Margaret E Rice

Publications of Margaret E Rice

• SKF-83566, a D1-dopamine receptor antagonist, inhibits the dopamine transporter.

  Authors: Melissa A Stouffer, Solav Ali, Maarten E A Reith, Jyoti C Patel, Federica Sarti, Kenneth D Carr, Margaret E Rice

  Journal of neurochemistry. 06/2011; 118(5):714-20.

  Dopamine (DA) is an important transmitter in both motor and limbic pathways. We sought to investigate the role of D(1)-receptor activation in axonal DA release regulation in dorsal striatum using aDopamine (DA) is an important transmitter in both motor and limbic pathways. We sought to investigate the role of D(1)-receptor activation in axonal DA release regulation in dorsal striatum using a D(1)-receptor antagonist, SKF-83566. Evoked DA release was monitored in rat striatal slices using fast-scan cyclic voltammetry. SKF-83566 caused a concentration-dependent increase in peak single-pulse evoked extracellular DA concentration, with a maximum increase of ∼ 65% in 5 μM SKF-83566. This was accompanied by a concentration-dependent increase in extracellular DA concentration clearance time. Both effects were occluded by nomifensine (1 μM), a dopamine transporter (DAT) inhibitor, suggesting that SKF-83566 acted via the DAT. We tested this by examining [(3)H]DA uptake into LLc-PK cells expressing rat DAT, and confirmed that SKF-83566 is a competitive DAT inhibitor with an IC(50) of 5.7 μM. Binding studies with [(3)H]CFT, a cocaine analog, showed even more potent action of SKF-83566 at the DAT cocaine binding site (IC(50) = 0.51 μM). Thus, data obtained using SKF-83566 as a D(1) DA-receptor antagonist may be confounded by concurrent DAT inhibition. More positively, however, SKF-83566 might be a candidate to attenuate cocaine effects in vivo because of the greater potency of this drug at the cocaine versus DA binding site of the DAT.

• Subsecond regulation of striatal dopamine release by pre-synaptic KATP channels.

  Authors: Jyoti C Patel, Paul Witkovsky, William A Coetzee, Margaret E Rice

  Journal of neurochemistry. 06/2011; 118(5):721-36.

  ATP-sensitive K(+) (K(ATP)) channels are composed of pore-forming subunits, typically Kir6.2 in neurons, and regulatory sulfonylurea receptor subunits. In dorsal striatum, activity-dependent H(2)O(2)ATP-sensitive K(+) (K(ATP)) channels are composed of pore-forming subunits, typically Kir6.2 in neurons, and regulatory sulfonylurea receptor subunits. In dorsal striatum, activity-dependent H(2)O(2) produced from glutamate receptor activation inhibits dopamine release via K(ATP) channels. Sources of modulatory H(2)O(2) include striatal medium spiny neurons, but not dopaminergic axons. Using fast-scan cyclic voltammetry in guinea-pig striatal slices and immunohistochemistry, we determined the time window for H(2)O(2)/K(ATP)-channel-mediated inhibition and assessed whether modulatory K(ATP) channels are on dopaminergic axons. Comparison of paired-pulse suppression of dopamine release in the absence and presence of glibenclamide, a K(ATP)-channel blocker, or mercaptosuccinate, a glutathione peroxidase inhibitor that enhances endogenous H(2)O(2) levels, revealed a time window for inhibition of 500-1000 ms after stimulation. Immunohistochemistry demonstrated localization of Kir6.2 K(ATP)-channel subunits on dopaminergic axons. Consistent with the presence of functional K(ATP) channels on dopaminergic axons, K(ATP)-channel openers, diazoxide and cromakalim, suppressed single-pulse evoked dopamine release. Although cholinergic interneurons that tonically regulate dopamine release also express K(ATP) channels, diazoxide did not induce the enhanced frequency responsiveness of dopamine release seen with nicotinic-receptor blockade. Together, these studies reveal subsecond regulation of striatal dopamine release by endogenous H(2)O(2) acting at K(ATP) channels on dopaminergic axons, including a role in paired-pulse suppression.

• H(2)O(2): a dynamic neuromodulator.

  Authors: Margaret E Rice

  The Neuroscientist : a review journal bringing neurobiology, neurology and psychiatry. 06/2011; 17(4):389-406.

