Control of spontaneous firing patterns by the selective coupling of calcium currents to calcium-activated potassium currents in striatal cholinergic interneurons

Corticotropin releasing factor alters the functional diversity of accumbal cholinergic interneurons

Cholinergic interneurons (ChIs) provide the main source of acetylcholine in the striatum and have emerged as a critical modulator of behavioral flexibility, motivation, and associative learning. In the dorsal striatum (DS), ChIs display heterogeneous firing patterns. Here, we investigated the spontaneous firing patterns of ChIs in the nucleus accumbens (NAc) shell, a region of the ventral striatum. We identified four distinct ChI firing signatures: regular single-spiking, irregular single-spiking, rhythmic bursting, and a mixed-mode pattern composed of bursting activity and regular single spiking. ChIs from females had lower firing rates compared with males and had both a higher proportion of mixed-mode firing patterns and a lower proportion of regular single-spiking neurons compared with males. We further observed that across the estrous cycle, the diestrus phase was characterized by higher proportions of irregular ChI firing patterns compared with other phases. Using pooled data from males and females, we examined how the stress-associated neuropeptide corticotropin releasing factor (CRF) impacts these firing patterns. ChI firing patterns showed differential sensitivity to CRF. This translated into differential ChI sensitivity to CRF across the estrous cycle. Furthermore, CRF shifted the proportion of ChI firing patterns toward more regular spiking activity over bursting patterns. Finally, we found that repeated stressor exposure altered ChI firing patterns and sensitivity to CRF in the NAc core, but not the NAc shell. These findings highlight the heterogeneous nature of ChI firing patterns, which may have implications for accumbal-dependent motivated behaviors.NEW & NOTEWORTHY Cholinergic interneurons (ChIs) within the dorsal and ventral striatum can exert a major influence on network output and motivated behaviors. However, the firing patterns and neuromodulation of ChIs within the ventral striatum, specifically the nucleus accumbens (NAc) shell, are understudied. Here, we report that NAc shell ChIs have heterogeneous ChI firing patterns that are labile and can be modulated by the stress-linked neuropeptide corticotropin releasing factor (CRF) and by the estrous cycle.

Corticotropin releasing factor alters the functional diversity of accumbal cholinergic interneurons

Cholinergic interneurons (ChIs) provide the main source of acetylcholine in the striatum and have emerged as a critical modulator of behavioral flexibility, motivation, and associative learning. In the dorsal striatum, ChIs display heterogeneous firing patterns. Here, we investigated the spontaneous firing patterns of ChIs in the nucleus accumbens (NAc) shell, a region of the ventral striatum. We identified four distinct ChI firing signatures: regular single-spiking, irregular single-spiking, rhythmic bursting, and a mixed-mode pattern composed of bursting activity and regular single spiking. ChIs from females had lower firing rates compared to males and had both a higher proportion of mixed-mode firing patterns and a lower proportion of regular single-spiking neurons compared to males. We further observed that across the estrous cycle, the diestrus phase was characterized by higher proportions of irregular ChI firing patterns compared to other phases. Using pooled data from males and females, we examined how the stress-associated neuropeptide corticotropin releasing factor (CRF) impacts these firing patterns. ChI firing patterns showed differential sensitivity to CRF. This translated into differential ChI sensitivity to CRF across the estrous cycle. Furthermore, CRF shifted the proportion of ChI firing patterns toward more regular spiking activity over bursting patterns. Finally, we found that repeated stressor exposure altered ChI firing patterns and sensitivity to CRF in the NAc core, but not the NAc shell. These findings highlight the heterogeneous nature of ChI firing patterns, which may have implications for accumbal-dependent motivated behaviors.

Large conductance voltage-and calcium-activated K+ (BK) channel in health and disease

Large Conductance Voltage- and Calcium-activated K+ (BK) channels are transmembrane pore-forming proteins that regulate cell excitability and are also expressed in non-excitable cells. They play a role in regulating vascular tone, neuronal excitability, neurotransmitter release, and muscle contraction. Dysfunction of the BK channel can lead to arterial hypertension, hearing disorders, epilepsy, and ataxia. Here, we provide an overview of BK channel functioning and the implications of its abnormal functioning in various diseases. Understanding the function of BK channels is crucial for comprehending the mechanisms involved in regulating vital physiological processes, both in normal and pathological conditions, controlled by BK. This understanding may lead to the development of therapeutic interventions to address BK channelopathies.

Cognitive effects of thalamostriatal degeneration are ameliorated by normalizing striatal cholinergic activity

The loss of neurons in parafascicular thalamus (Pf) and their inputs to dorsomedial striatum (DMS) in Lewy body disease (LBD) and Parkinson's disease dementia (PDD) have been linked to the effects of neuroinflammation. We found that, in rats, these inputs were necessary for both the function of striatal cholinergic interneurons (CINs) and the flexible encoding of the action-outcome (AO) associations necessary for goal-directed action, producing a burst-pause pattern of CIN firing but only during the remapping elicited by a shift in AO contingency. Neuroinflammation in the Pf abolished these changes in CIN activity and goal-directed control after the shift in contingency. However, both effects were rescued by either the peripheral or the intra-DMS administration of selegiline, a monoamine oxidase B inhibitor that we found also enhances adenosine triphosphatase activity in CINs. These findings suggest a potential treatment for the cognitive deficits associated with neuroinflammation affecting the function of the Pf and related structures.

Accumbal Histamine Signaling Engages Discrete Interneuron Microcircuits

BACKGROUND

Central histamine (HA) signaling modulates diverse cortical and subcortical circuits throughout the brain, including the nucleus accumbens (NAc). The NAc, a key striatal subregion directing reward-related behavior, expresses diverse HA receptor subtypes that elicit cellular and synaptic plasticity. However, the neuromodulatory capacity of HA within interneuron microcircuits in the NAc remains unknown.

METHODS

We combined electrophysiology, pharmacology, voltammetry, and optogenetics in male transgenic reporter mice to determine how HA influences microcircuit motifs controlled by parvalbumin-expressing fast-spiking interneurons (PV-INs) and tonically active cholinergic interneurons (CINs) in the NAc shell.

RESULTS

HA enhanced CIN output through an H2 receptor (H2R)-dependent effector pathway requiring Ca2+-activated small-conductance K+ channels, with a small but discernible contribution from H1Rs and synaptic H3Rs. While PV-IN excitability was unaffected by HA, presynaptic H3Rs decreased feedforward drive onto PV-INs via AC-cAMP-PKA (adenylyl cyclase-cyclic adenosine monophosphate-protein kinase A) signaling. H3R-dependent plasticity was differentially expressed at mediodorsal thalamus and prefrontal cortex synapses onto PV-INs, with mediodorsal thalamus synapses undergoing HA-induced long-term depression. These effects triggered downstream shifts in PV-IN- and CIN-controlled microcircuits, including near-complete collapse of mediodorsal thalamus-evoked feedforward inhibition and increased mesoaccumbens dopamine release.

CONCLUSIONS

HA targets H1R, H2R, and H3Rs in the NAc shell to engage synapse- and cell type-specific mechanisms that bidirectionally regulate PV-IN and CIN microcircuit activity. These findings extend the current conceptual framework of HA signaling and offer critical insight into the modulatory potential of HA in the brain.

A spiking computational model for striatal cholinergic interneurons

Cholinergic interneurons in the striatum, also known as tonically active interneurons or TANs, are thought to have a strong effect on corticostriatal plasticity and on striatal activity and outputs, which in turn play a critical role in modulating downstream basal ganglia activity and movement. Striatal TANs can exhibit a variety of firing patterns and responses to synaptic inputs; furthermore, they have been found to display various surges and pauses in activity associated with sensory cues and reward delivery in learning as well as with motor tic production. To help explain the factors that contribute to TAN activity patterns and to provide a resource for future studies, we present a novel conductance-based computational model of a striatal TAN. We show that this model produces the various characteristic firing patterns observed in recordings of TANs. With a single baseline tuning associated with tonic firing, the model also captures a wide range of TAN behaviors found in previous experiments involving a variety of manipulations. In addition to demonstrating these results, we explain how various ionic currents in the model contribute to them. Finally, we use this model to explore the contributions of the acetylcholine released by TANs to the production of surges and pauses in TAN activity in response to strong excitatory inputs. These results provide predictions for future experimental testing that may help with efforts to advance our understanding of the role of TANs in reinforcement learning and in motor disorders such as Tourette's syndrome.

