Left-right functional difference of the rat dorsal hippocampus for short-term memory and long-term memory.

The existence of left-right hemispheric differences has been suggested not only in humans but also in rodents. In recent studies, left-right anatomical and functional differences of the rodent hippocampus have been revealed. However, there is only one report investigating the left-right difference for short-term memory (STM), and the left-right difference for long-term memory (LTM) is not consistent among previous studies. Therefore, we examined the effects of unilateral hippocampal lesion and stimulation on the formation of STM and LTM in rats. Our results showed that the right, but not the left, hippocampal lesion impaired STM performance, evaluated by the alternation rate in the spontaneous alternation test and the novel-arm choice rate in the novelty preference test. In addition, electrical stimulation of the left, but not the right, hippocampus immediately before the tests impaired STM performance. On the other hand, the left, but not the right, hippocampal lesion impaired the LTM performance, evaluated by the discrimination index in the object recognition test. In addition, the stimulation of the left, but not the right, hippocampus impaired LTM performance. These results suggest that both the left and right hippocampi are involved in STM formation, and the right hippocampus has a facilitating role while the left hippocampus has a suppressing role for STM. On the other hand, LTM may be driven correctly only by the left hippocampus with appropriate level of neural activity. The left and right hippocampi of rodents may work in different mechanisms depending on the demand for STM and LTM.