Effects on sympathetic activity of 8-OHDPAT and clonidine in cat medullarylateral tegmental field

This study was designed to test the hypothesis that 8-hydroxy-2(di-n-propyl amino) tetralin (8-OHDPAT) and clonidine reduce sympathetic nerve discharge (SND) and mean arterial pressure (MAP), in part by actions in the medullar y lateral tegmental field (LTF). We microinjected these drugs bilaterally i nto the LTF of baroreceptor-innervated and -denervated cats anesthetized wi th Dial-urethane. Neither drug altered SND (as quantified by using power sp ectral analysis) or MAP when injected into the LTF of baroreceptor-denervat ed cats. However, cardiac-related power in SND was significantly increased to 148 +/- 12 (mean +/- SE) and 149 +/- 5% of control by microinjections of 8-OHDPAT (n = 5) and clonidine (n = 5), respectively, in baroreceptor-inne rvated cats whose MAP was kept constant; there was no change in 0- to 6-Hz power or total power. SND was significantly reduced by microinjection of th ese drugs into the rostral ventrolateral medulla of baroreceptor-innervated and -denervated cats. In conclusion, although 8-OHDPAT and clonidine did n ot reduce SND when injected into the LTF, they acted in this region to faci litate baroreceptor reflex control of SND, as evidenced by a selective incr ease in cardiac-related power.