Effects on sympathetic activity of 8-OHDPAT and clonidine in cat medullary lateral tegmental field.

This study was designed to test the hypothesis that 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) and clonidine reduce sympathetic nerve discharge (SND) and mean arterial pressure (MAP), in part by actions in the medullary lateral tegmental field (LTF). We microinjected these drugs bilaterally into the LTF of baroreceptor-innervated and -denervated cats anesthetized with Dial-urethane. Neither drug altered SND (as quantified by using power spectral analysis) or MAP when injected into the LTF of baroreceptor-denervated 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-innervated 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 these drugs into the rostral ventrolateral medulla of baroreceptor-innervated and -denervated cats. In conclusion, although 8-OHDPAT and clonidine did not reduce SND when injected into the LTF, they acted in this region to facilitate baroreceptor reflex control of SND, as evidenced by a selective increase in cardiac-related power.