Dopamine D(2)-like receptors selectively block N-type Ca(2+) channels to reduce GABA release onto rat striatal cholinergic interneurones

Parvalbumin interneuron deficiency in the prefrontal and motor cortices of spontaneously hypertensive rats: an attention-deficit hyperactivity disorder animal model insight

Background

Attention deficit hyperactivity disorder (ADHD) is characterized by impairments in developmental-behavioral inhibition, resulting in impulsivity and hyperactivity. Recent research has underscored cortical inhibition deficiencies in ADHD via the gamma-aminobutyric acid (GABA)ergic system, which is crucial for maintaining excitatory-inhibitory balance in the brain. This study explored postnatal changes in parvalbumin (PV) immunoreactivity, indicating GABAergic interneuron types, in the prefrontal (PFC) and motor (MC) cortices of spontaneously hypertensive rats (SHRs), an ADHD animal model.

Methods

Examining PV- positive (PV+) cells associated with dopamine D2 receptors (D2) and the impact of dopamine on GABA synthesis, we also investigated changes in the immunoreactivity of D2 and tyrosine hydroxylase (TH). Brain sections from 4- to 10-week-old SHRs and Wistar Kyoto rats (WKYs) were immunohistochemically analyzed, comparing PV+, D2+ cells, and TH+ fiber densities across age-matched SHRs and WKYs in specific PFC/MC regions.

Results

The results revealed significantly reduced PV+ cell density in SHRs: prelimbic (~20% less), anterior cingulate (~15% less), primary (~15% less), and secondary motor (~17% less) cortices. PV+ deficits coincided with the upregulation of D2 in prepubertal SHRs and the downregulation of TH predominantly in pubertal/postpubertal SHRs.

Conclusion

Reduced PV+ cells in various PFC regions could contribute to inattention/behavioral alterations in ADHD, while MC deficits could manifest as motor hyperactivity. D2 upregulation and TH deficits may impact GABA synthesis, exacerbating behavioral deficits in ADHD. These findings not only shed new light on ADHD pathophysiology but also pave the way for future research endeavors.

Acetylcholine release from striatal cholinergic interneurons is controlled differently depending on the firing pattern

How is the quantal size in neurotransmitter release adjusted for various firing levels? We explored the possible mechanisms that regulate acetylcholine (ACh) release from cholinergic interneurons using an ultra-mini superfusion system. After preloading [3 H]ACh in rat striatal cholinergic interneurons, the release was elicited by electrical stimulation under a condition in which presynaptic cholinergic and dopaminergic feedback was inhibited. [3 H]ACh release was reproducible at intervals of more than 10 min; shorter intervals resulted in reduced levels of ACh release. Upon persistent stimulation for 10 min, ACh release transiently increased, before gradually decreasing. Vesamicol, an inhibitor of the vesicular ACh transporter (VAChT), had no effect on the release induced by the first single pulse, but it reduced the release caused by subsequent pulses. Vesamicol also reduced the [3 H]ACh release evoked by multiple pulses, and the inhibition was enhanced by repetitive stimulation. The decreasing phase of [3 H]ACh release during persistent stimulation was accelerated by vesamicol treatment. Thus, it is likely that releasable ACh was slowly compensated for via VAChT during and after stimulation, changing the vesicular ACh content. In addition, ACh release per pulse decreased under high-frequency stimulation. The present results suggest that ACh release from striatal cholinergic interneurons may be adjusted by changes in the quantal size due to slow replenishment via VAChT, and by a reduction in release probability upon high-frequency stimulation. These two distinct processes likely enable the fine tuning of neurotransmission and neuroprotection/limitation against excessive output and have important physiological roles in the brain.

Presynaptic nigral GPR55 receptors stimulate [3 H]-GABA release through [3 H]-cAMP production and PKA activation and promote motor behavior

Striatal medium-sized spiny neurons express mRNA and protein of GPR55 receptors that stimulate neurotransmitter release; thus, GPR55 could be sent to nigral striatal projections, where it might modulate GABA release and motor behavior. Here we study the presence of GPR55 receptors at striato-nigral terminals, their modulation of GABA release, their signaling pathway, and their effect on motor activity. By double immunohistochemistry, we found the colocation of GPR55 protein and substance P in the dorsal striatum. In slices of the rat substantia nigra, the GPR55 agonists LPI and O-1602 stimulated [³ H]-GABA release induced by high K⁺ depolarization in a dose-dependent manner. The antagonists CID16020046 and cannabidiol prevented agonist stimulation in a dose-dependent way. The effect of GPR55 on nigral [³ H]-GABA release was prevented by lesion of the striatum with kainic acid, which was accompanied by a decrement of GPR55 protein in nigral synaptosomes, indicating the presynaptic location of receptors. The depletion of internal Ca²⁺ stores with thapsigargin did not prevent the effect of LPI on [³ H]-GABA release, but the remotion or chelation of external calcium did. Blockade of Gi, Gs, PLC, PKC, or dopamine D1 receptor signaling proteins did not prevent the effect of GPR55 on release. However, the activation of GPR55 stimulated [³ H]-cAMP accumulation and PKA activity. Intranigral unilateral injection of LPI induces contralateral turning. This turning was prevented by CID16020046, cannabidiol, and bicuculline but not by SCH 23390. Our data indicate that presynaptic GPR55 receptors stimulate [³ H]-GABA release at striato-nigral terminals through [³ H]-cAMP production and stimulate motor behavior. This article is protected by copyright. All rights reserved.

Dopamine D2L Receptor Deficiency Alters Neuronal Excitability and Spine Formation in Mouse Striatum

The striatum contains several types of neurons including medium spiny projection neurons (MSNs), cholinergic interneurons (ChIs), and fast-spiking interneurons (FSIs). Modulating the activity of these neurons by the dopamine D2 receptor (D2R) can greatly impact motor control and movement disorders. D2R exists in two isoforms: D2L and D2S. Here, we assessed whether alterations in the D2L and D2S expression levels affect neuronal excitability and synaptic function in striatal neurons. We observed that quinpirole inhibited the firing rate of all three types of striatal neurons in wild-type (WT) mice. However, in D2L knockout (KO) mice, quinpirole enhanced the excitability of ChIs, lost influence on spike firing of MSNs, and remained inhibitory effect on spike firing of FSIs. Additionally, we showed mIPSC frequency (but not mIPSC amplitude) was reduced in ChIs from D2L KO mice compared with WT mice, suggesting spontaneous GABA release is reduced at GABAergic terminals onto ChIs in D2L KO mice. Furthermore, we found D2L deficiency resulted in reduced dendritic spine density in ChIs, suggesting D2L activation plays a role in the formation/maintenance of dendritic spines of ChIs. These findings suggest new molecular and cellular mechanisms for causing ChIs abnormality seen in Parkinson’s disease or drug-induced dyskinesias.

Evaluation of radiolabeled acetylcholine synthesis and release in rat striatum

Cholinergic transmission underlies higher brain functions such as cognition and movement. To elucidate the process whereby acetylcholine (ACh) release is maintained and regulated in the central nervous system, uptake of [³ H]choline and subsequent synthesis and release of [³ H]ACh were investigated in rat striatal segments. Incubation with [³ H]choline elicited efficient uptake via high-affinity choline transporter-1, resulting in accumulation of [³ H]choline and [³ H]ACh. However, following inhibition of ACh esterase (AChE), incubation with [³ H]choline led predominantly to the accumulation of [³ H]ACh. Electrical stimulation and KCl depolarization selectively released [³ H]ACh but not [³ H]choline. [³ H]ACh release gradually declined upon repetitive stimulation, whereas the release was reproducible under inhibition of AChE. [³ H]ACh release was abolished after treatment with vesamicol, an inhibitor of vesicular ACh transporter. These results suggest that releasable ACh is continually replenished from the cytosol to releasable pools of cholinergic vesicles to maintain cholinergic transmission. [³ H]ACh release evoked by electrical stimulation was abolished by tetrodotoxin, but that induced by KCl was largely resistant. ACh release was Ca²⁺ dependent and exhibited slightly different sensitivities to N- and P-type Ca²⁺ channel toxins (ω-conotoxin GVIA and ω-agatoxin IVA, respectively) between both stimuli. [³ H]ACh release was negatively regulated by M2 muscarinic and D2 dopaminergic receptors. The present results suggest that inhibition of AChE within cholinergic neurons and of presynaptic negative regulation of ACh release contributes to maintenance and facilitation of cholinergic transmission, providing a potentially useful clue for the development of therapies for cholinergic dysfunction-associated disorders, in addition to inhibition of synaptic cleft AChE.

