Irene R.E. Taravini

Behavioral sensitization to different dopamine agonists in a parkinsonian rodent model of drug-induced dyskinesias

Repeated treatment with dopamine (DA) receptor agonists strongly potentiates contralateral turning behavior due to selective stimulation of D1 or D2-class receptors in 6-hydroxydopamine (6-OHDA)-lesioned rats. This phenomenon, referred to as sensitization, is believed to be related to the motor response complications (dyskinesias, on-off states) that occur during chronic administration of levodopa in Parkinsons disease patients. In recent years a new method for the evaluation of abnormal involuntary movements (AIMs) secondary to dopaminergic stimulation in 6-OHDA-lesioned rats was described. These AIMs resemble dyskinesias as seen in parkinsonian patients under levodopa therapy. Our objective was to evaluate the effects of repeated treatment with different regimes of DA agonists on turning behavior and on an AIMs scale in 6-OHDA lesioned rats, with the aim of discriminating between drugs with different dyskinesia-inducing potential. In addition, we explored the effects of a previous exposure to a DA agonist (priming) on the behavioral response to the subsequent administration of a DA agonist with the same or different pharmacologic profile. Our results show that in apomorphine-treated rats, rotational behavior and AIMs run a parallel course of enhancement, while in those receiving quinpirole there is a dissociation, suggesting that they could be mediated by different mechanisms. The finding of a significant priming effect on subsequent testing of 6-OHDA lesioned rats should be borne in mind as the use of these pharmacological tests in the screening of well lesioned animals could lead to an erroneous interpretation of further results on dyskinesias and rotational behavior.

Differential gene expression induced by chronic levodopa treatment in the striatum of rats with lesions of the nigrostriatal system

Levodopa, the major treatment for patients with Parkinsons disease, has been shown to induce a variety of compensatory effects, including facilitation of sprouting by dopaminergic neurons, in experimental animals with lesions leading to denervation of the striatum. To better understand the cellular and molecular environment where most of these compensatory changes take place, in particular elements that might contribute to the recovery of dopaminergic innervation, we have constructed a differential expression library enriched in transcripts from the striata of rats with lesions of the medial forebrain bundle treated with levodopa for 6 months. We have used this library to screen an expression array of rat genes representing the major cell functions, and have identified several that are involved in neurotrophic mechanisms and plasticity. We have confirmed the differential expression of selected transcripts by non‐radioactive in situ hybridization, and report that the growth factor pleiotrophin, myelin basic protein and calmodulin are overexpressed in the denervated striatum of levodopa‐treated rats.

Mapping the Effects of Three Dopamine Agonists with Different Dyskinetogenic Potential and Receptor Selectivity Using Pharmacological Functional Magnetic Resonance Imaging

The mechanisms underlying dopamine agonist-induced dyskinesia in Parkinsons disease remain poorly understood. Similar to patients, rats with severe nigrostriatal degeneration induced by 6-hydroxydopamine are more likely to show dyskinesia during chronic treatment with unselective dopamine receptor agonists than with D2 agonists, suggesting that D1 receptor stimulation alone or in conjunction with D2 receptor stimulation increases the chances of experiencing dyskinesia. As a first step towards disclosing drug-induced brain activation in dyskinesia, we examined the effects of dopamine agonists on behavior and blood oxygenation level-dependent (BOLD) signal in the striatum and motor cortex of rats with unilateral nigrostriatal lesions. Rats were rendered dyskinetic before pharmacologic functional magnetic resonance imaging by means of a repeated treatment regime with dopamine agonists. The unselective agonist apomorphine and the selective D1/D5 agonist SKF-81297 induced strong forelimb dyskinesia (FD) and axial dystonia and increased BOLD signal in the denervated striatum. Besides, SKF-81297 produced a significant but smaller BOLD increase in the intact striatum and a symmetric bilateral increase in the motor cortex. The D2 family agonist quinpirole, which induced mild dyskinesia on chronic treatment, did not produce BOLD changes in the striatum or motor cortex. Further evidence to support an association between BOLD changes and dyskinesia comes from a direct correlation between scores of FD and magnitude of drug-induced BOLD increases in the denervated striatum and motor cortex. Our results suggest that striatal and cortical activation induced by stimulation of D1/D5 receptors has a primary role in the induction of peak dose dyskinesia in parkinsonism.