  Increasing evidence implicates hydrogen peroxide (H(2)O(2)) as an intracellular and intercellular signaling molecule that can influence processes from embryonic development to cell death. MostIncreasing evidence implicates hydrogen peroxide (H(2)O(2)) as an intracellular and intercellular signaling molecule that can influence processes from embryonic development to cell death. Most research has focused on relatively slow signaling, on the order of minutes to days, via second messenger cascades. However, H(2)O(2) can also mediate subsecond signaling via ion channel activation. This rapid signaling has been examined most thoroughly in the nigrostriatal dopamine (DA) pathway, which plays a key role in facilitating movement mediated by the basal ganglia. In DA neurons of the substantia nigra, endogenously generated H(2)O(2) activates ATP-sensitive K(+) (K-ATP) channels that inhibit DA neuron firing. In the striatum, H(2)O(2) generated downstream from glutamatergic AMPA receptor activation in medium spiny neurons acts as a diffusible messenger that inhibits axonal DA release, also via K-ATP channels. The source of dynamically generated H(2)O(2) is mitochondrial respiration; thus, H(2)O(2) provides a novel link between activity and metabolism via K-ATP channels. Additional targets of H(2)O(2) include transient receptor potential (TRP) channels. In contrast to the inhibitory effect of H(2)O(2) acting via K-ATP channels, TRP channel activation is excitatory. This review describes emerging roles of H(2)O(2) as a signaling agent in the nigrostriatal pathway and basal ganglia neurons.

• Regulation of Substantia Nigra Pars Reticulata GABAergic Neuron Activity by H₂O₂ via Flufenamic Acid-Sensitive Channels and K(ATP) Channels.

  Authors: Christian R Lee, Paul Witkovsky, Margaret E Rice

  Frontiers in systems neuroscience. 01/2011; 5:14.

  Substantia nigra pars reticulata (SNr) GABAergic neurons are key output neurons of the basal ganglia. Given the role of these neurons in motor control, it is important to understand factors thatSubstantia nigra pars reticulata (SNr) GABAergic neurons are key output neurons of the basal ganglia. Given the role of these neurons in motor control, it is important to understand factors that regulate their firing rate and pattern. One potential regulator is hydrogen peroxide (H₂O₂), a reactive oxygen species that is increasingly recognized as a neuromodulator. We used whole-cell current clamp recordings of SNr GABAergic neurons in guinea-pig midbrain slices to determine how H₂O₂ affects the activity of these neurons and to explore the classes of ion channels underlying those effects. Elevation of H₂O₂ levels caused an increase in the spontaneous firing rate of SNr GABAergic neurons, whether by application of exogenous H₂O₂ or amplification of endogenous H₂O₂ through inhibition of glutathione peroxidase with mercaptosuccinate. This effect was reversed by flufenamic acid (FFA), implicating transient receptor potential (TRP) channels. Conversely, depletion of endogenous H₂O₂ by catalase, a peroxidase enzyme, decreased spontaneous firing rate and firing precision of SNr neurons, demonstrating tonic control of firing rate by H₂O₂. Elevation of H₂O₂ in the presence of FFA revealed an inhibition of tonic firing that was prevented by blockade of ATP-sensitive K(+) (K(ATP)) channels with glibenclamide. In contrast to guinea-pig SNr neurons, the dominant effect of H₂O₂ elevation in mouse SNr GABAergic neurons was hyperpolarization, indicating a species difference in H₂O₂-dependent regulation. Thus, H₂O₂ is an endogenous modulator of SNr GABAergic neurons, acting primarily through presumed TRP channels in guinea-pig SNr, with additional modulation via K(ATP) channels to regulate SNr output.

• Differential calcium dependence of axonal versus somatodendritic dopamine release, with characteristics of both in the ventral tegmental area.

  Authors: Billy T Chen, Jyoti C Patel, Kimberly A Moran, Margaret E Rice

  Frontiers in systems neuroscience. 01/2011; 5:39.

  Midbrain dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) exhibit somatodendritic release of DA. Previous studies indicate a difference between theMidbrain dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) exhibit somatodendritic release of DA. Previous studies indicate a difference between the Ca(2+) dependence of somatodendritic DA release in the SNc and that of axonal DA release in dorsal striatum. Here, we evaluated the Ca(2+) dependence of DA release in the VTA and nucleus accumbens (NAc) shell for comparison with that in the SNc and dorsal striatum. Release of DA was elicited by single-pulse stimulation in guinea-pig brain slices and monitored with subsecond resolution using carbon-fiber microelectrodes and fast-scan cyclic voltammetry. In dorsal striatum and NAc, DA release was not detectable at extracellular Ca(2+) concentrations ([Ca(2+)](o)) below 1 mM; however, a progressive increase in evoked extracellular DA concentration ([DA](o)) was seen with [Ca(2+)](o) ≥ 1.5 mM. By contrast, in SNc and VTA, robust increases in [DA](o) could be elicited in 0.25 mM [Ca(2+)](o) that were ∼60% of those seen in 1.5 mM [Ca(2+)](o). In SNc, a plateau in single-pulse evoked [DA](o) was seen at [Ca(2+)](o) ≥ 1.5 mM, mirroring the release plateau reported previously for pulse-train stimulation in SNc. In VTA, however, evoked [DA](o) increased progressively throughout the range of [Ca(2+)](o) tested (up to 3.0 mM). These functional data are consistent with the microanatomy of the VTA, which includes DA axon collaterals as well as DA somata and dendrites. Differences between axonal and somatodendritic release data were quantified using Hill analysis, which showed that the Ca(2+) dependence of axonal DA release is low affinity with high Ca(2+) cooperativity, whereas somatodendritic release is high affinity with low cooperativity. Moreover, this analysis revealed the dual nature of DA release in the VTA, with both somatodendritic and axonal contributions.

