Baroreflexes of the rat. IV. ADN-evoked responses at the NTS

In a long-term (7-21 days) neuromuscular blocked (NMB) rat preparation, using precise single-pulse aortic depressor nerve (ADN) stimulation and stable chronic evoked response (ER) recordings from the dorsal-medial solitary nucleus (dmNTS), two different response patterns were observed: continuous and discrete. For the continuous pattern, activity began ~3 ms after the stimulus and persisted for 45 ms; for the discrete pattern, two complexes were separated by a gap from ~17 to 25 ms. The early complex was probably transmitted via A-fibers: it had a low stimulus current threshold and an average conduction velocity (CV) of 0.58-5.5 m/s; the high threshold late (HTL) complex had a CV = 0.26-0.58 m/s. The average stimulus amplitude-ER magnitude transduction curves for the A and HTL complexes were sigmoidal. For individual rats, in the linear range, mean [r.sup.2] = 0.96 [+ or -] 0.03 for both complexes. The average stimulus amplitude vs. the systolic blood pressure change ([DELTA]sBP) transduction curve was also approximately linear; however, for individual rats, the relationship was not consistently reliable: mean [r.sup.2] = 0.48 [+ or -] 0.19. Approximately 90% of recording sites had respiratory, and 50% had cardiac synchronism. The NMB preparation is useful for studying central baroreflex mechanisms that operate on time scales of days or weeks, such as adaptation and other kinds of neural plasticity. baroreceptors; aortic depressor nerve; C-fiber; nucleus of the solitary tract; cardiac synchronism