The role of Ca2+ channels in the repetitive firing of striatal projection neurons

Dopamine D2 and Adenosine A2A Receptors Interaction on Ca2+ Current Modulation in a Rodent Model of Parkinsonism

Adenosine A1 and A2A receptors are expressed in striatal projection neurons (SPNs). A1 receptors are located in direct (dSPN) and indirect SPNs (iSNP). A2A receptors are only present in iSPNs. Dopamine D2 receptors are also expressed in iSPNs and interactions between D2 and A2A receptors have received attention. iSPNs activity increases during parkinsonism (PD) and A2A receptors may be responsible by enhancing Ca2+ currents (iCa2+). Therefore, A2A receptors blockade is a therapeutic approach. We asked whether A2A receptors need the interaction with D2 receptors (D2R) to exert their actions. By using isolated and identified iSPNs to avoid indirect influences, we show that D2R action habilitates A2A receptors (A2AR) modulation. iCa2+ through voltage gated Ca2+ channels (CaV) was used as a signal to observe this interaction. Voltage-clamp recordings in acutely dissociated iSPNs, current-clamp recordings in slices and calcium imaging in transgenic A2A-Cre mice, showed that D2R reduction in iCa2+ endows A2AR to restore iCa2+ on iSPNs showing an antagonistic interaction between D2 and A2A receptors. A2A receptors were blocked by the antagonist istradefylline, however, this blockade differed in control and dopamine-depleted iSPNs: istradefylline reduced D2R modulation in parkinsonian animals as compared to controls. Calcium imaging recordings show that istradefylline occludes D2R actions in the parkinsonian circuitry and this effect depends on the order of drugs application. Thus, while D2 activation enables A2A receptors action, blockade of A2AR induces a reduction in the action of D2 agonists, confirming a complex interaction.

Summary Statement

A2A receptor required previous D2 receptor activation to modulate Ca2+ currents. Istradefylline decreases pramipexole modulation on Ca2+ currents. Istradefylline reduces A2A + neurons activity in striatial microcircuit, but pramipexole failed to further reduce neuronal activity.

Firing Differences Between Adult Intralaminar Thalamo-striatal Neurons

Differences in the intrinsic properties of intralaminar thalamo-striatal neurons such as expressing low-threshold-spikes (LTS) or after hyperpolarizing potentials (AHPs) of different duration have been attributed to different maturation stages. However, two morphological types: "diffuse" and "bushy" have been described. Therefore, we explored whether electrophysiological differences persist in adult mice using whole cell recordings. Some recorded neurons were identified by intracellular labeling with biocytin and double labeling with retrograde or anterograde tracings using Cre-mice. We classified these neurons by their AHPs during spontaneous firing. Neurons with long duration AHPs, with fast and slow components, were mostly found in the parafascicular (Pf) nucleus. Neurons with brief AHPs were mainly found in the central lateral (CL) nucleus. However, neurons with both AHPs were found in both nuclei in different proportions. Firing frequency adaptation differed between these neuron classes: those with prolonged AHPs exhibited firing frequency adaptation with fast and slow time constants whereas those with brief AHPs were slow adapters. Neurons with more prolonged AHPs had significant higher input resistances than neurons with brief AHPs. Both cell classes could fire in two modes: trains of single action potentials at depolarized potentials or high frequency bursts on top of LTS at more hyperpolarized potentials. LTS were probably generated by Cav3 calcium channels since they were blocked by the selective antagonist TTA-P2. About 11% of neurons with brief AHPs and 55% of neurons with prolonged AHPs do not show LTS and bursts, even when potassium currents are blocked.

