OPEN-FIELD BEHAVIOR OF SPONTANEOUSLY HYPERTENSIVE RATS

The present experiment investigated the exploratory behavior of spontaneously hypertensive rats (SHR) of the Okamoto strain. A free-exploration open-field apparatus, permitting the animal to explore the novel environment from its home cage, was used for studying the open-field behavior of 10 prehypertensive spontaneously hypertensive rats and 10 normotensive control rats (NR) of the Wistar-Kyoto strain. In Experiment 1, each rat was run for one 15-min session on each of 10 consecutive days. The SHR gradually became much more active than the NR, showing shorter latency to leave the home cage, spending more time in the surrounding open field, and ambulating more. Experiment 2 involved introduction of a novel object. This resulted in a further increase in the activity of the SHR, developing gradually as in Experiment 1. The SHR explored the novel object more than the NR did, indicating that the hyperactivity observed in Experiments 1 and 2 was due to hyperexploration. Correlations between mean blood pressure as measured 2 months following the behavioral testing and mean total ambulatory activity showed that the SHR developing the most pronounced hypertension were the most active SHR. In contrast, the least active NR had higher blood pressures than the most active NR. The present behavioral results are interpreted as being due to increased effects of stimuli in SHR. It is suggested that increased neurohumoral load on the cardiovascular system due to increased effects of stimuli is important in the pathogenesis of hypertension in the SHR. © 1979 Academic Press, Inc.