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Synaptic Regulation of Proopiomelanocortin Neurons Can Occur Distal to the Arcuate Nucleus

Vollum Institute, Oregon Health and Science University, Portland, Oregon

Submitted 15 January 2007; accepted in final form 4 March 2007

Energy homeostasis is controlled to a large extent by various signals that are integrated in the hypothalamus. It is generally considered that neurons in each of the hypothalamic nuclei are regulated by afferent projections that terminate within the cell body region of the nucleus. However, here it is shown that hypothalamic proopiomelanocortin (POMC) neurons receive synaptic inputs onto distal dendrites that reside outside of the cell body region in the arcuate nucleus. Previous studies using whole cell recordings from identified neurons in brain slices have shown that cannabinoids reduce GABA release from inhibitory synapses onto the POMC cells. Here it was found that endocannabinoids inhibited GABAergic inhibitory postsynaptic currents in POMC neurons only in intact sagittal brain slices, but not coronal, horizontal, or sagittal slices that were truncated rostrally at the level of the optic chiasm. Thus endocannabinoids inhibited presynaptic GABA release only at an anatomically distinct subset of POMC–neuron dendrites that extends rostrally beyond the arcuate nucleus into preoptic hypothalamic regions. There are two key results. First, the activity of POMC neurons can be regulated by afferent input at sites much farther from the soma than previously recognized. Second, endocannabinoids can act to inhibit inputs only at selective dendrites. POMC neurons play a critical role in the maintenance of body weight. Therefore these data suggest that energy balance may be regulated, in part, by modulation of POMC neuron activity at sites outside of the arcuate nucleus.