Brain Research 882 2000 112–119 www.elsevier.com locate bres
Research report
Auditory noise can prevent increased extracellular acetylcholine levels in the hippocampus in response to aversive stimulation
¨ C.M. Thiel, C.P. Muller, J.P. Huston, R.K.W. Schwarting
¨ ¨
¨ Institute of Physiological Psychology I
, Heinrich-Heine-University of Dusseldorf, Universitats-str. 1, 40225 Dusseldorf, Germany Accepted 8 August 2000
Abstract
The intent of this study was to investigate neurochemical and behavioural effects of aversive stimulation and the impact of auditory background noise. Using in vivo microdialysis, hippocampal acetylcholine was extracted and subjected to HPLC analysis while male
Wistar rats were exposed to aversive stimulation similar to that used in conventional procedures for aversive conditioning. Three groups of animals were used. Animals in the first group were 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 shock. A third group was exposed to the same tone shock pairing and tone as the first group while being exposed to constant background 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 hippocampal ACh levels to tone shock stimulation. The unconditioned behavioural response, however,
was not prevented suggesting that hippocampal ACh is not a necessary correlate of behavioural activation or arousal. A second experiment intended to investigate the effects of background noise in a shuttle box avoidance learning paradigm where rats were trained to
avoid an aversive footshock, which was signalled by a tone. There, one group of rats was exposed to background noise during avoidance learning, and the other group was not exposed to noise. Whereas both groups learned to avoid the shock to some degree over training, the
noise exposed animals did not show improvement in escape performance over the course of training, indicating that the noise hindered development of an adaptive response to the shock. In summary, our data indicate that background noise can prevent increased
extracellular hippocampal ACh levels in response to an aversive stimulus, and can also lead to deficits in learning to escape from shock.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Hippocampus; Acetylcholine; Microdialysis; Avoidance learning; Aversive stimuli; Background noise
1. Introduction the functional role of hippocampal ACh in animals ex-
posed to various experimental manipulations. A number of Pharmacological, electrophysiological and lesion studies
variables have been shown to be associated with activation have implicated the septohippocampal cholinergic system
of hippocampal ACh. For example, some studies describe in a variety of functions ranging from arousal and atten-
a correlation between locomotor activity and increased tion to learning, memory and emotion [5,6,8,13,14,
hippocampal ACh levels [11,13]. It has also been found 18,24,43,49]. With the development of in vivo micro-
that simple sensory stimulation, including novel tactile, dialysis and sensitive assays, especially high-performance
visual and auditory stimuli, significantly increased hip- liquid chromatography with electrochemical detection
pocampal ACh [21]. Augmentation of hippocampal ACh HPLC–ED, it has become possible to monitor extracellu-
levels has also been obtained by various aversive or lar acetylcholine ACh levels in freely moving animals.
appetitive stimuli, and with neutral stimuli which had been Using such techniques, several studies have investigated
paired with
an aversive
stimulus [1,2,12,15,20,31,
35,36,48]. With respect to learning, increased cholinergic activation was reported in the hippocampus of rats during
Corresponding author. General and Physiological Psychology, Phillips-
performance of a discrimination learning task [51]. Using a
University of Marburg, Guttenburg-str. 18, 35032 Marburg, Germany.
simpler paradigm, namely habituation learning, we found
Tel.: 149-6421-282-3639; fax: 149-6421-282-3610.
increased hippocampal ACh levels when rats were exposed
E-mail address : schwartimailer.uni-marburg.de R.K.W. Schwart-
ing.
to a novel open field, but also when they showed be-
0006-8993 00 – see front matter
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C .M. Thiel et al. Brain Research 882 2000 112 –119
113
havioural habituation during re-exposure to the same open The rats were anaesthetised with a combination of Ketavet
field on the following day. Smaller cholinergic increases 0.9 ml kg and Rompun 0.4 ml kg, and were implanted
were, however, also found in a control group, which was with guide canulas, which consisted of a 15 mm thin wall
handled but not exposed to the open field, indicating that stainless steel canula 22 G with a thread in the top [7,41].
aversive aspects of handling might have contributed to The canula was aimed above the ventral hippocampus A:
increased extracellular ACh levels in this task [47]. 2
6.0, L: 64.8, V: 23.2 mm and was secured to the skull Therefore, the present study was intended to explore in
with dental cement. After surgery, the animals were more detail hippocampal cholinergic activity in relation to
handled daily and allowed to recover for at least 4 days aversive stimulation using a conditioning paradigm. A first
before the beginning of microdialysis. aim was to investigate the behavioural and neurochemical
effects of aversive stimulation a footshock, which was 2.1.2. Dialysis procedure
paired with a neutral stimulus a tone. Rats were exposed On the day preceding the experiment, a microdialysis
to a single tone shock pairing and a tone alone 2 h later, probe of concentric design length of membrane: 4 mm
while extracellular hippocampal ACh levels and behav- was inserted into the guide canula. The probes were
ioural activity were measured. A control group was only self-manufactured as described previously [41]. After
exposed to the tone. Behavioural testing in rats often insertion the probe was connected to a microinfusion pump
involves a constant masking background noise during the via a liquid swivel mounted on a balanced arm above the
1
experiment. Therefore, a second aim was to investigate cage. The probe was perfused with Ringer solution Na
1 11
2
whether such background noise might influence behav- 147 mmol, K
4 mmol, Ca 2.25 mmol, Cl
155 mmol ioural and neurochemical responses to aversive stimulation
at room temperature. In order to obtain detectable levels of in the above conditioning procedure. Thus, a third group of
ACh, 0.3 mM neostigmine was added to the Ringer rats was exposed to a single tone shock pairing and 2 h
solution. After probe insertion, the animal was placed later to a tone alone while being constantly exposed to
individually into the experimental cage 26328328 cm background noise.
which was situated in a sound attenuated chamber. Two The results showed that background noise prevented the
speakers were mounted on top of the cage for delivery of increase in hippocampal ACh levels to the tone shock
auditory stimuli. The animal was kept under a 12 h stimulation without, however, preventing the uncon-
light dark cycle luminous density: 60 Lux and had free ditioned behavioural response. We therefore performed a
access to food and water. second, behavioural experiment to investigate in more
On the next day, dialysis was performed between 9.30– detail the consequences of enduring background noise.
14.00 h. Samples were taken manually every 10 min. After Assuming that the septohippocampal cholinergic system is
collection of three baseline samples the animals were involved in learning, we asked whether the observed lack
exposed to the following procedures: of ACh activation in noise exposed animals would also
result in reduced learning. To keep conditions similar to 2.2. Experimental manipulations
the above in vivo microdialysis study, we chose shuttle box avoidance learning, a task, where rats are trained to
escape avoid an aversive footshock, which is signalled by •
Tone shock without background noise n57. Rats a tone. Half of the rats were exposed to constant back-
were exposed to a single tone shock pairing tone: ground noise during avoidance learning, while a control
1600 Hz for 3 s, 115 dB, followed immediately by a group was exposed to the learning paradigm without
60 s 0.3 mA scrambled footshock; constant current background noise. We hypothesised, that an attenuated
shock generator, 521 C, Campden Instruments; pa- cholinergic hippocampal reactivity in noise exposed ani-
rameters determined by piloting according to an mals would result in impaired active avoidance learning.
aversive conditioning procedure. Two hours later, the tone was presented again without shock. This 2 h time
window was chosen since hippocampal ACh levels
2. Materials and methods were expected to be at baseline 2 h after the tone
