BAROREFLEX FREQUENCY-RESPONSE CHARACTERISTICS TO AORTIC DEPRESSOR ANDCAROTID-SINUS NERVE-STIMULATION IN RATS

Dynamic cardiovascular regulation depends oil baroreflexes and the pro cessing of sensory information. We evaluated tl-ic influence of choice of anesthetic on the frequency-response characteristics of the barore flex of fats by electrical stimulation of two major baroreceptor-conta ining nerves, the carotid sinus (CSN) and aortic depressor nerves (ADN . The ADN contains baroreceptors alone, and the CSN has both chemorece ptors and baroreceptors. Most studies were performed under pentobarbit al sodium (PB: 65 mg/kg) anesthesia. We compared this to a combination of alpha-chloralose (80 mg/kg) and urethan (800 mg/kg) (CU). Stimulus trains were fixed at 60-s periods (0.1-ms shocks, supramaximal intens ities, 1-200 Hz) and delivered in steady and burst patterns. Unilatera l steady- frequency ADN stimulation in PB-anesthestized rats evoked re flex decreases in mean arterial pressure and heart rate that increased with frequencies between 1 and similar to 10 Hz before reaching a max imum. From 10 to 200 Hz, PB ADN reflex responses were sustained at the re maximal levels. Cutting the opposite ADN or both CSNs did not alter ADN baroreflex relationships. Heart rate and mean arterial pressure d epressor responses evoked by CSN stimulation In PB-anesthetized rats w ere smaller compared with ADN stimulation and were biphasic, with smal l plessor responses at 1 Hz. Maximal CSN depressor responses in PB-ane sthetized rats occurred at similar to 20 Hz and were sustained at 20-2 00 Hz. Baroreflex responses far ADN stimulation in CU-anesthetized rat s were similar to those in PB-anesthetized rats. In contrast, in CU-an esthetized rats, maximal CSN responses occurred at 20 Hz but declined at 50-200 Hz. Constant- and burst-stimulation responses were equivalen t. The results suggest that rat aortic baroreflex responses are sustai ned even at very high input frequencies (>100 Hz). The sustained high- frequency baroreflex responses seem to present a paradox in understand ing central integration because other studies show substantial depress ion of sensory transmission at the first synapse in the nucleus tractu s solitarius at frequencies as low as 10 Hz.