Role of serotonergic input to the ventrolateral medulla in expression of the 10-Hz sympathetic nerve rhythm. Am J Physiol Regul Integr Comp Physiol 294: R1435-R1444, 2008. First published March 12, 2008; doi:10.1152/ajpregu.00012.2008.-We studied the changes in inferior cardiac sympathetic nerve discharge ( SND) produced by unilateral microinjections of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists into the ventrolateral medulla (VLM) of urethane-anesthetized, baroreceptor-denervated cats. Microinjection of the 5-HT2 receptor antagonist LY-53857 ( 10 mM) into either the rostral or caudal VLM significantly reduced ( P <= 0.05) the 10-Hz rhythmic component of basal SND without affecting its lower-frequency, aperiodic component. The selective depression of 10- Hz power was accompanied by a statistically significant decrease in mean arterial pressure ( MAP). Microinjection of LY-53857 into the VLM also attenuated the increase in 10- Hz power that followed tetanic stimulation of depressor sites in the caudal medullary raphe nuclei. Microinjection of the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)2-amino-propane ( DOI;10 mu M) into the VLM selectively enhanced 10- Hz SND, and intravenous DOI ( 1 mg/kg) partially reversed the reduction in 10- Hz SND produced by 5-HT2 receptor blockade in the VLM. Microinjection of the 5-HT1A receptor agonist, 8-hydroxy-2( di-n-propylamino)tetralin ( 8-OHDPAT;10 mM), into either the rostral or caudal VLM also selectively attenuated 10- Hz SND and significantly reduced MAP. The reduction in 10- Hz SND produced by 8-OHDPAT was partially reversed by intravenous WAY-100635 ( 1 mg/kg), which selectively blocks 5-HT1A receptors. These results support the view that serotonergic inputs to the VLM play an important role in expression of the 10- Hz rhythm in SND.