Dietary protein content affects the profiles of extracellular amino acids in the medial preoptic area of freely moving rats

This study examined the effect of protein consumption on extracellular amino acid concentrations in the medial preoptic area (MPOA) of rats. Rats were given free access to diets containing 0, 25 or 50 % protein for 3-h duration, starting from the onset of dark cycle (1800 h). The microdialysis probe was implanted into the MPOA at 1500 h. Dialysates were collected every 20 min from 1700 h to 2100 h. Amino acid concentrations in dialysate samples were determined by reverse phase-HPLC. Extracellular amino acid concentrations in the MPOA were elevated by protein consumption within 20 to 40 min following the start of the meal. The 50 % protein diet resulted in increased (p<0.05) alanine, glutamine, isoleucine, leucine, methionine, tyrosine and valine concentrations, when compared with both baseline and the 0 % protein diet. When the 25 % protein diet was fed, amino acid concentrations in the MPOA were between those after the 0 and 50 % protein diets. The ratio of tryptophan to the total branched-chain amino acids in extracellular fluid was highest after the 0 % protein diet and increased with time. We conclude that extracellular amino acid profiles in the MPOA are affected by dietary protein content.