• Dysregulation of striatal dopamine release in a mouse model of dystonia.

  Authors: Li Bao, Jyoti C Patel, Ruth H Walker, Pullanipally Shashidharan, Margaret E Rice

  Journal of neurochemistry. 09/2010; 114(6):1781-91.

  Dystonia is a neurological disorder characterized by involuntary movements. We examined striatal dopamine (DA) function in hyperactive transgenic (Tg) mice generated as a model of dystonia. EvokedDystonia is a neurological disorder characterized by involuntary movements. We examined striatal dopamine (DA) function in hyperactive transgenic (Tg) mice generated as a model of dystonia. Evoked extracellular DA concentration was monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry in striatal slices from non-Tg mice, Tg mice with a positive motor phenotype, and phenotype-negative Tg littermates. Peak single-pulse evoked extracellular DA concentration was significantly lower in phenotype-positive mice than in non-Tg or phenotype-negative mice, but indistinguishable between non-Tg and phenotype-negative mice. Phenotype-positive mice also had higher functional D2 DA autoreceptor sensitivity than non-Tg mice, which would be consistent with lower extracellular DA concentration in vivo. Multiple-pulse (phasic) stimulation (five pulses, 10-100 Hz) revealed an enhanced frequency dependence of evoked DA release in phenotype-positive versus non-Tg or phenotype-negative mice, which was exacerbated when extracellular Ca(2+) concentration was lowered. Enhanced sensitivity to phasic stimulation in phenotype-positive mice was reminiscent of the pattern seen with antagonism of nicotinic acetylcholine receptors. Consistent with a role for altered cholinergic regulation, the difference in phasic responsiveness among groups was lost when nicotinic receptors were blocked by mecamylamine. Together, these data implicate compromised DA release regulation, possibly from cholinergic dysfunction, in the motor symptoms of this dystonia model.

• Glutamatergic signaling by mesolimbic dopamine neurons in the nucleus accumbens.

  Authors: Fatuel Tecuapetla, Jyoti C Patel, Harry Xenias, Daniel English, Ibrahim Tadros, Fulva Shah, Joshua Berlin, Karl Deisseroth, Margaret E Rice, James M Tepper, Tibor Koos

  The Journal of neuroscience : the official journal of the Society for Neuroscience. 05/2010; 30(20):7105-10.

  Recent evidence suggests the intriguing possibility that midbrain dopaminergic (DAergic) neurons may use fast glutamatergic transmission to communicate with their postsynaptic targets. Because ofRecent evidence suggests the intriguing possibility that midbrain dopaminergic (DAergic) neurons may use fast glutamatergic transmission to communicate with their postsynaptic targets. Because of technical limitations, direct demonstration of the existence of this signaling mechanism has been limited to experiments using cell culture preparations that often alter neuronal function including neurotransmitter phenotype. Consequently, it remains uncertain whether glutamatergic signaling between DAergic neurons and their postsynaptic targets exists under physiological conditions. Here, using an optogenetic approach, we provide the first conclusive demonstration that mesolimbic DAergic neurons in mice release glutamate and elicit excitatory postsynaptic responses in projection neurons of the nucleus accumbens. In addition, we describe the properties of the postsynaptic glutamatergic responses of these neurons during experimentally evoked burst firing of DAergic axons that reproduce the reward-related phasic population activity of the mesolimbic projection. These observations indicate that, in addition to DAergic mechanisms, mesolimbic reward signaling may involve glutamatergic transmission.

• Enhanced striatal dopamine transmission and motor performance with LRRK2 overexpression in mice is eliminated by familial Parkinson's disease mutation G2019S.

  Authors: Xianting Li, Jyoti C Patel, Jing Wang, Marat V Avshalumov, Charles Nicholson, Joseph D Buxbaum, Gregory A Elder, Margaret E Rice, Zhenyu Yue

  The Journal of neuroscience : the official journal of the Society for Neuroscience. 02/2010; 30(5):1788-97.

