A free-choice high-fat diet modulates the effects of a sucrose bolus on the expression of genes involved in glucose handling in the hypothalamus and nucleus accumbens.

The consumption of saturated fat and sucrose can have synergistic effects on the brain that do not occur when either nutrient is consumed by itself. In this study we hypothesize that saturated fat intake modulates glucose handling in the hypothalamus and nucleus accumbens, both brain areas highly involved in the control of food intake. To study this, male Wistar rats were given a free-choice high fat diet (fcHFD) or a control diet for two weeks. During the last seven days rats were given a daily bolus of either a 30% sucrose solution or water. Rats were sacrificed on day eight, 30 minutes after the onset of drinking. mRNA and protein levels of genes involved in glucose handling were assessed in the hypothalamus and nucleus accumbens. We found increased Glut3 and Glut4 mRNA in the hypothalamus of fcHFD-fed rats without an additional effect of the sucrose bolus. In the nucleus accumbens, the sucrose bolus increased Glut3 mRNA and decreased Glut4 mRNA independent of prior diet exposure. The ATP-sensitive potassium channel subunit Kir6.1 in the nucleus accumbens tended to be affected by the synergistic effects of a fcHFD and a sucrose bolus. These data suggest that acute glucose handling in the hypothalamus and nucleus accumbens may be affected by prior high fat exposure.

Neuropeptide Y Signaling in the Lateral Hypothalamus Modulates Diet Component Selection and is Dysregulated in a Model of Diet-Induced Obesity.

The preclinical multicomponent free-choice high-fat high-sucrose (fcHFHS) diet has strong validity to model diet-induced obesity (DIO) and associated maladaptive molecular changes in the central nervous system. fcHFHS-induced obese rats demonstrate increased sensitivity to intracerebroventricular infusion of the orexigenic Neuropeptide Y (NPY). The brain region-specific effects of NPY signaling on fcHFHS diet component selection are not completely understood. For example, fcHFHS-fed rats have increased intake of chow and fat following intracerebroventricular NPY infusion, whereas NPY administration in the nucleus accumbens, a key hub of the reward circuitry, specifically increases fat intake. Here, we investigated whether NPY infusion in the lateral hypothalamic area (LHA), which is crucially involved in the regulation of intake, regulates fcHFHS component selection, and if LHA NPY receptor subtypes 1 or 5 (NPYR1/5) are involved. Male Wistar rats were fed a chow or fcHFHS diet for at least seven days, and received intra-LHA vehicle or NPY infusions in a cross-over design. Diet component intake was measured two hours later. Separate experimental designs were used to test the efficacy of NPY1R- or NPY5R antagonism to prevent the orexigenic effects of intra-LHA NPY. Intra-LHA NPY increased caloric intake in chow- and fcHFHS-fed rats. This effect was mediated specifically by chow intake in fcHFHS-fed rats. The orexigenic effects of intra-LHA NPY were prevented by NPY1R and NPY5R antagonism in chow-fed rats, but only by NPY5R antagonism in fcHFHS-fed rats. Thus, NPY signaling has brain region-specific effects on fcHFHS component selection and LHA NPYR sensitivity is dysregulated during consumption of a fcHFHS diet.

Complete knockout of the nociceptin/orphanin FQ receptor in the rat does not induce compensatory changes in mu, delta and kappa opioid receptors.

The nociceptin/orphanin FQ (N/OFQ) opioid peptide receptor (NOPr) is a new member of the opioid receptor family consisting of mu, delta and kappa opioid receptors. The anti-opioid properties of its endogenous ligand, N/OFQ provide the receptor interesting potentials in symptoms and processes related to drug addiction, learning and memory, anxiety and depression, and nociception. Using target-selected N-ethyl-N-nitrosourea (ENU)-driven mutagenesis we recently generated a rat model bearing a premature stop codon in the opioid-like receptor (oprl1) gene, and here we describe the primary characterization of this novel model. Data revealed that [(3)H]N/OFQ binding to brain slices was completely absent in rats homozygous for the premature stop codon (oprl1(-/-)). Heterozygous rats displayed an intermediate level of NOPr binding. Oprl1 receptor transcript levels, as determined by Northern blot analysis, were reduced by approximately 50% in oprl1(-/-) rats compared to wild-type controls (oprl1(+/+)), and no alternative spliced transcripts were observed. Quantitative autoradiographic mapping of mu, delta and kappa opioid receptors using [(3)H]DAMGO, [(3)H]deltorphin and [(3)H]CI-977, respectively, did not show any changes in opioid receptor binding. In conclusion, we present a novel mutant rat lacking NOPr without compensatory changes in mu, delta and kappa opioid receptors. We anticipate that this mutant rat will have heuristic value to further understand the function of NOPr.

Characterization of the serotonin transporter knockout rat: a selective change in the functioning of the serotonergic system.

Serotonergic signaling is involved in many neurobiological processes and disturbed 5-HT homeostasis is implicated in a variety of psychiatric and addictive disorders. Here, we describe the functional characterization of the serotonin transporter (SERT) knockout rat model, that is generated by N-ethyl-N-nitrosurea (ENU)-driven target-selected mutagenesis. Biochemical characterization revealed that SERT mRNA and functional protein are completely absent in homozygous knockout (SERT-/-) rats, and that there is a gene dose-dependent reduction in the expression and function of the SERT in heterozygous knockout rats. As a result, 5-HT homeostasis was found to be severely affected in SERT-/- rats: 5-HT tissue levels and depolarization-induced 5-HT release were significantly reduced, and basal extracellular 5-HT levels in the hippocampus were ninefold increased. Interestingly, we found no compensatory changes in in vitro activity of tryptophan hydroxylase and monoamine oxidase, the primary enzymes involved in 5-HT synthesis and degradation, respectively. Similarly, no major adaptations in non-serotonergic systems were found, as determined by dopamine and noradrenaline transporter binding, monoamine tissue levels, and depolarization-induced release of dopamine, noradrenaline, glutamate and GABA. In conclusion, neurochemical changes in the SERT knockout rat are primarily limited to the serotonergic system, making this novel rat model potentially very useful for studying the behavioral and neurobiological consequences of disturbed 5-HT homeostasis.