Akihisa Mori

Modulation of GABAergic transmission in the striatopallidal system by adenosine A2A receptors A potential mechanism for the antiparkinsonian effects of A2A antagonists

The selective localization of adenosine A2A receptors to the striatopallidal system suggested a new therapeutic approach to the management of Parkinson’s disease (PD). The results of behavioral studies using A2A receptor-specific agents in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys highlight the therapeutic potential of A2A antagonists as a novel treatment for PD. However, little is known about the role of A2A receptors in basal ganglia function or their pathophysiologic role in PD. Recently, the authors found that presynaptic A2A receptors modulate GABAergic synaptic transmission in the striatum and globus pallidus (GP), suggesting an A2A receptor-mediated dual modulation of the striatopallidal system. Striatal A2A receptors may increase the excitability of medium spiny neurons (MSNs) by modulating an intrastriatal GABAergic network. In addition, pallidal modulation occurs at striatopallidal MSN terminals located at the GP, enhancing GABA release onto GP projection neurons and directly suppressing their activity. Blockade of these modulatory functions by A2A antagonists could counteract excessive striatopallidal neuronal activity provoked by striatal dopamine depletion in patients with PD, leading to a reversal of parkinsonian motor deficits.

Adenosine A2A receptor enhances GABAA‐mediated IPSCs in the rat globus pallidus

1 The actions of adenosine A2A receptor agonists were examined on GABAergic synaptic transmission in the globus pallidus (GP) in rat brain slices using whole‐cell patch‐clamp recording. GP neurones were characterized into two major groups, type I and type II, according to the degree of time‐dependent hyperpolarization‐activated inward rectification and the size of input resistance. 2 The A2A receptor agonist 2‐[p‐(2‐carboxyethyl)phenethylamino]‐5′‐N‐ethylcarboxamido‐ adenosine (CGS21680; 0.3‐3 μm) enhanced IPSCs evoked by stimulation within the GP. The actions of CGS21680 were blocked by the A2A antagonists (E)‐8‐(3,4‐dimethoxystyryl)‐1,3‐dipropyl‐7‐methylxanthine (KF17837) and 4‐(2‐[7‐amino‐2‐(2‐furyl)[1,2,4]triazolo[2,3‐a][1,3,5]triazin‐5‐ylamino]ethyl)phenol (ZM241385). 3 The CGS21680‐induced increase in IPSCs was associated with a reduction in paired‐pulse facilitation. CGS21680 (0.3 μm) increased the frequency of miniature IPSCs (mIPSCs) without affecting mIPSC amplitude. These observations demonstrated that the enhancement of IPSCs in the GP was attributable to presynaptic, but not postsynaptic, A2A receptors. 4 The results suggest that A2A receptors in the GP serve to inhibit GP neuronal activity, thereby disinhibiting subthalamic nucleus neurone activity. Thus, the A2A receptor‐mediated presynaptic regulation in the GP, together with the A2A receptor‐mediated intrastriatal presynaptic control of GABAergic neurotransmission described previously, may play a crucial role in controlling the neuronal functions of basal ganglia. This A2A receptor‐mediated presynaptic dual control in the striatopallidal pathway could also afford the mode of action of A2A antagonists for ameliorating the symptoms of Parkinsons disease in an animal model.

Presynaptic adenosine A2A receptors enhance GABAergic synaptic transmission via a cyclic AMP dependent mechanism in the rat globus pallidus

We previously reported a presynaptic facilitatory action of A2A receptors on GABAergic synaptic transmission in the rat globus pallidus (GP). In the present study we identify the intracellular signalling mechanisms responsible for this facilitatory action of A2A receptors, using biochemical and patch‐clamp methods in rat GP slices. The adenosine A2A receptor selective agonist CGS21680 (1, 10 μM) and the adenylyl cyclase activator forskolin (1, 10 μM) both significantly increased cyclic AMP accumulation in GP slices. The CGS21680 (1 μM)‐mediated increase in cyclic AMP was inhibited by the A2A receptor selective antagonist KF17837 (10 μM). In an analysis of miniature inhibitory postsynaptic currents (mIPSCs), forskolin (10 μM) increased the mIPSC frequency without affecting their amplitude distribution, a result similar to that previously reported with CGS21680. The adenylyl cyclase inhibitor 9‐(tetrahydro‐2‐furanyl)‐9H‐purin‐6‐amine (SQ22,536, 300 μM) abolished the CGS21680‐induced enhancement in the frequency of mIPSCs. H‐89 (10 μM), a selective inhibitor for cyclic AMP‐dependent protein kinase (PKA), blocked the CGS21680‐induced enhancement of the mIPSC frequency. The calcium channel blocker CdCl2 (100 μM) did not prevent CGS21680 from increasing the frequency of mIPSCs. These results indicate that A2A receptor‐mediated potentiation of mIPSCs in the GP involves the sequential activation of the A2A receptor, adenylyl cyclase, and then PKA, and that this facilitatory modulation could occur independently of presynaptic Ca2+ influx.

