Hippocampal lesions impaired spatial working memory system in rats

BACKGROUND AND AIMS: Working memory is based on multi-component system, which comprises short term memory, long term memory and Cognitive Skill Learning (CSL) that allows to perform task based on newly acquired information. In our study rats with bilateral hippocampal lesions were tested in the Allothetic Place Avoidance Alternation Test (APAAT) to verify functional and anatomical substrates of spatial working memory. METHODS: Lesions were made using ibotenic acid injections into the hippocampus. The task of rats in the APAAT was to remember and to avoid entering onto a 45° sector (described in the room frame coordinates) on the rotating arena, where shocks were given. It requires segregation of useful, room and misleading, arena frame information. The APAAT consists of four sessions (D1, 2, 3 and D21), which comprised of habituation, training 1, training 2 and test. Each session condition lasted 5 min. Effect of hippocampal lesion on working memory was evaluated by the maximum time spent on avoiding the shock sector (Tmax). Longer Tmax during training 2 than training 1 and habituation shows on effective memory functioning. CSL was determined by number of shocks/number of entrances ratio (S/E). Low value of S/E indicate that rats well know rules in the APAAT. Non-cognitive activity was determined by the total path length (TPL). RESULTS: Rats with hippocampal lesions presented shorter Tmax during training 2 in comparison with other session conditions (F3,33=9.55; P<0.0001). Moreover, S/E ratio was on higher level in operated rats than in control (F1,12 =10.37; P<0.007). All rats presented TPL on similar level (F1,11=0.15; P=0.7). Both groups walked the shortest distance on day 21. Hippocampal rats contrary to control had impaired ability to learn rules of the test (high S/E), to segregate information and to maintain the goal (short Tmax). CONCLUSION: The results indicate that short term memory and cognitive skill learning, components of spatial working memory system, strongly depend on hippocampus.