AMINO-ACID NEUROTRANSMITTERS IN NUCLEUS-TRACTUS-SOLITARIUS - AN IN-VIVO MICRODIALYSIS STUDY

Amino acid neurotransmitters in the nucleus tractus solitarius (NTS) a re thought to play a key role in the mediation of visceral reflexes an d glutamate has been proposed as the neurotransmitter of visceral affe rent nerves projecting to this region. The present studies sought to c haracterize the use of in vivo microdialysis to examine extracellular fluid levels of amino acids in the NTS of anesthetized rats. Using a m icrodialysis probe that was 450 mum in length and a sensitive HPLC ass ay for amino acids, amino acids could be measured in dialysate samples collected from the NTS. Perfusion of the microdialysis probe with 60 mM K+, to elicit depolarization of nerve terminals in the vicinity of the probe, resulted in increased dialysate fluid levels of aspartate, glutamate, glycine, taurine, and GABA. In contrast, glutamine and tyro sine were decreased and other amino acids were not significantly affec ted. Prior removal of the ipsilateral nodose ganglion did not alter th e K+-evoked changes in dialysate levels of any of these amino acids. E lectrical stimulation of the vagus nerves, using a variety of stimulus parameters, did not significantly alter dialysate levels of glutamate or any of the other amino acids that were measured. Blockade of gluta mate uptake with dihydrokainate increased dialysate levels of glutamat e, aspartate, and GABA, but in the presence of dihydrokainate vagal st imulation did not alter dialysate levels of these amino acids. The res ults show that in vivo microdialysis can be used to examine amino acid efflux in the rat NTS and provide further evidence for amino acidergi c neural transmission in the NTS. However, these studies fail to suppo rt the hypothesis that vagal afferents release glutamate or aspartate.