Auditory noise can prevent increased extracellular acetylcholine levels inthe hippocampus in response to aversive stimulation

The intent of this study was to investigate neurochemical and behavioural e ffects of aversive stimulation and the impact of auditory background noise. Using in vivo microdialysis, hippocampal acetylcholine was extracted and s ubjected to HPLC analysis while male Wistar rats were exposed to aversive s timulation similar to that used in conventional procedures for aversive con ditioning. Three groups of animals were used. Animals in the first group we re exposed to a single tone/footshock pairing followed by a tone alone 2 h later. Animals in the second group served as controls and were only exposed to the tone without shuck. A third group was exposed to the same tone/shoc k pairing and tone as the first group while being exposed to constant backg round noise during the whole experiment. The results showed. that the tone/ shock combination led to pronounced behavioral and cholinergic activation. In contrast, exposure to background noise prevented the increase in hippoca mpal ACh levels to tone/shock stimulation. The unconditioned behavioural re sponse, however, was nor prevented suggesting that hippocampal ACh is not a necessary correlate of behavioural activation or arousal. A second experim ent intended to investigate the effects of background noise in a shuttle bo x avoidance learning paradigm where rats were trained to avoid an aversive footshock, which was signalled by a tone. There, one group of rats was expo sed to background noise during avoidance learning, and the other group was not exposed to noise, Whereas both groups learned to avoid the shock to som e degree over training, the noise exposed animals did not show improvement in escape performance over the course of training, indicating that the nois e hindered development of an adaptive response to the shock. In summary, ou r data indicate that background noise can prevent increased extracellular h ippocampal ACh levels in response to an aversive stimulus, and can also len d to deficits in learning to escape from shock. (C) 2000 Elsevier Science B .V. All rights reserved.