Dopamine-Dependent Periadolescent Maturation of Corticostriatal Functional Connectivity in Mouse

Altered corticostriatal information processing associated with early dopamine systems dysfunction may contribute to attention deficit/hyperactivity disorder (ADHD). Mice with neonatal dopamine-depleting lesions exhibit hyperactivity that wanes after puberty and is reduced by psychostimulants, reminiscent of some aspects of ADHD. To assess whether the maturation of corticostriatal functional connectivity is altered by early dopamine depletion, we examined preadolescent and postadolescent urethane-anesthetized mice with or without dopamine-depleting lesions. Specifically, we assessed (1) synchronization between striatal neuron discharges and oscillations in frontal cortex field potentials and (2) striatal neuron responses to frontal cortex stimulation. In adult control mice striatal neurons were less spontaneously active, less responsive to cortical stimulation, and more temporally tuned to cortical rhythms than in infants. Striatal neurons from hyperlocomotor mice required more current to respond to cortical input and were less phase locked to ongoing oscillations, resulting in fewer neurons responding to refined cortical commands. By adulthood some electrophysiological deficits waned together with hyperlocomotion, but striatal spontaneous activity remained substantially elevated. Moreover, dopamine-depleted animals showing normal locomotor scores exhibited normal corticostriatal synchronization, suggesting that the lesion allows, but is not sufficient, for the emergence of corticostriatal changes and hyperactivity. Although amphetamine normalized corticostriatal tuning in hyperlocomotor mice, it reduced horizontal activity in dopamine-depleted animals regardless of their locomotor phenotype, suggesting that amphetamine modified locomotion through a parallel mechanism, rather than that modified by dopamine depletion. In summary, functional maturation of striatal activity continues after infancy, and early dopamine depletion delays the maturation of core functional capacities of the corticostriatal system.

Pleiotrophin over-expression provides trophic support to dopaminergic neurons in parkinsonian rats

BackgroundPleiotrophin is known to promote the survival and differentiation of dopaminergic neurons in vitro and is up-regulated in the substantia nigra of Parkinsons disease patients. To establish whether pleiotrophin has a trophic effect on nigrostriatal dopaminergic neurons in vivo, we injected a recombinant adenovirus expressing pleiotrophin in the substantia nigra of 6-hydroxydopamine lesioned rats.ResultsThe viral vector induced pleiotrophin over-expression by astrocytes in the substantia nigra pars compacta, without modifying endogenous neuronal expression. The percentage of tyrosine hydroxylase-immunoreactive cells as well as the area of their projections in the lesioned striatum was higher in pleiotrophin-treated animals than in controls.ConclusionsThese results indicate that pleiotrophin over-expression partially rescues tyrosine hydroxylase-immunoreactive cell bodies and terminals of dopaminergic neurons undergoing 6-hydroxydopamine-induced degeneration.

Cabergoline and pramipexole fail to modify already established dyskinesias in an animal model of parkinsonism.

Levodopa-induced dyskinesias are one of the major limiting side effects encountered in the treatment of Parkinsons disease. Dopamine agonists of the D2 family are less prone to induce these abnormal involuntary movements (AIMs), and in some instances it has been proposed that they could counteract them once already established. As differences in the plasma half-life of a given DA agonist could be related with a greater or lesser propensity to induce or to counteract AIMs, we compared the effects of two D2 agonists (cabergoline and pramipexole) with different half-lives, and levodopa, at doses producing similar improvement in purposeful forelimb use, in rats with severe nigrostriatal lesion, previously sensitized to levodopa. The same therapeutic regime was subsequently used in pharmacologically naïve rats. We found that: (i) prior induction of AIMs by levodopa administration primes rats for the occurrence of AIMs during mono-therapy with pramipexole (but not with cabergoline); (ii) an intervening period of D2 agonist mono-therapy does not modify the severity of AIMs induced by subsequent mono-therapy with levodopa; iii. de novo treatment with D2 agonists is associated with a lower risk of AIMs (regardless of the severity of the lesion) and does not modify AIMs during subsequent mono-therapy with levodopa. An unexpected finding was that prior levodopa therapy sensitized rats to the therapeutic effects of D2 agonists given in mono-therapy. In summary, the use of the rat with nigrostriatal lesion to model relevant therapeutic conditions does not support that D2 agonists prevent the development of AIMs during subsequent levodopa mono-therapy or can revert the dysfunction underlying it.

Immunodetection of heparin-binding growth associated molecule (pleiotrophin) in striatal interneurons

Pleiotrophin (PTN), a developmentally-regulated trophic factor, is over-expressed in the striatum of parkinsonian rats. Because striatal PTN can provide trophic support to dopamine neurons, we identified the cellular types containing PTN in the striatum of adult rats. By means of fluorescent double-immunolabeling, we found PTN to co-localize with a neuronal nuclei marker but not with glial fibrillary acidic protein. The number, distribution, and morphology of the PTN-immunolabeled cells suggested that they were interneurons. Further double-immunolabeling studies ruled out PTN localization to calretinin- and parvalbumin-containing interneurons. Instead, approximately 40% of the PTN-immunolabeled neurons contained nitric oxide synthase or somatostatin and approximately 60% expressed the vesicular acetylcholine transporter, supporting that they were GABAergic nitric oxide synthase/somatostatin-containing and cholinergic interneurons. Further work is necessary to determine if PTN from striatal interneurons can provide trophic support to dopamine neurons.

