Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A_2A receptors (A_2AR) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A_2A receptors in the basal ganglia. At present it is believed that A_2AR antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A_2AR antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A_2A adenosine receptors. Thus, the development of heteromer-specific A_2A receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.     

亲代谢型谷氨酸受体配基对帕金森病大鼠的神经保护作用

目的探讨PD模型大鼠黑质致密部亲代谢型谷氨酸受体(mGluRs)的蛋白表达及其配基的药物治疗作用。方法6-羟基多巴单侧黑质损毁法建立大鼠PD模型。免疫组织化学法观察黑质致密部mGluR1a,2/3,4,5,8和酪氨酸羟化酶(TH)的表达,并用Nissl和Fluoro-JadeB荧光双染法在激光共集焦显微镜下观察退行性变的神经元。结果6-羟基多巴导致损毁侧mGluRs和TH免疫活性下降。Ⅰ组mGluRs拮抗剂和Ⅱ,Ⅲ组mGluRs激动剂能使治疗组相应的受体表达升高,尤其是mGluR5,mGluR2/3和mGluR4的蛋白表达。其中,以APDC组(Ⅱ组mGluRs激动剂)的保护效应最为显著。5个非假手术组退行性变细胞与正常细胞的比值(D/V)均有不同程度地增高。结论mGluRs可能参与PD的发生、发展过程。Ⅰ组mGluRs拮抗剂和Ⅱ组mGluRs激动剂具有一定的神经保护功能。     

Astrocytes restrict discharge duration and neuronal sodium loads during recurrent network activity

Influx of sodium ions into active neurons is a highly energy‐expensive process which must be strictly limited. Astrocytes could play an important role herein because they take up glutamate and potassium from the extracellular space, thereby dampening neuronal excitation. Here, we performed sodium imaging in mouse hippocampal slices combined with field potential and whole‐cell patch‐clamp recordings and measurement of extracellular potassium ([K⁺]_o). Network activity was induced by Mg²⁺‐free, bicuculline‐containing saline, during which neurons showed recurring epileptiform bursting, accompanied by transient increases in [K⁺]_o and astrocyte depolarizations. During bursts, neurons displayed sodium increases by up to 22 mM. Astrocyte sodium concentration increased by up to 8.5 mM, which could be followed by an undershoot below baseline. Network sodium oscillations were dependent on action potentials and activation of ionotropic glutamate receptors. Inhibition of glutamate uptake caused acceleration, followed by cessation of electrical activity, irreversible sodium increases, and swelling of neurons. The gliotoxin NaFAc (sodium‐fluoroacetate) resulted in elevation of astrocyte sodium concentration and reduced glial uptake of glutamate and potassium uptake through Na⁺/K⁺‐ATPase. Moreover, NaFAc extended epileptiform bursts, caused elevation of neuronal sodium, and dramatically prolonged accompanying sodium signals, most likely because of the decreased clearance of glutamate and potassium by astrocytes. Our experiments establish that recurrent neuronal bursting evokes sodium transients in neurons and astrocytes and confirm the essential role of glutamate transporters for network activity. They suggest that astrocytes restrict discharge duration and show that an intact astrocyte metabolism is critical for the neurons' capacity to recover from sodium loads during synchronized activity. GLIA 2015;63:936–957     

Oxytocin Reduces Cocaine Seeking and Reverses Chronic Cocaine-Induced Changes in Glutamate Receptor Function

Background:

Oxytocin, a neurohypophyseal neuropeptide, is a potential mediator and regulator of drug addiction. However, the cellular mechanisms of oxytocin in drug seeking remain unknown.

Methods:

In the present study, we used a self-administration/reinstatement model to study the effects of oxytocin on cocaine seeking and its potential interaction with glutamate function at the receptor level.

