Role of brainstem and spinal noradrenergic and adrenergic neurons in the development and maintenance of hypertension in spontaneously hypertensive rats

Summary In young and adult spontaneously hypertensive rats (SHR), dopamine -hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT) activities in discrete areas of the brainstem and spinal cord were measured as indices of noradrenergic and adrenergic neuronal activities. In young SHR, the DBH activities were elevated in the locus coeruleus (LC), A2 cell area and thoracic intermediolateral cell area (IML). The elevation disappeared at adult SHR. In young SHR, no significant change of PNMT activity was observed in the A1, A2, nucleus tractus solitarii (NTS), LC and IML areas, while, in adult SHR, the PNMT activity in the A1 cell area and DBH activity in the NTS were elevated. Lowering of blood pressure by hydralazine decreased the PNMT activity elevated in the A1 cell area and elevated it in the NTS.Plasma levels of norepinephrine and epinephrine, as measured in blood samples collected via aortic cannula at resting state, were much lower than many reported values in blood collected from the decapitated trunk. In young SHR, a significant elevation of plasma norepinephrine and DBH levels was confirmed as signs of peripheral sympathetic nervous activation. The elevation disappeared at adult SHR. Plasma epinephrine levels raised under restraint stress were much higher in SHR at all ages than in normotensive controls.In young SHR, the selective activation of noradrenergic neurons of the IML, A2 and LC areas, accompanied by activation of the peripheral sympathetic nervous system, initiates the hypertension. In adult SHR, the activation of adrenergic neurons in the A1 cell area including the nucleus reticularis lateralis may not be involved in the maintenance of hypertension but may be the results of hypertension.