  PARK8/LRRK2 (leucine-rich repeat kinase 2) was recently identified as a causative gene for autosomal dominant Parkinson's disease (PD), with LRRK2 mutation G2019S linked to the most frequent familialPARK8/LRRK2 (leucine-rich repeat kinase 2) was recently identified as a causative gene for autosomal dominant Parkinson's disease (PD), with LRRK2 mutation G2019S linked to the most frequent familial form of PD. Emerging in vitro evidence indicates that aberrant enzymatic activity of LRRK2 protein carrying this mutation can cause neurotoxicity. However, the physiological and pathophysiological functions of LRRK2 in vivo remain elusive. Here we characterize two bacterial artificial chromosome (BAC) transgenic mouse strains overexpressing LRRK2 wild-type (Wt) or mutant G2019S. Transgenic LRRK2-Wt mice had elevated striatal dopamine (DA) release with unaltered DA uptake or tissue content. Consistent with this result, LRRK2-Wt mice were hyperactive and showed enhanced performance in motor function tests. These results suggest a role for LRRK2 in striatal DA transmission and the consequent motor function. In contrast, LRRK2-G2019S mice showed an age-dependent decrease in striatal DA content, as well as decreased striatal DA release and uptake. Despite increased brain kinase activity, LRRK2-G2019S overexpression was not associated with loss of DAergic neurons in substantia nigra or degeneration of nigrostriatal terminals at 12 months. Our results thus reveal a pivotal role for LRRK2 in regulating striatal DA transmission and consequent control of motor function. The PD-associated mutation G2019S may exert pathogenic effects by impairing these functions of LRRK2. Our LRRK2 BAC transgenic mice, therefore, could provide a useful model for understanding early PD pathological events.

• Immunocytochemical identification of proteins involved in dopamine release from the somatodendritic compartment of nigral dopaminergic neurons.

  Authors: Paul Witkovsky, Jyoti C Patel, Christian R Lee, Margaret E Rice

  Neuroscience. 09/2009;

  We examined the somatodendritic compartment of nigral dopaminergic neurons by immunocytochemistry and confocal microscopy, with the aim of identifying proteins that participate in dopamine packagingWe examined the somatodendritic compartment of nigral dopaminergic neurons by immunocytochemistry and confocal microscopy, with the aim of identifying proteins that participate in dopamine packaging and release. Nigral dopaminergic neurons were identified by location, cellular features and tyrosine hydroxylase immunoreactivity. Immunoreactive puncta of vesicular monoamine transporter type 2 and proton ATPase, both involved in the packaging of dopamine for release, were located primarily in dopaminergic cell bodies, but were absent in distal dopaminergic dendrites. Many presynaptic proteins associated with transmitter release at fast synapses were absent in nigral dopaminergic neurons, including synaptotagmin 1, syntaxin1, synaptic vesicle proteins 2a and 2b, synaptophysin and synaptobrevin 1 (VAMP 1). On the other hand, syntaxin 3, synaptobrevin 2 (VAMP 2) and SNAP-25-immunoreactivities were found in dopaminergic somata and dendrites Our data imply that the storage and exocytosis of dopamine from the somatodendritic compartment of nigral dopaminergic neurons is mechanistically distinct from transmitter release at axon terminals utilizing amino acid neurotransmitters.

• Mitochondria are the source of hydrogen peroxide for dynamic brain-cell signaling.

  Authors: Li Bao, Marat V Avshalumov, Jyoti C Patel, Christian R Lee, Evan W Miller, Christopher J Chang, Margaret E Rice

  The Journal of neuroscience : the official journal of the Society for Neuroscience. 08/2009; 29(28):9002-10.

  Hydrogen peroxide (H(2)O(2)) is emerging as a ubiquitous small-molecule messenger in biology, particularly in the brain, but underlying mechanisms of peroxide signaling remain an open frontier forHydrogen peroxide (H(2)O(2)) is emerging as a ubiquitous small-molecule messenger in biology, particularly in the brain, but underlying mechanisms of peroxide signaling remain an open frontier for study. For example, dynamic dopamine transmission in dorsolateral striatum is regulated on a subsecond timescale by glutamate via H(2)O(2) signaling, which activates ATP-sensitive potassium (K(ATP)) channels to inhibit dopamine release. However, the origin of this modulatory H(2)O(2) has been elusive. Here we addressed three possible sources of H(2)O(2) produced for rapid neuronal signaling in striatum: mitochondrial respiration, monoamine oxidase (MAO), and NADPH oxidase (Nox). Evoked dopamine release in guinea-pig striatal slices was monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry. Using direct fluorescence imaging of H(2)O(2) and tissue analysis of ATP, we found that coapplication of rotenone (50 nM), a mitochondrial complex I inhibitor, and succinate (5 mM), a complex II substrate, limited H(2)O(2) production, but maintained tissue ATP content. Strikingly, coapplication of rotenone and succinate also prevented glutamate-dependent regulation of dopamine release, implicating mitochondrial H(2)O(2) in release modulation. In contrast, inhibitors of MAO or Nox had no effect on dopamine release, suggesting a limited role for these metabolic enzymes in rapid H(2)O(2) production in the striatum. These data provide the first demonstration that respiring mitochondria are the primary source of H(2)O(2) generation for dynamic neuronal signaling.

• Mobilization of calcium from intracellular stores facilitates somatodendritic dopamine release.

  Authors: Jyoti C Patel, Paul Witkovsky, Marat V Avshalumov, Margaret E Rice

  The Journal of neuroscience : the official journal of the Society for Neuroscience. 06/2009; 29(20):6568-79.