Ionotropic Receptors as a Driving Force behind Human Synapse Establishment

The origin of nervous systems is a main theme in biology and its mechanisms are largely underlied by synaptic neurotransmission. One problem to explain synapse establishment is that synaptic orthologs are present in multiple aneural organisms. We questioned how the interactions among these elements evolved and to what extent it relates to our understanding of the nervous systems complexity. We identified the human neurotransmission gene network based on genes present in GABAergic, glutamatergic, serotonergic, dopaminergic, and cholinergic systems. The network comprises 321 human genes, 83 of which act exclusively in the nervous system. We reconstructed the evolutionary scenario of synapse emergence by looking for synaptic orthologs in 476 eukaryotes. The Human–Cnidaria common ancestor displayed a massive emergence of neuroexclusive genes, mainly ionotropic receptors, which might have been crucial to the evolution of synapses. Very few synaptic genes had their origin after the Human–Cnidaria common ancestor. We also identified a higher abundance of synaptic proteins in vertebrates, which suggests an increase in the synaptic network complexity of those organisms.

Pain Perception and Management: Where do We Stand?

BACKGROUND

Pain is often flammable, sharp and sometimes described as an electrical shock. It can be categorized in three different ways as nociceptive, neuropathic and inflammatory. Nociceptive pain always originates in specific situations such as in trauma. Neuropathic pain results in nerve damage. In inflammatory pain, inflammatory mediators are involved in the sensitization of nociceptors. It is important to control the pain as it affects the individual physically, mentally, and socially.

OBJECTIVE

Recognizing pain physiopathology and pain pathways, defining the relationship between receptor and transmitter is critical in developing new treatment strategies. In this review, current information on the definitions, classifications, and physiological and chemical mechanisms involved in pain are reviewed.

METHODS

Various search engines were used to gather related articles/information. Only peer-reviewed journals were considered. Additional, books/chapters of standard publishers were also included in the article.

RESULTS

With a better understanding of the physiological and chemical mechanisms that play a role in pain, significant improvements have been made in pain treatment. Various oral or intravenous drugs, local injection treatments, physical and occupational therapy, electrical stimulation, alternative medicine applications, psychological support, and surgical applications are routinely performed in the treatment, dependent upon the type, severity and cause of the pain.

CONCLUSION

Improved understanding of pain physiopathology will serve as the basis for future improvements in the delivery of efficacious and reliable treatments, and is likely to rely on novel technological innovations.

M1 muscarinic acetylcholine receptor-mediated inhibition of GABA release from striatal medium spiny neurons onto cholinergic interneurons

Acetylcholine (ACh) modulates neurotransmitter release in the central nervous system. Although GABAergic transmission onto the striatal cholinergic interneurons (ChIN) is modulated by dopamine receptors, cholinergic modulation of the same synapse is still unknown. In the present study, modulatory roles of ACh in the GABAergic transmission from striatal medium spiny neurons (MSNs) onto ChIN were investigated using optogenetics and whole-cell patch-clamp technique in juvenile and young-adult mice brain slices. GABA_A receptor-mediated inhibitory postsynaptic currents (IPSCs) were evoked by focal electrical- or blue-light stimulation. Bath application of carbachol, a muscarinic ACh receptor agonist, suppressed the amplitude of IPSCs in a concentration-dependent manner in both age groups. A choline esterase inhibitor, physostigmine, also suppressed the amplitude of IPSCs. In the presence of a membrane permeable M1 muscarine receptor antagonist, pirenzepine, carbachol-induced suppression of IPSCs was antagonized, whereas a M2 muscarine receptor antagonist, a M4 receptor antagonist, or a membrane impermeable M1 receptor antagonist did not antagonize carbachol-induced suppression of IPSCs. Retrograde cannabinoid cascade via cannabinoid receptor 1 was not involved in carbachol-induced inhibition. Furthermore, carbachol did not affect amplitude of inward currents induced by puff application of GABA, whereas coefficient of variation of IPSCs was significantly increased by carbachol. These results suggest that activation of presynaptic M1 muscarine receptors located on the GABAergic terminals including intracellular organelle of MSNs inhibits GABA release onto ChIN.

Lipid Rafts and Dopamine Receptor Signaling

The renal dopaminergic system has been identified as a modulator of sodium balance and blood pressure. According to the Centers for Disease Control and Prevention, in 2018 in the United States, almost half a million deaths included hypertension as a primary or contributing cause. Renal dopamine receptors, members of the G protein-coupled receptor family, are divided in two groups: D1-like receptors that act to keep the blood pressure in the normal range, and D2-like receptors with a variable effect on blood pressure, depending on volume status. The renal dopamine receptor function is regulated, in part, by its expression in microdomains in the plasma membrane. Lipid rafts form platforms within the plasma membrane for the organization and dynamic contact of molecules involved in numerous cellular processes such as ligand binding, membrane sorting, effector specificity, and signal transduction. Understanding all the components of lipid rafts, their interaction with renal dopamine receptors, and their signaling process offers an opportunity to unravel potential treatment targets that could halt the progression of hypertension, chronic kidney disease (CKD), and their complications.

Dopamine Receptor Type 2 and Ghrelin Receptor Coexpression Alters CaV2.2 Modulation by G Protein Signaling Cascades

Voltage-gated calcium channels type 2.2 (Ca_V2.2) are activated by action potentials at presynaptic terminals, and their calcium current induces neurotransmitter release. In this context, regulating Ca_V2.2 is critical, and one of the most important mechanisms for doing so is through its G protein-coupled receptor (GPCR) activity. Two such GPCRs are the ghrelin (GHSR) and the dopamine type 2 (D2R) receptors. We previously demonstrated that constitutive GHSR activity reduces Ca_V2.2 forward trafficking and that ghrelin-induced GHSR activity inhibits Ca_V2.2 currents. On the other hand, dopamine-induced D2R activity also inhibits Ca_V2.2 currents. It has been recently shown that D2R and GHSR form heteromers in hypothalamic neurons. This interaction profoundly changes the signaling cascades activated by dopamine and is necessary for dopamine-dependent anorexia. Here we explored how D2R-GHSR coexpression in HEK293T cells modulates the effect that each GPCR has on Ca_V2.2. We found that D2R-GHSR coexpression reduces the inhibition of Ca_V2.2 currents by agonist-induced D2R activation and added a new source of basal Ca_V2.2 current inhibition to the one produced by GHSR solely expression. We investigated the signaling cascades implicated and found that constitutive GHSR activity, G_q protein, and Gβγ subunit play a critical role in these altered effects. Moreover, we found that the effect of D2R agonist on native calcium currents in hypothalamic neurons is reduced when both D2R and GHSR are overexpressed. In summary, our results allow us to propose a novel mechanism for controlling Ca_V2.2 currents involving the coexpression of two physiologically relevant GPCRs.

