Diminished sympathetic silent period in spontaneously hypertensive rats

To determine if elevated sympathetic activity occurs in spontaneously hypertension, the silent period induced in splanchnic nerves following electrical stimulation of dorsal medullary sympathoexcitatory sites was compared in anesthetized normotensive Wistar Kyoto rats (WKYs) and Okamoto spontaneously hypertensive rats (SHRs). The strength of silent periods was defined as the degree of inhibition of responses to testing stimuli delivered at various latencies following conditioning trains, and it was assumed to be inversely related to the level of sympathetic activity. Weanling SHRs exhibited weaker silent periods than weanling WKYs although, at that age, the arterial pressures of the strains were not significantly different. Silent periods were also weaker in adult SHRs than in adult WKYs. This difference persisted after arterial pressures, which fell under anesthesia, were raised by phenylephrine infusions to the respective "normal" levels in each strain. These results support the hypothesis that elevated sympathetic activity exists during both the development and maintenance of spontaneous hypertension in rats.