Importance of glycinergic and glutamatergic synapses within the rostral ventrolateral medulla for blood pressure regulation in conscious rats

In this study we used a method that permits bilateral or unilateral microin jections of drugs into the rostral ventrolateral medulla (RVLM) of consciou s, freely moving rats. There is only limited information about how sympathe tic vasomotor tone is maintained by premotor RVLM neurons in conscious anim als, It has long been known that glycine microinjection into the RVLM regio n leads to a decrease in blood pressure (BP) in anesthetized animals. In th e present study we show that both unilateral and bilateral microinjection o f glycine at the same dose used for anesthetized rats (50 nmol, 50 nL) into the RVLM increases BP-in conscious animals. A similar response was also ob served when the excitatory amino acid L-glutamate was microinjected into th e RVLM. The microinjection of kynurenic acid into the RVLM did not change t he basal level of BP but blocked the increase in BP after glycine or glutam ate microinjection. A decrease in BP was only observed when low doses of gl ycine were used (1 to 10 nmol). We conclude that, in conscious animals, the hypertension occurring in response to high doses of glycine into the RVLM is dependent on glutamatergic synapses within the RVLM. A decrease in BP ob served when low doses of glycine were used shows that in conscious animals, the RVLM, in association with other premotor neurons, is probably responsi ble for the maintenance of sympathetic vasomotor tone, because glycine is l ess effective In decreasing BP under these circumstances than in anesthetiz ed animals.