Qu et al., 1996), and blockade of the MCH receptor New Haven, Connecticut 06520 with antagonists reduces body weight (Borowsky et al., 2 Columbus Children's Research Institute 2002). Food deprivation and leptin alter expression of Ohio State University MCH and c-fos mRNA in MCH neurons (Qu et al., 1996; Columbus, Ohio 43205 Huang et al., 1999). MCH peptide (Shimada et al., 1998) and receptor (Marsh et al., 2002) knockout mice eat less, have reduced body weights, and show increased Summary activity. It has been suggested that another orexigenic peptide, neuropeptide Y (NPY), which also stimulates Neurons that synthesize melanin-concentrating horfood intake, acts by stimulating the MCH neurons mone (MCH) may modulate arousal and energy ho-(Schwartz et al., 2000). MCH neurons may also play a meostasis. The scattered MCH neurons have been role in memory and depression (Borowsky et al., 2002). difficult to study, as they have no defining morphologi-As MCH neurons are scattered within the hypothalacal characteristics. We have developed a viral apmus and have no distinct morphological attributes, it proach with AAV for selective long-term reporter gene has been difficult to identify the live neurons in order (GFP) expression in MCH neurons, allowing the study to record from them. We have solved this problem by of their cellular physiology in hypothalamic slices. designing a recombinant adeno-associated virus (AAV) MCH neurons showed distinct membrane properties that expresses a reporter gene, GFP, only in MCH neucompared to other neurons infected with the same rons and appears to have no adverse effect on neuron virus with a cytomegalovirus promoter. Transmitters health. Although many cells may be infected by the virus, of extrahypothalamic arousal systems, including noronly those cells that contain immunoreactivity for the epinephrine, serotonin, and the acetylcholine agonist MCH peptide show reporter gene expression. The virus muscarine, evoked direct inhibitory actions. Orexishould work to identify MCH neurons in a wide variety genic neuropeptide Y was inhibitory by pre-and post-of species, as suggested by experiments here in both synaptic mechanisms; an anorexigenic melanocortin rats and mice. This approach has allowed us to study agonist had no effect. In contrast, the hypothalamic the membrane properties, synaptic activity, and mechaarousal peptide hypocretin/orexin evoked a direct innisms of transmitter responses in hypothalamic slices ward current and increased excitatory synaptic activfrom rodents injected in vivo with the recombinant virus, ity and spike frequency in the normally silent MCH and we have corroborated these findings with parallel neurons. Together, these data support the view that slice experiments from MCH-GFP transgenic mice. We MCH neurons may integrate information within the tested the responses of MCH neurons to two groups of arousal system in favor of energy conservation.neuromodulators that are found in axons in the LH: those that play a role in arousal, such as norepinephrine (NE),