Results:

Systemic oxytocin dose-dependently reduced cocaine self-administration during various schedules of reinforcement, including fixed ratio 1, fixed ratio 5, and progressive ratio. Oxytocin also attenuated reinstatement to cocaine seeking induced by cocaine prime or conditioned cues. Western-blot analysis indicated that oxytocin increased phosphorylation of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptor GluA1 subunit at the Ser 845 site with or without accompanying increases in phosphorylation of extracellular signal-regulated kinase, in several brain regions, including the prefrontal cortex, bed nucleus of the stria terminalis, amygdala, and dorsal hippocampus. Immunoprecipitation of oxytocin receptor and GluA1 subunit receptors further demonstrated a physical interaction between these 2 receptors, although the interaction was not influenced by chronic cocaine or oxytocin treatment. Oxytocin also attenuated sucrose seeking in a GluA1- or extracellular-signal-regulated kinase-independent manner.

Conclusions:

These findings suggest that oxytocin mediates cocaine seeking through interacting with glutamate receptor systems via second messenger cascades in mesocorticolimbic regions.     

BDNF Release Is Required for the Behavioral Actions of Ketamine

Background:

Recent studies demonstrate that the rapid antidepressant ketamine increases spine number and function in the medial prefrontal cortex (mPFC), and that these effects are dependent on activation of glutamate α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors and brain-derived neurotrophic factor (BDNF). In vitro studies also show that activation of AMPA receptors stimulates BNDF release via activation of L-type voltage-dependent calcium channels (VDCC).

Methods:

Based on this evidence, we examined the role of BDNF release and the impact of L-type VDCCs on the behavioral actions of ketamine.

Results:

The results demonstrate that infusion of a neutralizing BDNF antibody into the mPFC blocks the behavioral effects of ketamine in the forced swim test (FST). In addition, we show that pretreatment with nifedipine or verapamil, two structurally-different L-type calcium channel antagonists, blocks the behavioral effects of ketamine in the FST. Finally, we show that ketamine treatment stimulates BDNF release in primary cortical neurons and that this effect is blocked by inhibition of AMPA receptors or L-type VDCCs.

Conclusions:

Taken together, these results indicate that the antidepressant effects of ketamine are mediated by activation of L-type VDCCs and the release of BDNF. They further elucidate the cellular mechanisms underlying this novel rapid-acting antidepressant.     

Local anesthetics inhibit glutamate release from rat cerebral cortex synaptosomes

Local anesthetics have been widely used for regional anesthesia and the treatment of cardiac arrhythmias. Recent studies have also demonstrated that low‐dose systemic local anesthetic infusion has neuroprotective properties. Considering the fact that excessive glutamate release can cause neuronal excitotoxicity, we investigated whether local anesthetics might influence glutamate release from rat cerebral cortex nerve terminals (synaptosomes). Results showed that two commonly used local anesthetics, lidocaine and bupivacaine, exhibited a dose‐dependent inhibition of 4‐AP‐evoked release of glutamate. The effects of lidocaine or bupivacaine on the evoked glutamate release were prevented by the chelation of extracellular Ca²⁺ ions and the vesicular transporter inhibitor bafilomycin A1. However, the glutamate transporter inhibitor dl ‐threo‐beta‐benzyl‐oxyaspartate did not have any effect on the action of lidocaine or bupivacaine. Both lidocaine and bupivacaine reduced the depolarization‐induced increase in [Ca²⁺]_C but did not alter 4‐AP‐mediated depolarization. Furthermore, the inhibitory effect of lidocaine or bupivacaine on evoked glutamate release was prevented by blocking the Ca_v2.2 (N‐type) and Ca_v2.1 (P/Q‐type) channels, but it was not affected by blocking of the ryanodine receptors or the mitochondrial Na⁺/Ca²⁺ exchange. Inhibition of protein kinase C (PKC) and protein kinase A (PKA) also prevented the action of lidocaine or bupivacaine. These results show that local anesthetics inhibit glutamate release from rat cortical nerve terminals. This effect is linked to a decrease in [Ca²⁺]_C caused by Ca²⁺ entry through presynaptic voltage‐dependent Ca²⁺ channels and the suppression of the PKA and PKC signaling cascades. Synapse 67:568–579, 2013 . © 2013 Wiley Periodicals, Inc.     