ACh Transfers: Homeostatic Plasticity of Cholinergic Synapses

The field of homeostatic plasticity continues to advance rapidly, highlighting the importance of stabilizing neuronal activity within functional limits in the context of numerous fundamental processes such as development, learning, and memory. Most homeostatic plasticity studies have been focused on glutamatergic synapses, while the rules that govern homeostatic regulation of other synapse types are less understood. While cholinergic synapses have emerged as a critical component in the etiology of mammalian neurodegenerative disease mechanisms, relatively few studies have been conducted on the homeostatic plasticity of such synapses, particularly in the mammalian nervous system. An exploration of homeostatic mechanisms at the cholinergic synapse may illuminate potential therapeutic targets for disease management and treatment. We will review cholinergic homeostatic plasticity in the mammalian neuromuscular junction, the autonomic nervous system, central synapses, and in relation to pathological conditions including Alzheimer disease and DYT1 dystonia. This work provides a historical context for the field of cholinergic homeostatic regulation by examining common themes, unique features, and outstanding questions associated with these distinct cholinergic synapse types and aims to inform future research in the field.

Motor deficit and lack of overt dystonia in Dlx conditional Dyt1 knockout mice

DYT1 or DYT-TOR1A dystonia is early-onset generalized dystonia caused by a trinucleotide deletion of GAG in the TOR1A or DYT1 gene leads to the loss of a glutamic acid residue in the resulting torsinA protein. A mouse model with overt dystonia is of unique importance to better understand the DYT1 pathophysiology and evaluate preclinical drug efficacy. DYT1 dystonia is likely a network disorder involving multiple brain regions, particularly the basal ganglia. Tor1a conditional knockout in the striatum or cerebral cortex leads to motor deficits, suggesting the importance of corticostriatal connection in the pathogenesis of dystonia. Indeed, corticostriatal long-term depression impairment has been demonstrated in multiple targeted DYT1 mouse models. Pappas and colleagues developed a conditional knockout line (Dlx-CKO) that inactivated Tor1a in the forebrain and surprisingly displayed overt dystonia. We set out to validate whether conditional knockout affecting both cortex and striatum would lead to overt dystonia and whether machine learning-based video behavioral analysis could be used to facilitate high throughput preclinical drug screening. We generated Dlx-CKO mice and found no overt dystonia or motor deficits at 4 months. At 8 months, retesting revealed motor deficits in rotarod, beam walking, grip strength, and hyperactivity in the open field; however, no overt dystonia was visually discernible or through the machine learning-based video analysis. Consistent with other targeted DYT1 mouse models, we observed age-dependent deficits in the beam walking test, which is likely a better motor behavioral test for preclinical drug testing but more labor-intensive when overt dystonia is absent.

Ca2+- and Voltage-Activated K+ (BK) Channels in the Nervous System: One Gene, a Myriad of Physiological Functions

BK channels are large conductance potassium channels characterized by four pore-forming α subunits, often co-assembled with auxiliary β and γ subunits to regulate Ca2+ sensitivity, voltage dependence and gating properties. BK channels are abundantly expressed throughout the brain and in different compartments within a single neuron, including axons, synaptic terminals, dendritic arbors, and spines. Their activation produces a massive efflux of K+ ions that hyperpolarizes the cellular membrane. Together with their ability to detect changes in intracellular Ca2+ concentration, BK channels control neuronal excitability and synaptic communication through diverse mechanisms. Moreover, increasing evidence indicates that dysfunction of BK channel-mediated effects on neuronal excitability and synaptic function has been implicated in several neurological disorders, including epilepsy, fragile X syndrome, mental retardation, and autism, as well as in motor and cognitive behavior. Here, we discuss current evidence highlighting the physiological importance of this ubiquitous channel in regulating brain function and its role in the pathophysiology of different neurological disorders.

Developmental regulation of thalamus-driven pauses in striatal cholinergic interneurons

In response to salient sensory cues, the tonically active striatal cholinergic interneuron (ChI) exhibits a characteristic synchronized "pause" thought to facilitate learning and the execution of motivated behavior. We report that thalamostriatal-driven ChI pauses are enhanced in ex vivo brain slices from infantile (P10) mice, with decreasing expression in preadolescent (P28) and adult (P100) mice concurrent with waning excitatory input to ChIs. Our data are consistent with previous reports that the adult ChI pause is dependent on dopamine signaling, but we find that the robust pausing at P10 is dopamine independent. Instead, elevated expression of the noninactivating delayed rectifier Kv7.2/3 current promotes pausing in infantile ChIs. Because this current decreases over development, a parallel increase in Ih further attenuates pause expression. These findings demonstrate that cell intrinsic and circuit mechanisms of ChI pause expression are developmentally determined and may underlie changes in learning properties as the nervous system matures.

Synaptic determinants of cholinergic interneurons hyperactivity during parkinsonism

Parkinson’s disease is a neurodegenerative ailment generated by the loss of dopamine in the basal ganglia, mainly in the striatum. The disease courses with increased striatal levels of acetylcholine, disrupting the balance among these modulatory transmitters. These modifications disturb the excitatory and inhibitory balance in the striatal circuitry, as reflected in the activity of projection striatal neurons. In addition, changes in the firing pattern of striatal tonically active interneurons during the disease, including cholinergic interneurons (CINs), are being searched. Dopamine-depleted striatal circuits exhibit pathological hyperactivity as compared to controls. One aim of this study was to show how striatal CINs contribute to this hyperactivity. A second aim was to show the contribution of extrinsic synaptic inputs to striatal CINs hyperactivity. Electrophysiological and calcium imaging recordings in Cre-mice allowed us to evaluate the activity of dozens of identified CINs with single-cell resolution in ex vivo brain slices. CINs show hyperactivity with bursts and silences in the dopamine-depleted striatum. We confirmed that the intrinsic differences between the activity of control and dopamine-depleted CINs are one source of their hyperactivity. We also show that a great part of this hyperactivity and firing pattern change is a product of extrinsic synaptic inputs, targeting CINs. Both glutamatergic and GABAergic inputs are essential to sustain hyperactivity. In addition, cholinergic transmission through nicotinic receptors also participates, suggesting that the joint activity of CINs drives the phenomenon; since striatal CINs express nicotinic receptors, not expressed in striatal projection neurons. Therefore, CINs hyperactivity is the result of changes in intrinsic properties and excitatory and inhibitory inputs, in addition to the modification of local circuitry due to cholinergic nicotinic transmission. We conclude that CINs are the main drivers of the pathological hyperactivity present in the striatum that is depleted of dopamine, and this is, in part, a result of extrinsic synaptic inputs. These results show that CINs may be a main therapeutic target to treat Parkinson’s disease by intervening in their synaptic inputs.

BK channel properties correlate with neurobehavioral severity in three KCNMA1-linked channelopathy mouse models

KCNMA1 forms the pore of BK K+ channels, which regulate neuronal and muscle excitability. Recently, genetic screening identified heterozygous KCNMA1 variants in a subset of patients with debilitating paroxysmal non-kinesigenic dyskinesia, presenting with or without epilepsy (PNKD3). However, the relevance of KCNMA1 mutations and the basis for clinical heterogeneity in PNKD3 has not been established. Here, we evaluate the relative severity of three KCNMA1 patient variants in BK channels, neurons, and mice. In heterologous cells, BKN999S and BKD434G channels displayed gain-of-function (GOF) properties, whereas BKH444Q channels showed loss-of-function (LOF) properties. The relative degree of channel activity was BKN999S > BKD434G>WT > BKH444Q. BK currents and action potential firing were increased, and seizure thresholds decreased, in Kcnma1N999S/WT and Kcnma1D434G/WT transgenic mice but not Kcnma1H444Q/WT mice. In a novel behavioral test for paroxysmal dyskinesia, the more severely affected Kcnma1N999S/WT mice became immobile after stress. This was abrogated by acute dextroamphetamine treatment, consistent with PNKD3-affected individuals. Homozygous Kcnma1D434G/D434G mice showed similar immobility, but in contrast, homozygous Kcnma1H444Q/H444Q mice displayed hyperkinetic behavior. These data establish the relative pathogenic potential of patient alleles as N999S>D434G>H444Q and validate Kcnma1N999S/WT mice as a model for PNKD3 with increased seizure propensity.

