Effect of Bacopa monniera on amygdaloid neurons Vollala VR et al. CLINICS 2011;66(4):663-671 Figure 2 - Photomicrographs of silver-nitrate-impregnated basolateral amygdaloid neurons from control rats A) and rats treated with Bacopa monniera, BM, for 4 weeks at doses of 20 mg/kg B), 40 mg/kg C) and 80 mg/kg D). A significant increase in dendritic arborization in the BM 40 and 80 mg/kg groups is demonstrated. DISCUSSION Many studies have provided evidence for the involve- ment of the amygdala in different forms of memory storage and learning. Flood et al. (1995) stated that the amygdala is the most sensitive brain region for memory enhancement.41 The amygdala plays a crucial and preferential role in avoidance learning compared with spatial learning.42 The amygdala is involved in the formation of enhanced declarative memory for emotionally arousing events.43-45 According to Richter, Levin and Akirav (2000), the amygdala is involved in emotional responses and the formation of emotional memories.28 The amygdala is also involved in the formation of enhanced long-term mem- ory.46,47 This view is supported by Tabert et al. (2001), who reported that the amygdala facilitates long-term but not short-term memory consolidation of emotionally significant material,48 and it is the cholinergic neurons that project to the amygdala that play an important role in memory acquisition.49 The term ‘‘passive avoidance’’ is usually employed to describe experiments in which an animal learns to avoid a noxious event by suppressing a particular behavior. The passive avoidance test is a behavioral test that assesses memory retention. Rats quickly learn to avoid noxious stimuli, which evoke a highly emotional learning process, by suppressing an innate natural behavior, such as staying in dark corners. The passive avoidance test is a useful method for the assessment of an impairment in learning and for the detection of both the ability to retain this learned knowledge and its recall.50 In our earlier study, we examined the effect of BM on behavioral changes using the same rats that were used in the present study. The rats that were treated with all of the doses of BM showed a significant improvement in passive avoidance memory retention.34 This increased retention in 666