Dopamine response to a hedonic stimulus/natural reward: A PET study

Objectives To investigate the response of the dopaminergic sytem to a natural reward stimulus using positron emission tomography imaging (PET) with ¹¹C-raclopride.

Methods Sixteen healthy volunteers participated in the study (10 women, 6 men; age, 59.25 ± 10.64 yrs; body mass index, BMI, 24.66 ± 5.31 kg/m², range, 18.54 - 35.20). After being deprived of water and food to a varying degree on two occasions, they underwent either a baseline ¹¹C -raclopride (RAC) scan or a RAC scan during which they were administered the thirst satisfying milk shake/juice.Dopamine(DA) release was estimated as a difference between the BP_ND values estimated in the two scans and the subjective assessment of the drink effect on thirst and hunger was assessed with visual analogue scales.

Results A different response to the natural reward stimulus was observed in the dorsal and ventral striatum: in the caudate and putamen DA release was significantly related to the levels of thirst before the scan(positively) and levels of thirst after the scan(negatively). In the VS DA release was positively correlated with thirst levels and negatively correlated with hunger levels before the scan.

Conclusions Using PET we observed a different role of the caudate and putamen and ventral striatum in reward expectation/assessment: the response in the caudate and putamen are more likely to be related to reward value assessment consistent with (1). The response in the ventral striatum (i) reflects expectation rather than reward evaluation (ii) is greater, the greater the level of need for the anticipated reward (drink and thirst) (iii) is smaller, the greater the level of need for which there is no anticipated reward (drink and hunger). 1.Doherty et al.,Science 304, 452-454 (2004).

Research Support This work was supported by CIHR and MSFHR