Non-uniform distribution of dendritic nonlinearities differentially engages thalamostriatal and corticostriatal inputs onto cholinergic interneurons

The tonic activity of striatal cholinergic interneurons (CINs) is modified differentially by their afferent inputs. Although their unitary synaptic currents are identical, in most CINs cortical inputs onto distal dendrites only weakly entrain them, whereas proximal thalamic inputs trigger abrupt pauses in discharge in response to salient external stimuli. To test whether the dendritic expression of the active conductances that drive autonomous discharge contribute to the CINs’ capacity to dissociate cortical from thalamic inputs, we used an optogenetics-based method to quantify dendritic excitability in mouse CINs. We found that the persistent sodium (NaP) current gave rise to dendritic boosting, and that the hyperpolarization-activated cyclic nucleotide-gated (HCN) current gave rise to a subhertz membrane resonance. This resonance may underlie our novel finding of an association between CIN pauses and internally-generated slow wave events in sleeping non-human primates. Moreover, our method indicated that dendritic NaP and HCN currents were preferentially expressed in proximal dendrites. We validated the non-uniform distribution of NaP currents: pharmacologically; with two-photon imaging of dendritic back-propagating action potentials; and by demonstrating boosting of thalamic, but not cortical, inputs by NaP currents. Thus, the localization of active dendritic conductances in CIN dendrites mirrors the spatial distribution of afferent terminals and may promote their differential responses to thalamic vs. cortical inputs.

Electrophysiological characterization of the striatal cholinergic interneurons in Dyt1 ΔGAG knock-in mice

DYT1 dystonia is an inherited early-onset movement disorder characterized by sustained muscle contractions causing twisting, repetitive movements, and abnormal postures. Most DYT1 patients have a heterozygous trinucleotide GAG deletion mutation (ΔGAG) in DYT1/TOR1A, coding for torsinA. Dyt1 heterozygous ΔGAG knock-in (KI) mice show motor deficits and reduced striatal dopamine receptor 2 (D2R). Striatal cholinergic interneurons (ChIs) are essential in regulating striatal motor circuits. Multiple dystonia rodent models, including KI mice, show altered ChI firing and modulation. However, due to the errors in assigning KI mice, it is essential to replicate these findings in genetically confirmed KI mice. Here, we found irregular and decreased spontaneous firing frequency in the acute brain slices from Dyt1 KI mice. Quinpirole, a D2R agonist, showed less inhibitory effect on the spontaneous ChI firing in Dyt1 KI mice, suggesting decreased D2R function on the striatal ChIs. On the other hand, a muscarinic receptor agonist, muscarine, inhibited the ChI firing in both wild-type (WT) and Dyt1 KI mice. Trihexyphenidyl, a muscarinic acetylcholine receptor M1 antagonist, had no significant effect on the firing. Moreover, the resting membrane property and functions of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, μ-opioid receptors, and large-conductance calcium-activated potassium (BK) channels were unaffected in Dyt1 KI mice. The results suggest that the irregular and low-frequency firing and decreased D2R function are the main alterations of striatal ChIs in Dyt1 KI mice. These results appear consistent with the reduced dopamine release and high striatal acetylcholine tone in the previous reports.

Age-related changes in medial septal cholinergic and GABAergic projection neurons and hippocampal neurotransmitter receptors: relationship with memory impairment

The hippocampus, which provides cognitive functions, has been shown to become highly vulnerable during aging. One important modulator of the hippocampal neural network is the medial septum (MS). The present study attempts to determine how age-related mnemonic dysfunction is associated with neurochemical changes in the septohippocampal (SH) system, using behavioral and immunochemical experiments performed on young-adult, middle-aged and aged rats. According to these behavioral results, the aged and around 52.8% of middle-aged rats (within the "middle-aged-impaired" sub-group) showed both impaired spatial reference memory in the Morris water maze and habituation in the open field. Immunohistochemical studies revealed a significant decrease in the number of MS choline acetyltransferase immunoreactive cells in the aged and all middle-aged rats, in comparison to the young; however the number of gamma-aminobutyric acid-ergic (GABAergic) parvalbumin immunoreactive cells was higher in middle-aged-impaired and older rats compared to young and middle-aged-unimpaired rats. Western Blot analysis moreover showed a decrease in the level of expression of cholinergic, GABAergic and glutamatergic receptors in the hippocampus of middle-aged-impaired and aged rats in contrast to middle-aged-unimpaired and young rats. The present results demonstrate for the first time that a decrease in the expression level of hippocampal receptors in naturally aged rats with impaired cognitive abilities occurs in parallel with an increase in the number of GABAergic neurons in the MS, and it highlights the particular importance of inhibitory signaling in the SH network for memory function.

Structural and Functional Coupling of Calcium-Activated BK Channels and Calcium-Permeable Channels Within Nanodomain Signaling Complexes

Biochemical and functional studies of ion channels have shown that many of these integral membrane proteins form macromolecular signaling complexes by physically associating with many other proteins. These macromolecular signaling complexes ensure specificity and proper rates of signal transduction. The large-conductance, Ca²⁺-activated K⁺ (BK) channel is dually activated by membrane depolarization and increases in intracellular free Ca²⁺ ([Ca²⁺]_i). The activation of BK channels results in a large K⁺ efflux and, consequently, rapid membrane repolarization and closing of the voltage-dependent Ca²⁺-permeable channels to limit further increases in [Ca²⁺]_i. Therefore, BK channel-mediated K⁺ signaling is a negative feedback regulator of both membrane potential and [Ca²⁺]_i and plays important roles in many physiological processes and diseases. However, the BK channel formed by the pore-forming and voltage- and Ca²⁺-sensing α subunit alone requires high [Ca²⁺]_i levels for channel activation under physiological voltage conditions. Thus, most native BK channels are believed to co-localize with Ca²⁺-permeable channels within nanodomains (a few tens of nanometers in distance) to detect high levels of [Ca²⁺]_i around the open pores of Ca²⁺-permeable channels. Over the last two decades, advancement in research on the BK channel’s coupling with Ca²⁺-permeable channels including recent reports involving NMDA receptors demonstrate exemplary models of nanodomain structural and functional coupling among ion channels for efficient signal transduction and negative feedback regulation. We hereby review our current understanding regarding the structural and functional coupling of BK channels with different Ca²⁺-permeable channels.

A familiar study on self-limited childhood epilepsy patients using hIPSC-derived neurons shows a bias towards immaturity at the morphological, electrophysiological and gene expression levels

Background

Self-limited Childhood Epilepsies are the most prevalent epileptic syndrome in children. Its pathogenesis is unknown. In this disease, symptoms resolve spontaneously in approximately 50% of patients when maturity is reached, prompting to a maturation problem. The purpose of this study was to understand the molecular bases of this disease by generating and analyzing induced pluripotent stem cell-derived neurons from a family with 7 siblings, among whom 4 suffer from this disease.

Methods

Two affected siblings and, as controls, a healthy sister and the unaffected mother of the family were studied. Using exome sequencing, a homozygous variant in the FYVE, RhoGEF and PH Domain Containing 6 gene was identified in the patients as a putative genetic factor that could contribute to the development of this familial disorder. After informed consent was signed, skin biopsies from the 4 individuals were collected, fibroblasts were derived and reprogrammed and neurons were generated and characterized by markers and electrophysiology. Morphological, electrophysiological and gene expression analyses were performed on these neurons.

