Balanced synaptic currents underlie low‐frequency oscillations in the subiculum

Numerous synaptic and intrinsic membrane mechanisms have been proposed for generating oscillatory activity in the hippocampus. Few studies, however, have directly measured synaptic conductances and membrane properties during oscillations. The time course and relative contribution of excitatory and inhibitory synaptic conductances, as well as the role of intrinsic membrane properties in amplifying synaptic inputs, remains unclear. To address this issue, we used an isolated whole hippocampal preparation that generates autonomous low‐frequency oscillations near the theta range. Using 2‐photon microscopy and expression of genetically encoded fluorophores, we obtained on‐cell and whole‐cell patch recordings of pyramidal cells and fast‐firing interneurons in the distal subiculum. Pyramidal cell and interneuron spiking shared similar phase‐locking to local field potential oscillations. In pyramidal cells, spiking resulted from a concomitant and balanced increase in excitatory and inhibitory synaptic currents. In contrast, interneuron spiking was driven almost exclusively by excitatory synaptic current. Thus, similar to tightly balanced networks underlying hippocampal gamma oscillations and ripples, balanced synaptic inputs in the whole hippocampal preparation drive highly phase‐locked spiking at the peak of slower network oscillations.     

Excitatory extrinsic afferents to striatal interneurons and interactions with striatal microcircuitry

The striatum constitutes the main input structure of the basal ganglia and receives two major excitatory glutamatergic inputs, from the cortex and the thalamus. Excitatory cortico‐ and thalamostriatal connections innervate the principal neurons of the striatum, the spiny projection neurons (SPNs), which constitute the main cellular input as well as the only output of the striatum. In addition, corticostriatal and thalamostriatal inputs also innervate striatal interneurons. Some of these inputs have been very well studied, for example the thalamic innervation of cholinergic interneurons and the cortical innervation of striatal fast‐spiking interneurons, but inputs to most other GABAergic interneurons remain largely unstudied, due in part to the relatively recent identification and characterization of many of these interneurons. In this review, we will discuss and reconcile some older as well as more recent data on the extrinsic excitatory inputs to striatal interneurons. We propose that the traditional feed‐forward inhibitory model of the cortical input to the fast‐spiking interneuron then inhibiting the SPN, often assumed to be the prototype of the main functional organization of striatal interneurons, is incomplete. We provide evidence that the extrinsic innervation of striatal interneurons is not uniform but shows great cell‐type specificity. In addition, we will review data showing that striatal interneurons are themselves interconnected in a highly cell‐type‐specific manner. These data suggest that the impact of the extrinsic inputs on striatal activity critically depends on synaptic interactions within interneuronal circuitry.     

Integrated anatomical and physiological mapping of striatal afferent projections

The dorsomedial striatum, a key site of reward‐sensitive motor output, receives extensive afferent input from cortex, thalamus and midbrain. These projections are integrated by striatal microcircuits containing both spiny projection neurons and local circuit interneurons. To explore target cell specificity of these projections, we compared inputs onto D1‐dopamine receptor‐positive spiny neurons, parvalbumin‐positive fast‐spiking interneurons and somatostatin‐positive low‐threshold‐spiking interneurons, using cell type‐specific rabies virus tracing and optogenetic‐mediated projection neuron recruitment in mice. While the relative proportion of retrogradely labelled projection neurons was similar between target cell types, the convergence of inputs was systematically higher for projections onto fast‐spiking interneurons. Rabies virus is frequently used to assess cell‐specific anatomical connectivity but it is unclear how this correlates to synaptic connectivity and efficacy. To test this, we compared tracing data with target cell‐specific measures of synaptic efficacy for anterior cingulate cortex and parafascicular thalamic projections using novel quantitative optogenetic measures. We found that target‐specific patterns of convergence were extensively modified according to region of projection neuron origin and postsynaptic cell type. Furthermore, we observed significant divergence between cell type‐specific anatomical connectivity and measures of excitatory synaptic strength, particularly for low‐threshold‐spiking interneurons. Taken together, this suggests a basic uniform connectivity map for striatal afferent inputs upon which presynaptic–postsynaptic interactions impose substantial diversity of physiological connectivity.     