  Somatodendritic dopamine (DA) release in the substantia nigra pars compacta (SNc) shows a limited dependence on extracellular calcium concentration ([Ca(2+)](o)), suggesting the involvement ofSomatodendritic dopamine (DA) release in the substantia nigra pars compacta (SNc) shows a limited dependence on extracellular calcium concentration ([Ca(2+)](o)), suggesting the involvement of intracellular Ca(2+) stores. Here, using immunocytochemistry we demonstrate the presence of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 2 (SERCA2) that sequesters cytosolic Ca(2+) into the endoplasmic reticulum (ER), as well as inositol 1,4,5-triphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) in DAergic neurons. Notably, RyRs were clustered at the plasma membrane, poised for activation by Ca(2+) entry. Using fast-scan cyclic voltammetry to monitor evoked extracellular DA concentration ([DA](o)) in midbrain slices, we found that SERCA inhibition by cyclopiazonic acid (CPA) decreased evoked [DA](o) in the SNc, indicating a functional role for ER Ca(2+) stores in somatodendritic DA release. Implicating IP(3)R-dependent stores, an IP(3)R antagonist, 2-APB, also decreased evoked [DA](o). Moreover, DHPG, an agonist of group I metabotropic glutamate receptors (mGluR1s, which couple to IP(3) production), increased somatodendritic DA release, whereas CPCCOEt, an mGluR1 antagonist, suppressed it. Release suppression by mGluR1 blockade was prevented by 2-APB or CPA, indicating facilitation of DA release by endogenous glutamate acting via mGluR1s and IP(3)R-gated Ca(2+) stores. Similarly, activation of RyRs by caffeine increased [Ca(2+)](i) and elevated evoked [DA](o). The increase in DA release was prevented by a RyR blocker, dantrolene, and by CPA. Importantly, the efficacy of dantrolene was enhanced in low [Ca(2+)](o), suggesting a mechanism for maintenance of somatodendritic DA release with limited Ca(2+) entry. Thus, both mGluR1-linked IP(3)R- and RyR-dependent ER Ca(2+) stores facilitate somatodendritic DA release in the SNc.

• AMPA receptor-dependent H2O2 generation in striatal medium spiny neurons but not dopamine axons: one source of a retrograde signal that can inhibit dopamine release.

  Authors: Marat V Avshalumov, Jyoti C Patel, Margaret E Rice

  Journal of neurophysiology. 09/2008; 100(3):1590-601.

  Dopamine-glutamate interactions in the striatum are critical for normal basal ganglia-mediated control of movement. Although regulation of glutamatergic transmission by dopamine is increasingly wellDopamine-glutamate interactions in the striatum are critical for normal basal ganglia-mediated control of movement. Although regulation of glutamatergic transmission by dopamine is increasingly well understood, regulation of dopaminergic transmission by glutamate remains uncertain given the apparent absence of ionotropic glutamate receptors on dopaminergic axons in dorsal striatum. Indirect evidence suggests glutamatergic regulation of striatal dopamine release is mediated by a diffusible messenger, hydrogen peroxide (H2O2), generated downstream from glutamatergic AMPA receptors (AMPARs). The mechanism of H2O2-dependent inhibition of dopamine release involves activation of ATP-sensitive K+ (KATP) channels. However, the source of modulatory H2O2 is unknown. Here, we used whole cell recording, fluorescence imaging of H2O2, and voltammetric detection of evoked dopamine release in guinea pig striatal slices to examine contributions from medium spiny neurons (MSNs), the principal neurons of striatum, and dopamine axons to AMPAR-dependent H2O2 generation. Imaging studies of H2O2 generation in MSNs provide the first demonstration of AMPAR-dependent H2O2 generation in neurons in the complex brain-cell microenvironment of brain slices. Stimulation-induced increases in H2O2 in MSNs were prevented by GYKI-52466, an AMPAR antagonist, or catalase, an H2O2 metabolizing enzyme, but amplified by mercaptosuccinate (MCS), a glutathione peroxidase inhibitor. By contrast, dopamine release evoked by selective stimulation of dopamine axons was unaffected by GYKI-52466 or MCS, arguing against dopamine axons as a significant source of modulatory H2O2. Together, these findings suggest that glutamatergic regulation of dopamine release via AMPARs is mediated through retrograde signaling by diffusible H2O2 generated in striatal cells, including medium spiny neurons, rather than in dopamine axons.

• Dopamine spillover after quantal release: Rethinking dopamine transmission in the nigrostriatal pathway.

