Increased plasma and cerebrospinal fluid norepinephrine in older men: differential suppression by clonidine

To evaluate the effect of advanced age on central nervous system noradrenergic activity, cerebrospinal fluid (CSF) and plasma norepinephrine (NE) concentrations were measured concurrently in 14 older [mean, 65 +/- 9 (+/- SD) yr] and 33 younger (25 +/- 2 yr) normal men. CSF NE was significantly higher in older men than in young men [214 +/- 75 (+/- SD) vs. 164 +/- 56 pg/mL (1.26 +/- 0.44 vs. 0.97 +/- 0.33 nmol/L); P less than 0.02] as was plasma NE [282 +/- 103 vs. 211 +/- 63 pg/mL (1.67 +/- 0.61 vs. 1.25 +/- 0.37 nmol/L); P less than 0.02]. Subgroups of young and older men underwent two lumbar punctures, one of which was performed 100 min after the administration of 5 micrograms/kg oral clonidine. The young (n = 7) and older (n = 7) men had similar plasma clonidine levels [1.0 +/- 0.1 vs. 0.8 +/- 0.1 ng/mL (4.35 +/- 0.43 vs. 3.48 +/- 0.78 nmol/L)] and CSF clonidine levels [0.18 +/- 0.02 vs. 0.22 +/- 0.03 ng/mL (0.78 +/- 0.09 vs. 0.96 +/- 0.13 nmol/L)]. The suppression of CSF NE by clonidine was significantly greater (P less than 0.015) in young men [189 +/- 44 to 104 +/- 26 pg/mL (1.12 +/- 0.26 to 0.62 +/- 0.15 nmol/L)] than in older men [190 +/- 49 to 164 +/- 58 pg/mL (1.12 +/- 0.29 to 0.97 +/- 0.34 nmol/L)]. In contrast, the suppression of plasma NE by clonidine did not significantly differ between young [242 +/- 72 to 93 +/- 24 pg/mL (1.43 +/- 0.43 to 0.55 +/- 0.14)] and older men [285 +/- 102 to 167 +/- 84 pg/mL (1.68 +/- 0.60 to 0.99 +/- 0.50 nmol/L)]. These data suggest that decreased sensitivity of alpha 2-adrenergic mechanisms regulating CNS noradrenergic activity may contribute to increased CNS noradrenergic activity with aging.