Cholinergic interneurons in the Q140 knockin mouse model of Huntington's disease: Reductions in dendritic branching and thalamostriatal input

We have previously found that thalamostriatal axodendritic terminals are reduced as early as 1 month of age in heterozygous Q140 HD mice (Deng et al. [ 2013 ] Neurobiol Dis 60:89–107). Because cholinergic interneurons are a major target of thalamic axodendritic terminals, we examined the VGLUT2‐immunolabeled thalamic input to striatal cholinergic interneurons in heterozygous Q140 males at 1 and 4 months of age, using choline acetyltransferase (ChAT) immunolabeling to identify cholinergic interneurons. Although blinded neuron counts showed that ChAT⁺ perikarya were in normal abundance in Q140 mice, size measurements indicated that they were significantly smaller. Sholl analysis further revealed the dendrites of Q140 ChAT⁺ interneurons were significantly fewer and shorter. Consistent with the light microscopic data, ultrastructural analysis showed that the number of ChAT⁺ dendritic profiles per unit area of striatum was significantly decreased in Q140 striata, as was the abundance of VGLUT2⁺ axodendritic terminals making synaptic contact with ChAT⁺ dendrites per unit area of striatum. The density of thalamic terminals along individual cholinergic dendrites was, however, largely unaltered, indicating that the reduction in the areal striatal density of axodendritic thalamic terminals on cholinergic neurons was due to their dendritic territory loss. These results show that the abundance of thalamic input to individual striatal cholinergic interneurons is reduced early in the life span of Q140 mice, raising the possibility that this may occur in human HD as well. Because cholinergic interneurons differentially affect striatal direct vs. indirect pathway spiny projection neurons, their reduced thalamic excitatory drive may contribute to early abnormalities in movement in HD. J. Comp. Neurol. 524:3518–3529, 2016. © 2016 Wiley Periodicals, Inc.     

Plastic and behavioral abnormalities in experimental Huntington's disease: a crucial role for cholinergic interneurons

Huntington's disease (HD) is a fatal hereditary neurodegenerative disease causing degeneration of striatal spiny neurons, whereas cholinergic interneurons are spared. This cell-type specific pathology produces an array of abnormalities including involuntary movements, cognitive impairments, and psychiatric disorders. Although the genetic mutation responsible for HD has been identified, little is known about the early synaptic changes occurring within the striatal circuitry at the onset of clinical symptoms. We therefore studied the synaptic plasticity of spiny neurons and cholinergic interneurons in two animal models of early HD. As a pathogenetic model, we used the chronic subcutaneous infusion of the mitochondrial toxin 3-nitropropionic acid (3-NP) in rats. This treatment caused striatal damage and impaired response flexibility in the cross-maze task as well as defective extinction of conditioned fear suggesting a perseverative behavior. In these animals, we observed a loss of depotentiation in striatal spiny neurons and a lack of long-term potentiation (LTP) in cholinergic interneurons. These abnormalities of striatal synaptic plasticity were also observed in R6/2 transgenic mice, a genetic model of HD, indicating that both genetic and phenotypic models of HD show cell-type specific alterations of LTP. We also found that in control rats, as well as in wild-type (WT) mice, depotentiation of spiny neurons was blocked by either scopolamine or hemicholinium, indicating that reversal of LTP requires activation of muscarinic receptors by endogenous acetylcholine. Our findings suggest that the defective plasticity of cholinergic interneurons could be the primary event mediating abnormal functioning of striatal circuits, and the loss of behavioral flexibility typical of early HD might largely depend on cell-type specific plastic abnormalities.     

Downregulation of cannabinoid receptor 1 from neuropeptide Y interneurons in the basal ganglia of patients with Huntington's disease and mouse models

Cannabinoid receptor 1 (CB₁ receptor) controls several neuronal functions, including neurotransmitter release, synaptic plasticity, gene expression and neuronal viability. Downregulation of CB₁ expression in the basal ganglia of patients with Huntington's disease (HD) and animal models represents one of the earliest molecular events induced by mutant huntingtin (mHtt). This early disruption of neuronal CB₁ signaling is thought to contribute to HD symptoms and neurodegeneration. Here we determined whether CB₁ downregulation measured in patients with HD and mouse models was ubiquitous or restricted to specific striatal neuronal subpopulations. Using unbiased semi‐quantitative immunohistochemistry, we confirmed previous studies showing that CB₁ expression is downregulated in medium spiny neurons of the indirect pathway, and found that CB₁ is also downregulated in neuropeptide Y (NPY)/neuronal nitric oxide synthase (nNOS)‐expressing interneurons while remaining unchanged in parvalbumin‐ and calretinin‐expressing interneurons. CB₁ downregulation in striatal NPY/nNOS‐expressing interneurons occurs in R6/2 mice, Hdh^Q150/Q150 mice and the caudate nucleus of patients with HD. In R6/2 mice, CB₁ downregulation in NPY/nNOS‐expressing interneurons correlates with diffuse expression of mHtt in the soma. This downregulation also occludes the ability of cannabinoid agonists to activate the pro‐survival signaling molecule cAMP response element‐binding protein in NPY/nNOS‐expressing interneurons. Loss of CB₁ signaling in NPY/nNOS‐expressing interneurons could contribute to the impairment of basal ganglia functions linked to HD.     

