S. Ruiu

Selective MPP+ uptake into synaptic dopamine vesicles: possible involvement in MPTP neurotoxicity.

1 In the present study we provide evidence for a saturable, Mg2+/ATP‐ and temperature‐dependent, tetrabenazine‐, dopamine‐, and amphetamine‐sensitive uptake of 1‐methyl‐4‐phenylpyridinium ion (MPP+) in synaptic vesicles from mouse striatum. 2 Similarity in the properties of the vesicular uptake suggests that in the striatum dopamine and MPP+ share the vesicular carrier. 3 The presence of MPP+ vesicular uptake in dopamine‐rich regions such as striatum, olfactory, tubercles and hypothalamus, as well as its absence in cerebellum, cortex and pons‐medulla, suggest that monoamine vesicular carriers differ between highly and poorly dopamine‐innervated regions. 4 The restriction of active MPP+ uptake to the dopaminergic regions, which reflects the previously shown distribution of [3H]‐MPP+ binding sites in mouse brain membranes, indicates MPP+ as a marker of the vesicular carrier for dopamine in dopaminergic neurones. 5 A role in MPP+ neurotoxicity is suggested for this region‐specific, vesicular storage of the toxin.

Induction of fos-like-immunoreactivity in the central extended amygdala by antidepressant drugs

The induction of the early gene c‐fos was evaluated through Fos immunohistochemistry in areas belonging to the extended amygdala after acute administration of two antidepressants, citalopram and imipramine. Both citalopram and imipramine at the dose of 5 and 20 mg/kg, respectively, induced Fos‐like immunoreactivity (FLI) in the central amygdaloid nucleus, lateral division of the bed nucleus of the stria terminalis (BSTL), and interstitial nucleus of the posterior limb of the anterior commissure (IPAC). The shell of the nucleus accumbens, which forms a continuum with the central extended amygdala, showed a decrease of FLI after administration of either citalopram or imipramine. The mechanism of action and the brain areas affected by antidepressants are still a matter of debate. By showing that the central extended amygdala is a common site of action for two different antidepressant types, these results provide new insight into the mechanism of action of antidepressants. Synapse 31:1–4, 1999.

Characterization of a putatively vesicular binding site for [3H]MPP+ in mouse striatal membranes

[3H] N-Methyl-4-phenylpyridinium ion (MPP+) binds with a fully reversible, high affinity process to a population of sites mainly localized in the mouse striatum (Bmax = 168 +/- 15 fmol/mg protein, KD = 1.4 +/- 0.4 nM). The majority of specifically-bound radioactivity was localized in the synaptosomal fraction. Unilateral, striatal denervation with 6-hydroxydopamine (6-OHDA) markedly (by 65-70%) decreased the number of [3H]MPP+ sites. Besides dopamine, the vesicular markers tyramine, tetrabenazine and reserpine inhibited [3H]MPP+, while mazindol was a poor displacer. Adenosine triphosphate (ATP) and Mg(2+)-ions did not affect [3H]MPP+ binding. It is concluded that these sites may represent a marker of striatal storage vesicles for dopamine.

[3H]1‐Methyl‐4‐Phenyl‐2,3‐Dihydropyridinium Ion Binding Sites in Mouse Brain: Pharmacological and Biological Characterization

Abstract: Because 1‐methyl‐4‐phenyl‐2,3‐dihydropyridinium ion (MPP+) appears to damage the dopaminergic neuron and cause neuronal death, we characterized [3H]MPP+ binding sites in mouse brain membranes. Among several compounds tested, debrisoquin [3,4‐dihydro‐2(1 H)‐isoquinolinecarboxamidine] and some analogues were able to antagonize [3H]MPP+ binding. Debrisoquin is able to block adrenergic transmission and inhibit the activity of monoamine oxidase A (MAO‐A). We found a certain correlation between the ability of these agents to displace [3H]MPP+ from its binding sites and their capacity to inhibit MAO‐A activity. These data and the finding of a higher number of [3H]MPP+ binding sites in human placenta compared to mouse brain suggest that these sites may correspond to MAO‐A enzymes. Recently it has been demonstrated in human brain that neurons in regions rich in catecholamines are positive for MAO‐A. Accordingly, we suggest MAO‐A as a possible accumulation site of MPP+ within the dopaminergic neuron. We also indicate the chemical structural requirement associated with the best binding of debrisoquin analogues with [3H]MPP+ sites. It would be reasonable to test the effects of debrisoquinlike drugs able to pass the blood‐brain barrier on 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine toxicity.

Localization of MPP+ binding sites in the brain of various mammalian species

SummaryThe distribution and density of3H-MPP+ binding sites were studied by in vitro quantitative autoradiography in the brain of the mouse, rat and monkey. The highest levels of3H-MPP+ specific binding were observed in rat brain. The substantia nigra in rat and monkey, and the anterior caudate-putamen formation in mouse and monkey showed the lowest density of autoradiographic grains. The presence of a relatively high density of MPP+ sites in the hippocampus of all species studied could be of interest to explain some effects of MPTP administration on convulsions caused by chemoconvulsants.The finding of a 60–70% reduction of3H-MPP+ binding sites in the rat caudate-putamen, on the side of quinolinic acid infusion and no changes after 6-hydroxydopamine lesion of dopaminergic nigrostriatal neurons suggests the presence of these sites mainly on striatal cells.The results suggest that the distribution of MPP+ binding sites in brain would not seem to be related to MPTP toxicity.

Heterogeneity of monoaminergic vesicular carriers: pharmacological evidence using MPP+ as a marker

MPP-production and uptake by dopaminergic terminals are critical steps in MPTP-induced Parkinson-like disorder. We reported evidence for a specific uptake of MPP by synaptic vesicles from mouse striatum. Its regional distribution suggests it as a marker of the dopamine vesicular carrier. We decided to further characterize such an MPP uptake. Tetrabenazine inhibits the dopamine uptake both in the striatum and in the cerebellum with similar Km values suggesting an identify of the vesicular carrier in these areas. On the contrary, 3H-MPP vesicular uptake had in the striatum a t1/2 of 60 sec, but was not detectable at any time in the cerebellum. Moreover, MPP inhibited the uptake of 3H-DA (Ki: 1.6 +/- 0.03 microM) and 3H-NE (Ki 2.6 +/- 0.01 microM) in the striatum but not in the cerebellum, even at molar concentration. These pharmacological data indicate that in nondopaminergic areas the monoamine carrier may be similar but not identical from that located in dopaminergic areas.