As most neurons communicate neurochemically, the study of extracellular neurotransmitter content is essential in order to understand the relationships between brain processes and behaviour. The introduction of in-vivo brain microdialysis and the development of sensitive neurochemical assays has provided means to locally study neurotransmitter levels in the brain of freely moving animals. Thereby it has become possible to monitor neurochemical activity in relation to specific behavioural tasks or presentation of significant stimuli. On the neurochemical side, methodological aspects of the microdialysis technique have frequently been investigated, whereas methodological aspects of behavioural testing have often been neglected. Here, we present results that are part of a series of studies designed to investigate cerebral cholinergic activity during basic behavioural testing procedures.

In experiment 1, we monitored acetylcholine (ACh) levels in the hippocampus, frontal cortex and nucleus accumbens of rats that were taken out of their home cage and exposed to an open field, in comparison to animals that were returned to their home cage. The respective procedure was repeated on the next day. Behaviour (rearing, locomotion, grooming and quiescence) was videotaped and analyzed subsequently with the help of a semi-automated computer system. Our results show, for one, that open field exposure and handling can lead to cholinergic activation in various brain regions with site-specific and experience-dependent differences in the pattern of activation. On the other hand the results underline, that a simple interaction like handling, can increase ACh in all three forebrain regions. Interestingly, in the frontal cortex, the cholinergic increase in the "home cage"-group was even bigger than that observed in open field exposed animals. This indicates that not only handling but also post-handling experience (e.g. "unsafe" open field vs. "safe" home cage) contribute to ACh release. Since handling is part of many neurobehavioural procedures, handling induced changes can interact with the imposed independent variables under investigation, such as pharmacological manipulations, and should be considered in the interpretation of any experiment employing handling of subjects.

In experiment 2, ACh levels were monitored in the hippocampus, frontal cortex and nucleus accumbens in two groups of rats that were exposed to an aversive stimulus in their home cage (115dB/1600Hz tone followed by 60sec 0.3mA foot shock). In one group of animals, the home cage was situated in an environment with constant background noise (70dB/200-6300Hz), whereas in the other group no background noise was provided. Both groups showed an unconditioned behavioural response to the shock; however, an increase in ACh to the footshock was observed only in that group that was kept in the environment without background noise. Thus, situational factors, like background noise, which is often used during behavioural testing to mask other irregular noise, can block the neurochemical - but not behavioural - reaction to an aversive stimulus. In summary we conclude, that cortical, hippocampal and accumbal cholinergic activity is highly sensitive to "simple" behaviourally relevant stimuli and that this neurochemical activation is critically dependent on situational factors and experience.