Results

Bona fide induced pluripotent stem cells and derived neurons could be generated in all cases. Overall, there were no major shifts in neuronal marker expression among patient and control-derived neurons. Compared to two familial controls, neurons from patients showed shorter axonal length, a dramatic reduction in synapsin-1 levels and cytoskeleton disorganization. In addition, neurons from patients developed a lower action potential threshold with time of in vitro differentiation and the amount of current needed to elicit an action potential (rheobase) was smaller in cells recorded from NE derived from patients at 12 weeks of differentiation when compared with shorter times in culture. These results indicate an increased excitability in patient cells that emerges with the time in culture. Finally, functional genomic analysis showed a biased towards immaturity in patient-derived neurons.

Conclusions

We are reporting the first in vitro model of self-limited childhood epilepsy, providing the cellular bases for future in-depth studies to understand its pathogenesis. Our results show patient-specific neuronal features reflecting immaturity, in resonance with the course of the disease and previous imaging studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02658-2.

Dopaminergic co-transmission with sonic hedgehog inhibits abnormal involuntary movements in models of Parkinson's disease and L-Dopa induced dyskinesia

L-Dopa induced dyskinesia (LID) is a debilitating side effect of dopamine replacement therapy for Parkinson’s Disease. The mechanistic underpinnings of LID remain obscure. Here we report that diminished sonic hedgehog (Shh) signaling in the basal ganglia caused by the degeneration of midbrain dopamine neurons facilitates the formation and expression of LID. We find that the pharmacological activation of Smoothened, a downstream effector of Shh, attenuates LID in the neurotoxic 6-OHDA- and genetic aphakia mouse models of Parkinson’s Disease. Employing conditional genetic loss-of-function approaches, we show that reducing Shh secretion from dopamine neurons or Smoothened activity in cholinergic interneurons promotes LID. Conversely, the selective expression of constitutively active Smoothened in cholinergic interneurons is sufficient to render the sensitized aphakia model of Parkinson’s Disease resistant to LID. Furthermore, acute depletion of Shh from dopamine neurons through prolonged optogenetic stimulation in otherwise intact mice and in the absence of L-Dopa produces LID-like involuntary movements. These findings indicate that augmenting Shh signaling in the L-Dopa treated brain may be a promising therapeutic approach for mitigating the dyskinetic side effects of long-term treatment with L-Dopa.

Lauren Malave et al. examine the impact of sonic hedgehog signaling in the dorsal striatum in L-Dopa induced dyskinesia (LID) animal models. Their results suggest that increasing sonic hedgehog signaling can reduce the severity of LID and abnormal involuntary movements, suggesting future therapeutic approaches to mitigate dyskinetic comorbidities of long-term treatment with L-Dopa.

BK channel activation by L-type Ca2+ channels CaV1.2 and CaV1.3 during the subthreshold phase of an action potential

Mammalian circadian (24 h) rhythms are timed by the pattern of spontaneous action potential firing in the suprachiasmatic nucleus (SCN). This oscillation in firing is produced through circadian regulation of several membrane currents, including large-conductance Ca²⁺- and voltage-activated K⁺ (BK) and L-type Ca²⁺ channel (LTCC) currents. During the day steady-state BK currents depend mostly on LTCCs for activation, whereas at night they depend predominantly on ryanodine receptors (RyRs). However, the contribution of these Ca²⁺ channels to BK channel activation during action potential firing has not been thoroughly investigated. In this study, we used a pharmacological approach to determine that both LTCCs and RyRs contribute to the baseline membrane potential of SCN action potential waveforms, as well as action potential-evoked BK current, during the day and night, respectively. Since the baseline membrane potential is a major determinant of circadian firing rate, we focused on the LTCCs contributing to low voltage activation of BK channels during the subthreshold phase. For these experiments, two LTCC subtypes found in SCN (Ca_V1.2 and Ca_V1.3) were coexpressed with BK channels in heterologous cells, where their differential contributions could be separately measured. Ca_V1.3 channels produced currents that were shifted to more hyperpolarized potentials compared with Ca_V1.2, resulting in increased subthreshold Ca²⁺ and BK currents during an action potential command. These results show that although multiple Ca²⁺ sources in SCN can contribute to the activation of BK current during an action potential, specific BK-Ca_V1.3 partnerships may optimize the subthreshold BK current activation that is critical for firing rate regulation.NEW & NOTEWORTHY BK K⁺ channels are important regulators of firing. Although Ca²⁺ channels are required for their activation in excitable cells, it is not well understood how BK channels activate using these Ca²⁺ sources during an action potential. This study demonstrates the differences in BK current activated by Ca_V1.2 and Ca_V1.3 channels in clock neurons and heterologous cells. The results define how specific ion channel partnerships can be engaged during distinct phases of the action potential.

Physiology and Therapeutic Potential of SK, H, and M Medium AfterHyperPolarization Ion Channels

SK, HCN, and M channels are medium afterhyperpolarization (mAHP)-mediating ion channels. The three channels co-express in various brain regions, and their collective action strongly influences cellular excitability. However, significant diversity exists in the expression of channel isoforms in distinct brain regions and various subcellular compartments, which contributes to an equally diverse set of specific neuronal functions. The current review emphasizes the collective behavior of the three classes of mAHP channels and discusses how these channels function together although they play specialized roles. We discuss the biophysical properties of these channels, signaling pathways that influence the activity of the three mAHP channels, various chemical modulators that alter channel activity and their therapeutic potential in treating various neurological anomalies. Additionally, we discuss the role of mAHP channels in the pathophysiology of various neurological diseases and how their modulation can alleviate some of the symptoms.

Genomic Action of Sigma-1 Receptor Chaperone Relates to Neuropathic Pain

Sigma-1 receptors (Sig-1Rs) are endoplasmic reticulum (ER) chaperones implicated in neuropathic pain. Here we examine if the Sig-1R may relate to neuropathic pain at the level of dorsal root ganglia (DRG). We focus on the neuronal excitability of DRG in a "spare nerve injury" (SNI) model of neuropathic pain in rats and find that Sig-1Rs likely contribute to the genesis of DRG neuronal excitability by decreasing the protein level of voltage-gated Cav2.2 as a translational inhibitor of mRNA. Specifically, during SNI, Sig-1Rs translocate from ER to the nuclear envelope via a trafficking protein Sec61β. At the nucleus, the Sig-1R interacts with cFos and binds to the promoter of 4E-BP1, leading to an upregulation of 4E-BP1 that binds and prevents eIF4E from initiating the mRNA translation for Cav2.2. Interestingly, in Sig-1R knockout HEK cells, Cav2.2 is upregulated. In accordance with those findings, we find that intra-DRG injection of Sig-1R agonist (+)pentazocine increases frequency of action potentials via regulation of voltage-gated Ca2+ channels. Conversely, intra-DRG injection of Sig-1R antagonist BD1047 attenuates neuropathic pain. Hence, we discover that the Sig-1R chaperone causes neuropathic pain indirectly as a translational inhibitor.

Mechanisms of Antiparkinsonian Anticholinergic Therapy Revisited

Before the advent of L-DOPA, the gold standard symptomatic therapy for Parkinson's disease (PD), anticholinergic drugs (muscarinic receptor antagonists) were the preferred antiparkinsonian therapy, but their unwanted side effects associated with impaired extrastriatal cholinergic function limited their clinical utility. Since most patients treated with L-DOPA also develop unwanted side effects such as L-DOPA-induced dyskinesia (LID), better therapies are needed. Recent studies in animal models demonstrate that optogenetic and chemogenetic manipulation of striatal cholinergic interneurons (SCIN), the main source of striatal acetylcholine, modulate parkinsonism and LID, suggesting that restoring SCIN function might serve as a therapeutic option that avoids extrastriatal anticholinergics' side effects. However, it is still unclear how the altered SCIN activity in PD and LID affects the striatal circuit, whereas the mechanisms of action of anticholinergic drugs are still not fully understood. Recent animal model studies showing that SCINs undergo profound changes in their tonic discharge pattern after chronic L-DOPA administration call for a reexamination of classical views of how SCINs contribute to PD symptoms and LID. Here, we review the recent advances on the circuit implications of aberrant striatal cholinergic signaling in PD and LID in an effort to provide a comprehensive framework to understand the effects of anticholinergic drugs and with the aim of shedding light into future perspectives of cholinergic circuit-based therapies.

