A microdialysis study of the noradrenergic response in rat frontal cortex and hypothalamus to a conditioned cue for aversive, naturalistic environmental stimuli

Abstract 

Rationale: Extracellular noradrenaline concentration in the rat forebrain is increased by aversive environmental stimuli. This study investigated whether conditioned cues for such stimuli have the same effect. Methods: After training rats to associate a tone (conditioned cue) with transfer from a neutral zone to a brightly lit zone of a light/dark shuttle-box (unconditioned stimulus), microdialysis probes were implanted into the frontal cortex and hypothalamus under halothane anaesthesia. Changes in extracellular noradrenaline concentration were then monitored on exposure to the tone alone. Parallel experiments monitored rats’ behaviour in the light arena. Results: A single exposure to the light arena increased extracellular noradrenaline in the frontal cortex and the hypothalamus but neither a single, nor repeated, exposure to the tone alone had any effect. After conditioning trials, the tone alone increased extracellular noradrenaline in the frontal cortex but not the hypothalamus, whilst the tone+transfer to the light arena resulted in a prolonged increase in extracellular noradrenaline in both brain regions. The time that rats spent within the light arena was also prolonged. Conclusions: Noradrenergic neurones in the frontal cortex, but not the hypothalamus, respond to conditioned cues for aversive environmental stimuli. However, prolongation of the noradrenergic response in both brain regions could contribute to the behavioural adaptation to such unconditioned stimuli.