Elsevier

Neuropharmacology

Volume 95, August 2015, Pages 50-58
Neuropharmacology

Changes in the expression of genes encoding for mGlu4 and mGlu5 receptors and other regulators of the indirect pathway in acute mouse models of drug-induced parkinsonism

https://doi.org/10.1016/j.neuropharm.2015.02.024Get rights and content

Highlights

  • We studied the expression of striatal genes in acute mouse models of parkinsonism.

  • The products of these genes either activate or inhibit the indirect pathway.

  • Haloperidol and MPTP caused transient changes in the expression of these genes.

  • Among these genes are those encoding for mGlu4 and mGlu5 receptors.

  • Genetic deletion of mGlu4 receptors altered the response to haloperidol and MPTP.

Abstract

Neuroadaptive changes involving the indirect pathway of the basal ganglia motor circuit occur in the early phases of parkinsonism. The precise identification of these changes may shed new light into the pathophysiology of parkinsonism and better define the time window of pharmacological intervention. We examined some of these changes in mice challenged with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), or with the dopamine receptor blocker, haloperidol. These two models clearly diverge from Parkinson's disease (PD); however, they allow an accurate time-dependent analysis of neuroadaptive changes occurring in the striatum. Acute haloperidol injection caused a significant increase in the transcripts of mGlu4 receptors, CB1 receptors and preproenkephalin-A at 2 and 24 h, and a reduction in the transcripts of mGlu5 and A2A receptors at 2 h. At least changes in the expression of mGlu4 receptors might be interpreted as compensatory because haloperidol-induced catalepsy was enhanced in mGlu4−/− mice. Mice injected with 30 mg/kg of MPTP also showed an increase in the transcripts of mGlu4 receptors, CB1 receptors, and preproenkephalin-A at 3 d, and a reduction of the transcript of A2A receptors at 1 d in the striatum. Genetic deletion of mGlu4 receptors altered the functional response to MPTP, assessed by counting c-Fos+ neurons in the external globus pallidus and ventromedial thalamic nucleus. These findings offer the first evidence that changes in the expression of mGlu4 and mGlu5 receptors occur in acute models of parkinsonisms, and lay the groundwork for the study of these changes in models that better recapitulate the temporal profile of nigrostriatal dysfunction associated with PD.

Introduction

Human and animal studies suggest that compensatory mechanisms in the basal ganglia motor circuit occur in the presymptomatic stage of Parkinson's disease (PD), in which motor signs (i.e., bradykinesia, rigidity and resting tremor) are not yet manifest in spite of extensive degeneration of nigrostriatal dopaminergic neurons (reviewed by Bezard et al., 2003, Brotchie and Fitzer-Attas, 2009). Unraveling the molecular nature and temporal profile of these mechanisms may shed new light into the pathophysiology of parkinsonism, and lay the groundwork for new therapeutic interventions aimed at delaying the clinical onset of the disorder. Two different categories of compensatory mechanisms have been identified in early parkinsonism, i.e. those enhancing the activity of surviving dopaminergic neurons, and those restraining the activity of the indirect pathway of the basal ganglia motor circuit (reviewed by Brotchie and Fitzer-Attas, 2009). Examples of compensatory changes influencing the activity of dopaminergic neurons include enhanced levels of tyrosine hydroxylase and L-aromatic amino acid decarboxylase, and reduced levels of the high affinity dopamine transporter (Zigmond et al., 1984, Uhl et al., 1994, Lee et al., 2000, Sossi et al., 2007). Interventions aimed at amplifying these mechanisms may accelerate degeneration of dopaminergic fibers because of the production of reactive oxygen species from endogenous dopamine (see Brotchie and Fitzer-Attas, 2009). Changes occurring within the indirect pathway are more “safe” and perhaps occur earlier in the course of the disease. The indirect pathway connects the input station, i.e. the neostriatum, with the output stations of the basal ganglia motor circuit (the internal globus pallidus and the substantia nigra pars reticulata) via the external globus pallidus (GPext) and the subthalamic nucleus (STN). In the indirect pathway, striatal GABAergic projection neurons make synaptic contacts with GPext neurons, which, in turn, send GABAergic projections to the STN. STN neurons send excitatory axons to the output stations, thereby restraining the activity of ventral motor thalamic neurons projecting to the cerebral cortex (reviewed by Conn et al., 2005).