  Authors: Margaret E Rice, Stephanie J Cragg

  Brain research reviews. 04/2008;

  The predominance of dopamine (DA) receptors at extrasynaptic vs. synaptic sites implies that DA signaling is by diffusion-based volume transmission. In this review, we compare characteristics thatThe predominance of dopamine (DA) receptors at extrasynaptic vs. synaptic sites implies that DA signaling is by diffusion-based volume transmission. In this review, we compare characteristics that regulate extracellular DA behavior in substantia nigra pars compacta (SNc) and striatum, including regional differences in structure (a 40% greater extracellular volume fraction in SNc vs. striatum) and in dynamic DA uptake (a 200-fold greater DA uptake rate in striatum vs. SNc). Furthermore, we test the assumption of diffusion-based volume transmission for SNc and striatum by modeling dynamic DA behavior after quantal release using region-specific parameters for diffusion and uptake at 37 degrees C. Our model shows that DA uptake does not affect peak DA concentration within 1 mum of a release site in either SNc or striatum because of the slow kinetics of DATs vs. diffusion. Rather, diffusion and dilution are the dominant factors governing DA concentration after quantal release. In SNc, limited DAT efficacy is reflected in a lack of influence of uptake on either amplitude or time course of DA transients after quantal release up to 10 mum from a release site. In striatum, the lack of effect of the DAT within 1 mum of a release site means that perisynaptic DATs do not "gate" synaptic spillover. This contrasts with the conventional view of DA synapses, in which DATs efficiently recycle DA by re-uptake into the releasing axon terminal. However, the model also shows that a primary effect of striatal uptake is to curtail DA lifetime after release. In both SNc and striatum, effective DA radius after quantal release is ~2 mum for activation of low-affinity DA receptors and 7-8 mum for high-affinity receptors; the corresponding spheres of influence would encompass tens to thousands of synapses. Thus, the primary mode of intercellular communication by DA, regardless of region, is volume transmission.

• Inhibition of striatal dopamine release by CB1 receptor activation requires nonsynaptic communication involving GABA, H2O2, and KATP channels.

  Authors: Zsuzsanna Sidló, Patricia H Reggio, Margaret E Rice

  Neurochemistry international. 02/2008; 52(1-2):80-8.

  The main psychoactive component of marijuana, Delta9-tetrahydrocannabinol (THC), acts in the CNS via type 1 cannabinoid receptors (CB1Rs). The behavioral consequences of THC or synthetic CB1RThe main psychoactive component of marijuana, Delta9-tetrahydrocannabinol (THC), acts in the CNS via type 1 cannabinoid receptors (CB1Rs). The behavioral consequences of THC or synthetic CB1R agonists include suppression of motor activity. One explanation for movement suppression might be inhibition of striatal dopamine (DA) release by CB1Rs, which are densely localized in motor striatum; however, data from previous studies are inconclusive. Here we examined the effect of CB1R activation on locally evoked DA release monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry in striatal slices. Consistent with previous reports, DA release evoked by a single stimulus pulse was unaffected by WIN55,212-2, a cannabinoid receptor agonist. However, when DA release was evoked by a train of stimuli, WIN55,212-2 caused a significant decrease in evoked extracellular DA concentration ([DA]o), implicating the involvement of local striatal circuitry, with similar suppression seen in guinea pig, rat, and mouse striatum. Pulse-train evoked [DA]o was not altered by either AM251, an inverse CB1R agonist, or VCHSR1, a neutral antagonist, indicating the absence of DA release regulation by endogenous cannabinoids with the stimulation protocol used. However, both CB1R antagonists prevented and reversed suppression of evoked [DA]o by WIN55,212-2. The effect of WIN55,212-2 was also prevented by picrotoxin, a GABAA receptor antagonist, and by catalase, a metabolizing enzyme for hydrogen peroxide (H2O2). Furthermore, blockade of ATP-sensitive K+ (KATP) channels by tolbutamide or glybenclamide prevented the effect of WIN55,212-2 on DA release. Together, these data indicate that suppression of DA release by CB1R activation within striatum occurs via a novel nonsynaptic mechanism that involves GABA release inhibition, increased generation of the diffusible messenger H2O2, and activation of KATP channels to inhibit DA release. In addition, the findings suggest a possible physiological substrate for the motor effects of cannabinoid agonist administration.

• H2O2 signaling in the nigrostriatal dopamine pathway via ATP-sensitive potassium channels: issues and answers.

  Authors: Marat V Avshalumov, Li Bao, Jyoti C Patel, Margaret E Rice

  Antioxidants & redox signaling. 03/2007; 9(2):219-31.

  The role of reactive oxygen species (ROS) as signaling agents is increasingly appreciated. Studies of ROS functions in the central nervous system, however, are only in their infancy. Using fast-scanThe role of reactive oxygen species (ROS) as signaling agents is increasingly appreciated. Studies of ROS functions in the central nervous system, however, are only in their infancy. Using fast-scan cyclic voltammetry and fluorescence imaging in brain slices, the authors discovered that hydrogen peroxide (H2O2) is an endogenous regulator of dopamine release in the dorsal striatum. Given the key role of dopamine in motor, reward, and cognitive pathways, regulation by H2O2 has implications for normal dopamine function, as well as for dysfunction of dopamine transmission. In this review, data are summarized to show that H2O2 is a diffusible messenger in the striatum, generated downstream from glutamate receptor activation, and an intracellular signal in dopamine neurons of the substantia nigra, generated during normal pacemaker activity. The mechanism by which H2O2 inhibits dopamine release and dopamine cell activity is activation of ATP-sensitive K+ (KATP) channels. Characteristics of the neuronal and glial antioxidant networks required to permit H2O2 signaling, yet prevent oxidative damage, are also considered. Lastly, estimates of physiological H2O2 levels are discussed, and strengths and limitations of currently available methods for H2O2 detection, including fluorescence imaging using dichlorofluorescein (DCF) and the next generation of fluorescent probes, are considered.