Dopamine-mediated calcium channel regulation in synaptic suppression in L. stagnalis interneurons

D2 dopamine receptor-mediated suppression of synaptic transmission from interneurons plays a key role in neurobiological functions across species, ranging from respiration to memory formation. In this study, we investigated the mechanisms of D2 receptor-dependent suppression using soma-soma synapse between respiratory interneuron VD4 and LPeD1 in the mollusk Lymnaea stagnalis (L. stagnalis). We studied the effects of dopamine on voltage-dependent Ca²⁺ current and synaptic vesicle release from the VD4. We report that dopamine inhibits voltage-dependent Ca²⁺ current in the VD4 by both voltage-dependent and -independent mechanisms. Dopamine also suppresses synaptic vesicle release downstream of activity-dependent Ca²⁺ influx. Our study demonstrated that dopamine acts through D2 receptors to inhibit interneuron synaptic transmission through both voltage-dependent Ca²⁺ channel-dependent and -independent pathways. Taken together, these findings expand our understanding of dopamine function and fundamental mechanisms that shape the dynamics of neural circuit.

Modulation of Ion Channels in the Axon: Mechanisms and Function

The axon is responsible for integrating synaptic signals, generating action potentials (APs), propagating those APs to downstream synapses and converting them into patterns of neurotransmitter vesicle release. This process is mediated by a rich assortment of voltage-gated ion channels whose function can be affected on short and long time scales by activity. Moreover, neuromodulators control the activity of these proteins through G-protein coupled receptor signaling cascades. Here, we review cellular mechanisms and signaling pathways involved in axonal ion channel modulation and examine how changes to ion channel function affect AP initiation, AP propagation, and the release of neurotransmitter. We then examine how these mechanisms could modulate synaptic function by focusing on three key features of synaptic information transmission: synaptic strength, synaptic variability, and short-term plasticity. Viewing these cellular mechanisms of neuromodulation from a functional perspective may assist in extending these findings to theories of neural circuit function and its neuromodulation.

Substantia nigra dopaminergic neurons and striatal interneurons are engaged in three parallel but interdependent postnatal neurotrophic circuits

The striatum integrates motor behavior using a well-defined microcircuit whose individual components are independently affected in several neurological diseases. The glial cell line-derived neurotrophic factor (GDNF), synthesized by striatal interneurons, and Sonic hedgehog (Shh), produced by the dopaminergic neurons of the substantia nigra (DA SNpc), are both involved in the nigrostriatal maintenance but the reciprocal neurotrophic relationships among these neurons are only partially understood. To define the postnatal neurotrophic connections among fast-spiking GABAergic interneurons (FS), cholinergic interneurons (ACh), and DA SNpc, we used a genetically induced mouse model of postnatal DA SNpc neurodegeneration and separately eliminated Smoothened (Smo), the obligatory transducer of Shh signaling, in striatal interneurons. We show that FS postnatal survival relies on DA SNpc and is independent of Shh signaling. On the contrary, Shh signaling but not dopaminergic striatal innervation is required to maintain ACh in the postnatal striatum. ACh are required for DA SNpc survival in a GDNF-independent manner. These data demonstrate the existence of three parallel but interdependent neurotrophic relationships between SN and striatal interneurons, partially defined by Shh and GDNF. The definition of these new neurotrophic interactions opens the search for new molecules involved in the striatal modulatory circuit maintenance with potential therapeutic value.

Presynaptic Inhibition of Primary Nociceptive Signals to Dorsal Horn Lamina I Neurons by Dopamine

The dorsal horn of the spinal cord represents the first relay station in the pain pathway where primary nociceptive inputs are modulated by local circuits and by descending signals before being relayed to supraspinal nuclei. To determine whether dopamine can modulate primary nociceptive Aδ- and C-fiber signals, the effects of dopamine were tested on the excitatory postsynaptic currents (EPSCs) recorded from large lamina I neurons and from retrograde-labeled spinoparabrachial lamina I neurons upon stimulation of the L4/L5 dorsal root in horizontal spinal cord slices in vitro Dopamine inhibited the EPSCs in a dose-dependent manner, with substantial inhibition (33%) at 1 μm and maximum inhibition (∼70%) at 10-20 μm Dopamine reduced the frequency of miniature EPSCs recorded from large lamina I neurons, increased the paired pulse depression ratio of paired EPSCs, and induced similar inhibition of EPSCs after dialysis of large lamina I neurons with GDP-β-S, consistent with actions at presynaptic sites. Pharmacological experiments suggested that the inhibitory effects of dopamine were largely mediated by D4 receptors (53%). Similar inhibition (66%) by dopamine was observed on EPSCs recorded from ipsilateral large lamina I neurons 6 d after injection of complete Freund's adjuvant in the hindpaw, suggesting that dopamine downregulates primary nociceptive inputs to lamina I neurons during chronic inflammatory pain. We propose that presynaptic inhibition of primary nociceptive inputs to lamina I projection neurons is a mechanism whereby dopamine can inhibit incoming noxious stimuli to the dorsal horn of the spinal cord.SIGNIFICANCE STATEMENT Lamina I projection neurons represent the main output for the pain signals from the dorsal horn of the spinal cord to brainstem and thalamic nuclei. We found that dopamine inhibits the nociceptive Aδ- and C-fiber synaptic inputs to lamina I projection neurons via presynaptic actions. Similar inhibitory effects of dopamine on the EPSCs were observed in rats subjected to complete Freund's adjuvant to induce peripheral inflammation, suggesting that dopamine inhibits the synaptic inputs to lamina I neurons in the setting of injury. A better understanding of how primary nociceptive inputs to the dorsal horn of the spinal cord are modulated by descending monoaminergic signals may help in the development of new pharmacological strategies to selectively downregulate the output from lamina I projection neurons.

L-DOPA inhibits excitatory synaptic transmission in the rat nucleus tractus solitarius through release of dopamine

The mode of action of L-DOPA on excitatory synaptic transmission in second-order neurons of the nucleus tractus solitarius (NTS) was studied using the rat brainstem slices. Superfusion of L-DOPA (10μM) reduced the frequency of miniature excitatory postsynaptic currents (mEPSCs) without any effect on the amplitude. A low concentration (1μM) was ineffective on the mEPSCs, and the highest concentration (100μM) exerted a stronger inhibitory effect. L-DOPA (10μM) decreased the amplitude of EPSCs (eEPSCs) evoked by electrical stimulation of the tractus solitarius and increased the paired-pulse ratio. The inhibitory effects of L-DOPA on mEPSCs and eEPSCs were similar to those of dopamine (100μM). The effects of L-DOPA were blocked by a competitive antagonist, L-DOPA methyl ester (100μM) and also by a D₂ receptor antagonist, sulpiride (10μM), while those of dopamine were blocked by the latter but not by the former. In reserpine (5mg/kg, s.c.)-treated rats, the effects of L-DOPA on both mEPSCs and eEPSCs were completely abolished, but those of dopamine remained unchanged. The present results suggest a possibility that L-DOPA may induce the release of dopamine from the axon terminals in the NTS and the released dopamine suppresses the glutamatergic transmission through activation of the presynaptic D₂ receptors.

