Conceptual, spatial, and cue learning in the Morris water maze in fast or slow kindling rats: attention deficit comorbidity

Abstract

Rat lines selectively bred for differences in amygdala excitability, manifested by "fast" or "slow" kindling epileptogenesis, display several comorbid features related to anxiety and learning. To assess the nature of the learning deficits in fast kindling rats, performance was evaluated in several variants of a Morris water-maze test. Regardless of whether the location of the platform was fixed or varied over days (matching-to-place task), the fast rats displayed inferior performance, suggesting both working and reference memory impairments. Furthermore, when the position of the platform was altered after the response was acquired, fast rats were more persistent in emitting the previously acquired response. The poor performance of fast rats was also evident in both cued and uncued tasks, indicating that their disturbed learning was not simply a reflection of a spatial deficit. Moreover, fast rats could be easily distracted by irrelevant cues, suggesting that these animals suffered from an attentional disturbance. Interestingly, when rats received several training trials with the platform elevated, permitting them to develop the concept of facile escape, the performance of fast rats improved greatly. The performance disturbance in fast rats may reflect difficulties in forming a conceptual framework under conditions involving some degree of ambiguity, as well as greater distractibility by irrelevant cues. These various attributes of the fast rats may serve as a potentially useful animal model of disorders characterized by an attention deficit.

Fig. 1.

Mean (±SEM) acquisition latencies (left panel) to reach the platform on four consecutive days (averaged over 4 trials per day) among fast and slow rats tested in a fixed position Morris water-maze paradigm. Time spent in the goal quadrant (i.e., the quadrant in which the platform had been located previously) over four 15 sec periods is shown in theright panel.

Fig. 2.

The left panel shows the mean (±SEM) latencies to reach the platform on four consecutive acquisition days (averaged over 4 trials per day) among fast and slow rats tested in a fixed position Morris water-maze paradigm. The right panel shows the latencies on four consecutive trials among rats tested on a fifth day under conditions in which the platform was placed in a new location.

Fig. 3.

Mean (±SEM) latencies to reach the platform on eight consecutive trials over 6 d (in 2 d blocks) in a matching-to-place paradigm among fast and slow rats. In this paradigm the position of the platform varied over days so that rats were required to remember the position of the platform based on the initial trial(s) of that day.

Fig. 4.

Mean (±SEM) latencies to reach a platform cued by an overhanging stimulus on four consecutive days (averaged over 4 trials per day) among fast and slow rats. The cued platform was always in the same location (left panel) or varied on each trial of each day (right panel).

Fig. 5.

Mean (±SEM) latencies to reach the platform on eight consecutive trials over 6 d (in 2 d blocks) in a matching-to-place paradigm among fast and slow rats. On the day before the initiation of the training rats received either one, three, or eight training trials in which the platform was elevated (top, middle, or bottom panel, respectively).

Fig. 6.

Mean (±SEM) latencies to reach the platform on eight consecutive trials (collapsed over 4 d) in a matching-to-place paradigm among fast (left) and slow (right) rats. On the day before the commencement of testing, rats of each line received either (1) no treatment, (2) eight training trials with an elevated platform and distal cues present, or (3) eight training trials with an elevated platform and distal cues absent (achieved by surrounding the pool with an opaque curtain).

Fig. 7.

Mean (±SEM) latencies to reach a submerged platform (left panel) and the thigmotaxic tendency (proportional distance swum along the pool perimeter relative to the total distance traversed; right panel) on eight consecutive trials. The location of the platform varied from trial to trial and was unsignaled; therefore, learning the specific platform location was not possible.

Fig. 8.

Mean (±SEM) latencies to reach the platform on 4 consecutive days (4 trials per day) in a cued variable-position paradigm among fast and slow rats. Rats of each line were tested under conditions in which only the cue signaling the platform position was present (no distractor) or in which the platform was signaled but another overhanging stimulus, acting as a distractor, was also present on each trial (distractor).