Differential roles of NMDA and non-NMDA receptors in synaptic responses ofneurons in nucleus tractus solitarii of the rat

The relative role of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in synaptic responses of neurons in caudal nucleus tractus solitarii (cNTS) wa s delineated by immunohistochemical and electrophysiologic experiments in r ats. Double immunohistochemical staining in in vivo experiments revealed th at similar to 80% of cNTS neurons that showed Fos-like immunoreactivity ind uced by baroreceptor activation were generally also immunoreactive to nonNM DA receptor subunits GluR1 or GluR2. On the other hand, only 20% of Fos-lab eled cNTS neurons showed immunoreactivity to NMDA receptor subunits NMDAR1 or NMDAR2. Stimulation of the ipsilateral solitary tract at suprathreshold intensity in slice preparations induced Fos expression in the cNTS and evok ed either a single action potential or a complex synaptic response consisti ng of an initial action potential followed by a secondary slow depolarizati on. In a majority (70%) of cNTS neurons that exhibited the complex synaptic response, both the initial and secondary components were eliminated revers ibly by 6-cyano-7-nitroquinoxaline-2,3-dione (20 mu M). This non-NMDA antag onist also inhibited the single action potential manifested by the other po pulation of cNTS neurons. On the other hand, only the secondary slow depola rization was blocked by D(-)-2-amino-5-phosphonopentanoic acid (250 mu M) o r potentiated by NMDA (1.7 mu M). Our results suggested that NMDA and non-N MDA receptors are involved differentially in the synaptic responses of cNTS neurons. Non-NMDA receptors may be distributed predominantly on a majority of the second-order cNTS neurons that may receive primary baroreceptor aff erent inputs. On the other hand, NMDA receptors are located primarily on hi gher-order neurons, which may be connected reciprocally with the second-ord er cNTS neurons.