Cholinergic neurons constitutively engage the ISR for dopamine modulation and skill learning in mice

The integrated stress response (ISR) maintains proteostasis by modulating protein synthesis and is important in synaptic plasticity, learning, and memory. We developed a reporter, SPOTlight, for brainwide imaging of ISR state with cellular resolution. Unexpectedly, we found a class of neurons in mouse brain, striatal cholinergic interneurons (CINs), in which the ISR was activated at steady state. Genetic and pharmacological manipulations revealed that ISR signaling was necessary in CINs for normal type 2 dopamine receptor (D2R) modulation. Inhibiting the ISR inverted the sign of D2R modulation of CIN firing and evoked dopamine release and altered skill learning. Thus, a noncanonical, steady-state mode of ISR activation is found in CINs, revealing a neuromodulatory role for the ISR in learning.

Recurrent Implication of Striatal Cholinergic Interneurons in a Range of Neurodevelopmental, Neurodegenerative, and Neuropsychiatric Disorders

Cholinergic interneurons are "gatekeepers" for striatal circuitry and play pivotal roles in attention, goal-directed actions, habit formation, and behavioral flexibility. Accordingly, perturbations to striatal cholinergic interneurons have been associated with many neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. The role of acetylcholine in many of these disorders is well known, but the use of drugs targeting cholinergic systems fell out of favor due to adverse side effects and the introduction of other broadly acting compounds. However, in response to recent findings, re-examining the mechanisms of cholinergic interneuron dysfunction may reveal key insights into underlying pathogeneses. Here, we provide an update on striatal cholinergic interneuron function, connectivity, and their putative involvement in several disorders. In doing so, we aim to spotlight recurring physiological themes, circuits, and mechanisms that can be investigated in future studies using new tools and approaches.

Patch-clamp and multi-electrode array electrophysiological analysis in acute mouse brain slices

Patch-clamp and multi-electrode array electrophysiology techniques are used to measure dynamic functional properties of neurons. Whole-cell and cell-attached patch-clamp recordings in brain slices can be performed in voltage-clamp and current-clamp configuration to reveal cell-type-specific synaptic and cellular parameters governing neurotransmission. Multi-electrode array electrophysiology can provide spike activity recordings from multiple neurons, enabling larger sample sizes, and long-term recordings. We provide our guide to preparing acute rodent brain slices with example experiments and analyses intended for novice and expert electrophysiologists.

For complete details on the use and execution of this protocol, please refer to Manz et al. (2020b).

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Viable and efficient preparation of mouse brain tissue

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Comprehensive material source for acute brain slice electrophysiology

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Detailed approach to whole-cell patch-clamp recording of neurons

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Utility and application of multi-electrode array electrophysiology in mouse brain

Striatal cholinergic transmission. Focus on nicotinic receptors' influence in striatal circuits

The critical role of acetylcholine (ACh) in the basal ganglia is evident from the effect of cholinergic agents in patients suffering from several related neurological disorders, such as Parkinson's disease, Tourette syndrome, or dystonia. The striatum possesses the highest density of ACh markers in the basal ganglia underlying the importance of ACh in this structure. Striatal cholinergic interneurons (CINs) are responsible for the bulk of striatal ACh, although extrinsic cholinergic afferents from brainstem structures may also play a role. CINs are tonically active, and synchronized pause in their activity occurs following the presentation of salient stimuli during behavioral conditioning. However, the synaptic mechanisms involved are not fully understood in this physiological response. ACh modulates striatal circuits by acting on muscarinic and nicotinic receptors existing in several combinations both presynaptically and postsynaptically. While the effects of ACh in the striatum through muscarinic receptors have received particular attention, nicotinic receptors function has been less studied. Here, after briefly reviewing relevant results regarding muscarinic receptors expression and function, I will focus on striatal nicotinic receptor expressed presynaptically on glutamatergic and dopaminergic afferents and postsynaptically on diverse striatal interneurons populations. I will also review recent evidence suggesting the involvement of different GABAergic sources in two distinct nicotinic-receptor-mediated striatal circuits: the disynaptic inhibition of striatal projection neurons and the recurrent inhibition among CINs. A better understanding of striatal nicotinic receptors expression and function may help to develop targeted pharmacological interventions to treat brain disorders such as Parkinson's disease, Tourette syndrome, dystonia, or nicotine addiction.

Polysynaptic inhibition between striatal cholinergic interneurons shapes their network activity patterns in a dopamine-dependent manner

Striatal activity is dynamically modulated by acetylcholine and dopamine, both of which are essential for basal ganglia function. Synchronized pauses in the activity of striatal cholinergic interneurons (ChINs) are correlated with elevated activity of midbrain dopaminergic neurons, whereas synchronous firing of ChINs induces local release of dopamine. The mechanisms underlying ChIN synchronization and its interplay with dopamine release are not fully understood. Here we show that polysynaptic inhibition between ChINs is a robust network motif and instrumental in shaping the network activity of ChINs. Action potentials in ChINs evoke large inhibitory responses in multiple neighboring ChINs, strong enough to suppress their tonic activity. Using a combination of optogenetics and chemogenetics we show the involvement of striatal tyrosine hydroxylase-expressing interneurons in mediating this inhibition. Inhibition between ChINs is attenuated by dopaminergic midbrain afferents acting presynaptically on D2 receptors. Our results present a novel form of interaction between striatal dopamine and acetylcholine dynamics.

Alterations in the intrinsic properties of striatal cholinergic interneurons after dopamine lesion and chronic L-DOPA

Changes in striatal cholinergic interneuron (ChI) activity are thought to contribute to Parkinson's disease pathophysiology and dyskinesia from chronic L-3,4-dihydroxyphenylalanine (L-DOPA) treatment, but the physiological basis of these changes is unknown. We find that dopamine lesion decreases the spontaneous firing rate of ChIs, whereas chronic treatment with L-DOPA of lesioned mice increases baseline ChI firing rates to levels beyond normal activity. The effect of dopamine loss on ChIs was due to decreased currents of both hyperpolarization-activated cyclic nucleotide-gated (HCN) and small conductance calcium-activated potassium (SK) channels. L-DOPA reinstatement of dopamine normalized HCN activity, but SK current remained depressed. Pharmacological blockade of HCN and SK activities mimicked changes in firing, confirming that these channels are responsible for the molecular adaptation of ChIs to dopamine loss and chronic L-DOPA treatment. These findings suggest that targeting ChIs with channel-specific modulators may provide therapeutic approaches for alleviating L-DOPA-induced dyskinesia in PD patients.

Fast spiking interneuron activity in primate striatum tracks learning of attention cues

Cognitive flexibility depends on a fast neural learning mechanism for enhancing momentary relevant over irrelevant information. A possible neural mechanism realizing this enhancement uses fast spiking interneurons (FSIs) in the striatum to train striatal projection neurons to gate relevant and suppress distracting cortical inputs. We found support for such a mechanism in nonhuman primates during the flexible adjustment of visual attention in a reversal learning task. FSI activity was modulated by visual attention cues during feature-based learning. One FSI subpopulation showed stronger activation during learning, while another FSI subpopulation showed response suppression after learning, which could indicate a disinhibitory effect on the local circuit. Additionally, FSIs that showed response suppression to learned attention cues were activated by salient distractor events, suggesting they contribute to suppressing bottom-up distraction. These findings suggest that striatal fast spiking interneurons play an important role when cues are learned that redirect attention away from previously relevant to newly relevant visual information. This cue-specific activity was independent of motor-related activity and thus tracked specifically the learning of reward predictive visual features.

