Stressful environmental stimuli increase extracellular DOPAC levels in the prefrontal cortex of hypoemotional (Roman high-avoidance) but not hyperemotional (Roman low-avoidance) rats. An in vivo voltammetric study.

The effects of a variety of stressful environmental situations on dopamine metabolism in the prefrontal cortex (as assessed by in vivo voltammetry with carbon fiber electrodes) have been compared in two genetically selected lines of rat (Roman high (RHA/Verh) and low (RLA/Verh) avoidance) which differ drastically in their level of emotionality. Heart rate was continuously monitored in these animals (via chronically implanted subcutaneous electrodes) so as to index the emotional reaction to the stressors. An electrochemical signal corresponding to the oxidation of dihydroxyphenylacetic acid (DOPAC) was recorded in the deeper laminae of the anteromedial prefrontal cortex in both lines of rats. Under normal conditions, this signal was stable for at least 4 h and its amplitude was similar in both lines. Introduction of the animals into an unfamiliar environment (30 min), application of a mild tail pinch (10 min) or of a high-intensity loud noise (30 min) or immobilization (20 min) were all associated with an increase in extracellular cortical DOPAC levels in the hypoemotional RHA/Verh line but not in the hyperemotional RLA/Verh line. Similarly, forced locomotion on a rotarod (40 min) provoked a dramatic increase in the amplitude of the cortical DOPAC oxidation peak in RHA/Verh rats and only a mild increase in this parameter in RLA/Verh rats. In RHA/Verh rats, tolerance to this increase was observed when animals were subjected to forced locomotion every day for 5 days. All of the stressful situations investigated provoked an immediate augmentation of heart rate which resumed gradually after cessation of the stressful stimulus; the magnitude and duration of this increase were much greater in RLA/Verh than in RHA/Verh rats. Moreover, in all stress situations, RLA/Verh but not RHA/Verh rats showed behavioral signs of emotional response e.g. defecation, freezing and self-grooming. It is concluded that the increase in cortical dopamine metabolism induced by stress is not connected to the emotional reaction caused by the aversive nature of the stressor but may rather reflect a heightened attention of the animal or activation of cognitive processes in an attempt to cope with the stressor.