It is well established that neural input to BAT remains a critical feature of its functional recruitment. In the case of postprandial thermogenesis, activation of BAT sympathetic nerve activity following peripheral or central glucose infusion suggests a central nutrient-sensing mechanism in the regulation of BAT activity. It is hypothesised that BAT-directed neurons in discrete hypothalamic brain regions alter their electrophysiological properties in response to increased extracellular glucose concentration.
Injection of the GFP-tagged, transsynaptic retrograde virus, pseudorabies virus (PRV), into the interscapular BAT of Sprague-Dawley rats allowed for identification of neurons with a known polysynaptic projection to BAT. Whole-cell patch clamp recordings were performed on GFP+ neurons from coronal sections of the arcuate nucleus (ARC) and retrochiasmatic area (RCh). Increasing the extracellular glucose concentration from 1mM (“fasted”) to 5mM (“fed”) revealed both glucose-excited (6.00 ± 0.84mV; 0.63 ± 0.18Hz; n=14 (29%)) and glucose-inhibited (-5.34 ± 0.75mV; -0.34 ± 0.07 Hz; n=18 (37%)) BAT-directed neurons in the ARC. Similarly, there were also substantial numbers of glucose-excited (7.32 ± 2.20mV; 0.75 ± 0.22Hz; n=5 (45%)) and glucose-inhibited (-3.12 ± 2.24mV; -0.80 ± 0.44Hz; n=4 (36%)) neurons in the RCh that projected polysynaptically to BAT. Retrospective immunohistochemical analyses of biocytin-filled cells revealed both POMC+ (n=9) and POMC- (n=5) glucose-sensitive neurons in both regions.
Furthermore, in attempt to delineate the heterogeneity of glucose-sensitive neurons through their monosynaptic projections, Retrobeads (Rb) were injected into the paraventricular nucleus, lateral hypothalamus and spinal cord of rats, and the glucose sensitivity of ARC/RCh double-labelled (Rb+/PRV+) neurons was tested.
These data provide a basis for the postprandial regulation of BAT thermogenesis through glucose-sensing mechanisms in hypothalamic neurons. They also provide additional insights into the axonal trajectory of identified hypothalamic glucose-sensors, which may form the basis of the observed heterogeneity within these populations of glucose-responsive, BAT-directed neurons.