MICRODIALYSIS AS A TOOL TO MEASURE DIETARY AND REGIONAL EFFECTS ON THE COMPLETE PROFILE OF EXTRACELLULAR AMINO-ACIDS IN THE HYPOTHALAMUS OFRATS

Regional and dietary-induced changes in hypothalamic extracellular ami no acid concentrations were examined. Microdialysis probes were simult aneously implanted in the hypothalamic paraventricular nucleus (PVN) a nd the lateral hypothalamus (LH) of anesthetized rats and perfused at a rate of 2 mu l/min. Dialysates were collected every 20 min for 1 h p rior to gavage of a balanced amino acid mixture (0.85 g patterned afte r 1 g of chicken egg albumin) and then every 20 min for 3 h after trea tment. Tail vein blood samples were also collected. Marked changes in plasma levels of most amino acids were evident immediately following t he amino acid gavage. In the PVN, concentrations of isoleucine, leucin e, methionine and valine all increased within 40 min, whereas signific ant decreases in glutamine, histidine and taurine were observed in the LH. In a separate study, PVN extracellular amino acid concentrations were examined in awake, freely-behaving rats following gavage of equic aloric loads of a balanced amino acid mixture, glucose (0.89 g) or wat er. Dialysate levels of glutamate, isoleucine, leucine, methionine, th reonine, tyrosine and valine showed reliable increases after amino aci d treatment, although the overall time course of these effects differe d somewhat. The amino acid profile of the PVN was, in general, unaffec ted by glucose administration. These findings suggest that specific br ain regions may respond uniquely to amino acid ingestion and further i mply that dietary composition may influence the amino acid profiles of the extracellular fluid in brain.