Norepinephrine levels in discrete brain nuclei in borderline hypertensive rats exposed to compound stressors

The borderline hypertensive rat (BHR), the first generation of an inbred cross between the normotensive Wistar-Kyoto rat (WKY) and the spontaneously hypertensive rat (SHR), appears to be an appropriate model for investigating the role of the environment in producing hypertension.

The central nervous system has been demonstrated to play important role in the regulation of cardiovascular function. Intensive studies using several animal models suggest that pathways that regulate blood pressure involve nuclei in the brain stem and hypothalamus, which may mediate the baroreflex, or influence sympathetic nervous system outflow. One particular neurotransmitter system, the norepinephrine (NE) system, has been suggested to be involved in the regulation of blood pressure. Its cell groups and the connections between them cover several regions in the hypothalamus and medulla oblongata.

Previous studies have demonstrated that the BHR shows chronic blood pressure elevations to both stress and high salt intake. Other studies suggest that interactions between the brain and kidney play an important role in initiating this hypertension. The central noradrenergic system has been implicated in these effects, especially in the hypothalamus. Because exercise has been found to attenuate stress-induced hypertension in the BHR, the current study sought to examine the impact of compound stressors on central NE levels. Special attention was paid to the effects of exercise paired with other stressors (salt intake or stress).

Male BHR were exposed to either control, salt plus stress, salt plus exercise, or stress plus exercise conditions for either 2 or 6 months, beginning at 2 months of age. Following sacrifice, brain nuclei in the brain stem and hypothalamus were removed using the Palkovits micropunch technique. Punches were analyzed for NE content via high performance liquid chromatography with electrochemical detection.

Compared with the control condition, chronic salt plus stress led to reductions in NE content in selected brain nuclei, especially in the hypothalamus. Compared with salt plus stress, the exercise conditions were associated with elevated NE levels, especially in the early phases of exposure to the treatment. The possible role of exercise training in preventing a central nervous system trigger from inducing hypertension in the BHR is discussed.