Striatal and nigral muscarinic type 1 and type 4 receptors modulate levodopa-induced dyskinesia and striato-nigral pathway activation in 6-hydroxydopamine hemilesioned rats

Acetylcholine muscarinic receptors (mAChRs) contribute to both the facilitation and inhibition of levodopa-induced dyskinesia operated by striatal cholinergic interneurons, although the receptor subtypes involved remain elusive. Cholinergic afferents from the midbrain also innervate the substantia nigra reticulata, although the role of nigral mAChRs in levodopa-induced dyskinesia is unknown. Here, we investigate whether striatal and nigral M1 and/or M4 mAChRs modulate dyskinesia and the underlying striato-nigral GABAergic pathway activation in 6-hydroxydopamine hemilesioned rats. Reverse microdialysis allowed to deliver the mAChR antagonists telenzepine (M1 subtype preferring), PD-102807 and tropicamide (M4 subtype preferring), as well as the selective M4 mAChR positive allosteric modulator VU0152100 in striatum or substantia nigra, while levodopa was administered systemically. Dyskinetic movements were monitored along with nigral GABA (and glutamate) and striatal glutamate dialysate levels, taken as neurochemical correlates of striato-nigral pathway and cortico-basal ganglia-thalamo-cortical loop activation. We observed that intrastriatal telenzepine, PD-102807 and tropicamide alleviated dyskinesia and inhibited nigral GABA and striatal glutamate release. This was partially replicated by intrastriatal VU0152100. The M2 subtype preferring antagonist AFDX-116, used to elevate striatal acetylcholine levels, blocked the behavioral and neurochemical effects of PD-102807. Intranigral VU0152100 prevented levodopa-induced dyskinesia and its neurochemical correlates whereas PD-102807 was ineffective. These results suggest that striatal, likely postsynaptic, M1 mAChRs facilitate dyskinesia and striato-nigral pathway activation in vivo. Conversely, striatal M4 mAChRs can both facilitate and inhibit dyskinesia, possibly depending on their localization. Potentiation of striatal and nigral M4 mAChR transmission leads to powerful multilevel inhibition of striato-nigral pathway and attenuation of dyskinesia.

Dopaminergic Transmission Rapidly and Persistently Enhances Excitability of D1 Receptor-Expressing Striatal Projection Neurons

Substantia nigra dopamine neurons have been implicated in the initiation and invigoration of movement, presumably through their modulation of striatal projection neuron (SPN) activity. However, the impact of native dopaminergic transmission on SPN excitability has not been directly demonstrated. Using perforated patch-clamp recording, we found that optogenetic stimulation of nigrostriatal dopamine axons rapidly and persistently elevated the excitability of D1 receptor-expressing SPNs (D1-SPNs). The evoked firing of D1-SPNs increased within hundreds of milliseconds of stimulation and remained elevated for ≥ 10 min. Consistent with the negative modulation of depolarization- and Ca2+-activated K+ currents, dopaminergic transmission accelerated subthreshold depolarization in response to current injection, reduced the latency to fire, and transiently diminished action potential afterhyperpolarization. Persistent modulation was protein kinase A dependent and associated with a reduction in action potential threshold. Together, these data demonstrate that dopaminergic transmission potently increases D1-SPN excitability with a time course that could support subsecond and sustained behavioral control.

Calcium currents in striatal fast-spiking interneurons: dopaminergic modulation of CaV1 channels

BACKGROUND

Striatal fast-spiking interneurons (FSI) are a subset of GABAergic cells that express calcium-binding protein parvalbumin (PV). They provide feed-forward inhibition to striatal projection neurons (SPNs), receive cortical, thalamic and dopaminergic inputs and are coupled together by electrical and chemical synapses, being important components of the striatal circuitry. It is known that dopamine (DA) depolarizes FSI via D₁-class DA receptors, but no studies about the ionic mechanism of this action have been reported. Here we ask about the ion channels that are the effectors of DA actions. This work studies their Ca²⁺ currents.

