Elsevier

Neuroscience

Volume 311, 17 December 2015, Pages 519-538
Neuroscience

Pathological gamma oscillations, impaired dopamine release, synapse loss and reduced dynamic range of unitary glutamatergic synaptic transmission in the striatum of hypokinetic Q175 Huntington mice

https://doi.org/10.1016/j.neuroscience.2015.10.039Get rights and content

Highlights

  • Evidence for hypokinesia in the Q175 mouse model of Huntington’s disease.

  • Gamma oscillations in the striatum as a biomarker of the hypokinesia in HD.

  • Evidence for reduced striatal dopamine release in freely moving HD mice.

  • HD-related synapse loss in immunostained sections.

  • Reduced functional range of glutamatergic synaptic transmission.

Abstract

Huntington’s disease (HD) is a severe genetically inherited neurodegenerative disorder. Patients present with three principal phenotypes of motor symptoms: choreatic, hypokinetic-rigid and mixed. The Q175 mouse model of disease offers an opportunity to investigate the cellular basis of the hypokinetic-rigid form of HD. At the age of 1 year homozygote Q175 mice exhibited the following signs of hypokinesia: Reduced frequency of spontaneous movements on a precision balance at daytime (−55%), increased total time spent without movement in an open field (+42%), failures in the execution of unconditioned avoidance reactions (+32%), reduced ability for conditioned avoidance (−96%) and increased reaction times (+65%) in a shuttle box. Local field potential recordings revealed low-frequency gamma oscillations in the striatum as a characteristic feature of HD mice at rest. There was no significant loss of DARPP-32 immunolabeled striatal projection neurons (SPNs) although the level of DARPP-32 immunoreactivity was lower in HD. As a potential cause of hypokinesia, HD mice revealed a strong reduction in striatal KCl-induced dopamine release, accompanied by a decrease in the number of tyrosine hydroxylase-(TH)- and VMAT2-positive synaptic varicosities. The presynaptic TH fluorescence level was also reduced. Patch-clamp experiments were performed in slices from 1-year-old mice to record unitary EPSCs (uEPSCs) of presumed cortical origin in the absence of G-protein-mediated modulation. In HD mice, the maximal amplitudes of uEPSCs amounted to 69% of the WT level which matches the loss of VGluT1+/SYP+ synaptic terminals in immunostained sections. These results identify impairment of cortico-striatal synaptic transmission and dopamine release as a potential basis of hypokinesia in HD.

Introduction

Huntington’s disease (HD) is a severe monogenetic disorder with a pathological gain of function. Disease progression is associated with a broad range of alterations in motor, cognitive and emotional performances eventually leading to dementia and premature death within an average of 20 years after the appearance of first symptoms (see (Walker, 2007, Ross et al., 2014) for a review of clinical studies). A characteristic feature of HD at earlier stages is the appearance of uncontrolled muscle contractions (chorea), whereas Parkinson-like symptoms, including hypokinesia, increased muscle tone and occasional tremor, are key symptoms of advanced HD. In 8–10% of cases (the Westphal variety of HD) the afflicted patients present without chorea and exhibit hypokinesia from the very beginning. The degree of motor impairment (hypokinesia, rigor and bradykinesia) but not chorea correlates with the neuropathological scores of disease severity (Rosenblatt et al., 2003).

Although the focus of the present report will be placed on the cellular basis of hypokinesia, it is deemed necessary to first provide a set of functional indicators of motor impairment to further characterize the selected mouse model of HD, the Z-Q175-KI mouse (Menalled et al., 2012). In comparison with the better known R6/2 HD mice, Q175 mice display motor disturbances at an older age and also survive longer, although the number of CAG repeats is extremely high (on average 184). At the selected age of 1 year Q175 heterozygotes and homozygotes (HOMs) exhibit motor deficits for at least 4 months (Menalled et al., 2012, Heikkinen et al., 2012, Loh et al., 2013).

As for the cellular basis of the transition from chorea/hyperactivity to dystonia/hypokinesia we shall regard three principal possibilities: (i) sequential degeneration of neuronal subsets in the striatum, (ii) dopamine depletion and (iii) deficiency of corticostriatal synaptic transmission. Neuropathological studies of human HD brains suggested that the DRD2/enkephalin-expressing striatal projection neurons (SPNs) of the indirect pathway (iSPNs) undergo neurodegeneration before the DRD1/substance P-expressing SPNs of the direct pathway (dSPNs) decrease in number (Reiner et al., 1988, Sapp et al., 1995, Glass et al., 2000, Deng et al., 2004). According to the hypothesis of (Albin et al., 1989), the motor symptoms would then initially reflect the imbalance between the direct/indirect pathways with their facilitatory/suppressive role in motor activity and, later on, manifest the complete breakdown of signal transmission through the basal ganglia.

