Effect of repeated immobilization on serotonin metabolism in different ratbrain areas and on serum corticosterone

The effect of daily repeated 10 min immobilization on the serotoninergic ne urotransmission and serum corticosterone levels was studied. Male Lewis rat s were immobilized for a 10 min period daily once or on 5 consecutive days. Serotoninergic neurotransmission was followed using differential in vivo p ulse voltammetry with carbon fibre electrodes measuring extracellular 5-hyd roxyindoleacetic acid (5-HIAA) levels. Recordings were performed in brain a reas involved in the control of behaviour, mood, and stress response such a s the frontal cortex, the hippocampal CA-3 and dentate gyrus, the striatum, and the raphe nuclei dorsalis (NRD) and medialis (MRN). The first immobili zation resulted in an increase of the extracellular 5-HIAA levels in all ar eas under study, except the striatum where no reaction was observed. The ma jor effect was recorded in the frontal cortex, showing an increase of about 400% as compared to control, which lasted for 3h after the end of the immo bilization period. Beginning on day 2 in all areas, except the striatum. a consecutive habituation to the stressor seemed to occur, since the stress-i nduced increase in the voltammetric signal was found to be reduced after co nsecutive immobilization. Serum corticosterone levels were measured directl y after a single and after 5 daily immobilization periods. After single imm obilization the serum corticosterone level was found to be about 270 ng/ml. After the 5th immobilization about 300 ng/ml were detected. These differen ces were not found to be significant. In summary, our data indicate that th e serotonin metabolism shows habituation in nearly all brain areas after re peated immobilization, though the corticosterone level at the end of the im mobilization period was comparable after single and repeated immobilization .