RESULTS

Whole-cell recordings in acutely dissociated and identified FSI from PV-Cre transgenic mice were used to show that FSI express an array of voltage gated Ca²⁺ channel classes: Ca_V1, Ca_V2.1, Ca_V2.2, Ca_V2.3 and Ca_V3. However, Ca_V1 Ca²⁺ channel carries most of the whole-cell Ca²⁺ current in FSI. Activation of D₁-like class of DA receptors by the D₁-receptor selective agonist SKF-81297 (SKF) enhances whole-cell Ca²⁺ currents through Ca_V1 channels modulation. A previous block of Ca_V1 channels with nicardipine occludes the action of the DA-agonist, suggesting that no other Ca²⁺ channel is modulated by D₁-receptor activation. Bath application of SKF in brain slices increases the firing rate and activity of FSI as measured with both whole-cell and Ca²⁺ imaging recordings. These actions are reduced by nicardipine.

CONCLUSIONS

The present work discloses one final effector of DA modulation in FSI. We conclude that the facilitatory action of DA in FSI is in part due to Ca_V1 Ca²⁺ channels positive modulation.

The role of Ca2+ -dependent K+ - channels at the rat corticostriatal synapses revealed by paired pulse stimulation

Potassium channels play an important role in modulating synaptic activity both at presynaptic and postsynaptic levels. We have shown before that presynaptically located K_V and K_IR channels modulate the strength of corticostriatal synapses in rat brain, but the role of other types of potassium channels at these synapses remains largely unknown. Here, we show that calcium-dependent potassium channels BK-type but not SK-type channels are located presynaptically in corticostriatal synapses. We stimulated cortical neurons in rat brain slices and recorded postsynaptic excitatory potentials (EPSP) in medium spiny neurons (MSN) in dorsal neostriatum. By using a paired pulse protocol, we induced synaptic facilitation before applying either BK- or SK-specific toxins. Thus, we found that blockage of BK_Ca with iberiotoxin (10 nM) reduces synaptic facilitation and increases the amplitude of the EPSP, while exposure to SK-blocker apamin (100 nM) has no effect. Additionally, we induced train action potentials on striatal MSN by current injection before and after the exposure to K_Ca toxins. We found that the action potential becomes broader when the MSN is exposed to iberiotoxin, although it has no impact on frequency. In contrast, exposure to apamin results in loss of afterhyperpolarization phase and an increase of spike frequency. Therefore, we concluded that postsynaptic SK channels are involved in afterhyperpolarization and modulation of spike frequency while the BK channels are involved on the late repolarization phase of the action potential. Altogether, our results show that calcium-dependent potassium channels modulate both input towards and output from the striatum.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

Modulation of Ca2+-currents by sequential and simultaneous activation of adenosine A1 and A 2A receptors in striatal projection neurons

D(1)- and D(2)-types of dopamine receptors are located separately in direct and indirect pathway striatal projection neurons (dSPNs and iSPNs). In comparison, adenosine A(1)-type receptors are located in both neuron classes, and adenosine A(2A)-type receptors show a preferential expression in iSPNs. Due to their importance for neuronal excitability, Ca(2+)-currents have been used as final effectors to see the function of signaling cascades associated with different G protein-coupled receptors. For example, among many other actions, D(1)-type receptors increase, while D(2)-type receptors decrease neuronal excitability by either enhancing or reducing, respectively, CaV1 Ca(2+)-currents. These actions occur separately in dSPNs and iSPNs. In the case of purinergic signaling, the actions of A(1)- and A(2A)-receptors have not been compared observing their actions on Ca(2+)-channels of SPNs as final effectors. Our hypotheses are that modulation of Ca(2+)-currents by A(1)-receptors occurs in both dSPNs and iSPNs. In contrast, iSPNs would exhibit modulation by both A(1)- and A2A-receptors. We demonstrate that A(1)-type receptors reduced Ca(2+)-currents in all SPNs tested. However, A(2A)-type receptors enhanced Ca(2+)-currents only in half tested neurons. Intriguingly, to observe the actions of A(2A)-type receptors, occupation of A(1)-type receptors had to occur first. However, A(1)-receptors decreased Ca(V)2 Ca(2+)-currents, while A(2A)-type receptors enhanced current through Ca(V)1 channels. Because these channels have opposing actions on cell discharge, these differences explain in part why iSPNs may be more excitable than dSPNs. It is demonstrated that intrinsic voltage-gated currents expressed in SPNs are effectors of purinergic signaling that therefore play a role in excitability.

