A Novel Weight Lifting Task for Investigating Effort and Persistence in Rats

Abstract

Here we present a novel effort-based task for laboratory rats: the weight lifting task (WLT). Studies of effort expenditure in rodents have typically involved climbing barriers within T-mazes or operant lever pressing paradigms. These task designs have been successful for neuropharmacological and neurophysiological investigations, but both tasks involve simple action patterns. High climbing barriers may also present risk of injury to animals and/or issues with tethered recording equipment. In the WLT, a rat is placed in a large rectangular arena and tasked with pulling a rope 30 cm to trigger food delivery at a nearby spout; weights can be added to the rope in 45 g increments to increase the intensity of effort. As compared to lever pressing and barrier jumping, 30 cm of rope pulling is a multi-step action sequence requiring sustained effort. The actions are carried out on the single plane of the arena floor, making it safer for the animal and more suitable for tethered equipment and video tracking. A microcontroller and associated sensors enable precise timestamping of specific behaviors to synchronize with electrophysiological recordings. The rope and reward spout are spatially segregated to allow for spatial discrimination of the effort zone and the reward zone. We validated the task across five cohorts of rats (total n = 35) and report consistent behavioral metrics. The WLT is well-suited for neuropharmacological and/or in vivo neurophysiological investigations surrounding effortful behaviors, particularly when wanting to probe different aspects of effort expenditure (intensity vs. duration).

Dorsomedial prefrontal neural ensembles reflect changes in task utility that culminate in task quitting

“…Six rats performed a total of 30 sessions of the PW task (3-7 sessions per rat; average 5.0 ± 1.7 S.D.). Consistent with our previous studies (Porter and Hillman, 2019), rats were more likely to quit on heavier weights (Figure 2a). No rat was able to complete 10 successful pulls of 225 g. Due to the inter- and intra-individual variance of quit weights across sessions, each session varied in time duration and number of trials.…”

“…Prior to surgery, animals were trained in the WLT as previously described (Porter and Hillman, 2019). Briefly, the WLT is performed in a 120 x 90 x 60 cm arena (Figure 1a).…”

“…Rats completed two WLT paradigms: a progressive weight (PW) paradigm and a fixed weight (FW) paradigm (Porter and Hillman, 2019). In the PW paradigm, a session started with a two minute pre-baseline period where the rat was placed in the WLT arena but the rope was not extended into the environment.…”

“…Behavioral metrics were calculated as previously reported (Porter and Hillman, 2019). Briefly, for the PW paradigm, the quit weight was calculated as the highest weight in a given session on which the rat made at least one rope pulling attempt.…”

Stimulation in the Rat Anterior Insula and Anterior Cingulate During an Effortful Weightlifting Task

“…Before surgery, animals were trained in the WLT, a novel effort expenditure task that was recently developed and validated by our lab. For a detailed description of the WLT apparatus, materials, and full training procedures, see Porter and Hillman (2019); videos of the task in action are included in the supplementary material of that publication. In brief, the WLT consists of a 120 cm × 90 cm × 60 cm wooden open arena, painted black.…”