Claude Rouillard

Nurr1 Is Required for Maintenance of Maturing and Adult Midbrain Dopamine Neurons

Transcription factors involved in the specification and differentiation of neurons often continue to be expressed in the adult brain, but remarkably little is known about their late functions. Nurr1, one such transcription factor, is essential for early differentiation of midbrain dopamine (mDA) neurons but continues to be expressed into adulthood. In Parkinsons disease, Nurr1 expression is diminished and mutations in the Nurr1 gene have been identified in rare cases of disease; however, the significance of these observations remains unclear. Here, a mouse strain for conditional targeting of the Nurr1 gene was generated, and Nurr1 was ablated either at late stages of mDA neuron development by crossing with mice carrying Cre under control of the dopamine transporter locus or in the adult brain by transduction of adeno-associated virus Cre-encoding vectors. Nurr1 deficiency in maturing mDA neurons resulted in rapid loss of striatal DA, loss of mDA neuron markers, and neuron degeneration. In contrast, a more slowly progressing loss of striatal DA and mDA neuron markers was observed after ablation in the adult brain. As in Parkinsons disease, neurons of the substantia nigra compacta were more vulnerable than cells in the ventral tegmental area when Nurr1 was ablated at late embryogenesis. The results show that developmental pathways play key roles for the maintenance of terminally differentiated neurons and suggest that disrupted function of Nurr1 and other developmental transcription factors may contribute to neurodegenerative disease.

17β-estradiol at a physiological dose acutely increases dopamine turnover in rat brain

Ovariectomized rats injected with 17 beta-estradiol (30 ng, s.c.) showed an increase of the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with no change of dopamine concentrations in the striatum and nucleus accumbens. This increase was observed 30 min after the steroid injection and coincided with peak plasma 17 beta-estradiol concentrations. Plasma prolactin concentrations were not significantly elevated after 30 ng of 17 beta-estradiol. In ovariectomized rats with a unilateral lesion of the entopeduncular nucleus, the same dose of 17 beta-estradiol induced a postural deviation to the lesioned side with a maximum at 30 min. Thus, very small doses of estradiol were able to increase dopamine turnover. This effect was seen within minutes and was relatively short. It is probably non-genomic, presynaptic and similar to the effect of a small dose of a dopamine releasing agent.

Rotenone induces non-specific central nervous system and systemic toxicity.

We investigated the dopaminergic (DA) neuronal degeneration in animals subjected to systemic treatment of rotenone via subcutaneous delivery. Behavioral observations revealed a hypokinetic period in rats sacrificed at 3 and 5 days, and dystonic episodes in animals sacrificed at 8 days. Less than 20% of the total number of animals given rotenone depicted brain lesions after 8 days of treatment, as demonstrated by a significant loss of DA fibers in the striatum, but not of DA nigral neurons. Tyrosine hydroxylase‐negative striatal territories were characterized by post‐synaptic toxicity as demonstrated by a decreased number of interneurons labeled for choline acetyltransferase, NADPH‐diaphorase, parvalbumin, and projection neurons labeled for calbindin and nerve growth factor inducible‐B (NGFI‐B). Post‐synaptic neurodegeneration was demonstrated further by abundant striatal staining for Fluoro‐Jade. Decrease in the nuclear orphan receptor Nurr1 expression was the only significant change observed at the level of the substantia nigra. Autopsy reports confirmed that animals suffered from severe digestion problems. These data suggest that hypokinesia observed between 3 and 5 days is the result of general health problems rather than a specific motor deficit associated to Parkinsons disease (PD) symptoms. Overall, the effects of rotenone toxicity are widespread, and subcutaneous administration of this toxin does not provide the neuropathological and behavioral basis for a relevant and reliable PD model.

Docosahexaenoic acid reduces levodopa-induced dyskinesias in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine monkeys

The objective of the present study was to investigate the effect of docosahexaenoic acid (DHA), a polyunsaturated fatty acid (omega‐3), on levodopa‐induced dyskinesias (LIDs) in parkinsonian 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)–treated monkeys.

