Marc Marien

Time Course of Adaptations in Dopamine Biosynthesis, Metabolism, and Release Following Nigrostriatal Lesions: Implications for Behavioral Recovery from Brain Injury

Abstract: Alterations in neostriatal dopamine metabolism, release, and biosynthesis were determined 3, 5, or 18 days following partial, unilateral destruction of the rat nigrostriatal dopamine projection. Concentrations of dopamine and each of its metabolites, 3,4‐dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 3‐methoxytyra‐mine (3‐MT) were markedly decreased in the lesioned stri‐ata at 3, 5, or 18 days postoperation. The decline in striatal high‐affinity [3H]dopamine uptake closely matched the depletion of dopamine at 3 and 18 days postoperation. However, neither DOPAC, HVA, nor 3‐MT concentrations were decreased to as great an extent as dopamine at any time following lesions that depleted the dopamine innervation of the striatum by >80%. In these more severely lesioned animals, dopamine metabolism, estimated from the ratio of DOPAC or HVA to dopamine, was increased two‐ to fourfold in the injured hemisphere compared with the intact hemisphere. Dopamine release, estimated by the ratio of 3‐MT to dopamine, was more increased, by five‐ to sixfold. Importantly, the HVA/dopamine, DOPAC/dopamine, and 3‐MT/dopamine ratios did not differ between 3 and 18 days postlesioning. The rate of in vivo dopamine biosynthesis, as estimated by striatal DOPA accumulation following 3,4‐dihydroxyphenylalanine (DOPA) decarboxylase inhibition with NSD 1015, was increased by 2.6‐ to 2.7‐fold in the surviving dopamine terminals but again equally at 3 and 18 days postoperation. Thus, maximal increases in dopamine metabolism, release, and biosynthesis occur rapidly within neostriatal terminals that survive a lesion. This mobilization of dopaminergic function could contribute to the recovery from the behavioral deficits of partial denervation by increasing the availability of dopamine to neostriatal dopamine receptors. However, these presynaptic compensations are not sufficient to account for the protracted (at least 3‐week) time course of sensorimotor recovery that has been observed following partial nigrostriatal lesion.

Effects of locus coeruleus lesions on the release of endogenous dopamine in the rat nucleus accumbens and caudate nucleus as determined by intracerebral microdialysis

Bilateral 6-hydroxydopamine lesions of the rat locus coeruleus (a) depleted forebrain norepinephrine levels by 67%, (b) reduced the basal release of dopamine in the nucleus accumbens and caudate nucleus by 26% and 19%, respectively, and (c) reduced (+)-amphetamine-induced release in the nucleus accumbens and caudate nucleus. The locus coeruleus appears to exert a tonic excitatory influence on striatal and limbic dopamine release in vivo.

Poly (ADP-ribose) polymerase inhibitors protect against MPTP-induced depletions of striatal dopamine and cortical noradrenaline in C57B1/6 mice

Abstract Treatment of C57B1/6 mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) reduced striatal dopamine and cortical noradrenaline levels by 77–83% and 43–46%, respectively, at 7 days post-treatment. Co-treatments with five different inhibitors of poly(ADP-ribose) polymerase (PARP), including benzamide, significantly prevented the MPTP-induced catecholamine depletions. Benzamide was present in the striatum, 30 min after single i.p. injection, at low millimolar concentrations known to selectively inhibit PARP in vitro. The protective activities of benzamide and its derivatives paralleled their in vitro efficacies and potencies both as neuroprotective agents and as inhibitors of PARP, while the activity of 1,5-dihydroxyisoquinoline, a structurally-unrelated compound, did not. In naive animals, the PARP inhibitors by themselves did not alter striatal dopamine levels at 7 days post-treatment. However, in acute studies, 1,5-dihydroxyisoquinoline and nicotinamide caused marked alterations in striatal dopamine metabolite levels; on the contrary, benzamide and its amino-derivatives showed little or no effect on dopamine metabolism. These results indicate that, although these compounds might act at other sites in addition to PARP, PARP inhibitors possess neuroprotective potential in vivo and suggest a role for PARP in MPTP neurotoxicity.

Noradrenaline depletion exacerbates MPTP-induced striatal dopamine loss in mice

Injection of C57Bl/6 mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 4 x 10 mg/kg i.p. over 8 h) resulted in a partial (40%) striatal dopamine depletion at 7 days post-drug. Pretreatment with the selective noradrenergic neurotoxin N-[2-chloroethyl]-N-ethyl-2-bromobenzylamine (DSP-4; 40 mg/kg i.p.), while having no effect per se on striatal dopamine levels, exacerbated the MPTP-induced dopamine deficit to 60%. Results support the hypothesis that damage to the locus coeruleus-noradrenergic system, by removing a facilitatory influence on the nigrostriatal dopamine system, interferes with the ability of the nigrostriatal pathway to compensate for or recover from injury.