Reduced striatal acetylcholine efflux in the R6/2 mouse model of Huntington's disease: an examination of the role of altered inhibitory and excitatory mechanisms

Huntington's disease (HD) is a genetic neurodegenerative disorder that is characterized by the progressive onset of cognitive, psychiatric, and motor symptoms. In parallel, the neuropathology of HD is characterized by progressive loss of projection neurons in cortex and striatum; striatal cholinergic interneurons are relatively spared. Nonetheless, there is evidence that striatal acetylcholine (ACh) function is altered in HD. The present study is the first to examine striatal ACh function in awake, behaving animals, using the R6/2 mouse model of HD, which is transgenic for exon 1 of the mutant huntingtin gene. Physiological levels of extracellular striatal ACh were monitored in R6/2 mice and wild type controls using in vivo microdialysis. Results indicate that spontaneous ACh release is reduced in R6/2 mice relative to controls. Intrastriatal application of the GABA_A antagonist bicuculline methiodide (10.0 μM) significantly elevated ACh levels in both R6/2 mice and wild type controls, while overall ACh levels were reduced in the R6/2 mice compared to the wild type group. In contrast, systemic administration of the D₁ dopamine receptor partial agonist, SKF-38393 (10.0 mg/kg, IP), elevated ACh levels in control animals, but not R6/2 mice. Taken together, the present results suggest that GABA-mediated inhibition of striatal ACh release is intact in R6/2 mice, further demonstrating that cholinergic interneurons are capable of increased ACh release, whereas D₁ receptor-dependent activation of excitatory inputs to striatal cholinergic interneurons is dysfunctional in R6/2 mice. Reduced levels of extracellular striatal ACh in HD may reflect abnormalities in the excitatory innervation of cholinergic interneurons, which may have implications ACh-dependent processes that are altered in HD, including corticostriatal plasticity.     

Shank1 regulates excitatory synaptic transmission in mouse hippocampal parvalbumin‐expressing inhibitory interneurons

The Shank genes (SHANK1, 2, 3) encode scaffold proteins highly enriched in postsynaptic densities where they regulate synaptic structure in spiny neurons. Mutations in human Shank genes are linked to autism spectrum disorder and schizophrenia. Shank1 mutant mice exhibit intriguing cognitive phenotypes reminiscent of individuals with autism spectrum disorder. However, the molecular mechanisms leading to the human pathophysiological phenotypes and mouse behaviors have not been elucidated. In this study it is shown that Shank1 protein is highly localized in parvalbumin‐expressing (PV+) fast‐spiking inhibitory interneurons in the hippocampus. Importantly, a lack of Shank1 in hippocampal CA1 PV+ neurons reduced excitatory synaptic inputs and inhibitory synaptic outputs to pyramidal neurons. Furthermore, it is demonstrated that hippocampal CA1 pyramidal neurons in Shank1 mutant mice exhibit a shift in the excitatory and inhibitory balance (E–I balance), a pathophysiological hallmark of autism spectrum disorder. The mutant mice also exhibit lower expression of gephyrin (a scaffold component of inhibitory synapses), supporting the dysregulation of E–I balance in the hippocampus. These results suggest that Shank1 scaffold in PV+ interneurons regulates excitatory synaptic strength and participates in the maintenance of E–I balance in excitatory neurons.     