Sigma-1 Receptor Plays a Negative Modulation on N-type Calcium Channel

The sigma-1 receptor is a 223 amino acids molecular chaperone with a single transmembrane domain. It is resident to eukaryotic mitochondrial-associated endoplasmic reticulum and plasma membranes. By chaperone-mediated interactions with ion channels, G-protein coupled receptors and cell-signaling molecules, the sigma-1 receptor performs broad physiological and pharmacological functions. Despite sigma-1 receptors have been confirmed to regulate various types of ion channels, the relationship between the sigma-1 receptor and N-type Ca²⁺ channel is still unclear. Considering both sigma-1 receptors and N-type Ca²⁺ channels are involved in intracellular calcium homeostasis and neurotransmission, we undertake studies to explore the possible interaction between these two proteins. In the experiment, we confirmed the expression of the sigma-1 receptors and the N-type calcium channels in the cholinergic interneurons (ChIs) in rat striatum by using single-cell reverse transcription-polymerase chain reaction (scRT-PCR) and immunofluorescence staining. N-type Ca²⁺ currents recorded from ChIs in the brain slice of rat striatum was depressed when sigma-1 receptor agonists (SKF-10047 and Pre-084) were administrated. The inhibition was completely abolished by sigma-1 receptor antagonist (BD-1063). Co-expression of the sigma-1 receptors and the N-type calcium channels in Xenopus oocytes presented a decrease of N-type Ca²⁺ current amplitude with an increase of sigma-1 receptor expression. SKF-10047 could further depress N-type Ca²⁺ currents recorded from oocytes. The fluorescence resonance energy transfer (FRET) assays and co-immunoprecipitation (Co-IP) demonstrated that sigma-1 receptors and N-type Ca²⁺ channels formed a protein complex when they were co-expressed in HEK-293T (Human Embryonic Kidney -293T) cells. Our results revealed that the sigma-1 receptors played a negative modulation on N-type Ca²⁺ channels. The mechanism for the inhibition of sigma-1 receptors on N-type Ca²⁺ channels probably involved a chaperone-mediated direct interaction and agonist-induced conformational changes in the receptor-channel complexes on the cell surface.

Dopamine and Serotonin-Induced Modulation of GABAergic and Glutamatergic Transmission in the Striatum and Basal Forebrain

Catecholamine receptor-mediated modulation of glutamatergic or GABAergic transmission in the striatum as well as basal forebrain (BF) has been intensively studied during these two decades. In the striatum, activation of dopamine (DA) D2 receptors in GABAergic terminals inhibits GABA release onto cholinergic interneurons by selective blockade of N-type calcium channels. In the BF, glutamatergic transmission onto cholinergic projection neurons is inhibited via DA D1-like receptors by selective blockade of P/Q-type calcium channels. On the other hand, presynaptic inhibition of the GABA release onto cholinergic neurons mediated by D1-like receptors or 5-HT_1B receptors is independent of calcium influx. In addition, the DA receptor-mediated calcium influx dependent presynaptic inhibition mentioned above decreases with postnatal development, with selective coupling between DA receptors and each subtype of calcium channels being unchanged. Furthermore, the precise origin of these GABAergic or glutamatergic inputs to postsynaptic neurons can be identified by recent optogenetic approaches. Thus, modulatory mechanisms in specific synaptic connections between certain types of neurons in the striatum and BF are being identified.

Neuronal circuits and physiological roles of the basal ganglia in terms of transmitters, receptors and related disorders

The authors have reviewed recent research advances in basal ganglia circuitry and function, as well as in related disorders from multidisciplinary perspectives derived from the results of morphological, electrophysiological, behavioral, biochemical and molecular biological studies. Based on their expertise in their respective fields, as denoted in the text, the authors discuss five distinct research topics, as follows: (1) area-specific dopamine receptor expression of astrocytes in basal ganglia, (2) the role of physiologically released dopamine in the striatum, (3) control of behavioral flexibility by striatal cholinergic interneurons, (4) regulation of phosphorylation states of DARPP-32 by protein phosphatases and (5) physiological perspective on deep brain stimulation with optogenetics and closed-loop control for ameliorating parkinsonism.

Dopamine neurons control striatal cholinergic neurons via regionally heterogeneous dopamine and glutamate signaling

Midbrain dopamine neurons fire in bursts conveying salient information. Bursts are associated with pauses in tonic firing of striatal cholinergic interneurons. Although the reciprocal balance of dopamine and acetylcholine in the striatum is well known, how dopamine neurons control cholinergic neurons has not been elucidated. Here, we show that dopamine neurons make direct fast dopaminergic and glutamatergic connections with cholinergic interneurons, with regional heterogeneity. Dopamine neurons drive a burst-pause firing sequence in cholinergic interneurons in the medial shell of the nucleus accumbens, mixed actions in the accumbens core, and a pause in the dorsal striatum. This heterogeneity is due mainly to regional variation in dopamine-neuron glutamate cotransmission. A single dose of amphetamine attenuates dopamine neuron connections to cholinergic interneurons with dose-dependent regional specificity. Overall, the present data indicate that dopamine neurons control striatal circuit function via discrete, plastic connections with cholinergic interneurons.

GABAergic synaptic transmission onto striatal cholinergic interneurons in dopamine D2 receptor knock-out mice

Whole-cell or cell-attached analysis was carried out in dopamine (DA) D2 receptor (D2R) knock-out (KO) mice to elucidate the function of this receptor in the regulation of GABAergic synaptic transmission onto striatal cholinergic interneurons as well as their spontaneous firing. In slice preparation obtained from wild-type mice, evoked GABAergic inhibitory postsynaptic currents (IPSCs) showed frequency-dependent suppression, and this suppression significantly decreased in the presence of a D2-like receptor antagonist or in D2R KO mice. Contribution of N-type calcium channel was also significantly reduced in the striatal cholinergic interneurons of the D2R KO mice compared with that in the wild-type mice. Spontaneous firing of striatal cholinergic interneurons was inhibited by 5- or 10-Hz stimulation, and the suppression was decreased in the presence of a D2-like receptor antagonist or in D2R KO mice. These findings substantiate the physiological role of D2R in the regulation of GABAergic synaptic transmission onto striatal cholinergic interneurons as well as their excitability, confirming the tight coupling between D2R and N-type calcium channels in the regulation of GABA release.

Time-dependent effects of haloperidol on glutamine and GABA homeostasis and astrocyte activity in the rat brain

RATIONALE

Schizophrenia is a severe, persistent, and fairly common mental illness. Haloperidol is widely used and is effective against the symptoms of psychosis seen in schizophrenia. Chronic oral haloperidol administration decreased the number of astrocytes in the parietal cortex of macaque monkeys (Konopaske et al., Biol Psych 63:759-765, 2008). Since astrocytes play a key role in glutamate metabolism, chronic haloperidol administration was hypothesized to modulate astrocyte metabolic function and glutamate homeostasis.

OBJECTIVES

This study investigated the effects of chronic haloperidol administration on astrocyte metabolic activity and glutamate, glutamine, and GABA homeostasis.

METHODS

We used ex vivo ¹³C magnetic resonance spectroscopy along with high-performance liquid chromatography after [1-¹³C]glucose and [1,2-¹³C]acetate administration to analyze forebrain tissue from rats administered oral haloperidol for 1 or 6 months.

RESULTS

Administration of haloperidol for 1 month produced no changes in ¹³C labeling of glutamate, glutamine, or GABA, or in their total levels. However, a 6-month haloperidol administration increased ¹³C labeling of glutamine by [1,2-¹³C]acetate. Moreover, total GABA levels were also increased. Haloperidol administration also increased the acetate/glucose utilization ratio for glutamine in the 6-month cohort.

CONCLUSIONS

Chronic haloperidol administration in rats appears to increase forebrain GABA production along with astrocyte metabolic activity. Studies exploring these processes in subjects with schizophrenia should take into account the potential confounding effects of antipsychotic medication treatment.