In the neostriatum, the activity of projection neurons of the indirect pathway is negatively modulated by D2 dopamine (DA) receptors and positively regulated by A2A adenosine receptors, NMDA receptors, and mGlu5 metabotropic glutamate receptors. GABA release at the synapses between striatal projection neurons and GPext neurons is negatively modulated by mGlu4 receptors, as well as by MOR opiate receptors activated by enkephalins that are released from the terminals of striatal projection neurons (reviewed by Conn et al., 2005). CB1 cannabinoid receptors are co-localized with D2 receptors in striatal projection neurons of the indirect pathway, and are also expressed on axon terminals in the GPext (Gerfen et al., 1990, Mailleux and Vanderhaeghen, 1992, Szabo et al., 1998, Hermann et al., 2002, Mátyás et al., 2006, Crespo et al., 2008, Martín et al., 2008, Van Waes et al., 2012). The overall function of CB1 receptors is to inhibit the indirect pathway (Blume et al., 2013).

The following changes have been described in early stages of experimental parkinsonism: (i) a reduced membrane availability of the GluN2B subunit of NMDA receptors expressed by striatal projection neurons (Hallett et al., 2005); (ii) a reduced catabolism of endocannabinoids leading to an increased activation of CB1 receptors (Kreitzer and Malenka, 2007); and (iii) an enhanced production of enkephalins in axon terminals of striatal neurons projecting to the GPext (Herrero et al., 1995, Asselin et al., 1994, Nisbet et al., 1995, Gudehithlu et al., 1991). No data are available on the expression of mGlu4, mGlu5, A2A and CB1 receptors, in spite of the importance of these receptors as drug targets in the treatment of parkinsonism (Jones et al., 2013, Gasparini et al., 2013, Amalric et al., 2013).

Here, we report that parkinsonism induced by acute injection of haloperidol or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice is associated with transient changes in the expression of genes encoding for mGlu4 receptors, mGlu5 receptors, A2A receptors, CB1 receptors, and pre-proenkephalin-A (the peptide precursor of enkephalins), in the striatum.

Section snippets

Animals

C57BL mice (22–24 g, body weight) were purchased from Charles River (Calco, Italy). Grm4+/−(B6.129-Grm4tm1Hpn/J) mice, on a C57/BL6 background, were purchased from The Jackson Laboratory (Bar Harbor, ME). The Grm4−/− offspring of heterozygotes were used to establish colonies of Grm4−/− mice (see Fallarino et al., 2010). Mice were kept under environmentally controlled conditions (ambient temperature, 22 °C; humidity, 40%) on a 12 h light/dark cycle with food and water ad libitum. All experiments

Time-dependent changes in the expression of genes encoding for preproenkaphalin-A and mGlu4, mGlu5, CB1, and A2A receptors in the striatum following acute haloperidol injection

Mice treated with saline or haloperidol (1 mg/kg, s.c.) were killed at different time points for measurements of the transcripts encoding for preproenkephalin-A, mGlu4 receptors, mGlu5 receptors, CB1 receptors, and A2A adenosine receptors in the striatum. There was no difference in any of the transcripts between values obtained at 1 and 3 d following a single injection of saline. Thus, the two sets of values obtained in mice treated with saline were pooled and used for controls. Treatment with

Discussion

The two models we have used, i.e. mice challenged with MPTP or with a single injection of haloperidol, are far from human pathology but are useful for an accurate time-dependent analysis of changes in the expression of striatal genes occurring at early times following nigro-striatal damage or DA receptor blockade. We have found that striatal genes encoding for therapeutic targets in PD, such as A2A, mGlu4, and mGlu5 receptors (Conn et al., 2005, Johnson et al., 2009, Nicoletti et al., 2011,

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