Striatal spreading depolarization: Possible implication in levodopa-induced dyskinetic-like behavior.

OBJECTIVE: Spreading depolarization (SD) is a transient self-propagating wave of neuronal and glial depolarization coupled with large membrane ionic changes and a subsequent depression of neuronal activity. Spreading depolarization in the cortex is implicated in migraine, stroke, and epilepsy. Conversely, spreading depolarization in the striatum, a brain structure deeply involved in motor control and in Parkinson's disease (PD) pathophysiology, has been poorly investigated. METHODS: We characterized the participation of glutamatergic and dopaminergic transmission in the induction of striatal spreading depolarization by using a novel approach combining optical imaging, measurements of endogenous DA levels, and pharmacological and molecular analyses. RESULTS: We found that striatal spreading depolarization requires the concomitant activation of D1-like DA and N-methyl-d-aspartate receptors, and it is reduced in experimental PD. Chronic l-dopa treatment, inducing dyskinesia in the parkinsonian condition, increases the occurrence and speed of propagation of striatal spreading depolarization, which has a direct impact on one of the signaling pathways downstream from the activation of D1 receptors. CONCLUSION: Striatal spreading depolarization might contribute to abnormal basal ganglia activity in the dyskinetic condition and represents a possible therapeutic target. © 2019 International Parkinson and Movement Disorder Society.

Dopamine D2 receptor-mediated neuroprotection in a G2019S Lrrk2 genetic model of Parkinson's disease.

Parkinson's disease (PD) is a neurodegenerative disorder in which genetic and environmental factors synergistically lead to loss of midbrain dopamine (DA) neurons. Mutation of leucine-rich repeated kinase2 (Lrrk2) genes is responsible for the majority of inherited familial cases of PD and can also be found in sporadic cases. The pathophysiological role of this kinase has to be fully understood yet. Hyperactivation of Lrrk2 kinase domain might represent a predisposing factor for both enhanced striatal glutamatergic release and mitochondrial vulnerability to environmental factors that are observed in PD. To investigate possible alterations of striatal susceptibility to mitochondrial dysfunction, we performed electrophysiological recordings from the nucleus striatum of a G2019S Lrrk2 mouse model of PD, as well as molecular and morphological analyses of G2019S Lrrk2-expressing SH-SY5Y neuroblastoma cells. In G2019S mice, we found reduced striatal DA levels, according to the hypothesis of alteration of dopaminergic transmission, and increased loss of field potential induced by the mitochondrial complex I inhibitor rotenone. This detrimental effect is reversed by the D2 DA receptor agonist quinpirole via the inhibition of the cAMP/PKA intracellular pathway. Analysis of mitochondrial functions in G2019S Lrrk2-expressing SH-SY5Y cells revealed strong rotenone-induced oxidative stress characterized by reduced Ca2+ buffering capability and ATP synthesis, production of reactive oxygen species, and increased mitochondrial fragmentation. Importantly, quinpirole was able to prevent all these changes. We suggest that the G2019S-Lrrk2 mutation is a predisposing factor for enhanced striatal susceptibility to mitochondrial dysfunction induced by exposure to mitochondrial environmental toxins and that the D2 receptor stimulation is neuroprotective on mitochondrial function, via the inhibition of cAMP/PKA intracellular pathway. We suggest new possible neuroprotective strategies for patients carrying this genetic alteration based on drugs specifically targeting Lrrk2 kinase domain and mitochondrial functionality.

Epilepsy, amyloid-ß, and D1 dopamine receptors: a possible pathogenetic link?