Striatal parvalbuminergic neurons are lost in Huntington's disease: implications for dystonia

Although dystonia represents a major source of motor disability in Huntington's disease (HD), its pathophysiology remains unknown. Because recent animal studies indicate that loss of parvalbuminergic (PARV+) striatal interneurons can cause dystonia, we investigated if loss of PARV+ striatal interneurons occurs during human HD progression, and thus might contribute to dystonia in HD. We used immunolabeling to detect PARV+ interneurons in fixed sections, and corrected for disease‐related striatal atrophy by expressing PARV+ interneuron counts in ratio to interneurons co‐containing somatostatin and neuropeptide Y (whose numbers are unaffected in HD). At all symptomatic HD grades, PARV+ interneurons were reduced to less than 26% of normal abundance in rostral caudate. In putamen rostral to the level of globus pallidus, loss of PARV+ interneurons was more gradual, not dropping off to less than 20% of control until grade 2. Loss of PARV+ interneurons was even more gradual in motor putamen at globus pallidus levels, with no loss at grade 1, and steady grade‐wise decline thereafter. A large decrease in striatal PARV+ interneurons, thus, occurs in HD with advancing disease grade, with regional variation in the loss per grade. Given the findings of animal studies and the grade‐wise loss of PARV+ striatal interneurons in motor striatum in parallel with the grade‐wise appearance and worsening of dystonia, our results raise the possibility that loss of PARV+ striatal interneurons is a contributor to dystonia in HD.     

Increased GABAergic function in mouse models of Huntington's disease: reversal by BDNF

Huntington's disease (HD) is characterized by loss of striatal gamma-aminobutyric acid (GABA)ergic medium-sized spiny projection neurons (MSSNs), whereas some classes of striatal interneurons are relatively spared. Striatal interneurons provide most of the inhibitory synaptic input to MSSNs and use GABA as their neurotransmitter. We reported previously alterations in glutamatergic synaptic activity in the R6/2 and R6/1 mouse models of HD. In the present study, we used whole-cell voltage clamp recordings to examine GABAergic synaptic currents in MSSNs from striatal slices in these two mouse models compared to those in age-matched control littermates. The frequency of spontaneous GABAergic synaptic currents was increased significantly in MSSNs from R6/2 transgenics starting around 5-7 weeks (when the overt behavioral phenotype begins) and continuing in 9-14-week-old mice. A similar increase was observed in 12-15-month-old R6/1 transgenics. Bath application of brain-derived neurotrophic factor, which is downregulated in HD, significantly reduced the frequency of spontaneous GABAergic synaptic currents in MSSNs from R6/2 but not control mice at 9-14 weeks. Increased GABA current densities also occurred in acutely isolated MSSNs from R6/2 animals. Immunofluorescence demonstrated increased expression of the ubiquitous alpha1 subunit of GABA(A) receptors in MSSNs from R6/2 animals. These results indicate that increases in spontaneous GABAergic synaptic currents and postsynaptic receptor function occur in parallel to progressive decreases in glutamatergic inputs to MSSNs. In conjunction, both changes will severely alter striatal outputs to target areas involved in the control of movement.     

Cortical and thalamic inputs exert cell type-specific feedforward inhibition on striatal GABAergic interneurons

The classical view of striatal GABAergic interneuron function has been that they operate as largely independent, parallel, feedforward inhibitory elements providing inhibitory inputs to spiny projection neurons (SPNs). Much recent evidence has shown that the extrinsic innervation of striatal interneurons is not indiscriminate but rather very specific, and that striatal interneurons are themselves interconnected in a cell type-specific manner. This suggests that the ultimate effect of extrinsic inputs on striatal neuronal activity depends critically on synaptic interactions within interneuronal circuitry. Here, we compared the cortical and thalamic input to two recently described subtypes of striatal GABAergic interneurons, tyrosine hydroxylase-expressing interneurons (THINs), and spontaneously active bursty interneurons (SABIs) using transgenic TH-Cre and Htr3a-Cre mice of both sexes. Our results show that both THINs and SABIs receive strong excitatory input from the motor cortex and the thalamic parafascicular nucleus. Cortical optogenetic stimulation also evokes disynaptic inhibitory GABAergic responses in THINs but not in SABIs. In contrast, optogenetic stimulation of the parafascicular nucleus induces disynaptic inhibitory responses in both interneuron populations. However, the short-term plasticity of these disynaptic inhibitory responses is different suggesting the involvement of different intrastriatal microcircuits. Altogether, our results point to highly specific interneuronal circuits that are selectively engaged by different excitatory inputs.     