The role of L-type calcium channels in neuronal excitability and aging

Over the last two decades there has been significant progress towards understanding the neural substrates that underlie age-related cognitive decline. Although many of the exact molecular and cellular mechanisms have yet to be fully understood, there is consensus that alterations in neuronal calcium homeostasis contribute to age-related deficits in learning and memory. Furthermore, it is thought that the age-related changes in calcium homeostasis are driven, at least in part, by changes in calcium channel expression. In this review, we focus on the role of a specific class of calcium channels: L-type voltage-gated calcium channels (LVGCCs). We provide the reader with a general introduction to voltage-gated calcium channels, followed by a more detailed description of LVGCCs and how they serve to regulate neuronal excitability via the post burst afterhyperpolarization (AHP). We conclude by reviewing studies that link the slow component of the AHP to learning and memory, and discuss how age-related increases in LVGCC expression may underlie cognitive decline by mediating a decrease in neuronal excitability.

TMEM16A expression in cholinergic neurons of the medial habenula mediates anxiety-related behaviors

TMEM16A, a Ca2+ -activated Cl- channel, is known to modulate the excitability of various types of cells; however, its function in central neurons is largely unknown. Here, we show the specific expression of TMEM16A in the medial habenula (mHb) via RNAscope in situ hybridization, immunohistochemistry, and electrophysiology. When TMEM16A is ablated in the mHb cholinergic neurons (TMEM16A cKO mice), the slope of after-hyperpolarization of spontaneous action potentials decreases and the firing frequency is reduced. Reduced mHb activity also decreases the activity of the interpeduncular nucleus (IPN). Moreover, TMEM16A cKO mice display anxiogenic behaviors and deficits in social interaction without despair-like phenotypes or cognitive dysfunctions. Finally, chemogenetic inhibition of mHb cholinergic neurons using the DREADD (Designer Receptors Exclusively Activated by Designer Drugs) approach reveals similar behavioral phenotypes to those of TMEM16A cKO mice. We conclude that TMEM16A plays a key role in anxiety-related behaviors regulated by mHb cholinergic neurons and could be a potential therapeutic target against anxiety-related disorders.

New Insights Into Cholinergic Neuron Diversity

Cholinergic neurons comprise a small population of cells in the striatum but have fundamental roles in fine tuning brain function, and in the etiology of neurological and psychiatric disorders such as Parkinson’s disease (PD) or schizophrenia. The process of developmental cell specification underlying neuronal identity and function is an area of great current interest. There has been significant progress in identifying the developmental origins, commonalities in molecular markers, and physiological properties of the cholinergic neurons. Currently, we are aware of a number of key factors that promote cholinergic fate during development. However, the extent of cholinergic cell diversity is still largely underestimated. New insights into the biological basis of their specification indicate that cholinergic neurons may be far more diverse than previously thought. This review article, highlights the physiological features and the synaptic properties that segregate cholinergic cell subtypes. It provides an accurate picture of cholinergic cell diversity underlying their organization and function in neuronal networks. This review article, also discusses current challenges in deciphering the logic of the cholinergic cell heterogeneity that plays a fundamental role in the control of neural processes in health and disease.

Retinal Characterization of the Thy1-GCaMP3 Transgenic Mouse Line After Optic Nerve Transection

Purpose

GCaMP3 is a genetically encoded calcium indicator for monitoring intracellular calcium dynamics. We characterized the expression pattern and functional properties of GCaMP3 in the Thy1-GCaMP3 transgenic mouse retina.

Methods

To determine the specificity of GCaMP3 expression, Thy1-GCaMP3 (B6; CBA-Tg(Thy1-GCaMP3)6Gfng/J) retinas were processed for immunohistochemistry with anti-green fluorescent protein (anti-GFP, to enhance GCaMP3 fluorescence), anti-RBPMS (retinal ganglion cell [RGC]-specific marker), and antibodies against amacrine cell markers (ChAT, GABA, GAD67, syntaxin). Calcium imaging was used to characterize functional responses of GCaMP3-expressing (GCaMP+) cells by recording calcium transients evoked by superfusion of kainic acid (KA; 10, 50, or 100 μM). In a subset of animals, optic nerve transection (ONT) was performed 3, 5, or 7 days prior to calcium imaging.

Results

GFP immunoreactivity colocalized with RBPMS but not amacrine cell markers in both ONT and non-ONT (control) groups. Calcium transients evoked by KA were reduced after ONT (50 μM KA; ΔF/F0 [SD]; control: 1.00 [0.67], day 3: 0.50 [0.35], day 5: 0.31 [0.28], day 7: 0.35 [0.36]; P < 0.05 versus control). There was also a decrease in the number of GCaMP3+ cells after ONT (cells/mm2 [SD]; control: 2198 [453], day 3: 2224 [643], day 5: 1383 [375], day 7: 913 [178]; P < 0.05). Furthermore, the proportion of GCaMP3+ cells that responded to KA decreased after ONT (50 μM KA, 97%, 54%, 47%, and 58%; control, 3, 5, and 7 days, respectively).

Conclusions

Following ONT, functional RGC responses are lost prior to the loss of RGC somata, suggesting that anatomical markers of RGCs may underestimate the extent of RGC dysfunction.

Contribution of cholinergic interneurons to striatal pathophysiology in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder caused by the loss of nigral dopaminergic neurons innervating the striatum, the main input structure of the basal ganglia. This creates an imbalance between dopaminergic inputs and cholinergic interneurons (ChIs) within the striatum. The efficacy of anticholinergic drugs, one of the earliest therapy for PD before the discovery of L-3,4-dihydroxyphenylalanine (L-DOPA) suggests an increased cholinergic tone in this disease. The dopamine (DA)-acetylcholine (ACh) balance hypothesis is now revisited with the use of novel cutting-edge techniques (optogenetics, pharmacogenetics, new electrophysiological recordings). This review will provide the background of the specific contribution of ChIs to striatal microcircuit organization in physiological and pathological conditions. The second goal of this review is to delve into the respective contributions of nicotinic and muscarinic receptor cholinergic subunits to the control of striatal afferent and efferent neuronal systems. Special attention will be given to the role played by muscarinic acetylcholine receptors (mAChRs) in the regulation of striatal network which may have important implications in the development of novel therapeutic strategies for motor and cognitive impairment in PD.

Cholinergic modulation of striatal microcircuits

The purpose of this review is to bridge the gap between earlier literature on striatal cholinergic interneurons and mechanisms of microcircuit interaction demonstrated with the use of newly available tools. It is well known that the main source of the high level of acetylcholine in the striatum, compared to other brain regions, is the cholinergic interneurons. These interneurons provide an extensive local innervation that suggests they may be a key modulator of striatal microcircuits. Supporting this idea requires the consideration of functional properties of these interneurons, their influence on medium spiny neurons, other interneurons, and interactions with other synaptic regulators. Here, we underline the effects of intrastriatal and extrastriatal afferents onto cholinergic interneurons and discuss the activation of pre‐ and postsynaptic muscarinic and nicotinic receptors that participate in the modulation of intrastriatal neuronal interactions. We further address recent findings about corelease of other transmitters in cholinergic interneurons and actions of these interneurons in striosome and matrix compartments. In addition, we summarize recent evidence on acetylcholine‐mediated striatal synaptic plasticity and propose roles for cholinergic interneurons in normal striatal physiology. A short examination of their role in neurological disorders such as Parkinson's, Huntington's, and Tourette's pathologies and dystonia is also included.

