The sympathetic silent period in developing spontaneously hypertensive and normotensive rats

Several laboratories have described an abnormally high sympathetic nerve activity (SNA) in developing and adult Kyoto spontaneously hypertensive rats (SHR's). Further, SNA in this strain appears to be more resistant to baroreceptor inhibition. The present study examined a dynamic property of SNA, the sympathetic silent period (SP), in weanling and mature SHR's and normotensive Wistar Kyoto rats (WKY's). SHR's and WKY's, 30 and 100 days old, were anesthetized with chloral hydrate, intubated, paralyzed and artificially respired. A carotid artery was cannulated for measurement of arterial pressure. The left greater splanchnic nerve was cut proximal to the celiac ganglion and placed on bipolar electrodes. Splanchnic evoked potentials were elicited by stimulation of a pressor region just lateral or ventral to the solitary tract. The SP was characterized by delivering conditioning trains (100 Hz) of 1, 3, 5 and 10 pulses followed 100, 200, 400 and 800 ms by a test pulse. Statistically significant differences in the length of the SP's observed were as follows: mature SHR's < young SHR's < young or old WKY's. These data suggest that subnormal postexcitatory inhibition may contribute to the abnormally high SNA in SHR's.