Complexities and paradoxes in understanding the role of dopamine in incentive motivation and instrumental action: Exertion of effort vs. anhedonia

Instrumental behavior is a very complex and multifaceted process. Behavioral output during instrumental performance is influenced by a variety of factors, including associative conditioning, directional and activational aspects of motivation, affect, action selection and execution, and decision-making functions. Detailed assessments of instrumental behavior can focus on the temporal characteristics of instrumental behavior such as local frequency and response duration, and biophysical measures of response topography such as force output over time. Furthermore, engaging in motivated behavior can require exertion of effort and effort-based decision making. The present review provides an overview of research on the specific deficits in operant behavior induced by dopamine antagonism and depletion. Furthermore, it discusses research on effort-based decision making, and highlights the complexities and seeming paradoxes that are revealed when detailed analyses of operant behavior are conducted, and instrumental behavior is put in the context of factors such as primary or unconditioned food reinforcement, appetite, binge-like eating, and response choice. Although impairments in mesolimbic dopamine are sometimes labeled as being due to "anhedonia", a detailed deconstruction of the findings in this area of research point to a much more complex and nuanced picture of the role that dopamine plays in regulating instrumental behavior. Low doses of DA antagonists and accumbens dopamine depletions blunt the exertion of physical effort as measured by several different challenges in animal studies (e.g., lever pressing, barrier climbing, wheel running), and yet leave fundamental aspects of hedonic reactivity, food motivation, and reinforcement intact. Continued research on the specific features of instrumental behaviors that regulate the sensitivity to impaired dopamine transmission across a number of contexts is important for resolving some of the complexities that are evident in this area of inquiry. These investigations can also provide insights into psychomotor and motivational dysfunctions that are seen in neuropsychiatric conditions such as depression, schizophrenia, and Parkinson's disease.