Interaction between medullary and cervical regulation of renal sympathetic activity.

We have reported that electrical or glutamate stimulation of the dorsolateral surface of the cervical spinal cord elicits a 40-60% decrease in renal sympathetic activity (RSA) in anesthetized rats. Because evoked sympatho-inhibition was observed, however, only after transection of the cervical spinal cord at C1, we suggested that unidentified supraspinal neurons affect the regulation of RSA by cervical neurons. In the present experiments, we tested the hypothesis that the modulatory supraspinal neurons are located in the ventrolateral medulla by observing the effects of rostroventral, lateral, medullary (RVLM) injections of the GABAergic agonist, muscimol, on baseline RSA and on our ability to inhibit that activity by cervical stimulation. GABAergic inhibition in the RVLM of chlorolose anesthetized rats elicited changes in RSA that were similar to those observed after transection of the spinal cord, including a 41% decrease in mean arterial pressure and a 44% increase in RSA. Moreover, after muscimol inhibition of RVLM neurons, electrical or glutamate stimulation of the dorsolateral cervical spinal cord elicited a decrease in RSA in otherwise intact rats. These results suggest that neurons in the RVLM interact with neurons in the cervical spinal cord in the regulation of RSA.