The small-conductance calcium-activated potassium channel is a key modulator of firing and long-term depression in the dorsal striatum

The striatum is considered to be critical for the control of goal-directed action, with the lateral dorsal striatum (latDS) being implicated in modulation of habits and the nucleus accumbens thought to represent a limbic-motor interface. Although medium spiny neurons from different striatal subregions exhibit many similar properties, differential firing and synaptic plasticity could contribute to the varied behavioral roles across subregions. Here, we examined the contribution of small-conductance calcium-activated potassium channels (SKs) to action potential generation and synaptic plasticity in adult rat latDS and nucleus accumbens shell (NAS) projection neurons in vitro. The SK-selective antagonist apamin exerted a prominent effect on latDS firing, significantly decreasing the interspike interval. Furthermore, prolonged latDS depolarization increased the interspike interval and reduced firing, and this enhancement was reversed by apamin. In contrast, NAS neurons exhibited greater basal firing rates and less regulation of firing by SK inhibition and prolonged depolarization. LatDS neurons also had greater SK currents than NAS neurons under voltage-clamp. Importantly, SK inhibition with apamin facilitated long-term depression (LTD) induction in the latDS but not the NAS, without alterations in glutamate release. In addition, SK activation in the latDS prevented LTD induction. Greater SK function in the latDS than in the NAS was not secondary to differences in sodium or inwardly rectifying potassium channel function, and apamin enhancement of firing did not reflect indirect action through cholinergic interneurons. Thus, these data demonstrate that SKs are potent modulators of action potential generation and LTD in the dorsal striatum, and could represent a fundamental cellular mechanism through which habits are regulated.

Muscarinic M(1) modulation of N and L types of calcium channels is mediated by protein kinase C in neostriatal neurons

In some neurons, muscarinic M(1)-class receptors control L-type (Ca(V)1) Ca(2+)-channels via protein kinase C (PKC) or calcineurin (phosphatase 2B; PP-2B) signaling pathways. Both PKC and PP-2B pathways start with phospholipase C (PLC) activation. In contrast, P/Q- and N-type (Ca(V)2.1, 2.2, respectively) Ca(2+)-channels are controlled by M(2)-class receptors via G proteins that may act, directly, to modulate these channels. The hypothesis of this work is that this description is not enough to explain muscarinic modulation of Ca(2+) channels in rat neostriatal projection neurons. Thus, we took advantage of the specific muscarinic toxin 3 (MT-3) to block M(4)-type receptors in neostriatal neurons, and leave in isolation the M(1)-type receptors to study them separately. We then asked what Ca(2+) channels are modulated by M(1)-type receptors only. We found that M(1)-receptors do modulate L, N and P/Q-types Ca(2+) channels. This modulation is blocked by the M(1)-class receptor antagonist (muscarinic toxin 7, MT-7) and is voltage-independent. Thereafter, we asked what signaling pathways, activated by M(1)-receptors would control these channels. We found that inactivation of PLC abolishes the modulation of all three channel types. PKC activators (phorbol esters) mimic muscarinic actions, whereas reduction of intracellular calcium virtually abolishes all modulation. As expected, PKC inhibitors prevented the muscarinic reduction of the afterhyperpolarizing potential (AHP), an event known to be dependent on Ca(2+) entry via N- and P/Q-type Ca(2+) channels. However, PKC inhibitors (bisindolylmaleimide I and PKC-1936) only block modulation of currents through N and L types Ca(2+) channels; while the modulation of P/Q-type Ca(2+) channels remains unaffected. These results show that different branches of the same signaling cascade can be used to modulate different Ca(2+) channels. Finally, we found no evidence of calcineurin modulating these Ca(2+) channels during M(1)-receptor activation, although, in the same cells, we demonstrate functional PP-2B by activating dopaminergic D(2)-receptor modulation.

