MEDIATION BY N-METHYL-D-ASPARTATE AND NON-N-METHYL-D-ASPARTATE RECEPTORS IN THE EXPRESSION OF FOS PROTEIN AT THE NUCLEUS-TRACTUS-SOLITARII IN RESPONSE TO BARORECEPTOR ACTIVATION IN THE RAT

We investigated the role of glutamatergic synapses in the expression o f Fos protein al the nucleus tractus solitarii following baroreceptor activation in rats anaesthetized with pentobarbital sodium. Microinjec tion of L-glutamate (1 nmol) bilaterally into the nucleus tractus soli tarii elicited significant hypotension and bradycardia. There was a co ncurrent increase, as determined immunohistochemically, in the express ion of Fos protein at the commissural, medial and dorsomedial subnucle i of the caudal nucleus tractus solitarii. These effects were blunted when L-glutamate was co-administered with either the selective N-methy l-D-aspartate or non-N-methyl-D-aspartate glutamate receptor antagonis t, dizocilpine maleate (200 pmol) or 6-cyano-7-nitroquinoxaline-2,3-di one (8 pmol), into the caudal nucleus tractus solitarii. Repeated and scheduled transient hypertension evoked by phenylephrine (2.5, 5.0 or 10.0 mu g/kg, i.v.) also appreciably increased the number of Fos-immun oreactive neurons at the commissural, medial and dorsomedial subnuclei of the caudal nucleus tractus solitarii. The expression of Fos protei n in this fashion was reduced, simultaneous with a discernible depress ion in baroreceptor reflex response, when baroreceptor activation was coupled with microinjection bilaterally of dizocilpine maleate (200 pm ol) or 6-cyano-7-nitroquinoxaline-2,3-dione (8 pmol) into the nucleus tractus solitarii. Regression analysis showed that the depressive acti on on the baroreceptor reflex response by both glutamate receptor anta gonists correlated positively to the reduction in Fos-immunoreactivity in the nucleus tractus solitarii after baroreceptor activation. Doubl e immunohistochemical staining revealed that nucleus tractus solitarii neurons that showed Fos immunoreactivity were generally also immunore active to pha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunit 1. On the other hand, Fos immunoreactivity was usually absent from neurons in the nucleus tractus solitarii that were immunoreactive to N-methyl-D-aspartate receptor subunit 1. These results suggest tha t glutamatergic neurotransmission plays an active role, via comparable contributions from both N-methyl-D-aspartate and non-N-methyl-D-aspar tate receptors, in the expression of Fos protein at the caudal nucleus tractus solitarii in response to baroreceptor activation. (C) 1997 IB RO. Published by Elsevier Science Ltd.