The effect of reversible inactivation of raphe nuclus on learning and memory in rats

The role of raphe nucleus (R.N) and serotonin in some behaviors such as sleep, cognition, mood, and memory has previously been reported. The median raphe (MR) nucleus is a major serotonin-containing cell group within the brainstem and is one of the main sources of projections to the septum and hippocampus. The hippocampus is widely believed to be essential for context-conditioning learning. Moreover, the hippocampus is thought to have a temporary function in the storage of memory, because when the hippocampus is damaged, recent, but not past memories are impaired. In the present study, the effect of reversible inactivation of R.N on memory and learning was investigated using passive avoidance method. For this purpose, Wistar rats (220-250 g) were anesthetized with ketamine and cannula was implanted above the R.N according to atlas of Paxinos. Ten days after surgery, animals were divided into 4 groups. In three of the experimental groups, lidocaine (2%) was injected 0, 60, and 120 minutes after acquisition period and in 1 group, saline used as control. Forty-eight hours later, animals were placed in shuttle box and latency time before entering into the dark chamber and total duration being spent in darkness and lightness was recorded. The results indicated that the latency period before entering into the dark chamber increases up to 350%, 450% and 475% at 0, 60, and 120 minutes respectively, as compared to control group. In addition, time duration being spent in dark space reduced up to 46%, 31%, and 87% and time duration being spent in light space increased up to 64%, 100% and 90% at 0, 60, and 120 minutes as compared to control. It could be proposed that the interactions between the septo-hippocampal 5-HT and ACh in the modulation of learning and memory may be related with 5-HT-ergic origin (DRN or MRN). This study showed that injection of lidocaine (2%) into the raphe nucleus could improve learning and memory.