non-monotonic I-V relationship. These results demonstrate that the effect of Inap on Rin and {tau}m is stronger in potentials near the firing threshold, which could potentiate the temporal summation of the EPSPs increasing their temporal integration and facilitating action potential firing. Because of its negative slope conductance, INaP is more effective in increasing excitability near threshold than a depolarizing leak current.

biophysics↗