Loss of Striatonigral GABAergic Presynaptic Inhibition Enables Motor Sensitization in Parkinsonian Mice

SummaryDegeneration of dopamine (DA) neurons in Parkinson’s disease (PD) causes hypokinesia, but DA replacement therapy can elicit exaggerated voluntary and involuntary behaviors that have been attributed to enhanced DA receptor sensitivity in striatal projection neurons. Here we reveal that in hemiparkinsonian mice, striatal D1 receptor-expressing medium spiny neurons (MSNs) directly projecting to the substantia nigra reticulata (SNr) lose tonic presynaptic inhibition by GABAB receptors. The absence of presynaptic GABAB response potentiates evoked GABA release from MSN efferents to the SNr and drives motor sensitization. This alternative mechanism of sensitization suggests a synaptic target for PD pharmacotherapy

Expression of X-chromosome linked inhibitor of apoptosis protein in mature purkinje cells and in retinal bipolar cells in transgenic mice induces neurodegeneration

Transgenic mice with overexpression of the caspase-inhibitor, X-chromosome-linked inhibitor of apoptosis protein (XIAP) in Purkinje cell (PC) and in retinal bipolar cells (RBCs) were produced to study the regulation of cell death. Unexpectedly, an increased neurodegeneration was observed in the PCs in these L7-XIAP mice after the third postnatal week with the mice exhibiting severe ataxia. The loss of PCs was independent of Bax as shown by crossing the L7-XIAP mice with Bax gene–deleted mice. Electron microscopy revealed intact organelles in PCs but with the stacking of ER cisterns indicative of cell stress. Immunostaining for cell death proteins showed an increased phosphorylation of c-Jun in the PCs, suggesting an involvement in cell degeneration. Apart from PCs, the number of RBCs was decreased in adult retina in line with the expression pattern for the L7 promoter. The data show that overexpression of the anti-apoptotic protein XIAP in vulnerable neurons leads to enhanced cell death. The mechanisms underlying this neurodegeneration can be related to the effects of XIAP on cell stress and altered cell signaling.Supported by Sigrid Juselius Foundation, Academy of Finland, EU Biotech Grant, Liv och Hälsa, Maud Kuistila, Ylppö Foundation, Uppsala University and Minerva Foundation. We thank Dr. Urmas Arumäe for discussions, and Dr. Patrik Ernfors for the Bax KO mice, and Eeva Lehto for technical assistance. L7AUG was a kind gift from Dr. J. Oberdick, Ohio State University, USA.Peer reviewe

Molecular mechanisms underlying the actions of psychoactive drugs in the basal ganglia : Focus on cannabinoids and morphine

This thesis is centered on the identification of the molecular mechanisms involved in the psychomotor effects of cannabinoids and morphine. These drugs share the ability of acting at the level of the basal ganglia, a group of subcortical structures involved in the control of locomotion, as well as in cognitive and motivational aspects of motor function. In Paper I and II, we have examined the involvement of the dopamine- and cAMP-dependent phosphoprotein of 32 kDa (DARPP-32) in the motor depressant effect produced by activation of the neuronal CB1 receptor (CB1R). DARPP-32 is highly expressed in the medium spiny neurons of the striatum, which is the largest component of the basal ganglia. We found that administration of CP55,940, a selective CB1R agonist, or delta9-tetrahydrocannabinol (delta9-THC), the active component of marihuana or hashish, increased the state of phosphorylation of DARPP-32 at the cAMP-dependent protein kinase site (PKA), Thr34. Similar increases were observed with AM404, a blocker of the reuptake of endogenous cannabinoids (e.g. anandamide and 2-arachidonyl glycerol), or URB597, an inhibitor of the enzyme fatty acid amide hydrolase (FAAH), which is responsible for the degradation of endocannabinoids. The motor depressant effect (catalepsy) produced by CP55,940, was attenuated by genetic inactivation of DARPP-32. Point mutation of Thr34 on DARPP-32 produced a similar reduction in the effect of the CB1R agonist. Genetic inactivation either of dopamine D2 receptors (D2Rs) or of adenosine A2A receptors (A2ARs) reduced the phosphorylation of DARPP-32 at Thr34 and the motor depression produced by CP55,940. These data indicated that a considerable proportion of the psychomotor effect of cannabinoids is accounted for by a signaling cascade involving PKA-dependent phosphorylation of DARPP-32, achieved via CB1R-mediated modulation of D2R and A2AR transmission. In Paper III, we have examined the involvement of DARPP-32 in the short- and long-term effects of morphine. We found that acute administration of morphine increased DARPP-32 phosphorylation at Thr34 in both dorsal striatum and ventral striatum (nucleus accumbens). The ability of morphine to stimulate Thr34 phosphorylation was prevented by blockade of dopamine D1 receptors (D1Rs). Genetic inactivation of DARPP-32 or point mutation of Thr34 reduced the hyperlocomotor response to a single injection of morphine. In contrast, DARPP-32 mutant mice developed behavioral sensitization to morphine comparable to that of wild-type controls and displayed normal morphine conditioned place preference. These results demonstrated that dopamine D1R-mediated activation of the cAMP/DARPP-32 cascade in striatal MSNs is involved in the psychomotor action, but not in the rewarding properties, of morphine. Exposure to cues previously associated with intake of substances of abuse can promote drug related responses. In Paper IV, we have examined the effects of exposure to a drug-associated context on the psychomotor response to morphine. We found that the psychomotor sensitization produced by repeated administration of morphine was markedly increased in mice examined 4 weeks after the last drug injection. In addition, the withdrawal period was able to confer to the environment paired with morphine the ability to increase ERK phosphorylation in a specific compartment (i.e. the shell) of the nucleus accumbens. Using transgenic mice with enhanced green fluorescent protein (EGFP) expression under the control of the D1R (drd1a-EGFP) or D2R promoter (drd2-EGFP), we showed that context-dependent ERK phosphorylation was restricted to D1R-expressing MSNs. Furthermore, we found that this effect depended on D1R activation. This study showed that, following repeated morphine injections, a drug free period induced context-dependent phosphorylation of ERK in a discrete group of neurons within the nucleus accumbens shell. This activation was associated with enhanced psychomotor sensitization and could be implicated in context-elicited drug seeking induced by repeated exposure to drugs of abuse

