Insulin activates insulin receptors (InsRs) in the hypothalamus to signal satiety after a meal. However, the rising incidence of obesity, which results in chronically elevated insulin levels, implies that insulin may also act in brain centres that regulate motivation and reward. We report here that insulin can amplify action potential-dependent dopamine (DA) release in the nucleus accumbens (NAc) and caudate–putamen through an indirect mechanism that involves striatal cholinergic interneurons that express InsRs. Furthermore, two different chronic diet manipulations in rats, food restriction (FR) and an obesogenic (OB) diet, oppositely alter the sensitivity of striatal DA release to insulin, with enhanced responsiveness in FR, but loss of responsiveness in OB. Behavioural studies show that intact insulin levels in the NAc shell are necessary for acquisition of preference for the flavour of a paired glucose solution. Together, these data imply that striatal insulin signalling enhances DA release to influence food choices.

Background Treatment of Stimulant-Use Disorders remains a formidable challenge, and the dopamine transporter (DAT) remains a potential target for antagonist or agonist-like substitution therapies. Methods This review focuses on DAT ligands, such as benztropine, GBR 12909, modafinil, and DAT substrates derived from phenethylamine or cathinone that have atypical DAT-inhibitor effects, either in vitro or in vivo. The compounds are described from a molecular mechanistic, behavioral, and medicinal-chemical perspective. Results Possible mechanisms for atypicality at the molecular level can be deduced from the conformational cycle for substrate translocation. For each conformation, a crystal structure of a bacterial homolog is available, with a possible role of cholesterol, which is also present in the crystal of drosophila DAT. Although there is a direct relationship between behavioral potencies of most DAT inhibitors and their DAT affinities, a number of compounds bind to the DAT and inhibit dopamine uptake but do not share cocaine-like effects. Such atypical behavior, depending on the compound, may be related to slow DAT association, combined sigma-receptor actions, or bias for cytosol-facing DAT. Some structures are sterically small enough to serve as DAT substrates but large enough to also inhibit transport. Such compounds may display partial DA releasing effects, and may be combined with release or uptake inhibition at other monoamine transporters. Conclusions Mechanisms of atypical DAT inhibitors may serve as targets for the development of treatments for stimulant abuse. These mechanisms are novel and their further exploration may produce compounds with unique therapeutic potential as treatments for stimulant abuse.

The conformation and function of the dopamine transporter (DAT) can be affected by manipulating membrane cholesterol, yet there is no agreement as to the impact of cholesterol on the activity of lipid-raft localized DATs compared to non-raft DATs. Given the paucity of information regarding the impact of cholesterol on substrate efflux by the DAT, this study explores its influence on the kinetics of DAT-mediated DA efflux induced by dextroamphetamine, as measured by rotating disk electrode voltammetry (RDEV). Treatment with methyl-β-cyclodextrin (mβCD), which effectively depletes total membrane cholesterol- uniformly affecting cholesterol-DAT interactions in both raft and non-raft membrane domains- reduced both DA uptake and efflux rate. In contrast, disruption of raft localized DAT by cholesterol chelation with nystatin had no effect, arguing against a vital role for raft-localized DAT in substrate uptake or efflux. Supra-normal repletion of cholesterol depleted cells with the analogue desmosterol, a non-raft promoting sterol, was as effective as cholesterol itself in restoring transport rates. Further studies with Zn2+ and the conformationally-biased W84L DAT mutant supported the idea that cholesterol is important for maintaining the outward-facing DAT with normal rates of conformational interconversions. Collectively, these results point to a role for direct cholesterol-DAT interactions in regulating DAT function.

Food restriction (FR) and obesogenic (OB) diets are known to alter brain dopamine transmission and exert opposite modulatory effects on behavioral responsiveness to psychostimulant drugs of abuse. Mechanisms underlying these diet effects are not fully understood. In the present study we examined diet effects on expression and function of the dopamine transporter (DAT) in caudate-putamen (CPu), nucleus accumbens (NAc), and midbrain regions. DA uptake by CPu, NAc or midbrain synapto(neuro)somes was measured in vitro with rotating disk electrode voltammetry or with [3H]DA uptake and was found to correlate with DA transporter (DAT) surface expression, assessed by maximal [3H]CFT binding and surface biotinylation assays. FR and OB diets were both found to decrease DAT activity in CPu with a corresponding decrease in surface expression but had no effects in the NAc and midbrain. Diet treatments also affected sensitivity to insulin-induced enhancement of DA uptake, with FR producing an increase in CPu and NAc, likely mediated by an observed increase in insulin receptor (InsR) expression, and OB producing a decrease in NAc. The increased expression of InsR in NAc of FR rats was accompanied by increased DA D2 receptor (D2R) expression, and the decreased DAT expression and function in CPu of OB rats was accompanied by decreased D2R expression. These results are discussed as partial mechanistic underpinnings of diet-induced adaptations that contribute to altered behavioral sensitivity to psychostimulants that target the DAT.

With respect to feeding, insulin is typically thought of as a satiety hormone, acting in the hypothalamus to limit ingestive behavior. However, accumulating evidence suggests that insulin also has the ability to alter dopamine release in the striatum and influence food preferences. With increased access to high calorie foods, Western societies have a high prevalence of obesity, accompanied by insulin insensitivity. Little is known about how insulin is trafficked into the brain following food consumption and whether insulin insensitivity in the periphery is mirrored in the central nervous system. We investigated insulin receptor activation in the ventral striatum of rats receiving water or 16% glucose either orally or intragastrically. We also investigated whether glucose-induced insulin receptor activation was altered in food-restricted (FR) or diet-induced obesity (OB) rat models. Lastly, we examined whether insulin plays a significant role in flavor-nutrient preference learning. Glucose intake stimulated a rapid increase in insulin receptor activity in the ventral striatum of FR and ad libitum (AL) fed rats, but not OB rats. Similarly, both AL and FR, but not OB rats demonstrated significant flavor-nutrient preferences. However AL rats receiving brief inhibition of insulin activity during conditioning failed to acquire a significant flavor-nutrient preference. These findings suggest that impaired insulin receptor activation in the ventral striatum may result in inaccurate valuation of nutritive foods, which could lead to overconsumption of food or the selection of foods that don't accurately meet the body's current physiological needs.