Frequency characteristics of baroreflex responses to electrical
stimulation of aortic depressor and carotid sinus nerves in rats.
Fan, Wei, Patrick J. Reynolds, and Michael C. Andresen.
Department of Physiology and Pharmacology, Oregon Health Sciences
University, Portland, Oregon 97201-3098
APStracts 3:0214H, 1996.
Dynamic cardiovascular regulation depends on baroreflexes and the
processing of sensory information. We evaluated the influence of
choice of anesthetic on the frequency response characteristics of the
baroreflex of rats by electrical stimulation of two major
baroreceptor containing nerves, the carotid sinus (CSN) and aortic
depressor nerves (ADN). ADN contains baroreceptors alone and CSN has
both chemoreceptors and baroreceptors. Most studies were performed
under pentobarbital (PB, 65 mg/kg) anesthesia. We compared this to a
combination (CU) of chloralose (80mg/kg) and urethane (800 mg/kg).
Stimulus trains were fixed at 60 s periods (0.1 ms shocks,
supramaximal intensities, 1-200 Hz) and delivered in steady and burst
patterns. Unilateral, steady frequency ADN stimulation in PB rats
evoked reflex decreases in MAP and heart rate (HR) which increased
with frequency between 1 and about 10 Hz before reaching a maximum.
From 10 Hz to 200 Hz, PB ADN reflex responses were sustained at these
maximal levels. Cutting the opposite ADN or both CSNs did not alter
ADN baroreflex relations. HR and MAP depressor responses evoked by
CSN stimulation in PB rats were smaller compared to ADN and were
biphasic with small pressor responses at 1 Hz. Maximal CSN depressor
responses in PB rats occurred at about 20 Hz and were sustained at
20-200 Hz. Baroreflex responses for ADN stimulation in CU rats were
similar to PB rats. In contrast in CU rats, maximal CSN responses
occurred at 20 Hz but declined at 50-200 Hz. Constant and burst
stimulation responses were equivalent. The results suggest that rat
aortic baroreflex responses are sustained even at very high input
frequencies (>100 Hz). The sustained, high frequency baroreflex
responses seems to present a paradox in understanding central
integration since other studies show substantial depression of
sensory transmission at the first synapse in the nucleus tractus
solitarius at frequencies as low as 10 Hz.
Received 31 October 1995; accepted in final form 1 May 1996.
APS Manuscript Number H1023-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 June 96