Apparent turnover of norepinephrine in the intact rat brain estimated from its rate of appearance in the cerebroventricular compartment.

The apparent turnover of norepinephrine (NE) in the intact brain of urethane-anesthetized rats was estimated in vivo from the rate of appearance of endogenous NE in the cerebroventricular compartment. Ventriculocisternal perfusion, a radioenzymatic assay for endogenous NE and isotope dilution of 3H-NE were used to determine the rate of appearance (Ra) of NE. The effect of tranylcypromine, a monoamine oxidase (MAO) inhibitor and releasing agent on Ra of NE, was also studied. In control animals, the mean Ra value was 0.40 pmol/min. Pretreatment with tranylcypromine resulted in a threefold increase in the mean value for Ra, which was due to increased secretion of NE into the CSF. The results indicate that 90% of the NE entering the cerebroventricular compartment was removed. Pretreatment with tranylcypromine did not modify removal, thus suggesting that tranylcypromine has no effect on the physiological processes which regulate the removal of NE from the CSF.