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Department of Physiology and Pharmacology, Oregon Health Sciences University, Portland, Oregon 97201-3098
Electrical
activation of myelinated (A type) and nonmyelinated (C type)
baroreceptor axons (BR) in aortic depressor nerve (ADN) evoked
baroreflex changes in mean arterial pressure (MAP) in
chloralose-urethan-anesthetized rats. Low stimulation intensities (<3
V) activated only A-type BR electroneurograms (ENG). A-type selective
stimulus trains required minimum frequencies >10 Hz to evoke reflex
MAP decreases, and the largest MAP responses occurred at 50 Hz and
higher. In contrast, high stimulation intensities (18-20 V)
maximally activated two volleys in ADN ENG corresponding to A- and
C-type BR volleys. High-intensity trains decreased MAP at low frequency
(1 Hz) and largest reflex responses at 
5 Hz. Capsaicin (Cap) applied
periaxonally to ADN selectively blocked C-type ENG volleys but not
A-type volleys. Reflex curves with supramaximal intensity during Cap
were indistinguishable from the pre-Cap, low-intensity baroreflexes. In
comparison, vagus ENG showed graded Cap block of the C-fiber volley
(ED50 = 200 nM) without
significant attenuation of the A-type volley below 1 µM. However, 100 µM Cap blocked conduction in all myelinated vagal axons as well as
C-type axons. Thus Cap is selective for sensory C-type axons only at
low micromolar concentrations. Myelinated and nonmyelinated arterial BR
evoke characteristically different frequency-response reflex relations
that suggest distinct differences in sensory information processing
mechanisms.
sensory afferents; blood pressure; myelinated axons; nonmyelinated axons
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