Sex differences in the nicotinic excitation of principal neurons within the developing hippocampal formation

The hippocampal formation (HF) plays an important role to facilitate higher order cognitive functions. Cholinergic activation of heteromeric nicotinic acetylcholine receptors (nAChRs) within the HF is critical for the normal development of principal neurons within this brain region. However, previous research investigating the expression and function of heteromeric nAChRs in principal neurons of the HF is limited to males or does not differentiate between the sexes. We used whole-cell electrophysiology to show that principal neurons in the CA1 region of the female mouse HF are excited by heteromeric nAChRs throughout postnatal development, with the greatest response occurring during the first two weeks of postnatal life. Excitability responses to heteromeric nAChR stimulation were also found in principal neurons in the CA3, dentate gyrus, subiculum, and entorhinal cortex layer VI (ECVI) of young postnatal female HF. A direct comparison between male and female mice found that principal neurons in ECVI display greater heteromeric nicotinic passive and active excitability responses in females. This sex difference is likely influenced by the generally more excitable nature of ECVI neurons from female mice, which display a higher resting membrane potential, greater input resistance, and smaller afterhyperpolarization potential of medium duration (mAHP). These findings demonstrate that heteromeric nicotinic excitation of ECVI neurons differs between male and female mice during a period of major circuitry development within the HF, which may have mechanistic implications for known sex differences in the development and function of this cognitive brain region.

Inhibition of striatal cholinergic interneuron activity by the Kv7 opener retigabine and the nonsteroidal anti-inflammatory drug diclofenac

Striatal cholinergic interneurons provide modulation to striatal circuits involved in voluntary motor control and goal-directed behaviors through their autonomous tonic discharge and their firing "pause" responses to novel and rewarding environmental events. Striatal cholinergic interneuron hyperactivity was linked to the motor deficits associated with Parkinson's disease and the adverse effects of chronic antiparkinsonian therapy like l-DOPA-induced dyskinesia. Here we addressed whether Kv7 channels, which provide negative feedback to excitation in other neuron types, are involved in the control of striatal cholinergic interneuron tonic activity and response to excitatory inputs. We found that autonomous firing of striatal cholinergic interneurons is not regulated by Kv7 channels. In contrast, Kv7 channels limit the summation of excitatory postsynaptic potentials in cholinergic interneurons through a postsynaptic mechanism. Striatal cholinergic interneurons have a high reserve of Kv7 channels, as their opening using pharmacological tools completely silenced the tonic firing and markedly reduced their intrinsic excitability. A strong inhibition of striatal cholinergic interneurons was also observed in response to the anti-inflammatory drugs diclofenac and meclofenamic acid, however, this effect was independent of Kv7 channels. These data bring attention to new potential molecular targets and pharmacological tools to control striatal cholinergic interneuron activity in pathological conditions where they are believed to be hyperactive, including Parkinson's disease.

Striatal cholinergic interneurons and Parkinson's disease

Giant, aspiny cholinergic interneurons (ChIs) have long been known to be key nodes in the striatal circuitry controlling goal-directed actions and habits. In recent years, new experimental approaches, like optogenetics and monosynaptic rabies virus mapping, have expanded our understanding of how ChIs contribute to the striatal activity underlying action selection and the interplay of dopaminergic and cholinergic signaling. These approaches also have begun to reveal how ChI function is distorted in disease states affecting the basal ganglia, like Parkinson's disease (PD). This review gives a brief overview of our current understanding of the functional role played by ChIs in striatal physiology and how this changes in PD. The translational implications of these discoveries, as well as the gaps that remain to be bridged, are discussed as well.

The striatal cholinergic system in L-dopa-induced dyskinesias

Cholinergic signaling plays a key role in regulating striatal function. The principal source of acetylcholine in the striatum is the cholinergic interneurons which, although low in number, densely arborize to modulate striatal neurotransmission. This modulation occurs via strategically positioned nicotinic and muscarinic acetylcholine receptors that influence striatal dopamine, GABA and other neurotransmitter release. Cholinergic interneurons integrate multiple striatal synaptic inputs and outputs to regulate motor activity under normal physiological conditions. Consequently, an imbalance between these systems is associated with basal ganglia disorders. Here, we provide an overview of how striatal cholinergic interneurons modulate striatal activity under normal and pathological conditions. Numerous studies show that nigrostriatal damage such as that occurs with Parkinson's disease affects cholinergic receptor-mediated striatal activity. This altered cholinergic signaling is an important contributor to Parkinson's disease as well as to the dyskinesias that develop with L-dopa therapy, the gold standard for treatment. Indeed, multiple preclinical studies show that cholinergic receptor drugs may be beneficial for the treatment of L-dopa-induced dyskinesias. In this review, we discuss the evidence indicating that therapeutic modulation of the cholinergic system, particularly targeting of nicotinic cholinergic receptors, may offer a novel approach to manage this debilitating side effect of dopamine replacement therapy for Parkinson's disease.

Differential contribution of Ca2+ sources to day and night BK current activation in the circadian clock

Large conductance K⁺ (BK) channels are expressed widely in neurons, where their activation is regulated by membrane depolarization and intracellular Ca²⁺ (Ca²⁺_i). To enable this regulation, BK channels functionally couple to both voltage-gated Ca²⁺ channels (VGCCs) and channels mediating Ca²⁺ release from intracellular stores. However, the relationship between BK channels and their specific Ca²⁺ source for particular patterns of excitability is not well understood. In neurons within the suprachiasmatic nucleus (SCN)-the brain's circadian clock-BK current, VGCC current, and Ca²⁺_i are diurnally regulated, but paradoxically, BK current is greatest at night when VGCC current and Ca²⁺_i are reduced. Here, to determine whether diurnal regulation of Ca²⁺ is relevant for BK channel activation, we combine pharmacology with day and night patch-clamp recordings in acute slices of SCN. We find that activation of BK current depends primarily on three types of channels but that the relative contribution changes between day and night. BK current can be abrogated with nimodipine during the day but not at night, establishing that L-type Ca²⁺ channels (LTCCs) are the primary daytime Ca²⁺ source for BK activation. In contrast, dantrolene causes a significant decrease in BK current at night, suggesting that nighttime BK activation is driven by ryanodine receptor (RyR)-mediated Ca²⁺_i release. The N- and P/Q-type Ca²⁺ channel blocker ω-conotoxin MVIIC causes a smaller reduction of BK current that does not differ between day and night. Finally, inhibition of LTCCs, but not RyRs, eliminates BK inactivation, but the BK β2 subunit was not required for activation of BK current by LTCCs. These data reveal a dynamic coupling strategy between BK channels and their Ca²⁺ sources in the SCN, contributing to diurnal regulation of SCN excitability.

Enhanced mu opioid receptor-dependent opioidergic modulation of striatal cholinergic transmission in DYT1 dystonia

BACKGROUND

Mu opioid receptor activation modulates acetylcholine release in the dorsal striatum, an area deeply involved in motor function, habit formation, and reinforcement learning as well as in the pathophysiology of different movement disorders, such as dystonia. Although the role of opioids in drug reward and addiction is well established, their involvement in motor dysfunction remains largely unexplored.

METHODS

We used a multidisciplinary approach to investigate the responses to mu activation in 2 mouse models of DYT1 dystonia (Tor1a^+/Δgag mice, Tor1a^+/- torsinA null mice, and their respective wild-types). We performed electrophysiological recordings to characterize the pharmacological effects of receptor activation in cholinergic interneurons as well as the underlying ionic currents. In addition, an analysis of the receptor expression was performed both at the protein and mRNA level.

RESULTS

In mutant mice, selective mu receptor activation caused a stronger G-protein-dependent, dose-dependent inhibition of firing activity in cholinergic interneurons when compared with controls. In Tor1a^+/- mice, our electrophysiological analysis showed an abnormal involvement of calcium-activated potassium channels. Moreover, in both models we found increased levels of mu receptor protein. In addition, both total mRNA and the mu opioid receptor splice variant 1S (MOR-1S) splice variant of the mu receptor gene transcript, specifically enriched in striatum, were selectively upregulated.

CONCLUSION

Mice with the DYT1 dystonia mutation exhibit an enhanced response to mu receptor activation, dependent on selective receptor gene upregulation. Our data suggest a novel role for striatal opioid signaling in motor control, and more important, identify mu opioid receptors as potential targets for pharmacological intervention in dystonia. © 2017 International Parkinson and Movement Disorder Society.