However, hypokinesia and rigidity might occur in the absence of substantial neuron loss in the striatum, as is the case in patients with the idiopathic parkinson syndrome (IPS) where motor impairments are mainly caused by the degeneration of dopaminergic neurons in the brain stem. Is then hypokinesia in HD caused by insufficiency of dopamine signaling (see (Cepeda et al., 2014) for a comprehensive review of clinical and animal studies)? Indeed, functional tests in HD preparations in vitro demonstrated reduced DA release (Petersen et al., 2002, Johnson et al., 2006, Callahan and Abercrombie, 2011, Dallerac et al., 2015) and impaired D1-dependent modulation of glutamatergic synaptic transmission (Joshi et al., 2009). It should be mentioned that some of these findings were stage-dependent, consistent with the notion that chorea might be associated with hyperactive dopamine signaling (Jahanshahi et al., 2010, Dallerac et al., 2015). Unfortunately, still little is known on the state of dopaminergic innervation and the capacity for striatal dopamine release of intact HD mice exhibiting hypokinesia. To fill this gap of knowledge was the second aim of the present study.

Finally, it had been suggested that corticostriatal uncoupling could be a cause of motor impairment in advanced HD (Cepeda et al., 2007, Joshi et al., 2009, Hong et al., 2012). However, the exact mechanism of lost corticostriatal control is still far from being understood. Since, for technical reasons, HD-related differences in the release characteristics of corticostriatal synapses are difficult to prove it has remained unclear whether reduced corticostriatal coupling is expressed at the level of individual corticostriatal connections or mainly due to degeneration of cortical pyramidal neurons. A first answer to this question could be obtained by recording unitary EPSCs (uEPSCs) of presumed corticostriatal origin. Such connections can be identified on the basis of their characteristic short-term plasticity, as it was shown that corticostriatal but not thalamostriatal EPSCs exhibit paired-pulse facilitation (PPF) under standard conditions (Ding et al., 2008). The results of these experiments could then be confronted with the results of synapse counts using the vesicular glutamate transporter 1 (VGLUT1) as a marker of synaptic terminals of cortical origin (Deng et al., 2013).

On the whole, our material from 1-year-old Q175 mice renders support to the hypothesis that HD hypokinesia is associated with a reduction of dopaminergic and glutamatergic innervation in the striatum and occurs prior to the substantial loss of DARPP-32-labeled striatal projection cells.

Section snippets

Ethical approval

The present experiments were performed in fully adult mice from a colony of Z-Q175-KI provided by the CHDI (“Cure Huntington’s disease Initiative”) foundation. Every precaution was taken to minimize stress and to reduce the number of animals used in each series of experiments. The work described here has been carried out in accordance with the EU Directive 2010/63/EU for animal experiments and complies with the requirements for manuscripts submitted to biomedical journals. The work was

Signs of hypokinesia in Q175 mice

When evaluating the motor activities in HD mice it is necessary to take into account the circadian rhythms of activity and sleep (Loh et al., 2013), as HD-related deviations might be more prominent at night. The graph of Fig. 1 presents the motor activity as the number of deflections registered by a sensitive Sartorius balance throughout an entire light–dark (LD) cycle in a sound-proof cage. During the night period HD mice displayed hyperactivity in the form of a delayed decline of the initial

Hypokinesia in Q175 mice

The starting point of this study was the observation that Q175 homozygotes exhibited signs of hypokinesia, especially when tested during day-light. The term hypokinesia refers to impairment of movement initiation due to difficulty selecting and/or activating respective motor programs in the basal ganglia. Based on the comparison of WT and Q175 HOMs we have interpreted the following observations as signs of hypokinesia: Reduced number of spontaneous movements on a balance after exposure to light

Conclusion

Our results suggest that restoring synaptic dopamine and glutamate release should normalize striatal activity at rest, abolish pathological gamma oscillations and alleviate the symptom of hypokinesia.

Acknowledgment

This work is supported by the Cure Huntington’s Disease Initiative foundation (A-7815), the German Research Council (Gr986/10-1, Exc 257/1), Charité Research Funds (2015-040) and intramural funds of the Leibniz Institute of Neurobiology. We highly appreciate the expert advice of Dr. Hannes Schmidt at the Max Delbrück Center for Molecular Medicine Berlin and Dr. Svetlana Warton, retired from the University of Sydney. Jörg Rösner at the Neurocure Microscope Core Facility provided skilled

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    These authors equally contributed to the material of this paper.

    With deep sadness we dedicate this publication to the memory of Holger Stark who suddenly died on the 18th of February, 2015.

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