Protein interacting with C kinase and neurological disorders

Best known for its interaction with the α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor subunit GluA2 and for its influence on excitatory synapse activity, the protein interacting with C kinase, PICK1, is the focus of considerable attention from neurobiologists. Indeed, this PSD‐95/DlgA/ZO‐1 (PDZ) domain‐containing protein has been shown to interact with a wide variety of neurotransmitter receptors, transporters, and enzymes, including glutamate and nicotinic acetylcholine receptors, dopamine and glutamate transporters, and the enzyme serine racemase. Through its lipid binding domain, PICK1 is targeted to the inner surface of the cell membrane where it contributes to anchoring these partners and thereby influences their synaptic localization and function. Under pathological conditions, the regulation of some PICK1‐interacting partners is altered, pointing to an involvement of PICK1 in neurological disorders. Also, genetic or pharmacological manipulations of PICK1 expression, localization, or function have been shown to influence several physiological or pathological processes in which putative PICK1 partners are involved. This review will summarize recent experimental observations that highlight the involvement of PICK1 in neurological disorders, including schizophrenia, Parkinson's disease, epilepsy, chronic pain, drug abuse, and amyotrophic lateral sclerosis. Synapse 67:532–540, 2013. © 2013 Wiley Periodicals, Inc.     

A role for vesicular glutamate transporter 1 in synaptic vesicle clustering and mobility

Synaptic vesicles (SVs) from excitatory synapses carry vesicular glutamate transporters (VGLUTs) that fill the vesicles with neurotransmitter. Although the essential function of VGLUTs as glutamate transporters has been well established, the evidence for additional cell‐biological functions is more controversial. Both VGLUT1 and VGLUT2 disruptions in mice result in a reduced number of SVs away from release sites, flattening of SVs, and the appearance of tubular structures. Therefore, we analysed the morphology, biochemical composition and trafficking of SVs at synapses of VGLUT1^?/? mice in order to test for a function of VGLUTs in the formation or clustering of SVs. Analyses with high‐pressure freezing immobilisation and electron tomography pointed to a role of VGLUT1 transport function in the tonicity of excitatory SVs, explaining the aldehyde‐induced flattening of SVs observed in VGLUT1^?/? synapses. We confirmed the steep reduction in the number of SVs previously observed in VGLUT1^?/? presynaptic terminals, but did not observe accumulation of endocytotic intermediates. Furthermore, SV proteins of adult VGLUT1^?/? mouse brain tissue were expressed at normal levels in all subcellular fractions, suggesting that they were not displaced to another organelle. We thus assessed the mobility of the recently documented superpool of SVs. Synaptobrevin2–enhanced green fluorescent protein time lapse experiments revealed an oversized superpool of SVs in VGLUT1^?/? neurons. Our results support the idea that, beyond glutamate loading, VGLUT1 enhances the tonicity of excitatory SVs and stabilises SVs at presynaptic terminals.     

Cyto‐ and receptor architecture of area 32 in human and macaque brains

Human area 32 plays crucial roles in emotion and memory consolidation. It has subgenual (s32), pregenual (p32), dorsal, and midcingulate components. We seek to determine whether macaque area 32 has subgenual and pregenual subdivisions and the extent to which they are comparable to those in humans by means of NeuN immunohistochemistry and multireceptor analysis of laminar profiles. The macaque has areas s32 and p32. In s32, layer IIIa/b neurons are larger than those of layer IIIc. This relationship is reversed in p32. Layer Va is thicker and Vb thinner in s32. Area p32 contains higher kainate, benzodiazepine (BZ), and serotonin (5‐HT)_1A but lower N‐methyl‐D‐aspartate (NMDA) and α₂ receptor densities. Most differences were found in layers I, II, and VI. Together, these differences support the dual nature of macaque area 32. Comparative analysis of human and macaque s32 and p32 supports equivalences in cyto‐ and receptor architecture. Although there are differences in mean areal receptor densities, there are considerable similarities at the layer level. Laminar receptor distribution patterns in each area are comparable in the two species in layers III–Va for kainate, NMDA, γ‐aminobutyric acid (GABA)_B, BZ, and 5‐HT_1A receptors. Multivariate statistical analysis of laminar receptor densities revealed that human s32 is more similar to macaque s32 and p32 than to human p32. Thus, macaque 32 is more complex than hitherto known. Our data suggest a homologous neural architecture in anterior cingulate s32 and p32 in human and macaque brains. J. Comp. Neurol. 521:3272–3286, 2013. © 2013 Wiley Periodicals, Inc.