Other – Motivation
Tests of motivation frequently utilize an operant testing approach. Specifically, progressive ratio (PR) schedules of reinforcement allow the assessment of motivation to obtain the available reinforcer, such as a food pellet or evaporated milk. Under the PR schedule, the test subject is required to make increasing numbers of responses (lever presses or nosepokes) to obtain the reinforcer. For example, the first pellet may require 5 presses, the second pellet 10 presses, the third pellet 17 presses, etc. The number of pellets obtained, or the final response requirement met, is defined as the break-point. Decreased motivation is detected as a drop in break-point, and increased motivation may be detected as increased break-point.
For example, apathy, which can be defined as a lack of motivation characterized by diminished goal-oriented behavior, is a known symptom of Huntington’s disease and other neurodegenerative disorders. A typical and well controlled assay to probe this domain in rodents in the progressive ratio task, a procedure where food-restricted animals are required to expend progressively increasing effort to obtain successive food reinforcements. An animal with an apathetic phenotype might be expected to reduce or halt responding where only a moderate level of effort is required to obtain food, where a healthy wild-type might be expected to show more persistent responding.
BAC HD mice, a transgenic model of Huntington’s disease, show reduced motivation to work for food reinforcement in our progressive ratio task. The figure illustrates the mean response rate recorded per reinforcer earned for a group of 80 week BAC HD mice vs. littermate control animals. These data show comparable response levels in all animals where fewer than 20 responses are required per reinforcer, but then show a clear reduction in responding in the BAC HD mice relative to controls as the ratio becomes more demanding.
Similar to methylphenidate (Ritalin), d-amphetamine was highly efficacious.
