Towards Defining the Role of CNS Circuits in Control of Energy Homeostasis

Healthy individuals are able to maintain a steady body weight over a long period of time due to an active process called energy homeostasis in which food intake is matched to energy expenditure with great precision. Once this tightly regulated system becomes unbalanced, weight gain and, on a longer time scale, obesity will develop. The control of energy homeostasis is accomplished by neuronal circuits within the CNS. These cir- cuits‚ are able to modulate food intake and energy expenditure in response to various peripheral signals related to energy stores. Among the peripheral signals which have been found to directly act on the CNS circuits in control energy of homeostasis is the pancreas-derived hormone insulin (Bruning et al. , 2000). Although, most studies focused on the role of insulin in neurons of the arcuate nucleus of the hypothalamus (ARC), insulin receptors are also expressed in other brain areas involved in the control of energy homeostasis such as the ventromedial hypothalamus (VMH) and the mesencephalon (Havrankova et al. , 1978). In this thesis, the effect of insulin signaling on the electrophysiological properties of SF-1 neurons of the VMH and dopaminergic neurons of the mesencephalon was investigated. Perforated-patch recordings were performed to characterize insulin’s effect on the single-cell level and whole cell recordings were performed to investigated insulin signaling dependent alterations on a network level. In the VMH, insulin directly hyperpolarizes a subset of SF-1 neurons via a PI3-kinase signaling cascade. The reduction in firing rate is largely dependent on the activation of ATP-dependent potassium channels (KATP). In contrast, insulin did not reduce the firing rate of SF-1 neurons in which the insulin receptor was specifically deleted. On a network level, ablation of insulin receptors in SF-1 neurons alters the synaptic connectivity in mice subjected to a high-fat diet (HFD). Ablation of the insulin receptor results in the increase of excitatory drive on anorexigenic POMC neurons in the ARC. Accordingly, mice with a SF-1 neuron specific ablation of the insulin receptor are at least in part protected against HFD-induced alterations of metabolic parameters. In the mesencephalic dopaminergic circuitry, insulin stimulates the neuronal activity of a subset of dopaminergic neurons. The excitatory effect of insulin is cell-intrinsic and is mediated via PI3K-dependent signaling. Ablation of the insulin receptor in dopaminergic neurons abolished this response. Furthermore, this ablation decreases the excitatory input on dopaminergic neurons. Thus, insulin signaling is involved in the establishment or maintenance of excitatory synaptic connections in mesencephalic dopaminergic neurons. In spite of the dysregulation of fuel-related signals, variations of certain genetic factors are also associated with the development of obesity and obesity-associated co-morbidities like type 2 diabetes. Among those variations, single nucleotide polymorphisms within the FTO gene showed one of the most robust correlations with an increase in body mass index (Frayling et al. , 2007). The role of Fto was investigated in mesencephalic dopaminergic neurons using the perforated-patch configuration. Ablation of Fto in mesencephalic dopaminergic neurons leads to profound alterations in cocaine-evoked responses in such a way that the cocaine-induced inhibition was strongly reduced in Fto-deficient mice. This altered response is also seen in mice with a Fto-deletion specifically in dopaminergic neurons showing the effect of Fto is cell-autonomous. Further pharmacological characterization could demonstrate that Fto regulates the activity of dopaminergic neurons via alterations of dopamine receptor type 2 signaling. This notion is further supported by behavioral experiments and quantitative realtime PCR. In summary, these findings reveal an Fto-dependent alteration of the function of the mesencephalic dopaminergic circuitry.