The alpha2-adrenoceptor antagonist dexefaroxan enhances hippocampal neurogenesis by increasing the survival and differentiation of new granule cells.

The generation of new neurons in the hippocampus is a dynamic process regulated by environmental, endocrine, and pharmacological factors. Since enhancement of hippocampal neurogenesis has been associated with learning and memory, and the locus coeruleus–noradrenergic system has been shown to modulate these cognitive functions, we hypothesized that activation of noradrenergic neurotransmission might enhance neurogenesis in the adult hippocampus. To test this hypothesis in vivo, we induced the release of noradrenaline in the hippocampus by blocking presynaptic inhibitory autoreceptors with the selective alpha2-adrenoceptor antagonist dexefaroxan. Confocal microscopy showed that noradrenergic afferents make contact with proliferating and differentiating cells, suggesting a direct noradrenergic influence on neurogenesis. Chronic systemic treatment of rats with dexefaroxan did not affect cell proliferation per se in the dentate gyrus (as monitored by bromodeoxyuridine-labeling), but promoted the long-term survival of newborn neurons by reducing apoptosis. Dexefaroxan treatment also enhanced the number and complexity of the dendritic arborizations of polysialated neural cell adhesion molecule-positive neurons. The trophic effects of dexefaroxan on newborn cells might involve an increase in brain-derived neurotrophic factor, which was upregulated in afferent noradrenergic fiber projection areas and in neurons in the granule cell layer. By promoting the survival of new endogenously formed neurons, dexefaroxan treatment represents a potential therapeutic strategy for maintaining adult neurogenesis in neurodegenerative conditions, such as Alzheimers disease, that affect the hippocampus.

Suppression of nigrostriatal and mesolimbic dopamine release in vivo following noradrenaline depletion by DSP-4: A microdialysis study

Pretreatment of rats with the noradrenergic neurotoxin DSP-4 selectively reduced regional levels of noradrenaline in the brain by more than 75%, and decreased the concentration of endogenous DA in microdialysates of the caudate nucleus and nucleus accumbens by 52% and 28%, respectively. Results support the hypothesis that central noradrenergic mechanisms facilitate nigrostriatal and mesolimbic dopamine transmission in vivo.

Decreases in mouse brain NAD+ and ATP induced by 1-methyl-4-phenyl-1, 2,3,6-tetrahydropyridine (MPTP): prevention by the poly(ADP-ribose) polymerase inhibitor, benzamide.

Inhibitors of poly(ADP-ribose) polymerase (PARP), including benzamide, protect against 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP)-induced dopamine neurotoxicity in vivo [Cosi et al., Brain Res. 729 (1996) 264-269]. In vitro, the activation of PARP by free radical damaged DNA has been shown to be correlated with rapid decreases in the cellular levels of its substrate nicotinamide adenine dinucleotide (NAD+), and ATP. Here, we investigated in vivo whether MPTP acutely caused region- and time-dependent changes in brain levels of NAD+, ATP, ADP and AMP in C57BL/6N mice killed by head-focused microwave irradiation, and whether such effects were modified by treatments with neuroprotective doses of benzamide. At 1 h after MPTP injections (4x20 mg/kg i.p.), NAD+ was reduced by 11-13% in the striatum and ventral midbrain, but not in the frontal cortex. The ATP/ADP ratio was reduced by 10% and 32% in the striatum and cortex, respectively, but was unchanged in the midbrain. All of these regional changes were prevented by co-treatment with benzamide (2x160 mg/kg i.p.), which by itself did not alter regional levels of NAD+, ATP, ADP or AMP in control mice. In a time-course study, a single dose of MPTP (30 mg/kg i.p.) resulted in maximal and transient increases in striatal levels of MPP+ and 3-methoxytyramine (+540%) at 0.5-2 h, followed by maximal and coincidental decreases in NAD+ (-10%), ATP (-11%) and dopamine content (-39%) at 3 h. Benzamide (1x640 mg/kg i. p., 30 min before MPTP) partially reduced MPP+ levels by 30% with little or no effect on MPTP or MPDP+ levels, did not affect or even slightly potentiated the increase in 3-methoxytyramine, and completely prevented the losses in striatal NAD+, ATP and dopamine content, without by itself causing any changes in these latter parameters in control mice. These results (1) confirm that MPTP reduces striatal ATP levels [Chan et al., J. Neurochem. 57 (1991) 348-351.]; (2) show that MPTP causes a regionally-dependent (striatal and midbrain) loss of NAD+; (3) indicate that the PARP inhibitor benzamide can prevent these losses without interfering with MPTP-induced striatal dopamine release; and (4) provide further evidence to suggest an involvement of PARP in MPTP-induced neurotoxicity in vivo.

The phenotypic differentiation of locus ceruleus noradrenergic neurons mediated by brain-derived neurotrophic factor is enhanced by corticotropin releasing factor through the activation of a cAMP-dependent signaling pathway.