Contribution of spinal 5-HT1A and 5-HT7 receptors to locomotor-like movement induced by 8-OH-DPAT in spinal cord-transected mice

Growing evidence from in vitro studies suggests that spinal serotonin (5‐HT) receptor subtypes 5‐HTR1A and 5‐HTR7 are associated with an induction of central pattern generator activity. However, the possibility of a specific role for these receptor subtypes in locomotor rhythmogenesis in vivo remains unclear. Here, we studied the effects of a single dose (1 mg/kg, i.p.) of 8‐hydroxy‐2‐(di‐N‐propylamino)‐tetralin (8‐OH‐DPAT), a potent and selective 5‐HTR1A/7 agonist, in mice spinal cord transected at the low‐thoracic level (Th9/10). The results show that 8‐OH‐DPAT acutely induced, within 15 min, hindlimb movements that share some characteristics with normal locomotion. Paraplegic mice pretreated with the selective 5‐HTR1A antagonists, WAY100,135 or WAY100,635, displayed significantly less 8‐OH‐DPAT‐induced movement. A similar reduction of 8‐OH‐DPAT‐induced movements was found in animals pretreated with SB269970, a selective 5‐HTR7 antagonist. Moreover, a near complete blockade of 8‐OH‐DPAT‐induced movement was obtained in wild‐type mice pretreated with 5‐HTR1A and 5‐HTR7 antagonists, and in 5‐HTR7–/– mice pretreated with 5‐HTR1A antagonists. Overall, these results clearly demonstrate that 8‐OH‐DPAT potently induces locomotor‐like movement in the previously paralysed hindlimbs of low‐thoracic‐transected mice. The results, with selective antagonists and knockout animals, provide compelling evidence of a specific contribution of both receptor subtypes to spinal locomotor rhythmogenesis in vivo.

Dorsal raphe stimulation differentially modulates dopaminergic neurons in the ventral tegmental area and substantia nigra

The serotoninergic (5‐HT) input from the dorsal raphe nucleus (DRN) to midbrain dopamine (DA) neurons is one of the most prominent. In this study, using standard extracellular single cell recording techniques we investigated the effects of electrical stimulation of the DRN on the spontaneous activity of substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA) DA neurons in anesthetized rats. Poststimulus time histograms (PSTH) revealed two different types of response in both SNpc and VTA. Some cells exhibited an inhibition‐excitation response while in other DA neurons the initial response was an excitation followed by an inhibition. In SNpc, 56% of the DA cells recorded were initially inhibited and 31% of the DA cells were initially excited. In contrast, 63% of VTA DA cells were initially excited and 34% were initially inhibited. Depletion of endogenous 5‐HT by the neurotoxin, 5,7‐dihydroxytryptamine (5,7‐DHT), and the 5‐HT synthesis inhibitor para‐chlorophenylalanine (PCPA), almost completely eliminated the inhibition‐excitation response in both SNpc and VTA DA cells, without changing the percentage of DA cells initially excited. Consequently, the proportion of DA neurons that were not affected by DR stimulation increased after 5‐HT depletion (from 13% to 60% in SNpc and from 6% to 31% in VTA). In several DA cells, DRN stimulation caused important changes in firing rate and firing pattern. These data strongly suggest that the 5‐HT input from the DRN is mainly inhibitory. It also suggests that 5‐HT afferences modulate SNpc and VTA DA neurons in an opposite manner. Our results also suggest that non‐5‐HT inputs from DR can also modulate mesencephalic DA neurons. A differential modulation of VTA and SNpc DA neurons by 5‐HT afferences from the DRN could have important implications for the development of drugs to treat schizophrenia or other neurologic and psychiatric diseases in which DA neurons are involved. Synapse 35:281–291, 2000.

Contrasting Patterns and Cellular Specificity of Transcriptional Regulation of the Nuclear Receptor Nerve Growth Factor-Inducible B by Haloperidol and Clozapine in the Rat Forebrain

