Effects of pargyline on tele-methylhistamine and histamine in rat brain.

Rapid and complete inhibition of brain MAO produced linear increases in brain t-MH levels from 30 min to 4 hr after drug treatment at a rate of 0.26 nmole/g X hr, resulting in a 3-fold increase which persisted for at least 12 hr. HA levels were slightly elevated 1 and 2 hr after drug administration but quickly returned to control levels, suggestive of sensitive regulatory mechanisms in brain. Although the slight change in HA levels precludes steady-state assumptions, the rate of increase in brain t-MH levels after MAO inhibition provides a novel estimate of the half-life of endogenous brain HA (50 min). Despite the transient effect of pargyline on brain HA content, the effect of pargyline on brain t-MH levels suggests that MAO inhibitors may produce long-term alterations in brain histaminergic dynamics.