DIFFERENTIAL MODULATION OF DOPAMINERGIC SYSTEMS IN THE RAT-BRAIN BY DIETARY-PROTEIN

Rats that consume a diet 50% rich in protein exhibit hyperactivity and hyperresponsiveness to nociceptive stimuli, in which facilitation of dopaminergic activity has been implicated. We studied the regional cha nges in the concentrations of dopamine (DA) and its metabolites, dihyd roxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain s of rats that were maintained on high-protein (HP, 50% casein), norma l-protein (NP, 20% casein), and low-protein (LP, 8% casein) diets for 36 weeks. Brain nuclei that represented different DAergic systems were punch-dissected and analyzed using HPLC. In the substantia nigra, the striatum, and the dentate gyrus, DA concentrations decreased and incr eased, respectively, with a decrease and increase in dietary protein ( p < 0.05 compared to the NP diet). Similar trends in the effect of the HP diet were observed in the ventral tegmental area, amygdala, fronta l cortex, subiculum, centromedial nucleus (CM) of the thalamus, and in ferior colliculi (IC), although the differences in DA concentrations w ere not statistically significant. These brain areas also showed a pat tern of decreased DA concentration in association with the LP diet, an d the differences were statistically significant (p<0.05) in the CM an d IC. DA concentrations in most regions of the midbrain and brainstem were not different between the diet groups, nor were consistent trends observed in those regions. Also, there were no consistent relationshi ps between DOPAC/DA and HVA/DA ratios and dietary protein level. These data suggest that only discrete dopaminergic neuronal circuits in the rat forebrain were sensitive to changes in dietary protein level.