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This finding, combined with evidence that leptin receptors100 and POMC neurons are both present in the NTS (the only brain area other than the arcuate nucleus that expresses the POMC gene)101, indicates that the hindbrain and forebrain may both process information involved in energy homeostasis. Thus, the NTS or other brainstem areas may, like the arcuate nucleus, contain neurons that respond to leptin and, through ascending projections to key forebrain sites, contributetoadaptivefeedingresponsestochangesinbodyfatcontent. Further clarification of the mechanisms that integrate forebrain
and hindbrain circuitry involved in this process is an important area for future study. Monoamine neurotransmitters and food intake Noradrenaline Noradrenaline is synthesized in brainstem areas such as the dorsal vagal complex and the locus ceruleus. These areas project both caudally to the spinal cord and rostrally to the hypothalamus, thalamus and cortex. In some of these neurons, including those projecting to the PVN, noradrenaline is co-localized with NPY. Like NPY, injection of noradrenaline into the PVN increases food intake robustly and repeated injections can result in substantial weight gain102. The observation of elevated noradrenaline levels in the PVN of ob/ob mice103 indicates that leptin may inhibit noradrenaline release from terminals in this brain area, a possibility supported by in vitro studies using rat hypothalamus104. Increased noradrenaline signalling in the PVN or other hypothalamic areas may therefore contribute to hyperphagia induced by leptin deficiency, a hypothesis that implicates noradrenaline as an anabolic effector in the CNS controlofenergy homeostasis. Dopamine A critical dependency of food intake on CNS dopamine signalling is implied by the profound feeding deficits that result from both pharmacological depletion105 and genetic disruption106 of dopamine synthesis. The interpretation of this finding is complicated, however, by motor impairments associated with dopamine deficiency that may also affect feeding behaviour.