Experimental and clinical observations indicate that amyloid-ß1-42 (Aß1-42) peptide not only represents a major actor in neurodegenerative mechanisms but also induce hyperexcitation in individual neurons and neural circuits. In this abnormal excitability, possibly leading to seizures, the D1 dopamine (DA) receptors may play a role. Cerebrospinal fluid levels of Aß1-42 were measured in patients with late-onset epilepsy of unknown etiology. Moreover, the effect of amyloid peptide on the hippocampal epileptic threshold and synaptic plasticity and its link to D1 receptor function were tested in experimental mouse model of cerebral amyloidosis and in acute model of Aß1-42-induced neurotoxicity. Among 272 evaluated epileptic patients, aged >55 years, 35 suffered from late-onset epilepsy of unknown etiology. In these subjects, cerebrospinal fluid Aß1-42 levels were measured. The effects of Aß1-42, amyloid oligomers, and D1 receptor modulation on epileptic threshold were analyzed by electrophysiological recordings in the dentate gyrus of mice hippocampal slices. We found that Aß1-42 levels were significantly decreased in cerebrospinal fluid of patients with late-onset epilepsy of unknown etiology with respect to controls suggesting the cerebral deposition of this peptide in these patients. Aß1-42 enhanced epileptic activity in mice through a mechanism involving increased surface expression of D1 receptor, and this effect was mimicked by D1 receptor stimulation and blocked by SCH 23390, a D1 receptor antagonist. Aß1-42 may contribute to the pathophysiology of late-onset epilepsy of unknown origin. Our preclinical findings indicate that the D1 receptor is involved in mediating the epileptic effects of Aß1-42. This novel link between Aß1-42 and D1 receptor signaling might represent a potential therapeutic target.

Alpha-Synuclein Produces Early Behavioral Alterations via Striatal Cholinergic Synaptic Dysfunction by Interacting With GluN2D N-Methyl-D-Aspartate Receptor Subunit.

BACKGROUND: Advanced Parkinson's disease (PD) is characterized by massive degeneration of nigral dopaminergic neurons, dramatic motor and cognitive alterations, and presence of nigral Lewy bodies, whose main constituent is α-synuclein (α-syn). However, the synaptic mechanisms underlying behavioral and motor effects induced by early selective overexpression of nigral α-syn are still a matter of debate. METHODS: We performed behavioral, molecular, and immunohistochemical analyses in two transgenic models of PD, mice transgenic for truncated human α-synuclein 1-120 and rats injected with the adeno-associated viral vector carrying wild-type human α-synuclein. We also investigated striatal synaptic plasticity by electrophysiological recordings from spiny projection neurons and cholinergic interneurons. RESULTS: We found that overexpression of truncated or wild-type human α-syn causes partial reduction of striatal dopamine levels and selectively blocks the induction of long-term potentiation in striatal cholinergic interneurons, producing early memory and motor alterations. These effects were dependent on α-syn modulation of the GluN2D-expressing N-methyl-D-aspartate receptors in cholinergic interneurons. Acute in vitro application of human α-syn oligomers mimicked the synaptic effects observed ex vivo in PD models. CONCLUSIONS: We suggest that striatal cholinergic dysfunction, induced by a direct interaction between α-syn and GluN2D-expressing N-methyl-D-aspartate receptors, represents a precocious biological marker of the disease.

Endogenous 17ß-estradiol is required for activity-dependent long-term potentiation in the striatum: interaction with the dopaminergic system.

17ß-estradiol (E2), a neurosteroid synthesized by P450-aromatase (ARO), modulates various brain functions. We characterized the role of the locally synthesized E2 on striatal long-term synaptic plasticity and explored possible interactions between E2 receptors (ERs) and dopamine (DA) receptors in the dorsal striatum of adult male rats. Inhibition of E2 synthesis or antagonism of ERs prevented the induction of long-term potentiation (LTP) in both medium spiny neurons (MSNs) and cholinergic interneurons (ChIs). Activation of a D1-like DA receptor/cAMP/PKA-dependent pathway restored LTP. In MSNs exogenous E2 reversed the effect of ARO inhibition. Also antagonism of M1 muscarinic receptors prevented the D1-like receptor-mediated restoration of LTP confirming a role for ChIs in controlling the E2-mediated LTP of MSNs. A novel striatal interaction, occurring between ERs and D1-like receptors in both MSNs and ChIs, might be critical to regulate basal ganglia physiology and to compensate synaptic alterations in Parkinson's disease.

Ischemic-LTP in striatal spiny neurons of both direct and indirect pathway requires the activation of D1-like receptors and NO/soluble guanylate cyclase/cGMP transmission.