We have developed a model system of locus ceruleus (LC) neurons in culture, in which brain-derived neurotrophic factor (BDNF) induces the emergence of noradrenergic neurons attested by the presence of tyrosine hydroxylase (TH) and dopamine-β-hydroxylase and the absence of phenylethanolamine N-methyl-transferase. Although inactive in itself, the neuropeptide corticotropin releasing factor (CRF) strongly amplified the effect of BDNF, increasing the number of cells expressing TH and the active accumulation of noradrenaline by a factor of 2 to 3 via a mechanism that was nonmitogenic. CRF also acted cooperatively with neurotrophin-4, which like BDNF is a selective ligand of the TrkB tyrosine kinase receptor. The effect of CRF but not that of BDNF was prevented by astressin, a nonselective CRF-1/CRF-2 receptor antagonist. However, only CRF-1 receptor transcripts were detectable in LC cultures, suggesting that this receptor subtype mediated the effect of CRF. Consistent with the positive coupling of CRF-1 receptors to adenylate cyclase, the trophic action of CRF was mimicked by cAMP elevating agents. Epac, a guanine nucleotide exchange factor directly activated by cAMP, contributed to the effect of CRF through the stimulation of extracellular signal-regulated kinases (ERKs) 1/2. However, downstream of ERK1/2 activation by CRF, the phenotypic induction of noradrenergic neurons relied upon the stimulation of the phosphatidylinositol-3-kinase/Akt transduction pathway by BDNF. Together, our results suggest that CRF participates to the phenotypic differentiation of LC noradrenergic neurons during development. Whether similar mechanisms account for the high degree of plasticity of these neurons in the adult brain remains to be established.

Paraxanthine, the Primary Metabolite of Caffeine, Provides Protection against Dopaminergic Cell Death via Stimulation of Ryanodine Receptor Channels

Epidemiological evidence suggests that caffeine or its metabolites reduce the risk of developing Parkinsons disease, possibly by protecting dopaminergic neurons, but the underlying mechanism is not clearly understood. Here, we show that the primary metabolite of caffeine, paraxanthine (PX; 1, 7-dimethylxanthine), was strongly protective against neurodegeneration and loss of synaptic function in a culture system of selective dopaminergic cell death. In contrast, caffeine itself afforded only marginal protection. The survival effect of PX was highly specific to dopaminergic neurons and independent of glial cell line-derived neurotrophic factor (GDNF). Nevertheless, PX had the potential to rescue dopaminergic neurons that had matured initially with and were then deprived of GDNF. The protective effect of PX was not mediated by blockade of adenosine receptors or by elevation of intracellular cAMP levels, two pharmacological effects typical of methylxanthine derivatives. Instead, it was attributable to a moderate increase in free cytosolic calcium via the activation of reticulum endoplasmic ryanodine receptor (RyR) channels. Consistent with these observations, PX and also ryanodine, the preferential agonist of RyRs, were protective in an unrelated paradigm of mitochondrial toxin-induced dopaminergic cell death. In conclusion, our data suggest that PX has a neuroprotective potential for diseased dopaminergic neurons.

Implication of poly (ADP-ribose) polymerase (PARP) in neurodegeneration and brain energy metabolism. Decreases in mouse brain NAD+ and ATP caused by MPTP are prevented by the PARP inhibitor benzamide.

ABSTRACT: Poly(ADP‐ribose) polymerase (PARP) is a DNA binding protein that uses nicotinamide adenine dinucleotide (NAD+) as a substrate. Evidence from in vitro studies on nonneuronal cells in culture have shown that when fully activated by free radical‐induced DNA damage, PARP depletes cellular NAD+ and consequently adenosine triphosphate (ATP) levels within a matter of minutes, and that this depletion is associated with a cell death that can be prevented by PARP inhibitors. The present in vivo study utilized the 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐treated mouse, a model of central nigrostriatal dopamine neurotoxicity that recapitulates certain features of Parkinsons disease (PD), and one in which we have previously shown PARP inhibitors to be protective, 6 to examine whether MPTP acutely caused region‐ and time‐dependent changes in levels of NAD+ and ATP in the brain in vivo and whether such effects were modified by treatments with neuroprotective doses of the PARP inhibitor benzamide. The results confirm that MPTP reduces striatal ATP levels, as previously reported by Chan et al., 4 show that MPTP causes a regionally‐selective (striatal and midbrain) loss of NAD+, and indicate that the PARP inhibitor benzamide can prevent these losses without interfering with MPTP‐induced striatal dopamine release. These findings suggest an involvement of PARP in the control of brain energy metabolism during neurotoxic insult, provide further evidence in support of the participation of PARP in MPTP‐induced neurotoxicity in vivo and suggest that PARP inhibitors might be beneficial in the treatment of PD.