Parvalbumin‐producing striatal interneurons receive excitatory inputs onto proximal dendrites from the motor thalamus in male mice

In rodents, the dorsolateral striatum regulates voluntary movement by integrating excitatory inputs from the motor‐related cerebral cortex and thalamus to produce contingent inhibitory output to other basal ganglia nuclei. Striatal parvalbumin (PV)‐producing interneurons receiving this excitatory input then inhibit medium spiny neurons (MSNs) and modify their outputs. To understand basal ganglia function in motor control, it is important to reveal the precise synaptic organization of motor‐related cortical and thalamic inputs to striatal PV interneurons. To examine which domains of the PV neurons receive these excitatory inputs, we used male bacterial artificial chromosome transgenic mice expressing somatodendritic membrane–targeted green fluorescent protein in PV neurons. An anterograde tracing study with the adeno‐associated virus vector combined with immunodetection of pre‐ and postsynaptic markers visualized the distribution of the excitatory appositions on PV dendrites. Statistical analysis revealed that the density of thalamostriatal appositions along the dendrites was significantly higher on the proximal than distal dendrites. In contrast, there was no positional preference in the density of appositions from axons of the dorsofrontal cortex. Population observations of thalamostriatal and corticostriatal appositions by immunohistochemistry for pathway‐specific vesicular glutamate transporters confirmed that thalamic inputs preferentially, and cortical ones less preferentially, made apposition on proximal dendrites of PV neurons. This axodendritic organization suggests that PV neurons produce fast and reliable inhibition of MSNs in response to thalamic inputs and process excitatory inputs from motor cortices locally and plastically, possibly together with other GABAergic and dopaminergic dendritic inputs, to modulate MSN inhibition.     

Impaired Refinement of Kinematic Variability in Huntington Disease Mice on an Automated Home Cage Forelimb Motor Task

The effective development of novel therapies in mouse models of neurologic disorders relies on behavioral assessments that provide accurate read-outs of neuronal dysfunction and/or degeneration. We designed an automated behavioral testing system (PiPaw), which integrates an operant lever-pulling task directly into the mouse home cage. This task is accessible to group-housed mice 24 h per day, enabling high-throughput longitudinal analysis of forelimb motor learning. Moreover, this design eliminates the need for exposure to novel environments and minimizes experimenter interaction, significantly reducing two of the largest stressors associated with animal behavior. Male mice improved their performance of this task over 1 week of testing by reducing intertrial variability of reward-related kinematic parameters (pull amplitude or peak velocity). In addition, mice displayed short-term improvements in reward rate, and a concomitant decrease in movement variability, over the course of brief bouts of task engagement. We used this system to assess motor learning in mouse models of the inherited neurodegenerative disorder, Huntington disease (HD). Despite having no baseline differences in task performance, male Q175-FDN HD mice were unable to modulate the variability of their movements to increase reward on either short or long timescales. Task training was associated with a decrease in the amplitude of spontaneous excitatory activity recorded from striatal medium spiny neurons in the hemisphere contralateral to the trained forelimb in WT mice; however, no such changes were observed in Q175-FDN mice. This behavioral screening platform should prove useful for preclinical drug trials toward improved treatments in HD and other neurologic disorders.SIGNIFICANCE STATEMENT In order to develop effective therapies for neurologic disorders, such as Huntington disease (HD), it is important to be able to accurately and reliably assess the behavior of mouse models of these conditions. Moreover, these behavioral assessments should provide an accurate readout of underlying neuronal dysfunction and/or degeneration. In this paper, we used an automated behavioral testing system to assess motor learning in mice within their home cage. Using this system, we were able to study motor abnormalities in HD mice with an unprecedented level of detail, and identified a specific behavioral deficit associated with an underlying impairment in striatal neuronal plasticity. These results validate the usefulness of this system for assessing behavior in mouse models of HD and other neurologic disorders.     

Dopamine, acetylcholine and nitric oxide systems interact to induce corticostriatal synaptic plasticity