Somatostatinergic modulation of firing pattern and calcium-activated potassium currents in medium spiny neostriatal neurons

Somatostatin is synthesized and released by aspiny GABAergic interneurons of the neostriatum, some of them identified as low threshold spike generating neurons (LTS-interneurons). These neurons make synaptic contacts with spiny neostriatal projection neurons. However, very few somatostatin actions on projection neurons have been described. The present work reports that somatostatin modulates the Ca(2+) activated K(+) currents (K(Ca) currents) expressed by projection cells. These actions contribute in designing the firing pattern of the spiny projection neuron; which is the output of the neostriatum. Small conductance (SK) and large conductance (BK) K(Ca) currents represent between 30% and 50% of the sustained outward current in spiny cells. Somatostatin reduces SK-type K(+) currents and at the same time enhances BK-type K(+) currents. This dual effect enhances the fast component of the after hyperpolarizing potential while reducing the slow component. Somatostatin then modifies the firing pattern of spiny neurons which changed from a tonic regular pattern to an interrupted "stuttering"-like pattern. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) tissue expression analysis of dorsal striatal somatostatinergic receptors (SSTR) mRNA revealed that all five SSTR mRNAs are present. However, single cell RT-PCR profiling suggests that the most probable receptor in charge of this modulation is the SSTR2 receptor. Interestingly, aspiny interneurons may exhibit a "stuttering"-like firing pattern. Therefore, somatostatin actions appear to be the entrainment of projection neurons to the rhythms generated by some interneurons. Somatostatin is then capable of modifying the processing and output of the neostriatum.

Psychostimulants, madness, memory... and RGS proteins?

The ingestion of psychostimulant drugs by humans imparts a profound sense of alertness and well-being. However, repeated use of these drugs in some individuals will induce a physiological state of dependence, characterized by compulsive behavior directed toward the acquisition and ingestion of the drug, at the expense of customary social obligations. Drugs of abuse and many other types of experiences share the ability to alter the morphology and density of neuronal dendrites and spines. Dopaminergic modulation of corticostriatal synaptic plasticity is necessary for these morphological changes. Changes in the density of dendritic spines on striatal neurons may underlie the development of this pathological pattern of drug-seeking behavior. Identifying proteins that regulate dopaminergic signaling are of value. A family of proteins, the regulators of G protein signaling (RGS) proteins, which regulate signaling from G protein-coupled receptors, such as dopamine and glutamate, may be important in this regard. By regulating corticostriatal synaptic plasticity, RGS proteins can influence presynaptic activity, neurotransmitter release, and postsynaptic depolarization and thereby play a key role in the development of this plasticity. Pharmacological agents that modify RGS activity in humans could be efficacious in ameliorating the dependence on psychostimulant drugs.

A Reconfiguration of CaV2 Ca2+ Channel Current and Its Dopaminergic D2 Modulation in Developing Neostriatal Neurons

The modulatory effect of D2 dopamine receptor activation on calcium currents was studied in neostriatal projection neurons at two stages of rat development: postnatal day (PD)14 and PD40. D2-class receptor agonists reduced whole cell calcium currents by about 35% at both stages, and this effect was blocked by the D2 receptor antagonist sulpiride. Nitrendipine partially occluded this modulation at both stages, indicating that modulation of CaV1 channels was present throughout this developmental interval. Nevertheless, modulation of CaV1 channels was significantly larger in PD40 neurons. ω-Conotoxin GVIA occluded most of the Ca2+ current modulation in PD14 neurons. However, this occlusion was greatly decreased in PD40 neurons. ω-Agatoxin TK occluded a great part of the modulation in PD40 neurons but had a negligible effect in PD14 neurons. The data indicate that dopaminergic D2-mediated modulation undergoes a change in target during development: from CaV2.2 to CaV2.1 Ca2+ channels. This change occurred while CaV2.2 channels were being down-regulated and CaV2.1 channels were being up-regulated. Presynaptic modulation mediated by D2 receptors reflected these changes; CaV2.2 type channels were used for release in young animals but very little in mature animals, suggesting that changes took place simultaneously at the somatodendritic and the synaptic membranes.

