Tyrosine Hydroxylase‐Positive Neurons in the Hypothalamic Paraventricular Nucleus Modulate Glucose Homeostasis in Mice

Abstract

Tyrosine hydroxylase (TH)-positive neurons in the paraventricular nucleus of the hypothalamus (PVN), termed THPVN neurons, have emerged as key metabolic regulators. Recent studies showed that THPVN neurons are mostly Gamma-aminobutyric acid (GABA)ergic and/or dopaminergic. However, it is not known if THPVN neurons play a role in blood glucose regulation. We hypothesize that THPVN neurons are key regulators of blood glucose. To test our hypothesis, we used adeno-associated virus serotype 2 (AAV2)-mediated Cre-recombinase driven by a rat TH promoter (AAV2-TH-Cre), in combination with chemogenetic approach (Designer Receptors Exclusively Activated by Designer Drugs, DREADD) to activate or inhibit THPVN neurons and continuously monitored blood glucose levels using telemetry system. The hM3Dq-LoxP (excitatory-DREADD) and hM4Di-LoxP (inhibitory-DREADD) mice, 10-14 weeks old, were bilaterally microinjected with AAV2-TH-Cre into the PVN. One month after AAV injection, mice were implanted with telemetry glucose transmitters (HD-XG, DSI) to monitor blood glucose (BG) levels. To activate or inhibit the THPVN neuronal activity, Clozapine-N-oxide (CNO, 3 mg/Kg) or vehicle were administered (i.p.) and BG was monitored by telemetry. We found that activation of THPVN neurons by CNO in hM3Dq-LoxP mice decreased baseline blood glucose compared with the vehicle (ΔBG: -9.8 ± 0.2 mg/dL vs. 0.5 ± 0.1 mg/dL, n=5, p<0.0001); on the other hand, inhibition of THPVN neurons by CNO in the hM4Di-LoxP mice increased baseline blood glucose levels compared with the vehicle (ΔBG: 12.2 ± 0.2 mg/dL vs. 0.6 ± 0.3 mg/dL, n=3, p<0.0001). To exam the functional role of THPVN neurons on whole body glucose handling, mice were fasted overnight and simultaneously injected with glucose (1 g/Kg) and either CNO or vehicle. We found that activation of THPVN neurons by CNO improved glucose clearance after the glucose loading compared with vehicle (AUC: 3028.0±53.0 vs. 6021.0±102.0, p<0.001, n=3) in hM3Dq-LoxP mice. In summary, our data suggest that THPVN neurons negatively regulate blood glucose possibly by their GABAergic input to downstream neural circuits. We conclude that THPVN neurons play a functional role in the regulation of glucose homeostasis; while the mechanisms remain to be determined.