Leptin, by acting on leptin receptors (LEPRs) in the brain, exerts marked anti-obesity effects. Since the effects are large and specific, there is great interest in understanding their neural basis (the neurons and neurotransmitters that are involved). To identify the leptin-responsive neurons that initiate leptin's anti-obesity effects, we are genetically deleting LEPRs, in a neuron-specific fashion, and then assessing effects on energy balance. Our earlier studies established that POMC, AgRP and SF1 neurons are involved. However, it is also clear from these studies that a major part of the story is missing - other "first-order", leptin-responding neurons must also be playing an important role. To identify these "other" neurons, we are employing a novel approach - testing leptin-responsive, "first-order" neurons based upon the fast-acting neurotransmitter that they release (i.e. glutamate (excitatory) or GABA (inhibitory)). Towards these ends, we have generated mice that express cre-recombinase in either glutamatergic (VGLUT2-ires-Cre mice) or GABAergic neurons (VGAT-ires-Cre mice). After this, we then created mice that lack LEPRs on glutamatergic or GABAergic neurons. Our preliminary studies indicate that leptin's anti-obesity effects are mediated predominantly by LEPRs on GABAergic neurons. This finding suggests a new logic for piecing together leptin-regulated neural circuits (i.e. a key role for GABAergic inhibitory neurons). Specifically, we propose that leptin action on "local" GABAergic interneurons "indirectly" controls the activity of principle body weight-regulating projection neurons (POMC and possibly AgRP neurons in the arcuate nucleus). A number of approaches are being used to probe this novel hypothesis. These include: 1) Genetic manipulation of LEPRs on GABAergic neurons (and subsets of GABAergic neurons) (in Aims One and Two), 2) Anatomic and electrophysiological analyses to determine the location, identity and function of the relevant leptin-responsive GABAergic neurons (in Aims Two and Three), and 3) Channelrhodopsin-assisted circuit mapping (CRACM) to test the functional connectivity between "upstream" leptin-responsive GABAergic neurons and "downstream" body weight-regulating POMC neurons (in Aim Three). Our hypothesized model is of interest because leptin-responsive GABAergic neurons could be important substrates for nutritional programming and/or metabolic plasticity.
Neurocircuits in the brain control body fat stores. To develop anti-obesity therapies, we must first decipher the "wiring-diagrams" that underpin these circuits. We are using the following approaches to interrogate neural circuits engaged by the anti-obesity hormone, leptin: 1) neuron-specific gene manipulations, 2) optogenetics (light-activated neuronal stimulation) for probing circuit connectivity, and 3) electrical assessments of neuronal function.
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