Submitted on January 7, 2008
Accepted on March 10, 2008
Role of serotonergic input to the ventrolateral medulla
in expression of the 10-Hz sympathetic nerve rhythm
Hakan S Orer, Gerard L. Gebber1, and Susan M. Barman2*
1 Department of Pharmacology and Toxicology, Michigan State University, East Lansing,, Michigan, United States
2 Department of Pharmacology & Toxicology, Michigan State University, East Lansing, Michigan, United States
* To whom correspondence should be addressed. E-mail: barman{at}msu.edu.
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 µM) 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.
