effect of noise pollution on passive avoidance learning and the size of hippocampus in rat

introduction hippocampus is a well-defined brain region involved in learning and memory. in spite of the existing reports on hippocampal-dependent memory impairment due to noise stress, to our knowledge, there is no study on the effect of noise stress on the size of hippocampus. the present study aimed to investigate the likely effects of chronic noise exposure on learning and the size of rat hippocampus. methods twenty adult male wistar rats were randomly divided into two equal groups. animals in the experimental group were exposed to noise with intensity of 40 db for 20 minutes 3 times per day for a period of 3 months. the control animals were kept in the same laboratory conditions without being exposed to noise. learning function was assessed according to passive avoidance test. animals were then anesthetized and transcardially perfused with fixative solution. the brains were removed. the right hemispheres were selected; 25-μm thick sections were cut along the entire extent of the hippocampus. using systematic uniformly random sampling, one section from every twenty sections was analyzed. volume estimation was performed using cavalieri principle. results a decreased latency to enter the dark compartment was observed in noise-exposed animals, suggesting decreased learning function. this study also showed that the noise-exposed rats had smaller hippocampal formation than controls. conclusions reduced volume of hippocampus most probably reflects alterations in its neuronal circuits. these changes may be relevant to the learning and memory disturbances in human and animals following noise stress. keywords noise pollution, hippocampus, volume, stereology.