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

Rationale: Extracellular noradrenaline concentration in the rat forebrain i s increased by aversive environmental stimuli. This study investigated whet her conditioned cues for such stimuli have the same effect. Methods: After training rats to associate a tone (conditioned cue) with transfer from a ne utral zone to a brightly lit zone of a light/dark shuttle-box (unconditione d stimulus), microdialysis probes were implanted into the frontal cortex an d hypothalamus under halothane anaesthesia. Changes in extracellular noradr enaline concentration were then monitored on exposure to the tone alone. Pa rallel experiments monitored rats' behaviour in the light arena. Results: A single exposure to the light arena increased extracellular noradrenaline i n the frontal cortex and the hypothalamus but neither a single, nor repeate d, exposure to the tone alone had any effect. After conditioning trials, th e tone alone increased extracellular noradrenaline in the frontal cortex bu t not the hypothalamus, whilst the tone+transfer to the light arena resulte d in a prolonged increase in extracellular noradrenaline in both brain regi ons. The time that rats spent within the light arena was also prolonged. Co nclusions: Noradrenergic neurones in the frontal cortex, but not the hypoth alamus, respond to conditioned cues for aversive environmental stimuli. How ever, prolongation of the noradrenergic response in both brain regions coul d contribute to the behavioural adaptation to such unconditioned stimuli.