Effects of leptin on rat ventromedial hypothalamic neurons

Neurons in the ventromedial and arcuate hypothalamic nuclei (VMN and ARC, respectively) mediate many of leptin's effects on energy homeostasis. Some are also glucosensing, whereby they use glucose as a signaling molecule to regulate their firing rate. We used fura-2 calcium (Ca2+) imaging to determine the interactions between these two important mediators of peripheral metabolism on individual VMN neurons and the mechanisms by which leptin regulates neuronal activity in vitro. Leptin excited 24%, inhibited 20%, and had a biphasic response in 10% of VMN neurons. Excitation occurred with a EC50 of 5.2 fmol/liter and inhibition with a IC50 of 4.2 fmol/liter. These effects were independent of the ambient glucose levels, and both glucosensing and non-glucosensing neurons were affected by leptin. In contrast, the ARC showed a very different distribution of leptin-responsive neurons, with 40% leptin excited, 10% leptin inhibited, and 2% having a biphasic response (chi2=60.2; P<0.0001). Using pharmacological manipulations we found that leptin inhibits VMN neurons via activation of phosphoinositol-3 kinase and activation of the ATP-sensitive K+ channel. In addition, leptin inhibition was antagonized by 5'-AMP-activated protein kinase activation in 39% of neurons but was unaffected by 5'-AMP-activated protein kinase inhibition. No mechanism was delineated for leptin-induced excitation. Thus, within the physiological range of brain glucose levels, leptin has a differential effect on VMN vs. ARC neurons, and acts on both glucosensing and non-glucosensing VMN neurons in a glucose-independent fashion with inhibition primarily dependent upon activation of the ATP-sensitive K+ channel.