Impaired striatal D2 receptor function leads to enhanced GABA transmission in a mouse model of DYT1 dystonia

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Abstract

DYT1 dystonia is caused by a deletion in a glutamic acid residue in the C-terminus of the protein torsinA, whose function is still largely unknown. Alterations in GABAergic signaling have been involved in the pathogenesis of dystonia. We recorded GABA- and glutamate-mediated synaptic currents from a striatal slice preparation obtained from a mouse model of DYT1 dystonia. In medium spiny neurons (MSNs) from mice expressing human mutant torsinA (hMT), we observed a significantly higher frequency, but not amplitude, of GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) and miniature currents (mIPSCs), whereas glutamate-dependent spontaneous excitatory synaptic currents (sEPSCs) were normal. No alterations were found in mice overexpressing normal human torsinA (hWT). To identify the possible sources of the increased GABAergic tone, we recorded GABAergic Fast-Spiking (FS) interneurons that exert a feed-forward inhibition on MSNs. However, both sEPSC and sIPSC recorded from hMT FS interneurons were comparable to hWT and non-transgenic (NT) mice. In physiological conditions, dopamine (DA) D2 receptor act presynaptically to reduce striatal GABA release. Of note, application of the D2-like receptor agonist quinpirole failed to reduce the frequency of sIPSCs in MSNs from hMT as compared to hWT and NT mice. Likewise, the inhibitory effect of quinpirole was lost on evoked IPSCs both in MSNs and FS interneurons from hMT mice. Our findings demonstrate a disinhibition of striatal GABAergic synaptic activity, that can be at least partially attributed to a D2 DA receptor dysfunction.

Introduction

DYT1 dystonia is a severe form of inherited generalized dystonia, caused by a deletion in the DYT1 gene encoding the protein torsinA (Ozelius et al., 1997). The physiological function of torsinA is unclear, though it has been proposed to perform chaperone-like functions, assist in protein trafficking, membrane fusion and participate in secretory processing (Goodchild et al., 2005, Granata et al., 2007, Hewett et al., 2007). The neurochemical basis for primary dystonia is currently unknown, although abnormalities in striatal dopaminergic signalling have been proposed to play a role in the pathophysiology of this disorder (rev. Breakefield, et al., 2008). A reduction of dopamine (DA) levels was found in the putamen and caudate in one DYT1 patient (Furukawa et al., 2000). A subsequent study revealed no difference in total striatal DA content in three DYT1 brains, but an increased DOPAC/DA ratio, suggestive of an increased DA turnover, along with a trend toward decreased D1 and D2 receptor binding (Augood et al., 2002). In non-manifesting carriers of the DYT1 mutation, a PET study revealed a moderate reduction of striatal D2 receptor binding (Asanuma et al., 2005). Similar results have been reported in patients with focal dystonia (Perlmutter et al., 1997).

Evidence on the role of DA transmission emerged also from genetic mouse models of DYT1 dystonia. In mice overexpressing mutant torsinA (Sharma et al., 2005), basal striatal DA levels and binding of D1 and D2 receptors were unaffected (Balcioglu et al., 2007, Zhao et al., 2008). However, amphetamine-induced DA release was reduced. DA metabolite ratios were found either increased (Zhao et al., 2008), or unchanged (Balcioglu et al., 2007). In this same model, we have identified altered D2 receptor responses in striatal cholinergic interneurons (Pisani et al., 2006). Recently, abnormalities in serotonin, but not DA levels were found in another model of DYT1 dystonia (Grundmann et al., 2007).

GABAergic medium spiny neurons (MSNs) are the principal output neurons and the primary target of the dopaminergic nigrostriatal pathway. Inhibitory synaptic inputs to MSNs derive both from axon collaterals of other MSNs, and from GABAergic interneurons, primarily Fast-Spiking (FS), parvalbumin-immunoreactive interneurons (Tepper et al., 2004). Both synthesis and release of GABA are tonically inhibited by D2 receptors (Girault et al., 1986, Delgado et al., 2000). The cellular localization of DA receptors has been extensively studied, showing a predominant expression of D2 receptors on enkephalinergic striatopallidal MSNs (Gerfen et al., 1990). While it has been demonstrated that cholinergic interneurons express D2 receptors (Le Moine et al., 1990), the precise localization of D2 receptors on GABAergic interneurons remains to be established. In particular, it is unclear if GABAergic axon terminals expressing D2 receptors derive from collaterals of other MSNs or from interneurons. The critical role of such interaction led us to hypothesize that mutant torsinA might disrupt dopaminergic regulation of striatal GABA. We found an abnormally increased GABAergic synaptic activity in mice overexpressing the mutant torsinA (hMT), compared to wild-type littermates expressing normal torsinA (hWT). More importantly, D2 receptor activation failed to reduce GABA currents. These findings provide further evidence for a role of D2 receptors in the pathogenesis of dystonia.

Section snippets

Corticostriatal slice preparation

Experiments were conducted according to the EC guidelines (86/609/EEC) and approved by the University “Tor Vergata”. Mice (8–10 weeks old) were generated as described (Sharma et al., 2005). For each strain, non-transgenic littermates were utilized as controls, and are defined as non-transgenics (NT) (C57BL-6). Thus, the genetic background was identical between the control and experimental animals, with the only difference being the presence of the transgene. Thus, “hWT” indicates mice

Intrinsic membrane properties of striatal neuronal subtypes

Whole-cell recordings were made of MSNs from the striatum of NT (n = 85), hWT (n = 81) and hMT (n = 92) mice, identified by means of electrophysiological criteria (Kita et al., 1984). MSNs from either NT or hWT and hMT mice did not exhibit firing activity at rest and, upon depolarizing current pulses showed membrane rectification and tonic action potential discharge (Fig. 1). Resting membrane potential (RMP), action potential amplitude, input resistance were similar in the three strains (Fig. 1;

Discussion

The main finding of the present study is a significant abnormality of striatal GABAergic function in a transgenic mouse model of DYT1 dystonia. These mice do not exhibit overt dystonic symptoms, but display both impairment in motor function and biochemical abnormalities in DA neurotransmission (Sharma et al., 2005, Balcioglu et al., 2007, Zhao et al., 2008). In MSNs from hMT mice, we found an increased GABAergic signaling, measurable in the frequency of both sIPSCs and of mIPSCs. In

Acknowledgments

This work was supported by grants from Bachmann-Strauss Dystonia & Parkinson's Foundation and Dystonia Medical Research Foundation to AP; Ministero Salute (Progetto Finalizzato and Articolo 56) to GB and AP; Istituto Superiore Sanità (Malattie Rare) to AP; NIH grants NS37409 to DGS; NIH K08 NS044272 to NS.

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