Dopamine-releasing action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridine (MPP +) in the neostriatum of the rat as demonstrated in vivo by the push-pull perfusion technique: dependence on sodium but not calcium ions

Abstract

This study examined the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its metabolite, 1-methyl-4-phenylpyridine (MPP +) on the levels of dopamine (DA) and 3,4,-dihydroxyphenylacetic acid (DOPAC) in push-pull perfusates of the striatum in chloral hydrate-anaesthetized rats. In control animals the levels of DA and DOPAC remained stable for at least 6 h and responded rapidly to a depolarizing stimulus of 25 mM K +. This K +-induced DA release was Ca 2+-dependent since no stimulation was observed when the striatal sites were perfused with high K + in a Ca 2+-free medium containing 2 mM EGTA thus verifying that the striatal sites were functionally active. MPTP (0.025 and 0.05 μg/μl) stimulated DA release and inhibited DOPAC output in a dose-related manner, MPP + (0.01, 0.025 and 0.05 μg/μl) produced a more robust dose-dependent increase in DA levels in the perfusates; however, the level of suppression of DOPAC was similar to that in response to MPTP. The effect of MPP + on DA release was attenuated by 10 −6 M benztropine, the DA re-uptake blocker and completely inhibited by 10 μg/kg i.p. benztropine and 10 −4 M ouabain, the NA +,K +-ATPase (Na pump) inhibitor. However, although these substances prevented the MPP +-induced release of DA, the levels of DOPAC in the perfusates did not recover and remained completely suppressed suggesting that MPP + may inhibit extraneuronal rather than intraneuronal monoamine oxidase (MAO). Perfusion of the striatal sites with a Ca 2+-free medium containing 2 mM EGTA did not prevent the MPP +-induced DA release indicating that MPP + does not release DA from the striatal DA terminals by the Ca 2+-dependent process of exocytosis. The responses of DA and DOPAC to 25 mM K + were markedly suppressed in animals by the Ca 2+-dependent process these effects being most severe with the highest dose of MPP +. Moreover, this suppression of the K +-induced responses persisted in animals perfused with MPP + in the presence of benztropine or ouabain, thus suggesting that MPP + may have potent deleterious membrane effects. These studies have provided the first direct in vivo demonstration of the action of MPTP and MPP + and the neuropharmacology basis of this action on DA metabolism in the rat striatum. The results show that the elevated levels of DA in the striatal perfusates are due to a direct action of MPTP and MPP + on the nigrostriatal DA terminals and cannot be fully accounted for solely by their inhibition of MAO activity and/or inhibition of DA re-uptake. In support of previous data, we have demonstrated that MPP + is taken up by the striatal DA terminals by an energy-dependent process and this entry of MPP + into the DA terminals may be the first in a sequence of events which ultimately leads to a depletion of DA stores. The compound may interfere with a number of basic mechanisms regulating DA release which may reflect damage to the terminals. This is consistent with the neurotoxic effect of the compound on the nigrostriatal DA neuronal system.