Two distinct forms of synaptic plasticity have been described at corticostriatal synapses: long-term depression (LTD) and long-term potentiation (LTP). Both these enduring changes in the efficacy of excitatory neurotransmission in the striatum have a major impact on the physiological activity of the basal ganglia and are triggered by the stimulation of complex and independent cascades of intracellular second messenger systems. Along with the massive glutamatergic inputs originating from the cortex, striatal neurons receive a myriad of other synaptic contacts arising from different sources. In particular, while the nigrostriatal pathway provides this brain area with dopamine (DA), intrinsic circuits are the main source of acetylcholine (ACh) and nitric oxide (NO). The three neurotransmitter systems interact with each other to determine whether corticostriatal LTP or LTD is triggered in response to repetitive synaptic stimulation. Two distinct subtypes of striatal interneurons produce ACh and NO in the striatum. These interneurons are activated by the cortex during the induction phase of striatal plasticity, and stimulate, in turn, the intracellular changes in projection neurons required for LTD or LTP. Interneurons, therefore, exert a feedforward control of the excitability of striatal projection neurons by ensuring the coordinate expression of two alternative forms of synaptic plasticity at the same type of excitatory synapse. The integrative action exerted by striatal projection neurons on the converging information arising from the cortex, nigral DA neurons, and from ACh- and NO-producing interneurons dictates the final output of the striatum to the other structures of the basal ganglia.     

Selective inhibition of striatal fast-spiking interneurons causes dyskinesias

Fast-spiking interneurons (FSIs) can exert powerful control over striatal output, and deficits in this cell population have been observed in human patients with Tourette syndrome and rodent models of dystonia. However, a direct experimental test of striatal FSI involvement in motor control has never been performed. We applied a novel pharmacological approach to examine the behavioral consequences of selective FSI suppression in mouse striatum. IEM-1460, an inhibitor of GluA2-lacking AMPARs, selectively blocked synaptic excitation of FSIs but not striatal projection neurons. Infusion of IEM-1460 into the sensorimotor striatum reduced the firing rate of FSIs but not other cell populations, and elicited robust dystonia-like impairments. These results provide direct evidence that hypofunction of striatal FSIs can produce movement abnormalities, and suggest that they may represent a novel therapeutic target for the treatment of hyperkinetic movement disorders.     

Abnormal sensitivity to cannabinoid receptor stimulation might contribute to altered gamma-aminobutyric acid transmission in the striatum of R6/2 Huntington's disease mice.

BACKGROUND: One of the earliest neurochemical alterations observed in both Huntington's disease (HD) patients and HD animal models is the dysregulation of the endocannabinoid system, an alteration that precedes the development of identifiable striatal neuropathology. How this alteration impacts striatal synaptic transmission is unknown. METHODS: We measured the effects of cannabinoid receptor stimulation on gamma-aminobutyric acid (GABA)-ergic synaptic currents recorded from striatal neurons of R6/2 HD mice in the early phase of their disease. RESULTS: The sensitivity of striatal GABA synapses to cannabinoid receptor stimulation is severely impaired in R6/2 HD mice. In particular, whereas in control animals activation of cannabinoid CB1 receptors results in a significant inhibition of both evoked and spontaneous GABA-mediated synaptic events by a presynaptic mechanism, in R6/2 mice this treatment fails to reduce GABA currents but causes, in contrast, a slight increase of spontaneous inhibitory postsynaptic currents (sIPSCs). CONCLUSIONS: Experimental HD was also associated with enhanced frequency of sIPSCs, a result consistent with the conclusion that loss of cannabinoid-mediated control of GABA transmission might contribute to hyperactivity of GABA synapses in the striatum of HD mice. Accordingly, spontaneous excitatory postsynaptic currents, which were not upregulated in R6/2 mice, were still sensitive to cannabinoid receptor stimulation.     

Mechanisms underlying the enhancement of γ-aminobutyric acid responses in the external globus pallidus of R6/2 Huntington’s disease model mice

In Huntington's disease (HD), the output of striatal indirect pathway medium-sized spiny neurons (MSNs) is altered in its target region, the external globus pallidus (GPe). In a previous study we demonstrated that selective optogenetic stimulation of indirect pathway MSNs induced prolonged decay time of γ-aminobutyric acid (GABA) responses in GPe neurons. Here we identified the mechanism underlying this alteration. Electrophysiological recordings in slices from symptomatic R6/2 and wildtype (WT) mice were used to evaluate, primarily, the effects of GABA transporter (GAT) antagonists on responses evoked by optogenetic activation of indirect pathway MSNs. In addition, immunohistochemistry (IHC) and Western blots (WBs) were used to examine GAT-3 expression in HD and WT mice. A GAT-3 blocker (SNAP5114) increased decay time of GABA responses in WT and HD GPe neurons, but the effect was significantly greater in WT neurons. In contrast, a GAT-1 antagonist (NO-711) or a GABA_B receptor antagonist (CGP 54626) produced small increases in decay time but no differential effects between genotypes. IHC and WBs showed reduction of GAT-3 expression in the GPe of HD mice. Thus, reduced expression or dysfunction of GAT-3 could underlie alterations of GPe responses to GABA inputs from striatum and could be a target for therapeutic intervention.     