Adenosine modulates the striatal GABAergic inputs to the globus pallidus via adenosine A2A receptors in rats.

Previous studies have shown presynaptic modulation of adenosine A(2A) receptors for GABAergic synaptic transmission in the globus pallidus (GP). The pallidal A(2A) receptor-mediated modulation is caused by an action on the terminals of striatopallidal medium spiny neurons (MSNs) and/or axon collaterals of GP neurons. Herein, we examined the precise target neurons of the A(2A) receptor-mediated modulation. Activation of A(2A) receptors enhanced striatopallidal GABAergic transmission onto GP neurons, accompanied by a reduction in the paired-pulse facilitation, indicating the presynaptic contribution of A(2A) receptors at terminals of striatopallidal MSNs in the GP. Besides, no A(2A) receptor mRNA was detected in GP neurons by single-cell reverse transcription-polymerase chain reaction analysis, implying no contribution of axon collaterals of GP neurons to the A(2A) receptor regulation. These results demonstrate that the target neurons of adenosinergic modulation via A(2A) receptors in the GP are the striatopallidal MSNs.

Distribution of adenosine A2A receptor antagonist KW-6002 and its effect on gene expression in the rat brain

Abstract A novel adenosine A2A receptor selective antagonist, KW-6002 [(E)-1,3-diethyl-8-(3,4-dimethoxystyryl)-7-methyl-3,7-dihydro-1H-purine-2,6-dione], possesses antiparkinsonian activities in rodent and primate models. In the present study, we investigated the distribution of [14C]KW-6002 in forebrain after oral administration at pharmacologically effective doses. Also, we monitored the effects of the compound on preproenkephalin (PPE) and preprotachykinin (PPT) gene expression in rat striatum. The highest level of radioactivity was observed in the striatum after oral administration of [14C]KW-6002; 30 min after 0.1 and 0.3 mg/kg, the density values in the striatum were 2.45 and 2.43 times higher than those in a reference region (frontal cortex), respectively. At the dose of 3 mg/kg, p.o., the ratio was only 1.58 and the compound was distributed more extensively in the brain. The distribution pattern and intensity of radioactivity were maintained even 90 min after the administration of [14C]KW-6002. Oral administration of KW-6002 (0.3 and 3 mg/kg/day) to rats for 14 days reversed the increased gene expression of PPE in striatum that had been depleted of dopamine by prior treatment with 6-hydroxydopamine (6-OHDA). On the other hand, KW-6002 did not alter the decreased gene expression of PPT in 6-OHDA-treated rats. These results are the first to show directly that orally administered KW-6002 is distributed selectively to the striatum and that it modulates the activity of striatopallidal enkephalin-containing neurons but not striatonigral substance P-containing neurons.

Two distinct glutamatergic synaptic inputs to striatal medium spiny neurones of neonatal rats and paired-pulse depression.

Excitatory postsynaptic currents (EPSCs) were recorded from the medium spiny neurones of neonatal rat striatal slices using the whole‐cell patch clamp method. EPSCs were selectively elicited in the presence of picrotoxin with a glass stimulating pipette placed in the striatum. We found two distinct unitary EPSCs that were evoked by stimulation of single presynaptic fibres. The major type of EPSC, termed ‘S‐type’, failed frequently and had a small mean amplitude (2.05 pA). They probably represented cortical afferents. The other type of unitary EPSC, the ‘H‐type’, seldom failed and was 13 times larger than the S‐type. Spontaneous EPSCs with amplitudes similar to those of H‐type EPSCs could be induced. H‐type EPSCs were mediated by both non‐NMDA and NMDA receptors. The two types of EPSCs could be evoked in the same neurons. The intensity of stimulation for H‐type EPSCs was higher than that for S‐type EPSCs. H‐type EPSCs could be polysynaptically activated, suggesting the presence of glutamatergic interneurones in the striatum that generated H‐type EPSCs. H‐type EPSCs displayed particularly long‐lasting paired‐pulse depression, while that displayed by the S‐type EPSCs was short. The paired‐pulse depression of both EPSCs was Ca2+ dependent and involved presynaptic mechanisms. We have demonstrated that the medium spiny neurones of neonatal rats receive two different glutamatergic input systems having different amplitudes, origins and paired‐pulse depression, reminiscent of cerebellar Purkinje cells. This suggests that the two types of EPSCs also play distinctive roles in striatal neuronal circuitry.

