Abstract
Working memory is a process for short-term active maintenance of information. Behavioral neurophysiological studies in monkeys have demonstrated that the dorsolateral prefrontal cortex (dlPFC) is a key cortical region for working memory. The medial prefrontal cortex (mPFC) in rats is a cortical area similar to the dlPFC in monkeys in terms of anatomical connections, and is also required for behavioral performance on working-memory tasks. However, it is still controversial regarding whether and how mPFC neurons encode working memory. In the present study, we trained rats on a two-choice spatial delayed alternation task in Y maze, a typical working memory task for rodents, and investigated neuronal activities in the mPFC when rats performed the task. Our results show that, (1) inactivation of the mPFC severely impaired the performance of rats on the task, consistent with previous studies showing the importance of the mPFC for working-memory tasks; (2) 93.7% mPFC cells (449 in 479) exhibited changes in spiking frequency that were temporally locked with the task events, some of which, including delay-related cells, were tuned by spatial information; (3) differential delay activities in individual mPFC cells appeared transiently and sequentially along the delay, especially during the early phase of the delay; (4) some mPFC cells showed no change in discharge frequency but exhibited differential synchronization in firing during the delay. The present results suggest that mPFC neurons in rats are involved in encoding working memory, via increasing firing frequency or synchronization.
Highlights
Working memory is a short-term memory system for active maintenance and manipulation of information in order to guide behavior, and is considered as a core function of the prefrontal cortex (PFC) [1,2,3,4]
Studies on neural projections and functions of the prefrontal cortical subareas support a view that the medial PFC, termed prelimbic cortex (PrL), in rodents is similar to the dorsolateral PFC (dlPFC) in monkeys [9]
After the animals performed the task with correct rate of > 80% in two successive sessions, they were used for medial PFC (mPFC) inactivation or electrophysiological recording
Summary
Working memory is a short-term memory system for active maintenance and manipulation of information in order to guide behavior, and is considered as a core function of the prefrontal cortex (PFC) [1,2,3,4]. Studies on neural projections and functions of the prefrontal cortical subareas support a view that the medial PFC (mPFC), termed prelimbic cortex (PrL), in rodents is similar to the dlPFC in monkeys (for review see Seamans et al.) [9]. Previous studies showed that few neurons in the mPFC exhibited differential delay discharge when rats performed working-memory tasks. Jung et al [13,14] recorded neuronal activities in the mPFC when rats performed an eight-arm radial maze task or a Table 1 maze task. They found that mPFC neurons increased firing during the delay, but few cells discharged differentially. Baeg et al [15] reported that cell assemblies in the mPFC could predict spatial locations of delayed choice in the Table 1 maze, but majority of the cells making-up of the assemblies were not delay cells
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