Neuronal vulnerability following inhibition of mitochondrial complex II: a possible ionic mechanism for Huntington's disease

An impaired complex II (succinate dehydrogenase, SD) striatal mitochondrial activity is one of the prominent metabolic alterations in Huntington's disease (HD), and intoxication with 3-nitropropionic acid (3-NP), an inhibitor of mitochondrial complex II, mimics the motor abnormalities and the pathology of HD. We found that striatal spiny neurons responded to this toxin with an irreversible membrane depolarization/inward current, while cholinergic interneurons showed a hyperpolarization/outward current. Both these currents were sensitive to intracellular concentration of ATP. The 3-NP-induced depolarization was associated with an increased release of endogenous GABA, while acetylcholine levels were reduced. Moreover, 3-NP induced a higher depolarization in presymptomatic R6/2 HD transgenic mice compared to wild-type (WT) mice, showing an increased susceptibility to SD inhibition. Conversely, the hyperpolarization did not significantly differ from the one recorded in WT mice. The diverse membrane changes induced by SD inhibition may contribute to the cell-type-specific neuronal death in HD.     

Cholinergic Dysfunction Alters Synaptic Integration between Thalamostriatal and Corticostriatal Inputs in DYT1 Dystonia

Projections from thalamic intralaminar nuclei convey sensory signals to striatal cholinergic interneurons. These neurons respond with a pause in their pacemaking activity, enabling synaptic integration with cortical inputs to medium spiny neurons (MSNs), thus playing a crucial role in motor function. In mice with the DYT1 dystonia mutation, stimulation of thalamostriatal axons, mimicking a response to salient events, evoked a shortened pause and triggered an abnormal spiking activity in interneurons. This altered pattern caused a significant rearrangement of the temporal sequence of synaptic activity mediated by M(1) and M(2) muscarinic receptors in MSNs, consisting of an increase in postsynaptic currents and a decrease of presynaptic inhibition, respectively. Consistent with a major role of acetylcholine, either lowering cholinergic tone or antagonizing postsynaptic M(1) muscarinic receptors normalized synaptic activity. Our data demonstrate an abnormal time window for synaptic integration between thalamostriatal and corticostriatal inputs, which might alter the action selection process, thereby predisposing DYT1 gene mutation carriers to develop dystonic movements.     

Enhanced sensitivity to group II mGlu receptor activation at corticostriatal synapses in mice lacking the familial parkinsonism-linked genes PINK1 or Parkin

An altered glutamatergic input at corticostriatal synapses has been shown in experimental models of Parkinson's disease (PD). In the present work, we analyzed the membrane and synaptic responses of striatal neurons to metabotropic glutamate (mGlu) receptor activation in two different mouse models of inherited PD, linked to mutations in PINK1 or Parkin genes.Both in PINK1 and Parkin knockout (^−/−) mice, activation of group I mGlu receptors by 3,5-DHPG caused a membrane depolarization coupled to an increase in firing frequency in striatal cholinergic interneurons that was comparable to the response observed in the respective wild-type (WT) interneurons. The sensitivity to group II and III mGlu receptors was tested on cortically-evoked excitatory postsynaptic potentials (EPSPs) recorded from medium spiny neurons (MSNs). Both LY379268 and L-AP4, agonists for group II and III, respectively, had no effect on intrinsic membrane properties, but dose-dependently reduced the amplitude of corticostriatal EPSPs. However, both in PINK1^−/− and Parkin^−/− mice, LY379268, but not L-AP4, exhibited a greater potency as compared to WT in depressing EPSP amplitude. Accordingly, the dose–response curve for the response to LY379268 in both knockout mice was shifted leftward. Moreover, consistent with a presynaptic site of action, both LY379268 and L-AP4 increased the paired-pulse ratio either in PINK1^−/− and Parkin^−/− or in WT mice. Acute pretreatment with L-dopa did not rescue the enhanced sensitivity to LY379268.Together, these results suggest that the selective increase in sensitivity of striatal group II mGlu receptors represents an adaptive change in mice in which an altered dopamine metabolism has been documented.