Preference for palatable and energy-dense foods may be a risk factor for body weight gain and has both genetic and environmental components. Once obesity develops in an individual, weight loss is difficult to achieve. Indeed, obesity is often characterized by repeated attempts to reduce the overconsumption of energy-dense foods, followed by food craving and relapse to overconsumption. Relapse and loss of control over intake are observed also in drug addicts, and it has been shown that obesity and drug addiction not only share behavioural features but also neural circuitry, e.g. the mesolimbic dopamine pathway. In this thesis, we sought to investigate the mechanisms related to food preferences and craving using animal models previously used in addiction research. The risk of gaining weight may implicate behavioural traits and emotional states. We showed in rats that a risk-taking behavioural profile was associated both with increased preference for a high-fat (HF) diet and with increased motivational response to a palatable high-sucrose (HS) diet. Hypothalamic urocortin 2 expression was associated with the preference for the HF diet. We also tested the hypothesis that consumption of HS and HF diets separately or provided simultaneously (HFHS) affect anxiety-like behaviour and locomotion. Furthermore, we showed that withdrawal from HFHS food affects diet-induced obesity-prone (OP) and obesity-resistant (OR) animals differently. OP animals had increased motivation (craving) for HS food pellets as measured by the operant self-administration technique during withdrawal. Dopamine receptor expression in the striatum differed between OP and OR animals both at access to HFHS and during withdrawal. This strongly implicates dopaminergic signaling in the OP phenotype. In humans, food preferences may be monitored using questionnaires. We analyzed food preference data from parents of preschool children, and identified an inverse association of parental preference for high-fat high-protein food and overweight in children. In conclusion, we have employed animal models previously used in the addiction field to identify molecular mechanisms related both to food preference and vulnerability to obesity, and to food craving associated with withdrawal from palatable food. These findings add to our current understanding of obesity.

A key issue in neuroscience is to determine the connection between neuronal circuits and behaviour. In the adult brain, all neuronal circuits include a glutamatergic component. Three proteins designated Vesicular glutamate transporter 1-3 (VGLUT1-3) possess the capability of packaging glutamate into presynaptic vesicles for release of glutamate at the nerve terminal. The present study aimed at determining the role of VGLUT2 in neuronal circuits of higher brain function, emotion, and reward-pocessing. A conditional knockout (cKO) strategy was utilised, and three different mouse lines were produced to delete VGLUT2 in specific neuronal circuits in a temporally and spatially controlled manner. First, we produced a cKO mouse in which Vglut2 was deleted in specific subpopulations of the cortex, amygdala and hippocampus from preadolescence. This resulted in blunted aspects in cognitive, emotional and social behaviour in a schizophrenia-related phenotype. Furthermore, we showed a downstream effect of the targeted deletion on the dopaminergic system. In a subsequent analysis of the same cKO mice, we showed that female cKO mice were more affected their male counterparts, and we also found that female schizophrenia patients, but not male patients, had increased Vglut2 levels in the cortex.  Second, we produced and analysed cKO mice in which Vglut2 was deleted in the cortex, amygdala and hippocampus already from midgestation, and could show that this deletion affected emotional, but not cognitive, function. Third, we addressed the role of VGLUT2 in midbrain dopamine neurons by targeting Vglut2 specifically in these neurons. These cKO mice showed a blunted activational response to the psychostimulant amphetamine and increased operant self-administration of both sugar and cocaine reinforcers. Further, the cKO mice displayed strongly enhanced cocaine-seeking in response to cocaine-associated cues, a behaviour of relevance for addiction in humans. In summary, this thesis work has addressed the role of the presynaptic glutamatergic neuron in different neuronal circuits and shown that the temporal and spatial distribution of VGLUT2 is of great significance for normal brain function.