Abstract: This study was designed to investigate the possible involvement of members of the nuclear receptor family of transcription factors in the effects of antipsychotic drugs used in the treatment of schizophrenia. We have identified, using RT‐PCR screening, an important modulation of nerve growth factor‐inducible B (NGFI‐B) mRNA levels by typical and atypical neuroleptics in the rat forebrain. NGFI‐B, a member of the nuclear receptor family, can be observed in target structures of dopaminergic pathways. Using in situ hybridization, we also demonstrate that typical and atypical antipsychotics induced contrasting patterns of expression of NGFI‐B after both acute and chronic administration. An acute treatment with clozapine or haloperidol induces high NGFI‐B mRNA levels in the prefrontal and cingulate cortices and in the nucleus accumbens shell. However, haloperidol, but not clozapine, dramatically increases NGFI‐B expression in the dorsolateral striatum. In contrast, chronic treatment with clozapine reduces NGFI‐B expression below basal levels in the rat forebrain, whereas haloperidol still induces high NGFI‐B mRNA levels in the dorsolateral striatum. Finally, using a double in situ hybridization technique, we show that acute administration of both neuroleptics increases NGFI‐B expression in neurotensin‐containing neurons in the nucleus accumbens shell, whereas the effects of haloperidol in the dorsolateral striatum are mainly observed in enkephalin‐containing neurons. These results are the first demonstration that members of the nuclear receptor family of transcription factors could play an important role in the effects of antipsychotic drugs.

Differences between subacute and chronic MPTP mice models: investigation of dopaminergic neuronal degeneration and α‐synuclein inclusions

Animal models are invaluable tools to study neurodegenerative disorders but a general consensus on the most accurate rodent model of Parkinson’s disease has not been reached. Here, we examined how different methods of MPTP administration influence the degeneration of the dopaminergic (DA) system. Adult male C57BL/6 mice were treated with the same cumulative dose of MPTP following four distinct procedures: (i) subacute i.p. injections; (ii) 28‐day chronic s.c. infusion; (iii) 28‐day chronic i.p. infusion; and (iv) 14‐day chronic i.p. infusion. Subacute MPTP treatment significantly affected all aspects of the DA system within the nigral and striatal territories. In contrast, the 28‐day chronic s.c. infusion did not significantly alter any components of the DA system. The 28‐ and 14‐day chronic i.p. infusions induced loss of tyrosine hydroxylase (TH)‐positive cells correlated with a decrease in Nurr1 mRNA levels, but no significant decrease in the density of TH striatal fibers. Importantly, however, only the 14‐day chronic MPTP i.p. infusion protocol promoted the formation of neuronal inclusions as noted by the expression of α‐synuclein protein within the cytoplasm of TH nigral neurons. Overall, we found that the 14‐day chronic MPTP i.p. infusion reproduces more accurately the pathological characteristics of early stage Parkinson’s disease.

Nur77 and retinoid X receptors: crucial factors in dopamine-related neuroadaptation

Dopaminergic systems in the brain adapt in response to various stimuli from the internal and external world, but the mechanisms underlying this process are incompletely understood. Here, we review recent evidence that certain types of transcription factor of the nuclear receptor family, specifically Nur77 and retinoid X receptors, have important roles in adaptation and homeostatic regulation of dopaminergic systems. These findings call for a reassessment of our fundamental understanding of the molecular and cellular basis of dopamine-mediated transmission. Given that diseases such as Parkinsons disease and schizophrenia are thought to involve adaptation of dopamine signalling, these findings might provide new insight into these pathologies and offer new avenues for drug development.

Behavioral and biochemical evidence for a different effect of repeated administration of l-DOPA and bromocriptine on denervated versus non-denervated striatal dopamine receptors

The effect of repeated administration of bromocriptine and L-3,4-dihydroxyphenylalanine (L-DOPA) was studied behaviorally and biochemically in rats with a unilateral lesion of the nigrostriatal pathway. Groups of rats injected eight times with bromocriptine or L-DOPA significantly increased their contraversive circling. Rats receiving only two injections of bromocriptine did not. Animals receiving two injections of L-DOPA showed a slight but significant increase in circling. The affinity of the binding of [3H]spiperone to the dopamine receptors was unchanged by the lesion or the treatments, while the density of the binding was significantly modified. Chronic treatment with bromocriptine induced a significant decrease in the density of D2 dopamine receptors in the intact striata, while on the lesioned side, it remained unchanged. By contrast, chronic administration of L-DOPA induced a significant increase in density of the striatal dopamine receptors in the lesioned striata in addition to that caused by denervation, while the decrease on the intact side was not significant. It seems that contrary to the intact striatum, the lesioned side had a defective down-regulation mechanism in response to chronic treatment with a dopamine agonist. The results also show that L-DOPA was more potent than bromocriptine in inducing agonist supersensitivity in a denervated striatum. This may explain why chronic treatment with bromocriptine has a lesser tendency to induce dyskinesia in patients with Parkinsons disease.