5-Choice Serial Reaction Time Task

5-CSRTT was conducted in rat operant boxes (Coulbourn Instruments, Whitehall, PA, USA), which had five nose ports on the front panel, grid flooring, a back house light, and a food magazine port in the rear of the box. Each port was equipped with a light to serve as a signal and photobeam sensors to record entrances into the ports. Each box was stored within a sound-attenuating chamber with a fan for aeration and noise dampening. The input and output from each operant box were transmitted to Graphic State 4 software (Coulbourn Instruments, Whitehall, PA, USA) on an Optiplex 9020 computer.

The 5-CSRTT has been utilized extensively to measure attentional function and impulsivity in rodents, with significant work completed delineating the circuitry and pharmacology of the subcomponents of the task and its measures (Carli et al., 1983; Robbins, 2002). This task has been applied to examine age-related effects on attentional capacity as well (Jones et al., 1995; Muir et al., 1999; Grottick and Higgins, 2002; Guidi et al., 2015b) and thus is a sensitive measure to aging and pharmacological interventions. Rats were habituated to the operant boxes for 15 min, during which time the back house and food magazine lights were illuminated and 10 food pellets (45 mg), serving as reward, were contained within the food magazine. Once rats consumed 10 pellets, they proceeded to the next stage to develop the nose poking response, which entailed nose poking into the food magazine for food pellets on a FR-1 schedule at a rate of approximately three pellets per min for 30 min or until 100 pellets were acquired. After retrieving at least 30 pellets within a session, rats transitioned to the next training stage, which introduced the behavioral contingencies of nose poking to a light stimulus.

All subsequent training stages lasted 30 min (or until 100 trials were completed), and the back house light was illuminated to signal engagement in a trial. Rats initiated a trial with a nose poke into the food magazine, leading to an intertrial interval (ITI) of 5 s prior to illumination of a target hole. Upon nose poking into an illuminated port, the light extinguished, the food magazine was illuminated, and a pellet was delivered. Rats were required to earn 50 rewards for a single behavioral session prior to progressing onto the next training stage. Shaping of the nose poke response included all 5 holes being illuminated at once, with unlimited time to elicit a nose poke. The next training stage required rats to nose poke within 4 min to a single illuminated hole, with an additional minute to respond after the light was extinguished (limited hold period). Omissions and incorrect responses were followed by a time out period, which included all chamber lights extinguishing for 10 s. After rats could nose poke into the illuminated holes within 5 s after it extinguished, the signal duration was shortened and rats were trained to detect signals at 10 s, 2.5 s, and 0.5 s duration. Once rats performed at 50% accuracy on each signal duration individually, they underwent training with variable signal presentations, which included equal presentation of each signal duration across the five ports in a session. Rats performed this variable signal training until the accuracy on all signal durations was >50% with <10% omissions across 5 consecutive training days. Following baseline attentional performance on the variable signal training, rats were injected with LPS (1 mg/kg, i.p.) or saline (1 ml/kg, i.p.) once a week for 4 weeks in total. The weekly injection was delivered 48 h prior to behavioral training for that week to reduce the confounding effect of locomotor-related sickness behavior on attentional performance.

Behavioral performance was averaged across five consecutive training sessions (day 3–7 post-injection). The following behavioral measures were calculated and analyzed: accuracy for each signal type [i.e., (correct/correct+incorrect)*100], percent omissions for each signal type [(omitted trials/omitted+performed trials)*100], percent premature responses [(premature/premature+initiated trials)*100], number of initiated trials, and response and food retrieval latencies. The percent accuracy for each signal type represented sustained attentional capacity across various attentional loads, whereas premature responses indicated impulsivity. To determine if the drug treatment impacted motivation to perform the task, we analyzed the number of trials initiated and completed (i.e., omissions). In addition, latencies were examined to determine changes in motor capacity to perform the task following treatment.