Differential frequency-dependent reflex integration of myelinated and nonmyelinated rat aortic baroreceptors

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 greater than or equal to 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 block of the C-fiber volley (ED50) = 200 nM) without significant attenuation of the A-type volley below 1 mu M. However, 100 mu M 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 non-myelinated arterial BR evoke characteristically different frequency-response reflex relations that suggest distinct differences in sensory information processing mechanisms.