Alterations of Brain Monoamine Levels in Pigs Exposed to Acute Immobilization Stress*

Concentrations of noradrenaline (NA), adrenaline (A), dopamine (DA) and 5-hydroxytryptamine (5-HT), as well as DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and the main 5HT metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA), were measured using the HPLC technique in 15 brain areas of control and immobilized Duroc pigs. The animals were immobilized for 5, 15, 30 and 60 min in a prone position. Control pigs displayed patterns of regional distribution of brain monoamines similar to those described in other species, especially rats and dogs. However, absolute values of noradrenaline and adrenaline in the hypothalamus were much higher than in other species. Also, in most structures, the DOPAC/DA ratio was relatively high, in comparison to a relatively low HVA/DA ratio, which suggests a species-related difference in the turnover of dopamine. The most conspicuous changes produced by immobilization stress consisted of a substantial decrease in the hypothalamic levels of noradrenaline and adrenaline. Dopamine and 5-HT turnover was affected in the hippocampus (cornu Ammonis), and in the raphe nuclei. These structures are proposed to play a major role in the responsiveness of pigs to acute stress conditions.