• Regulation of postsynaptic Ca2+ influx in hippocampal CA1 pyramidal neurons via extracellular carbonic anhydrase.

  Authors: Nataliya Fedirko, Marat Avshalumov, Margaret E Rice, Mitchell Chesler

  The Journal of neuroscience : the official journal of the Society for Neuroscience. 02/2007; 27(5):1167-75.

  Synchronous neural activity causes rapid changes of extracellular pH (pH(e)) in the nervous system. In the CA1 region of the hippocampus, stimulation of the Schaffer collaterals elicits an alkalineSynchronous neural activity causes rapid changes of extracellular pH (pH(e)) in the nervous system. In the CA1 region of the hippocampus, stimulation of the Schaffer collaterals elicits an alkaline pH(e) transient in stratum radiatum that is limited by extracellular carbonic anhydrase (ECA). When interstitial buffering is diminished by inhibition of ECA, the alkalosis is enhanced and NMDA receptor (NMDAR)-mediated postsynaptic currents can be augmented. Accordingly, the dendritic influx of Ca2+ elicited by synaptic excitation may be expected to increase if ECA activity were blocked. We tested this hypothesis in the CA1 stratum radiatum of hippocampal slices from juvenile rats, using extracellular, concentric pH- and Ca2+-selective microelectrodes with response times of a few milliseconds, as well as Fluo-5F imaging of intracellular Ca2+ transients. Brief stimulation of the Schaffer collaterals elicited an alkaline pH(e) transient, a transient decrease in free extracellular Ca2+ concentration ([Ca2+]e), and a corresponding transient rise in free intracellular Ca2+ concentration ([Ca2+]i). Inhibition of ECA with benzolamide caused a marked amplification and prolonged recovery of the pH(e) and [Ca2+]e responses, as well as the dendritic [Ca2+]i transients. The increase in amplitude caused by benzolamide did not occur in the presence of the NMDAR antagonist APV, but the decay of the responses was still prolonged. These results indicate that ECA can shape dendritic Ca2+ dynamics governed by NMDARs by virtue of its regulation of concomitant activity-dependent pH(e) shifts. The data also suggest that Ca2+ transients are influenced by additional mechanisms sensitive to shifts in pH(e).

• Hydrogen Peroxide as a Diffusible Messenger: Evidence from Voltammetric Studies of Dopamine Release in Brain Slices

  Authors: Margaret E. Rice, Marat V. Avshalumov, Jyoti C. Patel

  01/2007;

  ISBN: 0849340756

• Limited regulation of somatodendritic dopamine release by voltage-sensitive Ca channels contrasted with strong regulation of axonal dopamine release.

  Authors: Billy T Chen, Kimberly A Moran, Marat V Avshalumov, Margaret E Rice

  Journal of neurochemistry. 03/2006; 96(3):645-55.

  The mechanism underlying somatodendritic release of dopamine (DA) appears to differ from that of axon-terminal release. Specifically, somatodendritic DA release in the substantia nigra pars compactaThe mechanism underlying somatodendritic release of dopamine (DA) appears to differ from that of axon-terminal release. Specifically, somatodendritic DA release in the substantia nigra pars compacta (SNc) persists in low extracellular Ca2+ concentrations that are insufficient to support axonal release in striatum, suggesting that limited Ca2+ entry is necessary to trigger somatodendritic release. Here, we compared the role of voltage-dependent Ca2+ channels in mediating DA release in striatum versus SNc using specific blockers of N-, P/Q-, T-, R- and L-type Ca2+ channels individually and in combination. Release of DA evoked by a single stimulus pulse in the dorsal striatum and SNc of guinea-pig brain slices was monitored in real time using carbon-fiber microelectrodes with fast-scan cyclic voltammetry. Single-pulse evoked DA release was shown to be independent of regulation by concurrently released glutamate or GABA acting at ionotropic receptors in both regions. Under these conditions, striatal DA release was completely prevented by an N-type channel blocker, omega-conotoxin GVIA (100 nm), and was decreased by 75% by the P/Q-type channel blocker omega-agatoxin IVA (200 nm). Blockade of T-type channels with Ni2+ (100 microm) or R-type channels with SNX-482 (100 nm) decreased axonal release in striatum by 25%, whereas inhibition of L-type channels with nifedipine (20 microm) had no effect. By contrast, none of these Ca2+-channel blockers altered the amplitude of somatodendritic DA release in the SNc. Even a cocktail of all blockers tested did not alter release-signal amplitude in the SNc, although the duration of the release response was curtailed. The limited involvement of voltage-dependent Ca2+ channels in somatodendritic DA release provides further evidence that minimal Ca2+ entry is required to trigger the release process, compared with that required for axon-terminal release.