Striatal medium-sized spiny neurons (MSNs) are highly vulnerable to ischemia. A brief ischemic insult, produced by oxygen and glucose deprivation (OGD), can induce ischemic long-term potentiation (i-LTP) of corticostriatal excitatory postsynaptic response. Since nitric oxide (NO) is involved in the pathophysiology of brain ischemia and the dopamine D1/D5-receptors (D1-like-R) are expressed in striatal NOS-positive interneurons, we hypothesized a relation between NOS-positive interneurons and striatal i-LTP, involving D1R activation and NO production. We investigated the mechanisms involved in i-LTP induced by OGD in corticostriatal slices and found that the D1-like-R antagonist SCH-23390 prevented i-LTP in all recorded MSNs. Immunofluorescence analysis confirmed the induction of i-LTP in both substance P-positive, (putative D1R-expressing) and adenosine A2A-receptor-positive (putative D2R-expressing) MSNs. Furthermore, i-LTP was dependent on a NOS/cGMP pathway since pharmacological blockade of NOS, guanylate-cyclase, or PKG prevented i-LTP. However, these compounds failed to prevent i-LTP in the presence of a NO donor or cGMP analog, respectively. Interestingly, the D1-like-R antagonism failed to prevent i-LTP when intracellular cGMP was pharmacologically increased. We propose that NO, produced by striatal NOS-positive interneurons via the stimulation of D1-like-R located on these cells, is critical for i-LTP induction in the entire population of MSNs involving a cGMP-dependent pathway.

A2A adenosine receptor antagonism enhances synaptic and motor effects of cocaine via CB1 cannabinoid receptor activation.

BACKGROUND: Cocaine increases the level of endogenous dopamine (DA) in the striatum by blocking the DA transporter. Endogenous DA modulates glutamatergic inputs to striatal neurons and this modulation influences motor activity. Since D2 DA and A2A-adenosine receptors (A2A-Rs) have antagonistic effects on striatal neurons, drugs targeting adenosine receptors such as caffeine-like compounds, could enhance psychomotor stimulant effects of cocaine. In this study, we analyzed the electrophysiological effects of cocaine and A2A-Rs antagonists in striatal slices and the motor effects produced by this pharmacological modulation in rodents. PRINCIPAL FINDINGS: Concomitant administration of cocaine and A2A-Rs antagonists reduced glutamatergic synaptic transmission in striatal spiny neurons while these drugs failed to produce this effect when given in isolation. This inhibitory effect was dependent on the activation of D2-like receptors and the release of endocannabinoids since it was prevented by L-sulpiride and reduced by a CB1 receptor antagonist. Combined application of cocaine and A2A-R antagonists also reduced the firing frequency of striatal cholinergic interneurons suggesting that changes in cholinergic tone might contribute to this synaptic modulation. Finally, A2A-Rs antagonists, in the presence of a sub-threshold dose of cocaine, enhanced locomotion and, in line with the electrophysiological experiments, this enhanced activity required activation of D2-like and CB1 receptors. CONCLUSIONS: The present study provides a possible synaptic mechanism explaining how caffeine-like compounds could enhance psychomotor stimulant effects of cocaine.

The distinct role of medium spiny neurons and cholinergic interneurons in the D2/A2A receptor interaction in the striatum: implications for Parkinson's disease.

A(2A) adenosine receptor antagonists are currently under investigation as potential therapeutic agents for Parkinson's disease (PD). However, the molecular mechanisms underlying this therapeutic effect is still unclear. A functional antagonism exists between A(2A) adenosine and D(2) dopamine (DA) receptors that are coexpressed in striatal medium spiny neurons (MSNs) of the indirect pathway. Since this interaction could also occur in other neuronal subtypes, we have analyzed the pharmacological modulation of this relationship in murine MSNs of the direct and indirect pathways as well in striatal cholinergic interneurons. Under physiological conditions, endogenous cannabinoids (eCBs) play a major role in the inhibitory effect on striatal glutamatergic transmission exerted by the concomitant activation of D(2) DA receptors and blockade of A(2A) receptors in both D(2)- and D(1)-expressing striatal MSNs. In experimental models of PD, the inhibition of striatal glutamatergic activity exerted by D(2) receptor activation did not require the concomitant inhibition of A(2A) receptors, while it was still dependent on the activation of CB(1) receptors in both D(2)- and D(1)-expressing MSNs. Interestingly, the antagonism of M1 muscarinic receptors blocked the effects of D(2)/A(2A) receptor modulation on MSNs. Moreover, in cholinergic interneurons we found coexpression of D(2) and A(2A) receptors and a reduction of the firing frequency exerted by the same pharmacological agents that reduced excitatory transmission in MSNs. This evidence supports the hypothesis that striatal cholinergic interneurons, projecting to virtually all MSN subtypes, are involved in the D(2)/A(2A) and endocannabinoid-mediated effects observed on both subpopulations of MSNs in physiological conditions and in experimental PD.