Population pharmacokinetic analysis of istradefylline in healthy subjects and in patients with Parkinson's disease.

This model‐based analysis quantifies the population pharmacokinetics (PK) of orally administered istradefylline, a selective adenosine A2A receptor antagonist, in healthy subjects and patients with Parkinsons disease, including the estimation of covariate effects on istradefylline PK parameters. Istradefylline plasma concentration data from 8 phase 1 and 8 phase 2/3 studies conducted in 1449 patients and normal, healthy volunteers aged from 18 to 87 years were best described by a 2‐compartment model with first‐order absorption parameterized in terms of apparent oral clearance (CL/F), apparent central volume of distribution (V2/F), apparent intercompartmental clearance (Q/F), apparent peripheral volume of distribution (V3/F) and a first‐order absorption rate‐constant (Ka). The typical population PK parameters were CL/F (5.76 L/h), V2/F (198 L), Q (21.6 L/h), V3/F (307 L), and Ka (0.464 h−1) for a 70‐kg, nonsmoking Caucasian who had 55.6 kg of lean body mass, no presence of CYP3A4 inhibitors, and unknown food status. Smoking and CYP3A4 inhibitors as concomitant medications were important predictors of istradefylline exposure. Istradefylline area under the concentration‐time curve at steady‐state increased 35% (95% confidence interval, 18%‐55%) in the presence of CYP3A4 inhibitors and decreased 38% (95% confidence interval, 26%‐50%) in smokers. The population PK model described the observed concentration data well and was deemed appropriate for further evaluation of the istradefylline exposure‐response relationship in patients with Parkinsons disease.

Quantal properties of H-type glutamatergic synaptic input to the striatal medium spiny neurons

The striatal medium spiny neuron receives two distinct glutamatergic synaptic inputs, S-type and H-type inputs, which have very different amplitudes. We report here that peak amplitudes of unitary H-type excitatory postsynaptic currents (EPSCs) could be reduced by decreasing external Ca2+, unlike S-type EPSCs. This suggests that H-type EPSCs are multiquantal, and supports the idea that the cellular origin of H-type EPSCs is distinct from that of the S-type EPSCs.

Physiology of Adenosine Receptors in the Striatum: Regulation of Striatal Projection Neurons

Publisher Summary This chapter focuses on A2A receptor modulation of inhibitory synaptic transmission and relates it to the more general role of adenosine in basal ganglia physiology. Along with this, a hypothetical mechanism of action of A2A receptor antagonists in the treatment of Parkinsons disease is also proposed. Considerable attention has been focused on the role of striatal adenosine and the effects caused by A2A receptor stimulation. This is not only due to the A2A receptor which is highly expressed in the striatum but also because of the findings that adenosine suppresses intrastriatal inhibitory synaptic transmission. The action of striatal adenosine on medium spiny neurons, A2A receptors have been shown to regulate and manage cholinergic interneurons. A2A receptor antagonists block the striatal A2A receptor-induced disinhibition among striatopallidal spiny cells, resulting in increased inhibition, thus suppressing excessive activation of indirect pathway and shifting the striatopallidal/striatonigral neuronal balance toward the normal state. The extensive works on striatal adenosine and basal ganglia physiology have introduced new insights into our understanding of the relationship between adenosine and motor control and suggest a new therapeutic approach for Parkinsons disease.

Quantal properties of S-type glutamatergic synaptic input to the striatal medium spiny neuron from neonate rat

We investigated the quantal properties of the small glutamatergic EPSPs (S-type EPSPs) in the striatal medium spiny neurons using the whole-cell recording method. Amplitudes of unitary glutamatergic EPSCs displayed a single peak at about 4.7 pA that was unaffected by decreasing external Ca2+. This indicates that afferent fibers contact the medium spiny neuron with only a few, possibly one, release sites which generate no more than one quantum of EPSCs at a time.