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

The mechanism underlying a hypercholinergic state in Parkinson's 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.

Loss of Homeostasis in the Direct Pathway in a Mouse Model of Asymptomatic Parkinson's Disease.

UNLABELLED: The characteristic slowness of movement in Parkinson's disease relates to an imbalance in the activity of striatal medium spiny neurons (MSNs) of the direct (dMSNs) and indirect (iMSNs) pathways. However, it is still unclear whether this imbalance emerges during the asymptomatic phase of the disease or if it correlates with symptom severity. Here, we have used in vivo juxtacellular recordings and transgenic mice showing MSN-type-specific expression of fluorescent proteins to examine striatal imbalance after lesioning dopaminergic neurons of the substantia nigra. Multivariate clustering analysis of behavioral data discriminated 2 groups of dopamine-lesioned mice: asymptomatic (42 ± 7% dopaminergic neuron loss) and symptomatic (85 ± 5% cell loss). Contrary to the view that both pathways have similar gain in control conditions, dMSNs respond more intensely than iMSNs to cortical inputs in control animals. Importantly, asymptomatic mice show significant functional disconnection of dMSNs from motor cortex without changes in iMSN connectivity. Moreover, not only the gain but also the timing of the pathways is altered in symptomatic parkinsonism, where iMSNs fire significantly more and earlier than dMSNs. Therefore, cortical drive to dMSNs decreases after partial nigrostriatal lesions producing no behavioral impairment, but additional alterations in the gain and timing of iMSNs characterize symptomatic rodent parkinsonism. SIGNIFICANCE STATEMENT: Prevailing models of Parkinson's disease state that motor symptoms arise from an imbalance in the activity of medium spiny neurons (MSNs) from the direct (dMSNs) and indirect (iMSNs) pathways. Therefore, it is hypothesized that symptom severity and the magnitude of this imbalanced activity are correlated. Using a mouse model of Parkinson's disease, we found that behaviorally undetectable nigrostriatal lesions induced a significant disconnection of dMSNs from the motor cortex. In contrast, iMSNs show an increased connectivity with the motor cortex, but only after a severe dopaminergic lesion associated with an evident parkinsonian syndrome. Overall, our data suggest that the lack of symptoms after a partial dopaminergic lesion is not due to compensatory mechanisms maintaining the activity of both striatal pathways balanced.

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.

Baclofen did not modify sexually dimorphic c-Fos expression during morphine withdrawal syndrome.

In previous studies, we have reported sex-related differences during morphine withdrawal. We have also shown that the GABA(B) agonist baclofen (BAC) was able to prevent the morphine withdrawal syndrome in male as well as in female mice. Considering that early gene expression is induced by drugs of abuse, we evaluated the expression of c-Fos in several brain areas, in mice of either sex during naloxone (NAL)-precipitated withdrawal, and after pretreatment with BAC. Swiss-Webster prepubertal mice were rendered dependent by i.p. injection of morphine (2 mg/kg), twice daily for 9 days. On the 10th day, dependent mice were divided into two groups: the withdrawal group received NAL (6 mg/kg, i.p.) after the last dose of morphine, while the prevention group received BAC (2 mg/kg, i.p.) before NAL. Thirty minutes after NAL, animals were sacrificed by transcardial perfusion. Brains were removed and slices were obtained to perform immunohistochemical studies. Our results show a significant decrease in c-Fos expression in hippocampal dentate gyrus, CA3, and CA1 areas of morphine withdrawn males, vs. their control group. Conversely, in females, the number of c-Fos positive nuclei was not modified in any of the areas studied. BAC pretreatment had no effect on the decreased c-Fos expression in morphine withdrawn males. The sexual dimorphism observed here confirms the greater sensitivity of males over females in their response to morphine. The preventive action of BAC on the expression of morphine withdrawal would not be related to an effect on c-Fos expression.

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.

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.

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 Parkinson's 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.

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 Parkinson's 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.

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.

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 Parkinson's 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.