A2A adenosine receptor antagonists protect the striatum against rotenone-induced neurotoxicity.

Adenosine A2A receptor has emerged as an attractive non-dopaminergic target in the experimental pharmacological therapy for Parkinson's disease (PD). Moreover, it has been postulated that A2A adenosine receptor antagonists exert neuroprotective effects in experimental models of PD and progressive supranuclear palsy (PSP). Interestingly, in both these pathological conditions a deficit of mitochondrial complex I has been found. Thus, utilizing extracellular and intracellular recordings from corticostriatal brain slices, we have tested the possible neuroprotective action of two A2A receptor antagonists, ST1535 and ZM241385, on the irreversible electrophysiological effects induced by the acute application of rotenone, a pesticide acting as a selective inhibitor of mitochondrial complex I activity. Both these antagonists reduced the rotenone-induced loss of corticostriatal field potential amplitude as well as the membrane depolarization caused by this toxin on striatal spiny neurons. The use of A2A receptor antagonists might represent a promising neuroprotective strategy in basal ganglia disorders involving a deficit of mitochondrial complex I activity.

Interaction of A2A adenosine and D2 dopamine receptors modulates corticostriatal glutamatergic transmission.

Adenosine and dopamine (DA) strongly modulate the neuronal activity in the striatum by pre- and postsynaptic mechanisms. As several behavioral and molecular studies indicate a functional antagonism between A2A adenosine and D2 DA receptors, compounds that are able to block A2A receptors are of particular interest as antiparkinsonian agents. To study the interaction of A2A and D2 receptors in the striatum, we performed intracellular recordings with sharp microelectrodes and whole-cell patch clamp recordings from spiny neurons in rat corticostriatal slices. The amplitude of the evoked excitatory postsynaptic potentials (EPSPs), as well as the frequency and the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs), were affected neither by the A2A receptor antagonists ST1535 and ZM241385, nor by the D2 receptor agonist quinpirole when applied in isolation. However, co-application of quinpirole and ST1535 or ZM241385 significantly reduced the EPSPs amplitude. This inhibitory effect was associated with an increased paired-pulse facilitation suggesting a presynaptic mechanism of action. Accordingly, whole-cell recordings showed that the concomitant activation of D2 receptors and the antagonism of A2A receptors decreased the frequency of sEPSCs without affecting their amplitude. These results suggest that A2A and D2 receptors converge in the control of corticostriatal glutamatergic transmission by exerting an opposite function.

Gas chromatography-tandem mass spectrometry analysis of gabapentin in serum.

A new highly sensitive analytical method for determining gabapentin [1-(aminomethyl) cyclohexaneacetic acid; Neurontin] in serum using gas chromatography/tandem mass spectrometry (GC-MS/MS) was developed. GC-MS/MS was applied to determine the levels of gabapentin in serum samples of mice at 1 and 6 h after oral or intraperitoneal treatment (300 mg/kg). At 1 h, the concentrations of the drug were 4.02 +/- 0.42 and 4.32 +/- 0.28 microg/mL in mice treated orally and intraperitoneally, respectively. At 6 h, drug levels decreased by about 66% in both groups. The method, coupling two stages of mass analysis, could be very useful in identifying the drug in complex mixtures such as blood and urine. Moreover, it is easy and rapid to perform, and sensitive enough to allow the presence of the drug to be determined at very low detection limits. It is a very reliable method for both clinical and experimental monitoring of gabapentin.