Activation of D2-like dopamine receptors inhibits GABA and glycinergic neurotransmission to pre-motor cardiac vagal neurons in the nucleus ambiguus

The parasympathetic control of heart rate arises from premotor cardiac vagal neurons (CVNs) located in the nucleus ambiguus (NA). Previous microinjection studies in NA show that dopamine evokes a decrease in heart rate, but the underlying mechanisms responsible for these responses were not identified. This study tested whether dopamine modulates inhibitory GABAergic and glycinergic and/or excitatory glutamatergic neurotransmission to CVNs. Retrogradely labeled CVNs were identified in an in vitro rat brainstem slice preparation and synaptic events were recorded using whole cell voltage clamp techniques. Bath application of dopamine (100 μM) had no effect on excitatory synaptic events, but reversibly inhibited the frequency (but not amplitude) of GABAergic inhibitory postsynaptic currents (IPSCs) in CVNs. Similarly, dopamine (10 μM and 100 μM) inhibited glycinergic IPSC frequency by ~50% and 70% respectively. The reduction in inhibitory neurotransmission to CVNs by dopamine was prevented by the sodium channel blocker TTX (1μM) indicating that the dopamine mediated effects were action potential dependent. Dopamine evoked responses were mimicked by the D2-like receptor agonist, Quinpirole but not D1-like receptor agonist, SKF 38393. In addition, the dopamine mediated depression of inhibitory synaptic responses were prevented by the D2-like receptor antagonist sulpiride, but not by D1-like or adrenergic or serotonergic receptor antagonists, suggesting that these responses were D2-like receptor mediated and not D1-like or adrenergic or 5-HT receptor mediated. These data suggest that dopamine acts via dis-inhibition, and diminishes inhibitory GABAergic and glycinergic neurotransmission to CVNs, which would be predicted to increase parasympathetic activity to the heart and evoke a bradycardia.

Neurochemical and behavioral features in genetic absence epilepsy and in acutely induced absence seizures

The absence epilepsy typical electroencephalographic pattern of sharp spikes and slow waves (SWDs) is considered to be due to an interaction of an initiation site in the cortex and a resonant circuit in the thalamus. The hyperpolarization-activated cyclic nucleotide-gated cationic I h pacemaker channels (HCN) play an important role in the enhanced cortical excitability. The role of thalamic HCN in SWD occurrence is less clear. Absence epilepsy in the WAG/Rij strain is accompanied by deficiency of the activity of dopaminergic system, which weakens the formation of an emotional positive state, causes depression-like symptoms, and counteracts learning and memory processes. It also enhances GABAA receptor activity in the striatum, globus pallidus, and reticular thalamic nucleus, causing a rise of SWD activity in the cortico-thalamo-cortical networks. One of the reasons for the occurrence of absences is that several genes coding of GABAA receptors are mutated. The question arises: what the role of DA receptors is. Two mechanisms that cause an infringement of the function of DA receptors in this genetic absence epilepsy model are proposed.

Short-term plasticity shapes activity pattern-dependent striato-pallidal synaptic transmission

The cortico-striato (Str)-globus pallidus external segment (GPe) projection plays major roles in the control of neuronal activity in the basal ganglia under both normal and pathological conditions. The present study used rat brain-slice preparations to address our hypothesis that the gain of this disynaptic projection is dynamically controlled by activations of short-term plasticity mechanisms of Str-GPe synapses. The Str-GPe projection neurons fire with very different frequency and firing patterns in vivo depending on the condition of the animal. The results show that the Str-GPe synapses have very strong short-term enhancement mechanisms and that repetitive burst activation of the Str-GPe synapses, which mimic oscillatory burst firing of Str neurons, can sustain enhanced states of synaptic transmission for tens of seconds. The results reveal that the short-term enhancement of Str-GPe synapses contributes to the generation of pauses in the firing of GPe neurons and that signal transfer function in the Str-GPe projection is highly dependent on the firing pattern of Str neurons.

Dopaminergic modulation of synaptic transmission in cortex and striatum

Among the many neuromodulators used by the mammalian brain to regulate circuit function and plasticity, dopamine (DA) stands out as one of the most behaviorally powerful. Perturbations of DA signaling are implicated in the pathogenesis or exploited in the treatment of many neuropsychiatric diseases, including Parkinson's disease (PD), addiction, schizophrenia, obsessive compulsive disorder, and Tourette's syndrome. Although the precise mechanisms employed by DA to exert its control over behavior are not fully understood, DA is known to regulate many electrical and biochemical aspects of neuronal function including excitability, synaptic transmission, integration and plasticity, protein trafficking, and gene transcription. In this Review, we discuss the actions of DA on ionic and synaptic signaling in neurons of the prefrontal cortex and striatum, brain areas in which dopaminergic dysfunction is thought to be central to disease.

Thromboxane receptor activation enhances striatal dopamine release, leading to suppression of GABAergic transmission and enhanced sugar intake

The extracellular dopamine level is regulated not only by synaptic inputs to dopamine neurons but also by local mechanisms surrounding dopaminergic terminals. However, much remains to be investigated for the latter mechanism. Thromboxane A(2) is one of the cyclooxygenase products derived from arachidonic acid, and acts on its cognate G protein-coupled receptor [thromboxane receptor (TP)]. We show here that TP in the striatum locally facilitates dopamine overflow. Intrastriatal injection of a TP agonist increased extracellular dopamine levels in the striatum as measured by in vivo microdialysis. TP stimulation also augmented electrically evoked dopamine overflow from striatal slices. Conversely, TP deficiency reduced dopamine overflow evoked by N-methyl-d-aspartic acid (NMDA) and acetylcholine in striatal slices. TP immunostaining showed that TP is enriched in vascular endothelial cells. Pharmacological blockade of nitric oxide (NO) synthesis and genetic deletion of endothelial NO synthase (eNOS) suppressed NMDA/acetylcholine-induced dopamine overflow. This involvement of NO was abolished in TP-deficient slices, suggesting a role for eNOS-derived NO synthesis in TP-mediated dopamine overflow. As a functional consequence of TP-mediated dopamine increase, a TP agonist suppressed GABAergic inhibitory postsynaptic currents in medium spiny neurons through a D2-like receptor-dependent mechanism. Finally, TP is involved in sucrose intake, a dopamine-dependent motivational behavior. These data suggest that TP stimulation in the striatum locally facilitates dopamine overflow evoked by synaptic inputs via NO synthesis in endothelial cells.

Altered profile and D2-dopamine receptor modulation of high voltage-activated calcium current in striatal medium spiny neurons from animal models of Parkinson's disease

In the present work we analyzed the profile of high voltage-activated (HVA) calcium (Ca2+) currents in freshly isolated striatal medium spiny neurons (MSNs) from rodent models of both idiopathic and familial forms of Parkinson's disease (PD). MSNs were recorded from reserpine-treated and 6-hydroxydopamine (6-OHDA)-lesioned rats, and from DJ-1 and PINK1 (PTEN induced kinase 1) knockout (-/-) mice. Our analysis showed no significant changes in total HVA Ca2+ current. However, we recorded a net increase in the L-type fraction of HVA Ca2+ current in dopamine-depleted rats, and of both N- and P-type components in DJ-1-/- mice, whereas no significant change in Ca2+ current profile was observed in PINK1-/- mice. Dopamine modulates HVA Ca2+ channels in MSNs, thus we also analyzed the effect of D1 and D2 receptor activation. The effect of the D1 receptor agonist SKF 83822 on Ca2+ current was not significantly different among MSNs from control animals or PD models. However, in both dopamine-depleted rats and DJ-1-/- mice the D2 receptor agonist quinpirole inhibited a greater fraction of HVA Ca2+ current than in the respective controls. Conversely, in MSNs from PINK1-/- mice we did not observe alterations in the effect of D2 receptor activation. Additionally, in both reserpine-treated and 6-OHDA-lesioned rats, the effect of quinpirole was occluded by the selective L-type Ca2+ channel blocker nifedipine, while in DJ-1-/- mice it was mostly occluded by ω-conotoxin GVIA, blocker of N-type channels. These results demonstrate that both dopamine depletion and DJ-1 deletion induce a rearrangement in the HVA Ca2+ channel profile, specifically involving those channels that are selectively modulated by D2 receptors.

