ROLE OF SEROTONERGIC NEURONS IN THE MAINTENANCE OF THE 10-HZ RHYTHM IN SYMPATHETIC-NERVE DISCHARGE

We studied the effects of serotonin (5-HT)-receptor agonists and antag onists on the naturally occurring 10-Hz rhythm in sympathetic nerve di scharge (SND) of urethan-anesthetized, baroreceptor-denervated cats. I ntravenous doses of the 6-HT1A-receptor agonists 8-hydroxy-2(di-n-prop ylamino)tetralin (8-OH-DPAT) and U-93385E, which inhibit the firing of serotonergic medullary raphe neurons, decreased the power in the 10-H z band of SND without affecting the power at frequencies less than or equal to 6 Hz. The inhibitory effects of 8-OH-DPAT and U-93385E were r eversed by the 5-HT1A-receptor antagonists spiperone and WAY-100135. M icroinjection of 8-OH-DPAT into medullary raphe nuclei also selectivel y eliminated the 10-Hz rhythm in SND. Intravenous administration of th e 5-HT2-receptor antagonist methysergide blocked the 10-Hz rhythm in S ND, whereas the 5-HT2-receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)- 2-amino-propane increased peak frequency and power in the 10-Hz band o f SND. Microinjection of N-methyl-D-aspartic acid into the medullary r aphe also enhanced the 10-Hz rhythm in SND. These data support the vie w that the naturally occurring discharges of serotonergic medullary ra phe neurons preferentially enhance the 10-Hz rhythm in SND.