No abstract

Rodents’ spatial memory is traditionally assessed in the radial arm-maze (RAM). An accurate response pattern in the RAM is described as the tendency to visit a new arm after each choice (i.e. win-shift strategy). When this response pattern is found, it is said that the animal remembers the places visited. In the present experiment, 12 hamsters were assessed in the RAM under two conditions: the depleting condition, in which feeders were not rebaited after each visit; and the replenishing condition, in which, feeders were rebaited. We registered the number of new arms visited (hits), the time spent in the central area of the maze, and the behaviors emitted in the central area before each arm choice. Results showed that, regardless of condition, animals were significantly more likely to visit new arms. However, more pre-choice behaviors and a longer center time were observed in the depleting condition than in the replenishing one. It is discussed that hamsters have a win-shift strategy for hoarding behavior even when they do not need to remember the places visited, though they exhibited more pre-choice behaviors when searching for food in the depleting condition.

Reduced response-inhibition capacity is a defining feature of attention-deficit hyperactivity disorder. The fixed minimum interval (FMI) schedule has been systematically validated to assess such capacity in rats. On each FMI trial, the first lever press initiates an inter-response time (IRT); a potentially consummatory response terminates the IRT; only IRTs longer than a target interval result in access to food. Despite task validity, steady-state FMI performance in the most common animal model of attention-deficit hyperactivity disorder, the spontaneously hypertensive rat (SHR), is similar to normotensive control performance, even though SHR performs at lower levels, especially during acquisition, in similar response-withholding tasks. To determine whether such limitations of the model are specific to stable-state performance, this experiment compared FMI 6-s performance in SHR and Wistar rats during acquisition and in steady state, and assessed the effect of acute D-amphetamine (AMP) administration (0.1, 0.5, and 1.0 mg/kg) on steady-state performance. Median latencies to first lever press were consistently shorter in SHR than in Wistar rats; IRTs were shorter for SHR than for Wistar rats during acquisition, but substantially less so during asymptotic performance. AMP dose-dependently reduced latencies, shortened IRTs, and, at the highest dose, increased the proportion of IRTs under schedule control. These results suggest that, relative to Wistar rats, SHR have a reduced capacity to learn to withhold a reinforced response; once the FMI is acquired, high doses of D-AMP disrupt withholding performance in both strains, but they also enhance the responsiveness of both strains to reinforcement contingencies.