The hypothalamic neuropeptide melanin-concentrating hormone acts in the nucleus accumbens to modulate feeding behavior and forced-swim performance

Abstract

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide with a prominent role in feeding and energy homeostasis. The rodent MCH receptor (MCH1R) is highly expressed in the nucleus accumbens shell (AcSh), a region that is important in the regulation of appetitive behavior. Here we establish a role for MCH and MCH1R in mediating a hypothalamic-limbic circuit that regulates feeding and related behaviors. Direct delivery of an MCH1R receptor antagonist to the AcSh blocked feeding and produced an antidepressant-like effect in the forced swim test, whereas intra-AcSh injection of MCH had the opposite effect. Expression studies demonstrated that MCH1R is present in both the enkephalin- and dynorphin-positive medium spiny neurons of the AcSh. Biochemical analysis in AcSh explants showed that MCH signaling blocks dopamine-induced phosphorylation of the AMPA glutamate receptor subunit GluR1 at Ser845. Finally, food deprivation, but not other stressors, stimulated cAMP response element-binding protein-dependent pathways selectively in MCH neurons of the hypothalamus, suggesting that these neurons are responsive to a specific set of physiologically relevant conditions. This work identifies a novel hypothalamic-AcSh circuit that influences appetitive behavior and mediates the antidepressant activity of MCH1R antagonists.

Figure 1.

MCH signaling in the AcSh modulates feeding behavior. MCH (1 μg/side) was injected into the AcSh of rats at the beginning of the dark cycle (a) or 4 h after the start of the dark cycle (b). The MCH1R antagonist (antag.) [1 μg/side (c) or 200 ng/side (d)] was injected into the AcSh at the beginning of the dark cycle. ^*p < 0.05; ^**p < 0.01; n = 6-10 per group. Error bars indicate SEM.

Figure 2.

Antidepressant-like effect of MCH1R antagonism. The effect on latency to immobility (a) and total swimming time (b) on day 1 of the FST was investigated in MCH-KO mice versus wild-type (WT) mice. The same analysis was performed in rats (day 2, FST) that received injections of MCH1R antagonist (antag.) [0.5 μg/side (c, d) or 1.0 μg/side (e, f)] directly into the AcSh. ^*p < 0.05; ^**p < 0.01; n = 5-9 per group. Error bars indicate SEM.

Figure 3.

MCH reduces performance in the FST. The effect of MCH [0.5 μg/side (a, b) or 1.0 μg/side (c, d)] injected into the AcSh in rats was tested in the FST. ^*p < 0.05; n = 7-10 per group. The effect of MCH (1 μg/side; e) or MCH1R antagonist (antag.) (1 μg/side; f) on locomotor activity was tested in separate groups of rats. Arrows indicate the days of MCH or MCH1R antagonist administration after habituation. Error bars indicate SEM.

Figure 4.

Location of microinjection sites within the AcSh (▪) and AcCo (•). Numbers are distances (in centimeters) anterior to bregma (Paxinos and Watson, 2004). Overlapping sites are eliminated for clarity. This figure was adapted from Paxinos and Watson (2004).

Figure 5.

MCH1R is expressed in both Dyn and Enk neurons within the AcSh. a, Fluorescent microscopy shows the expression pattern of MCH1R, Enk mRNA, and Dyn mRNA in the AcSh. Confocal microscopy shows the colocalization of MCH1R with Enk (b) or Dyn (c). Horizontal arrows indicate examples of colocalization; vertical arrows indicate examples of cells that express MCH1R but not the neuropeptide. AC, Anterior commissure; Cy, cyanine.

Figure 6.

MCH blocks the SKF 81297-induced, Ser⁸⁴⁵-phosphorylated form of GluR1 in AcSh explants. a, Summary densitometric analysis of phospho-Ser⁸⁴⁵/total GluR1 after incubations with MCH (10 μm), MCH1R antagonist (antag.) (10 μm), and the D₁ dopamine receptor agonist SKF 81297 (1 μm) as specified. ^*p < 0.05 for the control group; ^#p < 0.05 for the SKF 81297 group; data were pooled from five experiments with five animals per experiment. b, Representative Western blots showing the phospho-Ser⁸⁴⁵ (P-Ser⁸⁴⁵) and total GluR1 signal in lanes that correspond to the columns above in a. Error bars indicate SEM.

Figure 7.

Starvation activates CRE-mediated transcription selectively in MCH neurons. CRE-LacZ reporter mice were subjected to restraint stress for 1 h (R), footshock stress (FS), social stress (SS), repeated unpredictable stress (Rep), or starvation for 24 h (Starv). a, Summary of the percentage of β-Gal colocalization with MCH or orexin (Orx) neurons after the stressors. ^**p < 0.01. b, Representative immunostaining for β-Gal, MCH, and orexin in a CRE-LacZ mouse exposed to restraint stress. Cy, Cyanine. Error bars indicate SEM.