Dietary Protein and Central Monoamine Concentrations in the Rat

In a previous report we demonstrated that rats that consumed a high-protein diet (HP; 50% casein) for 36 weeks were hyperactive and hyperresponsive to nociceptive stimuli, compared to rats that consumed normal (NP; 20% casein) or low-protein (LP; 8% casein) diets. In addition, we have also previously, reported that dopamine concentrations in the nigrostriatal system of the rats were decreased and increased, respectively, with a decrease and increase in dietary protein. In the present study, rats were maintained on the HP, NP and LP diets and regional changes in the concentrations of norepinephrine (NE) and serotonin (5-hydroxytryptamine, 5-HT) were assessed. Concentrations of 5-HT in the medial raphe, dorsal raphe, and several of their target tissues, revealed no consistent effect of manipulating dietary protein over the range of 5-HT levels measured. NE concentrations in most of the brain regions innervated by neurons of the locus coeruleus and lateral tegmentum showed no significant differences among the diet groups. However, NE concentrations in the parietal cortex were significantly increased in rats that consumed the HP diet. The present study indicates that the brain NE pathways, particularly that innervating the parietal cortex, is susceptible to dietary protein manipulation.