Developmental increase in D1-like dopamine receptor-mediated inhibition of glutamatergic transmission through P/Q-type channel regulation in the basal forebrain of rats

Whole-cell patch-clamp recordings of non-N-methyl-d-aspartate glutamatergic excitatory postsynaptic currents (EPSCs) were carried out from cholinergic neurons in slices of basal forebrain (BF) of developing rats aged 21-42 postnatal days to elucidate postnatal developmental change in Ca(2+) channel subtypes involved in the transmission as well as that in dopamine D(1)-like receptor-mediated presynaptic inhibition. The amplitude of EPSCs was inhibited by bath application of omega-conotoxin GVIA (omega-CgTX; 3 microM) or omega-agatoxin-TK (omega-Aga-TK; 200 nM) throughout the age range examined, suggesting that multiple types of Ca(2+) channel are involved in the transmission. The EPSC fraction reduced by omega-CgTX decreased with age, whereas that reduced by omega-Aga-TK increased. Inhibition of the EPSCs by a D(1)-like receptor agonist, SKF 81297 (SKF; 30 microM) increased with age in parallel with the increase in omega-Aga-TK-induced inhibition. An activator of the adenylyl cyclase (AC) pathway, forskolin (FK; 10 microM) inhibited the EPSCs, and FK-induced inhibition also increased with age in parallel with the increase in SKF-induced inhibition. Throughout the age range examined, SKF showed no further inhibitory effect on the EPSCs after omega-Aga-TK- or FK-induced effect had reached steady-state. These findings suggest that D(1)-like receptor-mediated presynaptic inhibition of glutamate release onto cholinergic BF neurons increases with age, and that the change is coupled with a developmental increase in the contribution of P/Q-type Ca(2+) channels as well as a developmental increase in AC pathway contribution.

Chronic intermittent hypoxia affects integration of sensory input by neurons in the nucleus tractus solitarii

The autonomic nervous and respiratory systems, as well as their coupling, adapt over a wide range of conditions. Chronic intermittent hypoxia (CIH) is a model for recurrent apneas and induces alterations in breathing and increases in sympathetic nerve activity which may ultimately result in hypertension if left untreated. These alterations are believed to be due to increases in the carotid body chemoreflex pathway. Here we present evidence that the nucleus tractus solitarii (nTS), the central brainstem termination site of chemoreceptor afferents, expresses a form of synaptic plasticity that increases overall nTS activity following intermittent hypoxia. Following CIH, an increase in presynaptic spontaneous neurotransmitter release occurs under baseline conditions. Furthermore, during and following afferent stimulation there is an augmentation of spontaneous transmitter release that occurs out of synchrony with sensory stimulation. On the other hand, afferent evoked synchronous transmitter release is attenuated. Overall, this shift from synchronous to asynchronous transmitter release enhances nTS cellular discharge. The role of the neurotransmitter dopamine in CIH-induced plasticity is also discussed. Dopamine attenuates synaptic transmission in nTS cells by blockade of N-type calcium channels, and this mechanism occurs tonically following normoxia and CIH. This dopaminergic pathway, however, is not altered in CIH. Taken together, alterations in nTS synaptic activity may play a role in the changes of chemoreflex function and cardiorespiratory activity in the CIH apnea model.

N-type calcium channels mediate a GABA(B) presynaptic modulation in the corticostriatal synapse in turtle's paleostriatum augmentatum

Spikes population evoked by a paired pulse protocol were used to assess the influence of GABA(A) and GABA(B) receptors agonists and antagonists on the synaptic potentials and in the S2/S1 ratio in a paired pulse (PP) protocol in the cortico-paleostriatum augmentatum synapses of the turtle. GABA(A) agonist, muscimol, decreased the amplitude of synaptic responses whereas the facilitation produced with the PP protocol did not change, suggesting a postsynaptic action for GABA(A) receptors. GABA(B) agonist, baclofen, enhanced paired pulse ratio indicating a presynaptic modulation through the GABA(B) receptor. Selective antagonists for N- and P/Q-type Ca(2+)-channels also enhanced paired pulse ratio, suggesting that any of these channel types may be involved in neurotransmitter release. However, the strong paired pulse facilitation produced by baclofen was occluded by blocking the N-type Ca2+ channels with omega-conotoxin GVIA (1 microM), but not by the blockage of P/Q-type Ca2+ channels with omega-agatoxin TK (400 nM). These data suggest that N and P/Q channels participate in the neurotransmitter release, whereas only N-type Ca2+ channels are involved in the presynaptic modulation of GABA(B) in the corticostriatal synapse of the turtle.

Differential dopaminergic modulation of neostriatal synaptic connections of striatopallidal axon collaterals

Recent studies have demonstrated that GABAergic synaptic transmission among neostriatal spiny projection neurons (SPNs) is strongly modulated by dopamine with individual connections exhibiting either D(1) receptor (D(1)R)-mediated facilitation or D(2) receptor (D(2)R)-mediated inhibition and, at least in some preparations, a subset of connections exhibiting both of these effects. In light of the cell type-specific expression of D(1a)R in striatonigral and D(2)R in striatopallidal neurons and the differential expression of the other D(1) and D(2) family dopamine receptors, we hypothesize that the nature of the dopaminergic modulation is specific to the types of SPNs that participate in the connection. Here the biophysical properties and dopaminergic modulation of intrastriatal connections formed by striatopallidal neurons were examined. Contrary to previous expectation, synapses formed by striatopallidal neurons were biophysically and pharmacologically heterogeneous. Two distinct types of axon collateral connections could be distinguished among striatopallidal neurons. The more common, small-amplitude connections (80%) exhibited mean IPSC amplitudes several times smaller than their less frequent large-amplitude counterparts, principally because of a smaller number of release sites involved. The two types of connections were also differentially regulated by dopamine. Small-amplitude connections exhibited strong and exclusively D(2)R-mediated presynaptic inhibition, whereas large-amplitude connections were unresponsive to dopamine. Synaptic connections from striatopallidal to striatonigral neurons exhibited exclusively D(2)R-mediated presynaptic inhibition that was similar to the regulation of small-amplitude connections between pairs of striatopallidal cells. Together, these findings demonstrate a previously unrecognized complexity in the organization and dopaminergic control of synaptic communication among SPNs.

Dopaminergic signals in primary motor cortex

Brainstem monoamine areas such as the ventral tegmental area (VTA) send dopaminergic projections to the cerebral cortex that are widely distributed across different cortical regions. Whereas the projection to prefrontal areas (PFC) has been studied in detail, little is known about dopaminergic projections to primary motor cortex (M1). These projections have been anatomically characterized in rat and primate M1. Primates have even denser dopaminergic projections to M1 than rats. The physiological role, the effects of dopaminergic input on the activity of M1 circuits, and the behavioral function of this projection are unknown. This review explores the existing anatomical, electrophysiological and behavioral evidence on dopaminergic projections to M1 and speculates about its functional role. The projection may explain basic features of motor learning and memory phenomena. It is of clinical interest because of its potential for augmenting motor recovery after a brain lesion as well as for understanding the symptomatology of patients with Parkinson's disease. Therefore, targeted investigations are necessary.

Plasticity in glutamatergic NTS neurotransmission

Changes in the physiological state of an animal or human can result in alterations in the cardiovascular and respiratory system in order to maintain homeostasis. Accordingly, the cardiovascular and respiratory systems are not static but readily adapt under a variety of circumstances. The same can be said for the brainstem circuits that control these systems. The nucleus tractus solitarius (NTS) is the central integration site of baroreceptor and chemoreceptor sensory afferent fibers. This central nucleus, and in particular the synapse between the sensory afferent and second-order NTS cell, possesses a remarkable degree of plasticity in response to a variety of stimuli, both acute and chronic. This brief review is intended to describe the plasticity observed in the NTS as well as the locus and mechanisms as they are currently understood. The functional consequence of NTS plasticity is also discussed.

