Influences of excitatory amino acid receptor agonists on nucleus of the solitary tract neurons receiving aortic depressor nerve inputs

Neurons in the nucleus of the solitary tract (NTS) of the anesthetized rat were classified according to their responses to aortic depressor nerve stimulation: monosynaptic neurons (MSNs), polysynaptic neurons (PSNs) and non-aortic depressor nerve-evoked neurons (NENs). Agonists for excitatory amino acid (EAA) receptors were applied by microiontophoresis at currents of 5 to 40 nA. At these 'doses,' the nonselective EAA agonist glutamate (100 mM) increased the firing rate of some MSNs (5/9), PSNs (6/8) and NENs (16/20) (P < .01 for each group). Some neurons in each group were very resistant to glutamate, even at high ejecting currents. In addition, most NTS neurons were excited by selective EAA agonists, (RS)-alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (10 mM), kainate (10 mM), N-methyl-D-aspartic acid (100 mM) and trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (100 mM). As with glutamate, some NTS neurons in each class were also very resistant to selective EAA agonists. Statistical analysis indicated that N- methyl-D-aspartic acid, but not (RS)-alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid and kainate, was more potent on PSNs than on MSNs or NENs (P < .01 for each comparison). There was a trend for trans-(1S,3R)-1- amino-1,3-cyclopentanedicarboxylic acid to be more potent on MSNs than on PSNs or NENs (P = .09 and .07, respectively). Our results suggest that all EAA receptor subtypes are involved in baroreceptor afferent integration within NTS, and NTS neurons appear to possess different combinations of EAA receptor subtypes.