Cholinergic Control of Firing Pattern and Neurotransmission in Rat Neostriatal Projection Neurons: Role of CaV2.1 and CaV2.2 Ca2+ Channels

Besides a reduction of L-type Ca2+-currents (CaV1), muscarine and the peptidic M1-selective agonist, MT-1, reduced currents through CaV2.1 (P/Q) and CaV2.2 (N) Ca2+ channel types. This modulation was strongly blocked by the peptide MT-7, a specific muscarinic M1-type receptor antagonist but not significantly reduced by the peptide MT-3, a specific muscarinic M4-type receptor antagonist. Accordingly, MT-7, but not MT-3, blocked a muscarinic reduction of the afterhyperpolarizing potential (AHP) and decreased the GABAergic inhibitory postsynaptic currents (IPSCs) produced by axon collaterals that interconnect spiny neurons. Both these functions are known to be dependent on P/Q and N types Ca2+ channels. The action on the AHP had an important effect in increasing firing frequency. The action on the IPSCs was shown to be caused presynaptically as it coursed with an increase in the paired-pulse ratio. These results show: first, that muscarinic M1-type receptor activation is the main cholinergic mechanism that modulates Ca2+ entry through voltage-dependent Ca2+ channels in spiny neurons. Second, this muscarinic modulation produces a postsynaptic facilitation of discharge together with a presynaptic inhibition of the GABAergic control mediated by axon collaterals. Together, both effects would tend to recruit more spiny neurons for the same task.

Contrast enhancement: a physiological effect of striatal dopamine?

Dopamine functions as an important neuromodulator in the dorsal striatum and ventral striatum/nucleus accumbens. Evidence is accumulating for the idea that striatal neurons compete with each other for control over the animal's motor resources, and that dopamine plays an important modulatory role that allows a particular subset of neurons, encoding a specific behavior, to predominate in this competition. One means by which dopamine could facilitate selection among competing neurons is to enhance the contrast between stronger and weaker excitations (or to increase the "signal to noise ratio" among neurons, where the firing of the most excited neurons is assumed to transmit signal and the firing of the least excited to transmit noise). Here, we review the electrophysiological evidence for this hypothesis and discuss potential cellular mechanisms by which dopamine-mediated contrast enhancement could occur.

Spontaneous voltage oscillations in striatal projection neurons in a rat corticostriatal slice

In a rat corticostriatal slice, brief, suprathreshold, repetitive cortical stimulation evoked long-lasting plateau potentials in neostriatal neurons. Plateau potentials were often followed by spontaneous voltage transitions between two preferred membrane potentials. While the induction of plateau potentials was disrupted by non-NMDA and NMDA glutamate receptor antagonists, the maintenance of spontaneous voltage transitions was only blocked by NMDA receptor and L-type Ca2+ channel antagonists. The frequency and duration of depolarized events, resembling up-states described in vivo, were increased by NMDA and L-type Ca2+ channel agonists as well as by GABAA receptor and K+ channel antagonists. NMDA created a region of negative slope conductance and a positive slope crossing indicative of membrane bistability in the current-voltage relationship. NMDA-induced bistability was partially blocked by L-type Ca2+ channel antagonists. Although evoked by synaptic stimulation, plateau potentials and voltage oscillations could not be evoked by somatic current injection--suggesting a dendritic origin. These data show that NMDA and L-type Ca2+ conductances of spiny neurons are capable of rendering them bistable. This may help to support prolonged depolarizations and voltage oscillations under certain conditions.