CENTRAL CATECHOLAMINERGIC NEURONS ARE INVOLVED IN EXPRESSION OF THE 10-HZ RHYTHM IN SND

We studied the effects of adrenoceptor agonists and antagonists on sym pathetic nerve discharge (SND) of urethan-anesthetized, baroreceptor-d enervated cats. In cats in which a 10-Hz rhythm coexisted with irregul ar 2- to 6-Hz oscillations in SND, intravenous clonidine, an alpha(2)- adrenoceptor agonist, blocked the 10-Hz rhythm without affecting power at lower frequencies. In contrast, power at frequencies less than or equal to 6 Hz was depressed by clonidine in cats in which the 10-Hz rh ythm was absent. These effects were reversed by intravenous administra tion of alpha(2)-adrenoceptor antagonists, idazoxan and rauwolscine. R auwolscine is devoid of affinity for imidazoline receptors. Furthermor e, in cats untreated with clonidine, idazoxan and rauwolscine enhanced or induced the 10-Hz rhythm without affecting power at lower frequenc ies. Prazosin, an alpha(1)-adrenoceptor antagonist, selectively blocke d the 10-Hz rhythm in SND. Finally, the 10-Hz rhythm in SND was blocke d by microinjection of clonidine into the rostral or caudal ventrolate ral medulla. The results support the view that central catecholaminerg ic neurons play a role in expression of the 10-Hz rhythm in SND.