Differential sensitivity of operant behaviors to changes in the concentration of a sucrose reinforcer: effects of pimozide

Altered reward sensitivity to sucrose outcomes prior to drug exposure in alcohol preferring rats

Addiction involves key impairments in reward sensitivity (RS). The current study explored impaired RS to natural reward as a predisposing factor to addictive-like behavior. Alcohol preferring (P) rats are selectively bred based on significantly greater ethanol consumption and preference and offer the ability to inspect differences in subjects with a positive family history of addictive-like behavior. P rat's RS was compared to RS in the well-used Sprague-Dawley (SD) strain. To assess RS in a novel manner, instrumental incentive contrast, discrimination and consumption of sucrose solution were examined. Animals performed in a free operant situation for different sucrose concentration solutions using a block of 'mixed' trials with alternating outcome concentrations (e.g., 5 and 10 % sucrose) to change outcome value in a predictable manner. Animals also performed for reward in blocks of single outcome trials (5 or 10 or 20 or 40 % sucrose daily exposure) surrounding the mixed block. RS (e.g., reward discrimination and contrast effects between and within-sessions) was measured by changes in trials completed, instrumental response latency and consumption. P rats expressed an altered profile of RS with a greater tendency toward equivalent responding to different outcomes within the same session and an absence of incentive contrast from diverse reward comparisons. In contrast, SD animals expressed within-session reward discrimination and a subset of incentive contrast effects. These effects were moderated by food deprivation more consistently in SD compared to P rats. P rat alterations in processing natural rewards could predispose them to addictive-like behaviors including greater alcohol consumption and preference.

BMI modulates calorie-dependent dopamine changes in accumbens from glucose intake

Objective

Dopamine mediates the rewarding effects of food that can lead to overeating and obesity, which then trigger metabolic neuroadaptations that further perpetuate excessive food consumption. We tested the hypothesis that the dopamine response to calorie intake (independent of palatability) in striatal brain regions is attenuated with increases in weight.

Method

We used positron emission tomography with [¹¹C]raclopride to measure dopamine changes triggered by calorie intake by contrasting the effects of an artificial sweetener (sucralose) devoid of calories to that of glucose to assess their association with body mass index (BMI) in nineteen healthy participants (BMI range 21–35).

Results

Neither the measured blood glucose concentrations prior to the sucralose and the glucose challenge days, nor the glucose concentrations following the glucose challenge vary as a function of BMI. In contrast the dopamine changes in ventral striatum (assessed as changes in non-displaceable binding potential of [¹¹C]raclopride) triggered by calorie intake (contrast glucose – sucralose) were significantly correlated with BMI (r = 0.68) indicating opposite responses in lean than in obese individuals. Specifically whereas in normal weight individuals (BMI <25) consumption of calories was associated with increases in dopamine in the ventral striatum in obese individuals it was associated with decreases in dopamine.

Conclusion

These findings show reduced dopamine release in ventral striatum with calorie consumption in obese subjects, which might contribute to their excessive food intake to compensate for the deficit between the expected and the actual response to food consumption.

Repeated maternal separation does not alter sucrose-reinforced and open-field behaviors

Repeated separation of rat pups from their mothers has been reported to increase behavioral fearfulness and hypothalamic-pituitary-adrenal (HPA) response to stress. Recently, it was suggested that it might also alter behavioral responses to natural and drug rewards. Here, we studied whether maternal separation (MS) would alter behavioral responses to a sucrose reward. We also tested whether MS would alter behavioral responses in an open-field test using a novel method of analysis [Software for the Exploration of Exploration (SEE)]. Long-Evans rat pups were exposed to either 180 min of MS, 15 min of separation [early handling (EH)] or left undisturbed [nonhandled (NH)] from postnatal day (PND) 3 to 14. The adult male offspring were tested for sucrose solution preference using a two-bottle free-choice test, operant response for sucrose under fixed ratio and progressive ratio (PR) schedules of reinforcement and response to a novel environment (open-field test). MS had no effect on sucrose preference or operant responding for sucrose reward. In the open-field test, NH rats showed a brief decrease in locomotor response, but MS rats did not differ from the NH and EH groups in the other behavioral measures. Thus, under the conditions of the present study, MS did not appear to alter reward-related processes and also had a minimal effect on open-field behavior.

Quinpirole- and amphetamine-induced hyperdipsia: influence of fluid palatability and behavioral cost

Daily administration of moderate doses of amphetamine or of the dopaminergic D2 agonist quinpirole is associated with the development of excessive, non-regulatory drinking. Here we compared the influence of manipulating fluid palatability and behavioral cost on the development of this drinking augmentation. Experiment 1 was based on the phenomenon of contrafreeloading (CFL): animals work for a resource even though the same resource is freely available. The effects of 15 daily injections of amphetamine (1.0 and 1.7 mg/kg i.p. ) or quinpirole (0.1 and 0.56 mg/kg i.p.) were evaluated in mildly water-deprived rats. For the first 6 days the rats obtained water by lever pressing (FR3) only; over the following 9 days water was also freely available (CFL). Initially, 0.56 mg/kg quinpirole reduced lever pressing for water. A complete recover of responding was then obtained, and was followed by a progressive increment in the amount water obtained by lever pressing during the CFL phase (from 10 to 50%). Amphetamine did not affect percent CFL, but at the highest dose (1.7 mg/kg) reduced total water intake during the last 3 days of treatment. In experiment 2 the rats had free access to two bottles, one of which contained tap water, and the other contained either an ethanol (6%) or a sucrose (5%) solution. After habituation to this regimen, the rats received 10 daily i.p. injections of vehicle, amphetamine (1.0 or 3 mg/kg), or quinpirole (0.1 or 0.56 mg/kg). Quinpirole 0.56 mg/kg enhanced daily fluid intake under both sucrose and ethanol conditions, but selectively reduced ethanol preference. The higher amphetamine dose reduced fluid intake and sucrose preference. In conclusion, chronic exposure to a dopaminergic D2 agonist, but not to amphetamine, produced an increment of drinking that was resistant to manipulation of either palatability or the behavioral cost of the fluid.