Decreased microRNA levels lead to deleterious increases in neuronal M2 muscarinic receptors in Spinal Muscular Atrophy models

Spinal Muscular Atrophy (SMA) is caused by diminished Survival of Motor Neuron (SMN) protein, leading to neuromuscular junction (NMJ) dysfunction and spinal motor neuron (MN) loss. Here, we report that reduced SMN function impacts the action of a pertinent microRNA and its mRNA target in MNs. Loss of the C. elegans SMN ortholog, SMN-1, causes NMJ defects. We found that increased levels of the C. elegans Gemin3 ortholog, MEL-46, ameliorates these defects. Increased MEL-46 levels also restored perturbed microRNA (miR-2) function in smn-1(lf) animals. We determined that miR-2 regulates expression of the C. elegans M2 muscarinic receptor (m2R) ortholog, GAR-2. GAR-2 loss ameliorated smn-1(lf) and mel-46(lf) synaptic defects. In an SMA mouse model, m2R levels were increased and pharmacological inhibition of m2R rescued MN process defects. Collectively, these results suggest decreased SMN leads to defective microRNA function via MEL-46 misregulation, followed by increased m2R expression, and neuronal dysfunction in SMA.

DOI:http://dx.doi.org/10.7554/eLife.20752.001

Spinal muscular atrophy is a genetic disease that causes muscles to gradually weaken. In people with the disease, the nerve cells that control the movement of muscles – called motor neurons – deteriorate over time, hindering the person’s mobility and shortening their life expectancy. Spinal muscular atrophy is usually caused by genetic faults affecting a protein called SMN (which is short for “Survival of motor neuron”) and recent research suggested that disrupting this protein alters the function of short pieces of genetic material called microRNAs. However, the precise role that microRNAs play in the disease and their connection to the SMN protein was not clear.

MicroRNAs interfere with the production of proteins by disrupting molecules called messenger RNAs, which are temporary strings of genetic code that carry the instructions for making protein. By disrupting messenger RNAs, microRNAs can delay or halt the production of specific proteins. This is an important part of the normal behavior of a cell, but disturbing the activity of microRNAs can lead to an unwanted rise or fall in crucial proteins.

O’Hern et al. made use of engineered nematode worms and mice that share genetic features with spinal muscular atrophy patients, including disruption of the gene responsible for producing the SMN protein. These animal models of the disease were used to examine the relationship between decreased SMN levels and microRNAs in motor neurons. The experiments showed that reduced SMN activity affects a specific microRNA, which in turn causes motor neurons to produce more of a protein called m2R. This protein is a receptor for a molecule, called acetylcholine, which motor neurons use to send signals to muscle cells.

Increased m2R may be detrimental to motor neurons. As such, O’Hern et al. decreased m2R protein activity to determine whether this could reverse the defects in motor neurons that arise in the animal models of the disease. Indeed, blocking this receptor rescued some of the defects seen in the animal models, supporting the link to spinal muscular atrophy.

Several treatments that block m2R are already available to treat other conditions. As such, the next step is to determine whether these existing treatments are able to protect mice models of spinal muscular atrophy against muscle deterioration or increase their lifespan. If successful, this could open new avenues for the development of treatments in people.

DOI:http://dx.doi.org/10.7554/eLife.20752.002

Cholinergic Interneurons Underlie Spontaneous Dopamine Release in Nucleus Accumbens

The release of dopamine from terminals in the NAc is regulated by a number of factors, including voltage-gated ion channels, D2-autoreceptors, and nAChRs. Cholinergic interneurons (CINs) drive dopamine release through activation of nAChRs on dopamine terminals. Using cyclic voltammetry in mouse brain slices, nAChR-dependent spontaneous dopamine transients and the mechanisms underlying the origin were examined in the NAc. Spontaneous events were infrequent (0.3 per minute), but the rate and amplitude were increased after blocking Kv channels with 4-aminopyridine. Although the firing frequency of CINs was increased by blocking glutamate reuptake with TBOA and the Sk blocker apamin, only 4-aminopyridine increased the frequency of dopamine transients. In contrast, inhibition of CIN firing with the μ/δ selective opioid [Met⁵]enkephalin (1 μm) decreased spontaneous dopamine transients. Cocaine increased the rate and amplitude of dopamine transients, suggesting that the activity of the dopamine transporter limits the detection of these events. In the presence of cocaine, the rate of spontaneous dopamine transients was further increased after blocking D2-autoreceptors. Blockade of muscarinic receptors had no effect on evoked dopamine release, suggesting that feedback inhibition of acetylcholine release was not involved. Thus, although spontaneous dopamine transients are reliant on nAChRs, the frequency was not strictly governed by the activity of CINs. The increase in frequency of spontaneous dopamine transients induced by cocaine was not due to an increase in cholinergic tone and is likely a product of an increase in detection resulting from decreased dopamine reuptake.SIGNIFICANCE STATEMENT The actions of dopamine in the NAc are thought to be responsible for endogenous reward and the reinforcing properties of drugs of abuse, such as psychostimulants. The present work examines the mechanisms underlying nAChR-induced spontaneous dopamine release. This study demonstrates that spontaneous dopamine release is (1) dependent of the activation of nicotinic receptors, (2) independent on the spontaneous activity of cholinergic interneurons, and (3) that cocaine increased the detection of dopamine transients by prolonging the presence and increasing the diffusion of dopamine in the extracellular space. The release of acetylcholine is therefore responsible for spontaneous dopamine transients, and cocaine augments dopamine tone without altering activity of cholinergic interneurons.

Cholinergic Interneurons Amplify Corticostriatal Synaptic Responses in the Q175 Model of Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder characterized by deficits in movement control that are widely viewed as stemming from pathophysiological changes in the striatum. Giant, aspiny cholinergic interneurons (ChIs) are key elements in the striatal circuitry controlling movement, but whether their physiological properties are intact in the HD brain is unclear. To address this issue, the synaptic properties of ChIs were examined using optogenetic approaches in the Q175 mouse model of HD. In ex vivo brain slices, synaptic facilitation at thalamostriatal synapses onto ChIs was reduced in Q175 mice. The alteration in thalamostriatal transmission was paralleled by an increased response to optogenetic stimulation of cortical axons, enabling these inputs to more readily induce burst-pause patterns of activity in ChIs. This adaptation was dependent upon amplification of cortically evoked responses by a post-synaptic upregulation of voltage-dependent Na⁺ channels. This upregulation also led to an increased ability of somatic spikes to invade ChI dendrites. However, there was not an alteration in the basal pacemaking rate of ChIs, possibly due to increased availability of Kv4 channels. Thus, there is a functional "re-wiring" of the striatal networks in Q175 mice, which results in greater cortical control of phasic ChI activity, which is widely thought to shape the impact of salient stimuli on striatal action selection.

Nicotinic and opioid receptor regulation of striatal dopamine D2-receptor mediated transmission

In addition to dopamine neuron firing, cholinergic interneurons (ChIs) regulate dopamine release in the striatum via presynaptic nicotinic receptors (nAChRs) on dopamine axon terminals. Synchronous activity of ChIs is necessary to evoke dopamine release through this pathway. The frequency-dependence of disynaptic nicotinic modulation has led to the hypothesis that nAChRs act as a high-pass filter in the dopaminergic microcircuit. Here, we used optogenetics to selectively stimulate either ChIs or dopamine terminals directly in the striatum. To measure the functional consequence of dopamine release, D2-receptor synaptic activity was assessed via virally overexpressed potassium channels (GIRK2) in medium spiny neurons (MSNs). We found that nicotinic-mediated dopamine release was blunted at higher frequencies because nAChRs exhibit prolonged desensitization after a single pulse of synchronous ChI activity. However, when dopamine neurons alone were stimulated, nAChRs had no effect at any frequency. We further assessed how opioid receptors modulate these two mechanisms of release. Bath application of the κ opioid receptor agonist U69593 decreased D2-receptor activation through both pathways, whereas the μ opioid receptor agonist DAMGO decreased D2-receptor activity only as a result of cholinergic-mediated dopamine release. Thus the release of dopamine can be independently modulated when driven by either dopamine neurons or cholinergic interneurons.