Abstract

By using the methylamine method, the influence of a transmembrane pH gradient on the distribution of the antidepressant imipramine and the neuroleptic haloperidol was assessed in synaptosomes. Resting synaptosomes concentrated methylamine 8- to 9-fold at pH 7.4, as compared with a 400- to 600-fold concentration of the lipophilic weak bases imipramine and haloperidol. Ignoring compartmentation, the methylamine ratio is consistent with an internal pH of 6.5. Weak acid partition yielded an internal pH of 7.1. In agreement with the pH partition hypothesis, elevation of the internal pH by NH4Cl reduced whereas internal acidification enhanced imipramine and haloperidol uptake. Release of both drugs could also be elicited with excess K+ or veratridine. The underlying mechanism is a depolarization-induced rise of the internal pH by 0.12 to 0.25 units. In polarized synaptosomes, about 70% of the uptake of imipramine and haloperidol depended on the transmembrane pH gradient. A predominant localization in synaptic vesicles in situ is derived from the proportional release of methylamine, imipramine, and endogenous dopamine by nigericin. It is concluded that the acidic internal pH plays an important role for the passive concentration of lipophilic bases like imipramine and haloperidol in the brain, thereby profoundly influencing the extra- and intracellular free concentrations. Binding to cellular constituents contributes to internal accumulation. Especially high concentrations are attained in acidic vesicles.