Increased release of acetylcholine and noradrenaline in rat medial prefrontal cortex during contingent and non-contingent performance of a visual attentional task
J.W. Dalley, J. McGaughy, R.N. Cardinal, L. Levita, B.J. Everitt, T.W. Robbins
Dept Exp Psychol, Univ Cambridge, Cambridge CB2 3EB, United Kingdom

The ascending cholinergic and noradrenergic systems have been implicated in arousal and specific forms of attention. Acetylcholine (ACh) and noradrenaline (NA) are hypothesized to enhance the signal-to-noise resolution of afferent signals in cortical networks. However, few studies have examined how this modulation at a cellular level relates to specific behavioral processes. In this study we measured ACh and NA efflux in rat prefrontal cortex using in-vivo microdialysis during performance of a 5-choice serial reaction time task, which assesses sustained visual attention. Cholinergic, but not noradrenergic, lesions are known to produce deficits on the baseline version of the task. To explore the possibility that the noradrenergic system may be activated by unpredictable rewards we included a yoked control group in the study. Behaviourally, yoked rats extinguished responding for target stimuli and made more entries into the food magazine compared to masters. ACh efflux increased by approximately 250% in masters during task performance. This response was attenuated in yoked controls. NA efflux increased transiently in masters during task onset but showed sustained elevations in yoked controls. These data support the hypothesis that cortical ACh contributes to the efficacy of stimulus detection and processing whereas cortical NA functions to maintain attentional selectivity in a changing sensory environment.

Supported by: The Wellcome Trust (UK). This study was completed within the Medical Research Council (UK) Cooperative in Brain, Behavior and Neuropsychiatry.

Key words: AROUSAL, CATECHOLAMINE*, CHOLINERGIC*, DIALYSIS