Attenuated methamphetamine induced neurotoxicity by modafinil administration in mice

Methamphetamine (METH) is a highly addictive drug that might induce neurotoxicity. Clinical trials have reported that modafinil, a wake‐promoting agent used to treat sleep disorders, may have some efficacy for the treatment of psychostimulant addiction. In this study we tested possible neuroprotective effects of modafinil after toxic METH administration in mice. We evaluated the effect of modafinil (two injections of either 90 or 180 mg/kg) and METH binge (3 × 7 mg/kg i.p. injections, 3‐h apart) coadministration on DA striatal content, TH immunoreactivity in striatal areas and spontaneous locomotor activity. We also investigated acute locomotor activity and stereotypy profile in mice treated with a single METH dose (2 and 7 mg/kg) pretreated with modafinil (90 and 180 mg/kg). We found that mice treated with a METH binge showed a marked decrease in DA and dopaminergic metabolites as well as lower levels of TH immunoreactivity in the dorsal striatum. Pretreatment with modafinil (both 90 and 180 mg/kg) attenuated these effects but did not prevent METH induced decrease in locomotion. We also found that groups that received the combination of both modafinil and single dose METH showed a decrease in total distance traveled in an open field compared with METH groups. We observed an increment in the time mice expended doing stereotypic movements (continuous sniffing) in the group that received the combination of both METH and modafinil (i.e., decreasing locomotion). Our results suggest a possible protective role of modafinil against METH acute striatal toxicity. Synapse 2011.

Altered Corticostriatal Connectivity and Exploration/Exploitation Imbalance Emerge as Intermediate Phenotypes for a Neonatal Dopamine Dysfunction

Findings showing that neonatal lesions of the forebrain dopaminergic system in rodents lead to juvenile locomotor hyperactivity and learning deficits have been taken as evidence of face validity for the attention deficit hyperactivity disorder. However, the core cognitive and physiological intermediate phenotypes underlying this rodent syndrome remain unknown. Here we show that early postnatal dopaminergic lesions cause long-lasting deficits in exploitation of shelter, social and nutritional resources, and an imbalanced exploratory behavior, where nondirected local exploration is exacerbated, whereas sophisticated search behaviors involving sequences of goal directed actions are degraded. Importantly, some behavioral deficits do not diminish after adolescence but instead worsen or mutate, particularly those related to the exploration of wide and spatially complex environments. The in vivo electrophysiological recordings and morphological reconstructions of striatal medium spiny neurons reveal corticostriatal alterations associated to the behavioral phenotype. More specifically, an attenuation of corticostriatal functional connectivity, affecting medial prefrontal inputs more markedly than cingulate and motor inputs, is accompanied by a contraction of the dendritic arbor of striatal projection neurons in this animal model. Thus, dopaminergic neurons are essential during postnatal development for the functional and structural maturation of corticostriatal connections. From a bottom-up viewpoint, our findings suggest that neuropsychiatric conditions presumably linked to developmental alterations of the dopaminergic system should be evaluated for deficits in foraging decision making, alterations in the recruitment of corticostriatal circuits during foraging tasks, and structural disorganization of the frontostriatal connections.

Decrease of a Current Mediated by Kv1.3 Channels Causes Striatal Cholinergic Interneuron Hyperexcitability in Experimental Parkinsonism

The mechanism underlying a hypercholinergic state in Parkinsons disease (PD) remains uncertain. Here, we show that disruption of the Kv1 channel-mediated function causes hyperexcitability of striatal cholinergic interneurons in a mouse model of PD. Specifically, our data reveal that Kv1 channels containing Kv1.3 subunits contribute significantly to the orphan potassium current known as IsAHP in striatal cholinergic interneurons. Typically, this Kv1 current provides negative feedback to depolarization that limits burst firing and slows the tonic activity of cholinergic interneurons. However, such inhibitory control of cholinergic interneuron excitability by Kv1.3-mediated current is markedly diminished in the parkinsonian striatum, suggesting that targeting Kv1.3 subunits and their regulatory pathways may have therapeutic potential in PD therapy. These studies reveal unexpected roles of Kv1.3 subunit-containing channels in the regulation of firing patterns of striatal cholinergic interneurons, which were thought to be largely dependent on KCa channels.