Pharmacological evidence for the implication of noradrenaline in effort

The trade-off between effort and reward is one of the main determinants of behavior and its alteration is at the heart of major disorders such as depression or Parkinsons disease. Monoaminergic neuromodulators are thought to play a key role in this trade-off, but their relative contribution remains unclear. Rhesus monkeys (Macaca mulatta) performed a choice-task requiring a trade-off between the volume of fluid reward and the amount of force to be exerted on a grip. In line with a causal role of noradrenaline in effort, decreasing noradrenaline levels with systemic clonidine injections (0.01mg/kg) decreased exerted force and enhanced the weight of upcoming force on choices, without any effect on reward sensitivity. Computational modeling demonstrated that NA acted on a single hidden variable, which affected both choices (as a variable of decision) and force production (as a driving force). Thus, our data strongly support noradrenalines implication in effort processing.