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As they age, Sprague-Dawley (SD) rats develop elevated systolic blood pressure associated with impaired baroreflex sensitivity (BRS) for control of heart rate. We previously demonstrated in young hypertensive (mRen2)27 rats that impaired BRS is restored by CB1 cannabinoid receptor blockade in the solitary tract nucleus (NTS), consistent with elevated content of the endocannabinoid 2-arachidonoylglycerol (2-AG) in dorsal medulla relative to normotensive SD rats. There is no effect of CB1 receptor blockade in young SD rats. We now report in older SD rats that dorsal medullary 2-AG levels are two-fold higher at 70 versus 15 weeks of age (4.22 +/- 0.61 vs. 1.93 +/- 0.22 ng/mg tissue; P < 0.05). Furthermore, relative expression of CB1 receptor mRNA is significantly lower in aged rats, while CB2 receptor mRNA is significantly higher. In contrast to young adult SD rats, microinjection of the CB1 receptor antagonist SR141716A (36 pmol) into the NTS of older SD rats normalized BRS in animals exhibiting impaired baseline BRS (0.56 +/- 0.06 baseline vs. 1.06 +/- 0.05 ms/mmHg after 60 min; P < 0.05). Therefore, this study provides evidence for alterations in the endocannabinoid system within the NTS of older SD rats that contribute to age-related impairment of BRS
