RESPIRATORY MODULATION OF SYMPATHETIC-NERVE ACTIVITY - EFFECT OF MK-801

The modulation of splanchnic sympathetic nerve activity (SNA) by brain stem neural networks generating the respiratory rhythm was examined i n decerebrate, unanesthetized, vagotomized, artificially ventilated ra ts before and after blockade of the N-methyl-D-aspartate (NMDA) channe l with intravenous administration of dizocilpine (MK-801). NMDA channe l blockade 1) prolonged inspiration and reduced the phrenic nerve ampl itude, 2) reduced SNA to 40% of control levels, and 3) decreased mean arterial pressure by 20 mmHg. A strong synchronization of SNA to the c entral respiratory cycle (monitored by the activity on the phrenic ner ve) was maintained after MK-801 administration, although a brief inhib ition of SNA during early inspiration and a sympathetic excitation dur ing early expiration were eliminated. These results suggest 1) the exi stence of an NMDA-independent mechanism by which some elements of the brain stem respiratory network excite sympathetic outflow 2) that the NMDA-mediated influence of specific classes of brain stem respiratory neurons can modulate this excitation during portions of the respirator y cycle, and 3) that an NMDA-dependent excitation in the brain stem or spinal cord plays a significant role in maintaining basal levels of s planchnic SNA.