Adenosine pressor and depressor systems in the nucleus tractus solitarius of rats

Abstract

This report discusses the neuronal and cardiovascular responses elicited by adenosine in two subareas of the nucleus tractus solitarius (NTS) of normotensive and spontaneously hypertensive (SHR) rats. Pressor and tachycardic responses were obtained from the rostral NTS (adenosine pressor system), and depressor and bradycardic responses were obtained from the caudal NTS (adenosine depressor system). In both areas, adenosine inhibited the firing rate of barosensitive neurons. Male Sprague-Dawley (SD), SHR, and Wistar-Kyoto rats (WKY) were anesthetized with urethane and prepared for blood pressure and heart rate recording, stereotaxic microinjection of adenosine into the NTS, and extracellular recording of single-unit neuronal activity of NTS neurons. Chemical identification of the targeted neuronal pool was made by L-glutamate (5 nmol) and confirmed by histology. Adenosine elicited dose-related neuronal and cardiovascular responses. However, SHR exhibited differential alterations in both adenosine systems. Compared with WKY, SHR exhibited attenuated pressor, tachycardic, and neuronal responses mediated by the adenosine pressor system and exaggerated depressor, bradycardic, and neuronal responses mediated by the adenosine depressor system. The responses elicited by adenosine were virtually abolished by theophylline (10 mg/kg IV) and by microinjection of 8(p-sulfophenyl)theophylline) into the same area, suggesting that these responses were mediated by adenosine receptors in the NTS. Furthermore, the theophylline-evoked increase in blood pressure was twofold higher in SHR compared with WKY. These findings suggest: 1) adenosine pressor and depressor systems involve different neuronal pools in the NTS, and 2) the differential alterations in adenosine pressor and depressor systems in the NTS of SHR may be implicated in the pathophysiology of hypertension.