• Partial mitochondrial inhibition causes striatal dopamine release suppression and medium spiny neuron depolarization via H2O2 elevation, not ATP depletion.

  Authors: Li Bao, Marat V Avshalumov, Margaret E Rice

  The Journal of neuroscience : the official journal of the Society for Neuroscience. 11/2005; 25(43):10029-40.

  Mitochondrial dysfunction is a potential causal factor in Parkinson's disease. We show here that acute exposure to the mitochondrial complex I inhibitor rotenone (30-100 nM; 30 min) causesMitochondrial dysfunction is a potential causal factor in Parkinson's disease. We show here that acute exposure to the mitochondrial complex I inhibitor rotenone (30-100 nM; 30 min) causes concentration-dependent suppression of single-pulse evoked dopamine (DA) release monitored in real time with carbon-fiber microelectrodes in guinea pig striatal slices, with no effect on DA content. Suppression of DA release was prevented by the sulfonylurea glibenclamide, implicating ATP-sensitive K+ (KATP) channels; however, tissue ATP was unaltered. Because KATP channels can be activated by hydrogen peroxide (H2O2), as well as by low ATP, we examined the involvement of rotenone-enhanced H2O2 generation. Confirming an essential role for H2O2, the inhibition of DA release by rotenone was prevented by catalase, a peroxide-scavenging enzyme. Striatal H2O2 generation during rotenone exposure was examined in individual medium spiny neurons using fluorescence imaging with dichlorofluorescein (DCF). An increase in intracellular H2O2 levels followed a similar time course to that of DA release suppression and was accompanied by cell membrane depolarization, decreased input resistance, and increased excitability. Extracellular catalase markedly attenuated the increase in DCF fluorescence and prevented rotenone-induced effects on membrane properties; membrane changes were also largely prevented by flufenamic acid, a blocker of transient receptor potential (TRP) channels. Thus, partial mitochondrial inhibition can cause functional DA denervation via H2O2 and KATP channels, without DA or ATP depletion. Furthermore, amplified H2O2 levels and TRP channel activation in striatal spiny neurons indicate potential sources of damage in these cells. Overall, these novel factors could contribute to parkinsonian motor deficits and neuronal degeneration caused by mitochondrial dysfunction.

• Endogenous hydrogen peroxide regulates the excitability of midbrain dopamine neurons via ATP-sensitive potassium channels.

  Authors: Marat V Avshalumov, Billy T Chen, Tibor Koós, James M Tepper, Margaret E Rice

  The Journal of neuroscience : the official journal of the Society for Neuroscience. 05/2005; 25(17):4222-31.

  ATP-sensitive K+ (K(ATP)) channels link metabolic state to cell excitability. Here, we examined regulation of K(ATP) channels in substantia nigra dopamine neurons by hydrogen peroxide (H2O2), whichATP-sensitive K+ (K(ATP)) channels link metabolic state to cell excitability. Here, we examined regulation of K(ATP) channels in substantia nigra dopamine neurons by hydrogen peroxide (H2O2), which is produced in all cells during aerobic metabolism. Blockade of K(ATP) channels by glibenclamide (100 nM) or depletion of intracellular H2O2 by including catalase, a peroxidase enzyme, in the patch pipette increased the spontaneous firing rate of all dopamine neurons tested in guinea pig midbrain slices. Using fluorescence imaging with dichlorofluorescein to visualize intracellular H2O2, we found that moderate increases in H2O2 during partial inhibition of glutathione (GSH) peroxidase by mercaptosuccinate (0.1-0.3 mM) had no effect on dopamine neuron firing rate. However, with greater GSH inhibition (1 mM mercaptosuccinate) or application of exogenous H2O2, 50% of recorded cells showed K(ATP) channel-dependent hyperpolarization. Responsive cells also hyperpolarized with diazoxide, a selective opener for K(ATP) channels containing sulfonylurea receptor SUR1 subunits, but not with cromakalim, a selective opener for SUR2-based channels, indicating that SUR1-based K(ATP) channels conveyed enhanced sensitivity to elevated H2O2. In contrast, when endogenous H2O2 levels were increased after inhibition of catalase, the predominant peroxidase in the substantia nigra, with 3-amino-1,2,4-triazole (1 mM), all dopamine neurons responded with glibenclamide-reversible hyperpolarization. Fluorescence imaging of H2O2 indicated that catalase inhibition rapidly amplified intracellular H2O2, whereas inhibition of GSH peroxidase, a predominantly glial enzyme, caused a slower, smaller increase, especially in nonresponsive cells. Thus, endogenous H2O2 modulates neuronal activity via K(ATP) channel opening, thereby enhancing the reciprocal relationship between metabolism and excitability.