Physiological approaches to assess diminished sympathetic activity in the conscious rat

The purpose of this study was to evaluate functional measures of diminished sympathetic activity after postganglionic neuronal loss in the conscious rat. To produce variable degrees of sympathetic postganglionic neuronal loss, adult rats were treated daily with toxic doses of guanethidine (100 mg/kg) for either 5 days or 11 days, followed by a recovery period of at least 18 days. Heart rate, blood pressure, cardiac baroreflex responsiveness, urinalysis (for catecholamine metabolite, 3-methoxy-4-hydroxyphenylethylenglycol; MHPG), and pupillometry were performed during the recovery period. At the end of the recovery period stereology of superior cervical ganglia (SCG) was performed to determine the degree of neuronal loss. Total number of SCG neurons was correlated to physiological outcomes using regression analysis. Whereas guanethidine treatment for 11 days caused significant reduction in the number of neurons (15,646 ± 1460 vs. 31,958 ± 1588), guanethidine treatment for 5 days caused variable levels of neuronal depletion (26,009 ± 3518). Regression analysis showed that only changes in urinary MHPG levels and systolic blood pressure significantly correlated with reduction of SCG neurons (r2 = 0.45 and 0.19, both p < 0.05). Although cardiac baroreflex-induced reflex tachycardia (345.7 ± 19.6 vs. 449.7 ± 20.3) and pupil/iris ratio (0.50 ± 0.03% vs. 0.61 ± 0.02%) were significantly attenuated in the 11-day guanethidine treated rats there was no significant relationship between these measurements and the number of remaining SCG neurons after treatment (p > 0.05). These data suggest that basal systolic blood pressure and urinary MHPG levels predict drug-induced depletion of sympathetic activity in vivo.