The measurement of central noradrenergic activity in spontaneously hypertensive rats: a comparison of free 3,4-dihydroxyphenylethyleneglycol levels with FLA-63 induced noradrenaline depletion.

Noradrenergic activity was measured in the brainstem, hypothalamus and thoracic spinal cord of male and female spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats at 6 and 28-36 weeks of age. Two techniques were used, measurement of a major noradrenaline (NA) metabolite, free 3,4-dihydroxyphenylethyleneglycol (DHPG), and measurement of the rate of decline in brain NA levels following dopamine-beta-hydroxylase (DBH) inhibition by FLA-63. There was a good correlation between the changes with age in NA turnover measured by the two techniques. NA levels and NA turnover measured by both techniques fell with age in brainstem and thoracic spinal cord in both SHR and WKY rats. In both strains these falls in turnover were associated with increases in blood pressure. However, the increase in blood pressure in the SHR was greater than in the WKY, even though NA turnover fell to a similar extent in both strains. These data show a difference in the pattern of change in NA levels and turnover in the brainstem and thoracic spinal cord compared to other brain regions and may therefore be related to the development of higher levels of blood pressure in older rats in both strains. They do not offer a simple explanation for the much higher blood pressures seen at all ages in the SHR.