Loss of Striatonigral GABAergic Presynaptic Inhibition Enables Motor Sensitization in Parkinsonian Mice

Degeneration of dopamine (DA) neurons in Parkinson's disease (PD) causes hypokinesia, but DA replacement therapy can elicit exaggerated voluntary and involuntary behaviors that have been attributed to enhanced DA receptor sensitivity in striatal projection neurons. Here we reveal that in hemiparkinsonian mice, striatal D1 receptor-expressing medium spiny neurons (MSNs) directly projecting to the substantia nigra reticulata (SNr) lose tonic presynaptic inhibition by GABAB receptors. The absence of presynaptic GABAB response potentiates evoked GABA release from MSN efferents to the SNr and drives motor sensitization. This alternative mechanism of sensitization suggests a synaptic target for PD pharmacotherapy

Critical involvement of cAMP/DARPP-32 and extracellular signal-regulated protein kinase signaling in L-DOPA-induced dyskinesia

The molecular basis of L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID), one of the major hindrances in the current therapy for Parkinson's disease, is still unclear. We show that attenuation of cAMP signaling in the medium spiny neurons of the striatum, achieved by genetic inactivation of the dopamine and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), reduces LID. We also show that, in dyskinetic mice, sensitized cAMP/cAMP-dependent protein kinase/DARPP-32 signaling leads to phosphorylation/activation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). The increase in ERK1/2 phosphorylation associated with dyskinesia results in activation of mitogen- and stress-activated kinase-1 (MSK- 1) and phosphorylation of histone H3, two downstream targets of ERK involved in transcriptional regulation. In line with these observations, we found that c- Fos expression is abnormally elevated in the striata of mice affected by LID. Persistent enhancement of the ERK signaling cascade is implicated in the generation of LID. Thus, pharmacological inactivation of ERK1/2 achieved using SL327 (alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl) benzeneacetonitrile), an inhibitor of the mitogen-activated kinase/ERK kinase, MEK, during chronic L-DOPA treatment counteracts the induction dyskinesia. Together, these results indicate that a significant proportion of the abnormal involuntary movements developed in response to chronic L-DOPA are attributable to hyperactivation in striatal medium spiny neurons of a signaling pathway including sequential phosphorylation of DARPP-32, ERK1/2, MSK-1, and histone H3

Voluntary adolescent drinking enhances excitation by low levels of alcohol in a subset of dopaminergic neurons in the ventral tegmental area.

Enhanced dopamine (DA) neurotransmission from the ventral tegmental area (VTA) to the ventral striatum is thought to drive drug self-administration and mediate positive reinforcement. We examined neuronal firing rates in slices of mouse midbrain following adolescent binge-like alcohol drinking and find that prior alcohol experience greatly enhanced the sensitivity to excitation by ethanol itself (10-50 mM) in a subset of ventral midbrain DA neurons located in the medial VTA. This enhanced response after drinking was not associated with alterations of firing rate or other measures of intrinsic excitability. In addition, the phenomenon appears to be specific to adolescent drinking, as mice that established a drinking preference only after the onset of adulthood showed no change in alcohol sensitivity. Here we demonstrate not only that drinking during adolescence induces enhanced alcohol sensitivity, but also that this DA neuronal response occurs over a range of alcohol concentrations associated with social drinking in humans

Cannabinoid action depends on phosphorylation of dopamine- and cAMP-regulated phosphoprotein of 32 kDa at the protein kinase A site in striatal projection neurons.

International audienceHerbal cannabis, smoked in the form of marihuana or hashish, is the most common illicit drug consumed in the Western world. In the brain, cannabinoids interact with neuronal CB1 receptors, thereby producing a marked reduction of motor activity. Here, we report that the motor depressant effect produced by the cannabinoid receptor agonist (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]trans-4-(3-hydroxypropyl)cyclohexanol (CP55,940) is attenuated by genetic inactivation of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), which is abundantly expressed in the medium spiny neurons of the striatum. Point mutation of Thr34, the protein kinase A (PKA) phosphorylation site of DARPP-32, produces a similar reduction in the effect of the CB1 agonist. In contrast, point mutation of Thr75, a site on DARPP-32 specifically phosphorylated by cyclin-dependent kinase 5, does not affect the behavioral response to CP55,940. Activation of CB1 receptors, either by an agonist or by inhibition of reuptake of endogenous cannabinoids, stimulates phosphorylation at Thr34, thereby converting DARPP-32 into an inhibitor of protein phosphatase-1. Genetic inactivation either of dopamine D2 receptors or of adenosine A2A receptors reduces the phosphorylation of DARPP-32 at Thr34 and the motor depression produced by CP55,940. Our data indicate that a considerable proportion of the psychomotor effect of cannabinoids can be accounted for by a signaling cascade in striatal projection neurons involving PKA-dependent phosphorylation of DARPP-32, achieved via modulation of dopamine D2 and adenosine A2A transmission