Dopamine inhibits N-type channels in visceral afferents to reduce synaptic transmitter release under normoxic and chronic intermittent hypoxic conditions

Glutamatergic synaptic currents elicited in second-order neurons in the nucleus of the solitary tract (nTS) by activation of chemosensory and other visceral afferent fibers are severely reduced following 10 days of chronic intermittent hypoxia (CIH). The mechanism by which this occurs is unknown. A strong candidate for producing the inhibition is dopamine, which is also released from the presynaptic terminals and which we have shown exerts a tonic presynaptic inhibition on glutamate release. We postulated that tonic activation of the D2 receptors inhibits presynaptic calcium currents to reduce transmitter release and that in CIH this occurs in conjunction with an increase in the dopamine inhibitory response due to the increase in presynaptic D2 receptors or an increase in dopamine release further suppressing the evoked excitatory postsynaptic current (eEPSC). Thus we predicted that blockade of the D2 receptors would return the EPSC to values of animals maintained under normoxic conditions. We found that dopamine and quinpirole, the selective D2-like agonist, inhibit calcium currents via the D2 receptors by acting on the N-type calcium channel in presynaptic neurons and their nTS central terminals. However, in brain slice studies from CIH animals, although the D2 antagonist sulpiride increased the CIH-reduced amplitude of synaptic currents, EPSCs were not restored to normal levels. This indicates that while the dopamine inhibitory effect remains intact in CIH, most of the reduction in the eEPSC amplitude occurs via alternative mechanisms.

A blocker of N- and T-type voltage-gated calcium channels attenuates ethanol-induced intoxication, place preference, self-administration, and reinstatement

There is a clear need for new therapeutics to treat alcoholism. Here, we test our hypothesis that selective inhibitors of neuronal calcium channels will reduce ethanol consumption and intoxication, based on our previous studies using knock-out mice and cell culture systems. We demonstrate that pretreatment with the novel mixed N-type and T-type calcium channel antagonist 1-(6,6-bis(4-fluorophenyl)hexyl)-4-(3,4,5-trimethoxybenzyl)piperazine (NP078585) reduced ethanol intoxication. NP078585 also attenuated the reinforcing and rewarding properties of ethanol, measured by operant self-administration and the expression of an ethanol conditioned place preference, and abolished stress-induced reinstatement of ethanol seeking. NP078585 did not affect alcohol responses in mice lacking N-type calcium channels. These results suggest that selective calcium channel inhibitors may be useful in reducing acute ethanol intoxication and alcohol consumption by human alcoholics.

Regional changes in the cholinergic system in mice lacking monoamine oxidase A

Elevated brain monoamine concentrations resulting from monoamine oxidase A genetic ablation (MAOA knock-out mice) lead to changes in other neurotransmitter systems. To investigate the consequences of MAOA deficiency on the cholinergic system, we measured ligand binding to the high-affinity choline transporter (CHT1) and to muscarinic and nicotinic receptors in brain sections of MAOA knock-out (KO) and wild-type mice. A twofold increase in [(3)H]-hemicholinium-3 ([(3)H]-HC-3) binding to CHT1 was observed in the caudate putamen, nucleus accumbens, and motor cortex in MAOA KO mice as compared with wild-type (WT) mice. There was no difference in [(3)H]-HC-3 labeling in the hippocampus (dentate gyrus) between the two genotypes. Binding of [(125)I]-epibatidine ([(125)I]-Epi), [(125)I]-alpha-bungarotoxin ([(125)I]-BGT), [(3)H]-pirenzepine ([(3)H]-PZR), and [(3)H]-AFDX-384 ([(3)H]-AFX), which respectively label high- and low-affinity nicotinic receptors, M1 and M2 muscarinic cholinergic receptors, was not modified in the caudate putamen, nucleus accumbens, and motor cortex. A small but significant decrease of 19% in M1 binding densities was observed in the hippocampus (CA1 field) of KO mice. Next, we tested acetylcholinesterase activity and found that it was decreased by 25% in the striatum of KO mice as compared with WT mice. Our data suggest that genetic deficiency in MAOA enzyme is associated with changes in cholinergic activity, which may account for some of the behavioral alterations observed in mice and humans lacking MAOA.

Recurrent inhibitory network among striatal cholinergic interneurons

The striatum plays a central role in sensorimotor learning and action selection. Tonically active cholinergic interneurons in the striatum give rise to dense axonal arborizations and significantly shape striatal output. However, it is not clear how the activity of these neurons is regulated within the striatal microcircuitry. In this study, using rat brain slices, we find that stimulation of intrastriatal cholinergic fibers evokes polysynaptic GABA(A) IPSCs in cholinergic interneurons. These polysynaptic GABA(A) IPSCs were abolished by general nicotinic acetylcholine receptor antagonists and also by a specific antagonist of nicotinic receptors containing beta2 subunits. Dopamine receptor antagonists or dopamine depletion failed to block polysynaptic IPSCs, indicating that phasic dopamine release does not directly mediate the polysynaptic transmission. Dual recording from pairs of cholinergic interneurons revealed that activation of a single cholinergic interneuron is capable of eliciting polysynaptic GABA(A) IPSCs both in itself and in nearby cholinergic interneurons. Although polysynaptic transmission arising from a single cholinergic interneuron was depressed during repetitive 2 Hz firing, intrastriatal stimulation reliably evoked large polysynaptic IPSCs by recruiting many cholinergic fibers. We also show that polysynaptic GABAergic inhibition leads to a transient suppression of tonic cholinergic interneuron firing. We propose a novel microcircuit in the striatum, in which cholinergic interneurons are connected to one another through GABAergic interneurons. This may provide a mechanism to convert activation of cholinergic interneurons into widespread recurrent inhibition of these neurons via nicotinic excitation of striatal GABAergic neurons.

Dopamine and cyclic-AMP regulated phosphoprotein-32-dependent modulation of prefrontal cortical input and intercellular coupling in mouse accumbens spiny and aspiny neurons

The roles of dopamine and cyclic-AMP regulated phosphoprotein-32 (DARPP-32) in mediating dopamine (DA)-dependent modulation of corticoaccumbens transmission and intercellular coupling were examined in mouse accumbens (NAC) neurons by both intracellular sharp electrode and whole cell recordings. In wild-type (WT) mice bath application of the D2-like agonist quinpirole resulted in 73% coupling incidence in NAC spiny neurons, compared with baseline (9%), whereas quinpirole failed to affect the basal coupling (24%) in slices from DARPP-32 knockout (KO) mice. Thus, D2 stimulation attenuated DARPP-32-mediated suppression of coupling in WT spiny neurons, but this modulation was absent in KO mice. Further, whole cell recordings revealed that quinpirole reversibly decreased the amplitude of cortical-evoked excitatory postsynaptic potentials (EPSPs) in spiny neurons of WT mice, but this reduction was markedly attenuated in KO mice. Bath application of the D1/D5 agonist SKF 38393 did not alter evoked EPSP amplitude in WT or KO spiny neurons. Therefore, DA D2 receptor regulation of both cortical synaptic (chemical) and local non-synaptic (dye coupling) communications in NAC spiny neurons is critically dependent on intracellular DARPP-32 cascades. Conversely, in fast-spiking interneurons, blockade of D1/D5 receptors produced a substantial decrease in EPSP amplitude in WT, but not in KO mice. Lastly, in putative cholinergic interneurons, cortical-evoked disynaptic inhibitory potentials (IPSPs) were attenuated by D2-like receptor stimulation in WT but not KO slices. These data indicate that DARPP-32 plays a central role in 1) modulating intercellular coupling, 2) cortical excitatory drive of spiny and aspiny GABAergic neurons, and 3) local feedforward inhibitory drive of cholinergic-like interneurons within accumbens circuits.

Presynaptic receptors for dopamine, histamine, and serotonin

Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca(2+) -dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions.Presynaptic dopamine, histamine and serotonin receptors are involved in various (patho)physiological conditions. Examples are the following:Dopamine autoreceptors play a role in Parkinson's disease, schizophrenia and drug addiction. Dopamine heteroreceptors affecting the release of acetylcholine and of amino acid neurotransmitters in the basal ganglia are also relevant for Parkinson's disease. Peripheral dopamine heteroreceptors on postganglionic sympathetic terminals influence heart rate and vascular resistance through modulation of noradrenaline release. Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C fiber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. Upregulation of presynaptic serotonin autoreceptors is probably involved in the pathogenesis of major depression. Correspondingly, antidepressant treatments can be linked with a reduced density of 5-HT autoreceptors. 5-HT Heteroreceptor activation diminishes acetylcholine and GABA release and may therefore increase anxiety. In the periphery, presynaptic 5-HT heteroreceptor agonists shorten migraine attacks by inhibition of the release of neuropeptides from trigeminal afferents, apart from their constrictive action on meningeal vessels.

