Exercise Training and Chemogenetic Activation of the Dorsal Motor Nucleus of the Vagus Rescue the Changes in the Hepatic Vein Glucose Concentration by Central Action of Insulin in SHR.

The Dorsal Motor Nucleus of the Vagus (DMNV) is the main parasympathetic output from CNS as part of the brain-liver axis that participates on glucose homeostasis. Previously, we have shown that icv injection of insulin elicited a decrease the hepatic vein glucose concentration (HVGC) of Wistar, but not in SHR. Here we aimed to investigate whether the deficiency of central action of insulin to control the HVGC of SHR is due to a dysfunction in the cholinergic neurotransmission in the DMNV and which putative neurotransmitter could play a role in this circuitry. To this, we used in vivo and in vitro studies using chemogenetic tools, and the whole-cell patch-clamp recordings. Ethical approval: CEUA/ICB-USP #8336131119). IN VIVO STUDIES: Male SHR (12 weeks old) received an injection of DREADD (LVV-Gq-hM3q-PRSx8-eGFP) to selectively activate de DMNV neurons with clozapine-N-oxide (CNO). After 10 days of CNO treatment, the animals received icv injection of insulin and the HVGC was monitored in freely moving rats. IN VITRO STUDIES: to identify liver-projecting DMVN neurons, age-matched (12 weeks old) of Wistar and sedentary and exercised trained SHR (SHRt) received, 5 days prior to the experiments, injections of Fluorogold (FG) on the liver parenchyma. At the day of the electrophysiological studies, the FG-labeled DMVN neurons was visualized and recorded in whole-cell patch-clamp mode, before and after application of oxytocin (OT) in the bath. The results of in vivo studies have shown that in the DREADD group the icv insulin was able to reduce the HVGC when compared to the control group (Control: 113,6±14,9 mg/dL n=5; vs DREADD: 74,2±6,16mg/dL; p≤0,05). As for the in vitro studies, the application of OT in liver projecting-DMNV depolarized the resting membrane potential (Basal: -59.4± 0.7 mV vs OT: -56.2 ± 1.2 mV, n = 10; t = 2.403, df = 9; P = 0.0397) and increased the firing frequency (baseline: 1.9±0.8 vs OT: 3.0 ± 0.9 Hz, n=10; P = 0.002) in Wistar, but not in sedentary SHR. Interestingly, in SHR trained for 4 weeks, the OT caused depolarization of resting membrane potential (baseline: -61.3 ± 1.3 vs OT: -59.6 ± 1.5 mV, n=18; P=0.0177) and an increase in the firing frequency (baseline: 3.9±0.7 vs OT: 4.7±0.6 Hz, n=18, P=0.0012). As conclusion, the selective activation of parasympathetic preganglionic neurons of the DMNV rescues the changes in the HVGC elicited by central action of insulin, which used to be defective in SHR, indicating that the circuitry involving DMNV-liver pathway is somehow dysfunctional, which could be due to the autonomic unbalance observed in this strain. Moreover, OT elicits similar changes in the electrophysiological properties of liver-projecting DMNV neurons of Wistar and exercise-trained but not sedentary SHR. These results indicate that exercise training can restore the sensitivity of liver-projecting DMNV neurones of exercise trained SHR to OT.