D2-like dopamine receptors modulate SKCa channel function in subthalamic nucleus neurons through inhibition of Cav2.2 channels

The activity patterns of subthalamic nucleus (STN) neurons are intimately related to motor function/dysfunction and modulated directly by dopaminergic neurons that degenerate in Parkinson's disease (PD). To understand how dopamine and dopamine depletion influence the activity of the STN, the functions/signaling pathways/substrates of D2-like dopamine receptors were studied using patch-clamp recording. In rat brain slices, D2-like dopamine receptor activation depolarized STN neurons, increased the frequency/irregularity of their autonomous activity, and linearized/enhanced their firing in response to current injection. Activation of D2-like receptors in acutely isolated neurons reduced transient outward currents evoked by suprathreshold voltage steps. Modulation was inhibited by a D2-like receptor antagonist and occluded by voltage-dependent Ca2+ (Cav) channel or small-conductance Ca2+-dependent K+ (SKCa) channel blockers or Ca2+-free media. Because Cav channels are targets of G(i/o)-linked receptors, actions on step- and action potential waveform-evoked Cav channel currents were studied. D2-like receptor activation reduced the conductance of Cav2.2 but not Cav1 channels. Modulation was mediated, in part, by direct binding of Gbetagamma subunits because it was attenuated by brief depolarization. D2 and/or D3 dopamine receptors may mediate modulation because a D4-selective agonist was ineffective and mRNA encoding D2 and D3 but not D4 dopamine receptors was detectable. Brain slice recordings confirmed that SKCa channel-mediated action potential afterhyperpolarization was attenuated by D2-like dopamine receptor activation. Together, these data suggest that D2-like dopamine receptors potently modulate the negative feedback control of firing that is mediated by the functional coupling of Cav2.2 and SKCa channels in STN neurons.

Dopamine D(1) receptor facilitation of depolarization-induced release of gamma-amino-butyric acid in rat striatum is mediated by the cAMP/PKA pathway and involves P/Q-type calcium channels

Transmission in the "direct" pathway through the basal ganglia, which has an important role in the control of motor movement, is markedly facilitated by the concurrent activation of dopamine D(1) receptors. Consistent with this, Ca(2+)-dependent, depolarization-induced release of [(3)H]-GABA from striatal slices from rats pretreated with reserpine was greatly increased in the presence of 1 microM SKF 38393, a dopamine D(1)-like receptor agonist. The effect of SKF 38393 was mimicked by 1 mM 8-bromo-cyclic AMP (Br-cAMP) and inhibited by the protein kinase A (PKA) inhibitor H-89, mean inhibition 92% +/- 4% with 10 microM H-89 (n = 3). The effects of SKF 38393 and Br-cAMP were not additive. The stimulatory effects of SKF 38393 and Br-cAMP were practically abolished in the presence of the histamine H(3) receptor agonist immepip (1 microM). The depolarization-induced release of [(3)H]-GABA in the presence of SKF 38393 was not significantly inhibited by 5 microM nimodipine, an L-type Ca(2+) channel blocker, or by 0.3 microM omega-conotoxin MVIIA, a selective blocker of N-type channels. However, preincubation of the slices with 0.95 microM omega-agatoxin TK, a P/Q-type channel blocker, followed by washing before changing to a depolarizing medium containing SKF 38393, resulted in a marked inhibition of the stimulated release of [(3)H]-GABA, mean 68% +/- 4% (n = 3). These observations provide evidence that dopamine D(1) agonist facilitation of the depolarization-induced release of GABA from striatal terminals is mediated by the cAMP/PKA pathway and involves mainly P/Q-type Ca(2+) channels.

D1-like dopamine receptors selectively block P/Q-type calcium channels to reduce glutamate release onto cholinergic basal forebrain neurones of immature rats

Whole-cell patch-clamp recordings of non-NMDA glutamatergic EPSCs were made from identified cholinergic neurones in slices of basal forebrain (BF) of young rats (P13-P18), to investigate the subtypes of calcium channels involved in dopamine D(1)-like receptor-mediated presynaptic inhibition of the EPSCs. The BF cholinergic neurones were pre-labelled by intracerebroventricular injection of a fluorescent marker, Cy3-192IgG. A D(1)-like receptor agonist, SKF 81297 (30 microM) suppressed the EPSCs reversibly by about 30%, and this inhibition was reproducible. Calcium channel subtypes involved in the glutamatergic transmission were elucidated using selective Ca(2+) channel blockers. The N-type Ca(2+) channel blocker omega-conotoxin (omega-CgTX, 3 microM) suppressed the EPSCs by 57.5%, whereas the P/Q-type channel selective blocker omega-agatoxin-TK (omega-Aga-TK, 200 nM) suppressed the EPSCs by 68.9%. Simultaneous application of both blockers suppressed the EPSCs by 96.1%. The R-type Ca(2+) channel blocker SNX-482 (300 nM) suppressed the EPSCs by 18.4%, whereas nifedipine, the L-type Ca(2+) channel blocker (10 microM), had little effect. In the presence of omega-Aga-TK, SKF 81297, a dopamine D(1)-like receptor agonist, had no effect on the EPSCs. On the other hand, SKF 81297 could still inhibit the EPSCs in the presence of either omega-CgTX, SNX-482 or nifedipine. SKF 81297 had no further effect on the EPSCs when external Ca(2+) concentration was raised to 7.2 mM in the presence of omega-Aga-TK, but could still inhibit the EPSCs in high Ca(2+) solution after omega-CgTX application. Forskolin (FK, 10 microM), an activator of adenylyl cyclase pathway, suppressed the EPSCs, and the FK-induced effect was mostly blocked in the presence of omega-Aga-TK but not that of omega-CgTX. These results suggest that D(1)-like receptor activation selectively blocks P/Q-type calcium channels to reduce glutamate release onto BF cholinergic neurones.

Presynaptic modulation of GABA release in the basal ganglia

Presynaptic receptors provide plasticity to GABAergic synapses in the basal ganglia network, in which GABA neurons outnumber all other neurons. Presynaptic receptors, mostly of the metabotropic type, enhance or reduce the strength of synaptic inhibition and are activated by ligands being released from the GABA terminals themselves (autoreceptors) or by ligands coming from other sources (heteroreceptors), including the target neurons innervated by the GABA terminals. The latter mechanism, termed retrograde signaling, is given particular emphasis as far as it occurs in substantia nigra.

Effects of local infusions of apomorphine on the extracellular citrulline level in the striatum: Involvement of D1 and D2 dopamine receptors

Studies using vital microdialysis and high-performance liquid chromatography showed that local infusion of the NO synthase inhibitor N-nitro-L-arginine (1 mM) into the striatum decreased, while infusion of the dopamine receptor agonist apomorphine (100 microM) increased the level of citrulline (a side product of nitric oxide synthesis) in the intercellular space of this structure in Sprague-Dawley rats. The increase in the citrulline level induced by infusions of apomorphine was completely prevented by local infusions of N-nitro-L-arginine (1 mM) and raclopride (10 microm), a dopamine D2 receptor blocker, but not by infusion of SCH-23390 (50 microm), a dopamine D1 receptor blocker. These data suggest that the increase in extracellular citrulline in the striatum induced by dopaminergic stimulation results from local increases in NO synthase activity and that this effect involves D2, but not D1 dopamine receptors.